Monthly Archives: March 2020

Research on solar energy harvesting is one of the most active areas within chemistry, and photovoltaics technology is one of the main directions for turning solar energy into electricity. Today, three different generations of photovoltaics exist.(1,2) The third and most recent generation includes materials able to overcome the Shockley–Queisser limit (∼33%).(3,4) This generation includes singlet exciton fission photovoltaics, or shortly singlet fission, i.e., a process where one photon of light, absorbed by a molecule, is used to create two excitons of triplet multiplicity in two molecules or in two chromophores linked intramolecularly.(5,6)

In singlet fission, a molecule in its first singlet excited-state (S₁) kickbacks some of its energy, transferring it to a neighboring ground-state (S₀) molecule, and both end up in the lowest triplet state (T₁). For this process, one needs molecules with their lowest excited states arranged according to at least two key criteria: the S₁ state and second triplet excited (T₂)-state should have energies higher than twice that of the T₁ state, i.e., 2E(T₁) E(S₁) and 2E(T₁) E(T₂). The first criterion guarantees that the singlet fission is exergonic and the second that the T₁ + T₁ → T₂ annihilation is slow and endergonic.(5−7) Ideally, E(S₁) should also lie 0.1–0.2 eV above 2E(T₁). Desirably, the T₂ state should be at an energy higher than the S₁ state, i.e., E(S₁) E(T₂), so as to hamper intersystem crossing from S₁ to T₂.(8) Combined, this means that 2E(T₁) E(S₁) E(T₂) for a singlet fission chromophore. Finally, to maximize the efficiency in energy conversion, E(T₁) should preferably be close to or slightly above 1 eV as this facilitates triplet exciton transfer and charge injection from the molecule into a silicon electrode, which has a bandgap of 1.11 eV.(9) Such a process was recently accomplished by tetracene (E(T₁)=1.25 eV), enabling a combined exciton yield of 133% ± 13%.(10) This and several other recent studies reveal that efficient combinations of singlet exciton fission with semiconductor technology can be accomplished to achieve triplet exciton harvesting and charge injection into electrodes, inducing photocurrents.(11−15)

A number of aspects need to be taken into account for the enhanced design of singlet fission solar cells.(16−18) We need, for example, deepened mechanistic understanding of the exciton fission process, better control of intra- and intermolecular singlet fission rates, and improved understanding of how singlet fission chromophores interact with each other as well as with a semiconductor surface or quantum dot. To fit many different technological setups we should also compose a library of compounds that expands outside the acenes suitable for singlet fission, and chromophores with higher E(T₁) than the present ones should be identified.(17,18) However, how can further chromophores with the desired characteristics be discovered?

The search for materials suitable for singlet fission is not a straightforward task, particularly as the experimental determination of E(T₁) is less facile than for E(S₁). However, with the help of quantum chemical analyses, the search has evolved considerably over the last years.(8,19−27) The first experimental evidence of efficient singlet fission was observed for anthracene crystals, and research then expanded to tetracene and pentacene materials.(28−30) 1,3-Diphenylisobenzofuran (DPB) was the first compound observed to undergo singlet fission that had been predicted by theoretical modeling.(31,32)

Recently, Bronstein, Musser, Friend, and co-workers used for the first time the concept of T₁-state Baird-aromaticity(33−36) to analyze singlet fission chromophores.(37) They studied indolonaphthyridine thiophene (INDT) derivatives, which they described as influenced by a resonance structure with a 4π-electron Baird-aromatic cycle. On the basis of quantum chemical computations of nearly 10000 INDT derivatives, they concluded that a significant part of these are influenced by aromaticity to various extents in their T₁ states and have the appropriate relative arrangement of E(T₁) and E(S₁) to function in singlet fission photovoltaics. Significantly higher photostabilities of two INDT derivatives compared to TIPS-pentacene were also observed,(37) a feature of excited-state aromatic molecules with 4nπ-electron cycles earlier noted by Wan and Shukla for dibenz[b,f]oxepins.(38,39)

Now, what are the explicit scopes of T₁-state Baird-aromaticity for the design of singlet fission chromophores, and what are the limitations? It was argued by Ryerson et al.,(40) when analyzing the INDT compounds by Fallon et al.,(37) that the intermediate triplet-state aromaticity yielding the E(S₁)=2E(T₁) situation needs to be found through inspection of calculated adiabatic excitation energies. Thus, a qualitative tool should be desirable. Herein, we report on approaches for the correlated tuning of the E(T₁) and E(S₁) of Baird-aromatic chromophores by exploiting an earlier observed similarity in the electronic structures of the T₁ and S₁ states of Baird-aromatic molecules.(39,41) Are their ways by which E(T₁) and E(S₁) can be tuned predictably in a similar manner while E(T₂) is left unaffected or changes in another way than E(T₁) and E(S₁)? How are compounds with E(T₁) slightly above 1 eV identified, allowing for the combination of the singlet fission process with existing silicon solar cell technology? We develop a general design strategy by combining qualitative theory on excited-state Baird- and ground-state Hückel-aromaticity with quantum chemical computations. We arrive at approaches that could be applied broadly to identify a plethora of excited-state Baird-aromatic compounds and compound classes potentially suitable for singlet fission photovoltaics.

The validity of the design strategy is first examined on fulvenes (pentafulvenes), as they represent a compound class in which the (anti)aromatic character of both the S₀ state and the T₁ and S₁ states can be varied extensively.(45−49) Our computations were mostly run with DFT at (U)M06-2X and TD-M06-2X levels,(65) but CASPT2//CASSCF multireference computations were also performed.(66) We primarily used a computational scheme derived by Zeng, Hoffmann, and Ananth (ZHA), which uses adiabatic excitations to the T₁ state and vertical excitations to the S₁ (T₂) states from the S₀ (T₁) optimized structures. This approach gives the correct ordering of the T₁, S₁, and T₂ states of pentacene,(67) and it gives the E(S₁)/E(T₁) ratios for pentacene and DPB above or close to 2 (2.33 and 1.96 with M06-2X, see Table S1). However, at times, we also used either (i) vertically excited E(T₁), labeled as E(T₁)_v as opposed to the adiabatic ones labeled E(T₁)_a, or (ii) adiabatically excited E(S₁) labeled as E(S₁)_a. When evaluated against the experimental E(T₁) and E(S₁) of pentacene, tetracene, DPB, and the four fulvenes of Figure 2, it becomes clear that the approach with vertical E(T₁) and E(S₁) underestimates E(S₁)/E(T₁) when compared to experiments while the ZHA approach exaggerates the ratio. The extent of geometric relaxation in the S₁ versus T₁ states is important to take into consideration, as this varies between the compound classes.

Kaupp and co-workers previously used a TD-DFT-based protocol for the screening of singlet fission chromophores and, provided there is no evidence of multireference character, they proposed vertical excitation energies computed with local hybrid functionals.(68) However, M06-2X was found to perform similarly. For the T₁ energies and optimized geometries of substituted fulvenes, we recently observed a good agreement between CASPT2 and M06-2X,(49) and we therefore used (U)M06-2X. For method assessments on CBD, benzene, DPB, and pentacene, see the Supporting Information. In short, the T₁-, S₁-, and T₂-state orderings are the same with (TD-)M06-2X and CASPT2, and the E(S₁)_v/E(T₁)_a with the two methods using the ZHA scheme resemble each other. The (anti)aromatic character of fulvenes was determined through nucleus-independent chemical shifts (NICSs)(69,70) computed with the GIAO method(71) in the S₀ and T₁ states at optimized geometries. NICS calculations in the S₁ state are cumbersome, and we instead used the electronic multicenter index (MCI)(72) for selected compounds. The geometry-based HOMA index(73,74) is not ideal for small T₁ aromatic molecules (HOMA equals 0.73 for ³Cp⁺ and 0.45 for ³CBD, thus not reflecting the aromatic ideal of 1.0), and we avoid it for the fulvenes. In contrast, HOMA was used to assess the aromaticity of individual 4nπ-electrons rings of polycyclic compounds as the extent of aromaticity of individual rings in such systems cannot be determined by NICS as each ring is influenced by several diatropic or paratropic circuits.

Excited-state Baird-aromaticity in combination with Hückel ground-state aromaticity can be used in the design of singlet fission chromophores, but it is a double-edged sword. While several compounds that formally can be written with Baird-aromatic resonance structures fulfill the singlet fission criteria, the specific compound within a certain compound class that is most suitable, having an E(T₁) of ∼1 eV, is not always strongly T₁-state Baird-aromatic. In order to identify the compounds within a certain compound class that satisfy the singlet fission criteria and that have E(T₁) in a suitable energy range, we have developed a general and qualitative design model.

If the T₁ and S₁ states throughout a compound class are (i) dominated by the singly excited HOMO → LUMO configuration, (ii) HOMO and LUMO each are of the same character, and (iii) distributed spatially in the same manner then the energy difference between the two states equals twice the exchange integral, ΔE(S₁ – T₁)=2K_H,L (H=HOMO and L=LUMO). As a result, the relative energies of the two states are influenced by substituents (electronic or steric) and benzannelation to similar extents. With this, we develop a geometric model to identify a region along a tuning coordinate (an ∼aromaticity/antiaromaticity scale or increased benzannelation), within which 2E(T₁) E(S₁) (orange region, Figure 1). It is noteworthy that when E(T₁) goes below a certain threshold, the E(S₁) E(T₂) criterion generally becomes satisfied (Figures 5B and 13A). However, the model also has its limitations, the most notable being variations in K_H,L and thus ΔE(S₁ – T₁), among individual compounds in a compound class.

Taken together, our study outlines the scope, limitations, and complications in the usage of T₁-state Baird-aromaticity for the design of new singlet fission chromophores. The design approach and the qualitative (geometric) model developed provide a novel design rational. We show that through careful selection of (stable) compound classes influenced by Baird-aromaticity in T₁ to various extents and with suitable E(T₁) and 2K_H,L, one can tailor many new candidates for novel types of singlet fission chromophores.

All S₀– and T₁-optimized geometries were obtained using the M06-2X functional(65) and 6-311+G(d,p) basis set.(98) Additionally, for benzene and CBD, CASPT2 optimizations were carried out using the atomic natural orbital (ANO-RCC-VDZP) basis set with a contraction [B, C, N, O, F/3s2p1d, Si/4s3p2d, H/2s1p].(99−101) For vertical excitations of the substituted fulvenes, two methods were used: TD-M06-2X/def2-TZVPD//M06-2X/6-311+G(d,p) and CASPT2-SA3/ANO-RCC-VDZP//M06-2X/6-311+G(d,p) at the S₀– or T₁-optimized geometries. As a multiconfigurational indicator, the percentage of the perturbative triplet correction (T) to the total CCSD(T) atomization energy (%TAE_e[(T)])(102) was evaluated at the CCSD(T)/aug-cc-pVDZ//M06-2X/6-311+G(d,p) level. Spin–orbit coupling (SOC) elements were computed within the TD-M06-2X framework using PySOC(80) DFT, TD-DFT, and CCSD(T) calculations performed using Gaussian 16 revision B.01,(103) and CASPT2 computations were carried out with OpenMolcas packages.(101) Aromaticity was evaluated in terms of the nucleus-independent chemical shift (NICS),(69) ACID plots,(104) harmonic oscillator model of aromaticity (HOMA),(73,74) and multicenter index (MCI)(72) computed at the M06-2X/6-311+G(d,p) optimized geometries. NICS values were calculated at 1.0 Å above the ring centers (NICS(1)_zz)(70) using the gauge-independent atomic orbital (GIAO) method.(71) NICS-XY scans were performed using the Aroma package, scanning from 1.7 Å above the plane of the molecule.(105) Distinctly negative NICS(1)_zz values indicate the aromatic character, values close to one indicate the nonaromatic character, and distinctly positive values indicate the antiaromatic character. For six PAAHs, NICS-XY scans were also computed using B3LYP-(106) and CAM-B3LYP-(107) optimized geometries (see Figures S19 and S18). MCI is an electronic index that provides a measure of electron sharing among the atoms considered: the higher the MCI value, the more aromatic the ring. MCI values were retrieved with the ESI-3D program(108) using the overlaps between the occupied molecular orbitals obtained by AIMAll.(109) The diradical and tetraradical character have been calculated using the spin-projected spin-unrestricted Hartree–Fock (PUHF) proposed by Yamaguchi(110) (see more details in the Supporting Information).

The authors declare no competing financial interest.

This article references 110 other publications.

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   A review. Singlet fission is a photophys. reaction in which a singlet excited electronic state splits into two spin-triplet states. Singlet fission was discovered more than 50 years ago, but the interest in this process has gained a lot of momentum in the past decade due to its potential as a way to boost solar cell efficiencies. This review presents and discusses the most recent advances with respect to the theor. and computational studies on the singlet fission phenomenon. The work revisits important aspects regarding electronic states involved in the process, the evaluation of fission rates and interstate couplings, the study of the excited state dynamics in singlet fission, and the advances in the design and characterization of singlet fission compds. and materials such as mol. dimers, polymers, or extended structures. Finally, the review tries to pinpoint some aspects that need further improvement and proposes future lines of research for theor. and computational chemists and physicists in order to further push the understanding and applicability of singlet fission.

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   A review. A theory of singlet fission is discussed along with its relation to photovoltaics. The literature is reviewed dealing with singlet fission in org. chromophores from the time of initial discovery of singlet fission to the present day.

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   A review. A survey is provided of recent progress in the understanding of singlet fission, a spin-allowed process in which a singlet excited mol. shares its energy with a ground-state neighbor to produce two triplet excited mols. It has been obsd. to occur in single-crystal, polycryst., and amorphous solids, on timescales from 80 fs to 25 ps, producing triplet yields as high as 200%. Photovoltaic devices using the effect have shown external quantum efficiencies in excess of 100%. Almost all the efficient materials are alternant hydrocarbons of the acene series or their simple derivs., and it is argued that a wider structural variety would be desirable. The current state of the development of mol. structure design rules, based on first-principles theor. considerations, is described along with initial examples of implementation.

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    Silicon dominates contemporary solar cell technologies1. But when absorbing photons, silicon (like other semiconductors) wastes energy in excess of its bandgap2. Reducing these thermalization losses and enabling better sensitivity to light is possible by sensitizing the silicon solar cell using singlet exciton fission, in which two excited states with triplet spin character (triplet excitons) are generated from a photoexcited state of higher energy with singlet spin character (a singlet exciton)3-5. Singlet exciton fission in the mol. semiconductor tetracene is known to generate triplet excitons that are energetically matched to the silicon bandgap6-8. When the triplet excitons are transferred to silicon they create addnl. electron-hole pairs, promising to increase cell efficiencies from the single-junction limit of 29 per cent to as high as 35 per cent9. Here we reduce the thickness of the protective hafnium oxynitride layer at the surface of a silicon solar cell to just eight angstroms, using elec.-field-effect passivation to enable the efficient energy transfer of the triplet excitons formed in the tetracene. The max. combined yield of the fission in tetracene and the energy transfer to silicon is around 133 per cent, establishing the potential of singlet exciton fission to increase the efficiencies of silicon solar cells and reduce the cost of the energy that they generate.

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    2679
    ISSN: .

    The efficiency of a conventional solar cell may be enhanced if one incorporates a molecular material capable of singlet fission, that is, the production of two triplet excitons from the absorption of a single photon. To implement this, we need to successfully harvest the two triplets from the singlet fission material. Here we show in the tetracene (Tc)/copper phthalocyanine (CuPc) model system that triplets produced from singlet fission in the former can transfer to the later on the timescale of 45±5 ps. However, the efficiency of triplet energy transfer is limited by a loss channel due to faster formation (400±100 fs) and recombination (2.6±0.5 ps) of charge transfer excitons at the interface. These findings suggest a design principle for efficient energy harvesting from singlet fission: one must reduce interfacial area between the two organic chromophores to minimize charge transfer/recombination while optimizing light absorption, singlet fission and triplet rather than singlet transfer.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A280%3ADC%252BC2c%252FoslahtQ%253D%253D&md5=20b2d7d911943ab83aaf3a5f09f33aee

13. 13

    Allardice, J. R.; Thampi, A.; Dowland, S.; Xiao, J.; Gray, V.; Zhang, Z.; Budden, P.; Petty, A. J., II; Davis, N. J. K. L.; Greenham, N. C.; Anthony, J. E.; Rao, A. Engineering Molecular Ligand Shells on Quantum Dots for Quantitative Harvesting of Triplet Excitons Generated by Singlet Fission. J. Am. Chem. Soc. 2019, 141, 12907– 12915, DOI: 10.1021/jacs.9b06584

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    13

    Engineering Molecular Ligand Shells on Quantum Dots for Quantitative Harvesting of Triplet Excitons Generated by Singlet Fission

    Allardice, Jesse R.; Thampi, Arya; Dowland, Simon; Xiao, James; Gray, Victor; Zhang, Zhilong; Budden, Peter; Petty, Anthony J., II; Davis, Nathaniel J. L. K.; Greenham, Neil C.; Anthony, John E.; Rao, Akshay

    Journal of the American Chemical Society
    (2019),
    141
    (32),
    12907-12915CODEN:
    JACSAT;
    ISSN: 0002-7863.

    (American Chemical Society)

    Singlet fission is an exciton multiplication process in org. mols. in which a photogenerated spin-singlet exciton is rapidly and efficiently converted to two spin-triplet excitons. This process offers a mechanism to break the Shockley-Queisser limit by overcoming the thermalization losses inherent to all single-junction photovoltaics. One of the most promising methods to harness the singlet fission process is via the efficient extn. of the dark triplet excitons into quantum dots (QDs) where they can recombine radiatively, thereby converting high-energy photons to pairs of low-energy photons, which can then be captured in traditional inorg. PVs such as Si. Such a singlet fission photon multiplication (SF-PM) process could increase the efficiency of the best Si cells from 26.7% to 32.5%, breaking the Shockley-Queisser limit. However, there has been no demonstration of such a singlet fission photon multiplication (SF-PM) process in a bulk system to date. Here, we demonstrate a soln.-based bulk SF-PM system based on the singlet fission material TIPS-Tc combined with PbS QDs. Using a range of steady-state and time-resolved measurements combined with anal. modeling we study the dynamics and mechanism of the triplet harvesting process. We show that the system absorbs>95% of incident photons within the singlet fission material to form singlet excitons, which then undergo efficient singlet fission in the soln. phase (135 ± 5%) before quant. harvesting of the triplet excitons (95 ± 5%) via a low concn. of QD acceptors, followed by the emission of IR photons. We find that in order to achieve efficient triplet harvesting it is crit. to engineer the surface of the QD with a triplet transfer ligand and that bimol. decay of triplets is potentially a major loss pathway which can be controlled via tuning the concn. of QD acceptors. We demonstrate that the photon multiplication efficiency is maintained up to solar fluence. Our results establish the soln.-based SF-PM system as a simple and highly tunable platform to understand the dynamics of a triplet energy transfer process between org. semiconductors and QDs, one that can provide clear design rules for new materials.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXhsVemu77F&md5=0035163d83d3d8f1bb3a23955ce078fd

14. 14

    Pun, A. B.; Sanders, S. N.; Kumarasamy, E.; Sfeir, M. Y.; Congreve, D. N.; Campos, L. M. Triplet Harvesting from Intramolecular Singlet Fission in Polytetracene. Adv. Mater. 2017, 29, 1701416, DOI: 10.1002/adma.201701416

15. 15

    Castrucci, J. S.; Josey, D. S.; Thibau, E.; Lu, Z.-H.; Bender, T. P. Boron Subphthalocyanines as Triplet Harvesting Materials within Organic Photovoltaics. J. Phys. Chem. Lett. 2015, 6, 3121– 3125, DOI: 10.1021/acs.jpclett.5b01254

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    15

    Boron subphthalocyanines as triplet harvesting materials within org. photovoltaics

    Castrucci, Jeffrey S.; Josey, David S.; Thibau, Emmanuel; Lu, Zheng-Hong; Bender, Timothy P.

    Journal of Physical Chemistry Letters
    (2015),
    6
    (15),
    3121-3125CODEN:
    JPCLCD;
    ISSN: 1948-7185.

    (American Chemical Society)

    Singlet fission, the generation of two excited triplet states from a single absorbed photon, is currently an area of significant interest to photovoltaic researchers. In this Letter, we outline how a polychlorinated boron subphthalocyanine, previously hypothesized to be an effective harvester of singlet fission derived triplets from pentacene, is relatively efficient at facilitating the process. As expected, we found a major increase in photocurrent generation at the expense of device voltage. For a direct point of comparison, we also have paired the same polychlorinated boron subphthalocyanine with α-sexithiophene to probe the alternative technique of complementary absorption engineering. The sum of these efforts have let us present new guidelines for the mol. design of boron subphthalocyanine for org. photovoltaic applications.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXht1SgsbrM&md5=f21988bdfb17ab3c9f745414b70f711c

16. 16

    Gish, M. K.; Pace, N. A.; Rumbles, G.; Johnson, J. C. Emerging Design Principles for Enhanced Solar Energy Utilization with Singlet Fission. J. Phys. Chem. C 2019, 123, 3923– 3934, DOI: 10.1021/acs.jpcc.8b10876

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    16

    Emerging Design Principles for Enhanced Solar Energy Utilization with Singlet Fission

    Gish, Melissa K.; Pace, Natalie A.; Rumbles, Garry; Johnson, Justin C.

    Journal of Physical Chemistry C
    (2019),
    123
    (7),
    3923-3934CODEN:
    JPCCCK;
    ISSN: 1932-7447.

    (American Chemical Society)

    Singlet fission (SF), the generation of two triplet excitons per the absorption of one photon, is a promising strategy for increasing the efficiency of solar cells beyond the theor. Shockley-Queisser limit of 34%. Upon photon absorption by a SF mol., the initially created singlet excited state (S1) interacts with a neighboring chromophore and is first transformed into a triplet pair (TT), which can be subsequently sepd. into independent triplet excitons (2T1). These independent triplet excitons can be harvested through triplet charge extn. or triplet energy transfer to an acceptor. Research on SF systems has revealed rates and efficiencies of triplet formation and triplet pair decorrelation that are strongly dependent on interchromophore coupling, which is dictated by mol. structure and the resulting geometrical arrangement of chromophores adopted in covalent (e.g., dimers) and noncovalent (e.g., films and crystals) systems. Incorporation of SF materials into realistic device architectures introduces a host of new challenges to consider regarding the efficient extn. of triplets generated through SF. In this Feature Article, we review our work that has led to some degree of understanding and control of inter- and intramol. SF rates placed in the context of solar energy harvesting architectures, including dye-sensitized solar cells, conjugated polymer films, and ligand-exchanged quantum dots. We emphasize the importance of understanding and manipulating interactions between SF mols. with each other and with the charge or energy collectors across an interface in order to strike a kinetic balance that leads to efficient utilization of triplet excitons.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXitFGgsL0%253D&md5=d2e9383954aec4be5cb9c269b4455ec9

17. 17

    Krishnapriya, K. C.; Musser, A. J.; Patil, S. Molecular Design Strategies for Efficient Intramolecular Singlet Exciton Fission. ACS Energy Lett. 2019, 4, 192– 202, DOI: 10.1021/acsenergylett.8b01833

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    17

    Molecular Design Strategies for Efficient Intramolecular Singlet Exciton Fission

    Krishnapriya, K. C.; Musser, Andrew J.; Patil, Satish

    ACS Energy Letters
    (2019),
    4
    (1),
    192-202CODEN:
    AELCCP;
    ISSN: 2380-8195.

    (American Chemical Society)

    The process of carrier multiplication via singlet fission can potentially exceed the Shockley-Queisser limit on the efficiency of single-junction photovoltaics. In the recent past, theor. anal. provided the principal guidelines on mol. design strategies for singlet fission. In this Perspective, we focus instead on correlating exptl. results for different classes of reported singlet fission materials to identify principles to aid in the design of new mols. for efficient intramol. singlet fission. Building on an evaluation of several series of multichromophoric and polymeric singlet fission materials, we ext. new suggested strategies for mol. design.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXitlWhtL%252FK&md5=d36cf31ac171611a9e6cf6c8ec1a1189

18. 18

    Basel, B. S.; Papadopoulos, I.; Thiel, D.; Casillas, R.; Zirzlmeier, J.; Clark, T.; Guldi, D. M.; Tykwinski, R. R. Pentacenes: A Molecular Ruler for Singlet Fission. Trends in Chem. 2019, 1, 11– 21, DOI: 10.1016/j.trechm.2019.02.002

19. 19

    Ito, S.; Nagami, T.; Nakano, M. Molecular Design for Efficient Singlet Fission. J. Photochem. Photobiol., C 2018, 34, 85– 120, DOI: 10.1016/j.jphotochemrev.2018.01.002

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    19

    Molecular design for efficient singlet fission

    Ito, Soichi; Nagami, Takanori; Nakano, Masayoshi

    Journal of Photochemistry and Photobiology, C: Photochemistry Reviews
    (2018),
    34
    (),
    85-120CODEN:
    JPPCAF;
    ISSN: 1389-5567.

    (Elsevier B.V.)

    A review. Singlet fission is a photophys. process in mols. and mol. aggregates, in which a singlet exciton generated by irradn. splits into two triplet excitons. Recently, singlet fission has attracted a great deal of attention from the viewpoint of applications in org. photovoltaic cells, where singlet fission has a possibility of improving the photoelec. conversion efficiency. Although singlet fission was first obsd. about 50 years ago in anthracene crystals, and the mechanism has been investigated in detail for a small no. of mol. systems such as tetracene and pentacene crystals, the relationships between mol. or crystal structures and singlet fission efficiency are yet to be precisely clarified. Thus, mol. structure – singlet fission relationships and mol. or crystal design guidelines for efficient singlet fission are intensely desired for realizing efficient photovoltaic energy conversion, and exptl. and theor. investigations advance rapidly. We introduce three investigation steps, which are based on bottom-up theor. modeling from a mol. to a mol. aggregate or crystal. The modeling involves energy level matching at the mol. level, electronic coupling at the aggregate level, and singlet fission dynamics including exciton-phonon (vibronic) coupling, by emphasizing the importance of interplay between each step. From the modeling, we present several design guidelines for efficient singlet fission, which is together with practical mol. structures, chem. modifications and mol. configurations.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXivFCku74%253D&md5=4f57ac0f40a9bc49a0b6f79ad9533fb9

20. 20

    Minami, T.; Ito, S.; Nakano, M. Fundamental of Diradical-Character-Based Molecular Design for Singlet Fission. J. Phys. Chem. Lett. 2013, 4, 2133– 2137, DOI: 10.1021/jz400931b

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    20

    Fundamental of Diradical-Character-Based Molecular Design for Singlet Fission

    Minami, Takuya; Ito, Soichi; Nakano, Masayoshi

    Journal of Physical Chemistry Letters
    (2013),
    4
    (13),
    2133-2137CODEN:
    JPCLCD;
    ISSN: 1948-7185.

    (American Chemical Society)

    The fundamental of diradical-character-based mol. design for singlet fission is clarified through the correlation between the diradical character, the first singlet (S1) and triplet (T1) excitation energies, the frontier orbital energy gap, and the energy level matching condition (2E(T1) – E(S1) ≈ 0 or

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXptlCgsr4%253D&md5=4395e6adc01babfe1515a66d311512d7

21. 21

    Zeng, T.; Goel, P. Design of Small Intramolecular Singlet Fission Chromophores: An Azaborine Candidate and General Small Size Effects. J. Phys. Chem. Lett. 2016, 7, 1351– 1358, DOI: 10.1021/acs.jpclett.6b00356

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    21

    Design of Small Intramolecular Singlet Fission Chromophores: An Azaborine Candidate and General Small Size Effects

    Zeng, Tao; Goel, Prateek

    Journal of Physical Chemistry Letters
    (2016),
    7
    (7),
    1351-1358CODEN:
    JPCLCD;
    ISSN: 1948-7185.

    (American Chemical Society)

    We report the first attempt to design small intramol. singlet fission chromophores, with the aid of quantum chem. and explicitly simulating the time evolution of state populations using quantum dynamics method. We start with three previously proposed azaborine-substituted intermol. singlet fission chromophores. Through analyzing their frontier orbital amplitudes, we select a BN-substituted azulene as the building block. Covalently connecting two such monomers and tuning their relative configuration, we examine three dimers. One dimer is found to be an eminent candidate: the triplet-pair state is quickly formed within 1 silicon::polycryst.,, and the two triplets are ready to be disentangled. We elucidate the general small size effects in intramol. singlet fission and focus on specific aspects which should be taken care of when manipulating the fission rate through steric hindrance.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XkslGqt70%253D&md5=bdb0554f3fc3eb17b45c7664b3d8ebb1

22. 22

    Match, C.; Perkins, J.; Schreckenbach, G. Simple Computational Screening of Potential Singlet Fission Molecules. Theor. Chem. Acc. 2018, 137, 109, DOI: 10.1007/s00214-018-2290-4

23. 23

    Nakano, M. Open-Shell-Character-Based Molecular Design Principles: Applications to Nonlinear Optics and Singlet Fission. Chem. Rec. 2017, 17, 27– 62, DOI: 10.1002/tcr.201600094

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    23

    Open-Shell-Character-Based Molecular Design Principles: Applications to Nonlinear Optics and Singlet Fission

    Nakano, Masayoshi

    Chemical Record
    (2017),
    17
    (1),
    27-62CODEN:
    CRHEAK;
    ISSN: 1528-0691.

    (Wiley-VCH Verlag GmbH & Co. KGaA)

    Open-shell character, e.g., diradical character, is a quantum chem. well-defined quantity in ground-state mol. systems, which is not an observable but can quantify the degree of effective bond weakness in the chem. sense or electron correlation strength in the phys. sense. Because this quantity also correlates to specific excited states, physicochem. properties concerned with those states are expected to strongly correlate to the open-shell character. This feature enables us to open a new path to revealing the mechanism of these properties as well as to realizing new design principles for efficient functional mol. systems. This account explains the open-shell-character-based mol. design principles and introduces their applications to the rational design of highly efficient nonlinear optical and singlet fission mol. systems.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XhtlShurfE&md5=1b07f1a1af532cc83425079f045d2cdc

24. 24

    Ito, S.; Nakano, M. Theoretical Molecular Design of Heteroacenes for Singlet Fission: Tuning the Diradical Character by Modifying π-Conjugation Length and Aromaticity. J. Phys. Chem. C 2015, 119, 148– 157, DOI: 10.1021/jp5103737

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    24

    Theoretical Molecular Design of Heteroacenes for Singlet Fission: Tuning the Diradical Character by Modifying π-Conjugation Length and Aromaticity

    Ito, Soichi; Nakano, Masayoshi

    Journal of Physical Chemistry C
    (2015),
    119
    (1),
    148-157CODEN:
    JPCCCK;
    ISSN: 1932-7447.

    (American Chemical Society)

    A theor. mol. design for efficient singlet fission (SF) is performed for several heteroacene models involving nitrogen (N) atoms based on the diradical character criterion of the energy level matching conditions. This criterion is found to be closely related to the relative contributions of diradical and zwitterionic resonance structures of the heteroacenes, i.e., the aromaticity of the central ring(s). From the anal. of the diradical characters of these heteroacene models, the increase in the aromaticity of the central ring(s) is found to prefer the diradical form to the zwitterionic form. From the comparison of the excitation energies evaluated by multireference second-order perturbation theory calcns., two promising candidates, chosen based on the diradical character criterion, are found to satisfy the energy level matching conditions and to possess high triplet energies of ∼1.1 eV, which are suitable for an application in org. photovoltaic cells. The proposed two candidates are shown to have mutually different types of the first excited singlet states, which are distinguished by the primary excitation configurations. These results suggest that the proposed two candidates exhibit different singlet fission dynamics due to the different amplitude of the electronic coupling.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXitV2ju7fE&md5=ae8b1b61f3c332de221cecaede8c7113

25. 25

    Zeng, T.; Ananth, N.; Hoffmann, R. Seeking Small Molecules for Singlet Fission: A Heteroatom Substitution Strategy. J. Am. Chem. Soc. 2014, 136, 12638– 12647, DOI: 10.1021/ja505275m

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    25

    Seeking Small Molecules for Singlet Fission: A Heteroatom Substitution Strategy

    Zeng, Tao; Ananth, Nandini; Hoffmann, Roald

    Journal of the American Chemical Society
    (2014),
    136
    (36),
    12638-12647CODEN:
    JACSAT;
    ISSN: 0002-7863.

    (American Chemical Society)

    We design theor. small mol. candidates for singlet fission chromophores, aiming to achieve a balance between sufficient diradical character and kinetic persistence. We develop a perturbation strategy based on the captodative effect to introduce diradical character into small π-systems. Specifically, this can be accomplished by replacing pairs of not necessarily adjacent C atoms with isoelectronic and isosteric pairs of B and N atoms. Three rules of thumb emerge from our studies to aid further design: (i) Lewis structures provide insight into likely diradical character; (ii) formal radical centers of the diradical must be well-sepd.; (iii) stabilization of radical centers by a donor (N) and an acceptor (B) is essential. Following the rules, we propose candidate mols. Employing reliable multireference calcns. for excited states, we identify three likely candidate mols. for SF chromophores. These include a benzene, a naphthalene, and an azulene, where four C atoms are replaced by a pair of B and a pair of N atoms.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhsVSnsrfP&md5=252ab72cf2c676144b0f4e7d0d05f51f

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    Ito, S.; Minami, T.; Nakano, M. Diradical Character Based Design for Singlet Fission of Condensed-Ring Systems with 4nπ Electrons. J. Phys. Chem. C 2012, 116, 19729– 19736, DOI: 10.1021/jp3072684

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    26

    Diradical Character Based Design for Singlet Fission of Condensed-Ring Systems with 4nπ Electrons

    Ito, Soichi; Minami, Takuya; Nakano, Masayoshi

    Journal of Physical Chemistry C
    (2012),
    116
    (37),
    19729-19736CODEN:
    JPCCCK;
    ISSN: 1932-7447.

    (American Chemical Society)

    By applying the diradical character based mol. design guideline for singlet fission (SF), we investigate the feasibility of efficient SF in condensed-ring π-conjugated mols. with 4nπ electrons (n=4, 5, …), i.e., antiarom. polycyclic hydrocarbons composed of five- and six-membered rings. The multiple diradical character (yi), which takes a value between 0 (closed shell) and 1 (pure open shell), is defined as the occupation no. of the lowest unoccupied natural orbital (LUNO) + i (i=0, 1, …) calcd. using the approx. spin-projected spin-UHF method. The excitation energies are also evaluated using the tuned long-range cor. time-dependent d. functional theory method with the Tamm-Dancoff approxn. to examine the energy level matching conditions for SF: (i) 2E(T1) – E(S1) ∼ 0 or ≤ 0 and (ii) 2E(T1) – E(T2)

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38Xht1KmsL7E&md5=71e724b13bea11298ec80ba071215c44

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    Chen, Y.; Shen, L.; Li, X. Effects of Heteroatoms of Tetracene and Pentacene Derivatives on Their Stability and Singlet Fission. J. Phys. Chem. A 2014, 118, 5700– 5708, DOI: 10.1021/jp503114b

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    Effects of Heteroatoms of Tetracene and Pentacene Derivatives on Their Stability and Singlet Fission

    Chen, Yuhan; Shen, Li; Li, Xiyou

    Journal of Physical Chemistry A
    (2014),
    118
    (30),
    5700-5708CODEN:
    JPCAFH;
    ISSN: 1089-5639.

    (American Chemical Society)

    The effects of the introduction of an sp2-hybridized nitrogen atom (=N-) and thiophene ring on the structure geometries, frontier MO energies, and excited state energies related to singlet fission (SF) for some tetracene and pentacene derivs. were theor. investigated by quantum chem. methods. The introduction of a nitrogen atom significantly decreases the energies of frontier MOs and hence improves their stabilities in air and light illumination. More importantly, it is helpful for reducing the energy loss of the exothermic singlet fission of pentacene derivs. For fused benzene-thiophene structures, the (α, β) connection pattern could stabilize the frontier MOs, while the (β, β) connection pattern can promote the thermodn. driving force of singlet fission. These facts provide a theor. ground for rational design of SF materials.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhtFens7bN&md5=2e6ee780697588cecb98355257bf03d9

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    Singh, S.; Jones, W. J.; Siebrand, W.; Stoicheff, B. P.; Schneider, W. G. Laser Generation of Excitons and Fluorescence in Anthracene Crystals. J. Chem. Phys. 1965, 42, 330– 342, DOI: 10.1063/1.1695695

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    Laser generation of excitons and fluorescence in anthracene crystals

    Singh, S.; Jones, W. J.; Siebrand, W.; Stoicheff, B. P.; Schneider, W. G.

    Journal of Chemical Physics
    (1965),
    42
    (1),
    330-42CODEN:
    JCPSA6;
    ISSN: 0021-9606.

    Exptl. and theoretical studies are reported of the short-lived and delayed fluorescence of anthracene single crystals, excited by single- and double-proton absorption. A giant-pulse ruby laser provides the primary source of radiation of 14,400 cm.-1 (up to 1027 photons/cm.2 sec.) and is also used to generate 2nd-harmonic radiation from adenosine 5′-diphosphate, as well as stimulated Raman radiation of 12,800 and 17,500 cm.-1 from liquid O. The time dependence of the fluorescence intensity is studied as a function of laser intensity, crystal temp., and excitation wavelength. The very intense fast fluorescence with a half-life of 30 nsec. at 300°K., characteristic of singlet exciton decay, and the relatively weak delayed fluorescence which involves intermediate triplet states, are sepd. by using sectored disks. The triplet state at 14,750 cm.-1 can be populated (1) by direct absorption of laser photons, involving an activation energy of 350 cm.-1; (2) via 2-photon absorption, presumably leading to a vibrationally excited state of the 1B2u exciton, followed by intersystem crossing; (3) via 1-photon (2nd-harmonic) excitation from levels ≥700 cm.-1 into the singlet absorption band, followed by conversion of the singlet exciton into a triplet pair. The latter process is suggested by the observed activation energy of 700 cm.-1 In agreement with these interpretations, the delayed fluorescence intensity varies with the 2nd to 4th power of the laser intensity, depending on the exptl. conditions. Also, light of 17,500 cm.-1 leads exclusively to process (1), light of 12,800 cm.-1 exclusively to (2). Triplet lifetimes from 2-17 msec. are obtained, depending on crystal purity, which indicates that unimol. triplet decay is an extrinsic, radiationless process. A singlet-triplet intersystem crossing rate const. of ∼3 × 10-5 sec.-1 is estd. The triplet-triplet annihilation rate const. is ∼5 × 10-11 cm.3 sec.-1 This value, considered together with the triplet-pair creation process, suggests a triplet exchange rate ⪆1013 sec.-1 and a triplet diffusion const. ⪆5 × 10-4 cm.2/sec.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaF2MXhvFyjtQ%253D%253D&md5=7a939ac766c41d53bcc49f62422007a6

29. 29

    Groff, R. P.; Avakian, P.; Merrifield, R. E. Coexistence of Exciton Fission and Fusion in Tetracene Crystals. Phys. Rev. B 1970, 1, 815– 817, DOI: 10.1103/PhysRevB.1.815

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    Burgos, J.; Pope, M.; Swenberg, Ch. E.; Alfano, R. R. Heterofission in Pentacene-Doped Tetracene Single Crystals. Phys. Status Solidi B 1977, 83, 249– 256, DOI: 10.1002/pssb.2220830127

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    Heterofission in pentacene-doped tetracene single crystals

    Burgos, J.; Pope, M.; Swenberg, C. E.; Alfano, R. R.

    Physica Status Solidi B: Basic Research
    (1977),
    83
    (1),
    249-56CODEN:
    PSSBBD;
    ISSN: 0370-1972.

    The magnetic field and the temp. dependence of both guest and host fluorescence in pentacene-doped tetracene single crystals was studied. From an anal. of the exptl. results pentacene enters substitutionally into the host lattice sites and thermally activated heterofission is site-dependent with activation energies ≈ (0.13 ± 0.01) eV and .ltorsim.0.06 eV. From a kinetic fit to the temp. dependence of the green and red fluorescence, a thermally activated host singlet exciton diffusion coeff. of ≈ 2 × 10-2 cm2/s is inferred; its activation energy is (0.018 ± 0.001) eV. The heterofission rate of ≈ (4.7 ± 0.5) × 10-10 cm3 s-1 was detd. and location of the pentacene triplet level is found at (0.86 ± 0.03) eV.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaE2sXlslGjtL0%253D&md5=934556b5642fb2fe13cd77706d3b48c9

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    Johnson, J. C.; Nozik, A. J.; Michl, J. High Triplet Yield from Singlet Fission in a Thin Film of 1,3-Diphenylisobenzofuran. J. Am. Chem. Soc. 2010, 132, 16302– 16303, DOI: 10.1021/ja104123r

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    High Triplet Yield from Singlet Fission in a Thin Film of 1,3-Diphenylisobenzofuran

    Johnson, Justin C.; Nozik, Arthur J.; Michl, Josef

    Journal of the American Chemical Society
    (2010),
    132
    (46),
    16302-16303CODEN:
    JACSAT;
    ISSN: 0002-7863.

    (American Chemical Society)

    Direct observation of triplet absorption and ground-state depletion upon pulsed excitation of a polycryst. thin solid film of 1,3-diphenylisobenzofuran at 77 K revealed a 200 ± 30% triplet yield, which was attributed to singlet fission.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXhtlOnsLvM&md5=e715b469a3c20c180c8fa1819457801e

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    Schrauben, J. N.; Ryerson, J. L.; Michl, J.; Johnson, J. C. Mechanism of Singlet Fission in Thin Films of 1,3-Diphenylisobenzofuran. J. Am. Chem. Soc. 2014, 136, 7363– 7373, DOI: 10.1021/ja501337b

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    32

    Mechanism of Singlet Fission in Thin Films of 1,3-Diphenylisobenzofuran

    Schrauben, Joel N.; Ryerson, Joseph L.; Michl, Josef; Johnson, Justin C.

    Journal of the American Chemical Society
    (2014),
    136
    (20),
    7363-7373CODEN:
    JACSAT;
    ISSN: 0002-7863.

    (American Chemical Society)

    In order to elucidate the mechanism of singlet fission in thin films of 1,3-diphenylisobenzofuran (1) the authors have performed ultrafast transient absorption spectroscopy as a function of sample temp. and excitation fluence on polycryst. thin films composed of two polymorphs. The authors earlier investigations revealed that films enriched in a particular polymorph of 1 displayed near 200% efficiency for triplet formation at 77 K, while films composed primarily of a second polymorph had a very low triplet quantum yield. Present data confirm the triplet yield disparities in the two polymorphs and demonstrate the distinct fates of the initially prepd. singlets in films of different structure. Singlet fission is inhibited in the more stable polymorph due to rapid excimer formation and trapping. The less stable polymorph undergoes highly efficient singlet fission with a dominant time const. of 10-30 ps and without strong thermal activation. Transient absorption measurements with varying excitation fluence indicate that singlet-singlet annihilation is a primary competitor of singlet fission at higher fluence and that fission from higher-lying states can also contribute to the triplet formation process. Measurements employing different excitation energies and sample temps. reveal the role that trapping processes play in attenuating the triplet quantum yield to produce the complex temp. dependence of the singlet fission yield. The rate consts. for singlet fission itself are essentially temp. independent.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXmt1SgsL8%253D&md5=ec61c22a867d92d4100c371d0a6b692c

33. 33

    Baird, N. C. Quantum organic photochemistry. II. Resonance and Aromaticity in the Lowest ³ ππState of Cyclic Hydrocarbons. J. Am. Chem. Soc. 1972, 94, 4941– 4948, DOI: 10.1021/ja00769a025

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    Quantum organic photochemistry. II. Resonance and aromaticity in the lowest 3ππstate of cyclic hydrocarbons

    Baird, N. Colin

    Journal of the American Chemical Society
    (1972),
    94
    (14),
    4941-8CODEN:
    JACSAT;
    ISSN: 0002-7863.

    The concepts of aromaticity, antiaromaticity, and Dewar resonance energy are extended to the lowest ππtriplet state of conjugated hydrocarbons by comparing the bonding energy of triplets to the most stable biradical ref. structure. Arguments based upon simple perturbation theory indicate that the rules for ground state aromaticity are reversed in the 3ππstate (4n rings display “aromatic” character whereas 4n + 2 systems display “antiaromaticity”). Semiempirical SCF-LCAO-MO calcns. by the neglect-of-nonbonded-differential-overlap method confirm these predictions, and predict transition, bonding, and stabilization energies for a wide range of triplets including those for cyclobutadiene and derivs., cyclooctatetraene and derivs., nonclassical polyenes, 3-7-membered rings contg. exocyclic C atoms, benzenoid hydrocarbons, butalene, azulene, and cyclodecapentaene. The preference of certain hydrocarbon triplets for a completely planar rather than 90° twisted structure (such as methylenecyclo-propene, fulvene, and heptafulvene) is analyzed by perturbation theory. The consequences of aromatic and antiaromatic character to the exothermicity of ortho addn. are explored for several hydrocarbon triplets.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaE38XkslCksb0%253D&md5=fd74d4499d8dfcca1fe6d296285623a6

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    Ottosson, H. Organic photochemistry: Exciting Excited-State Aromaticity. Nat. Chem. 2012, 4, 969– 971, DOI: 10.1038/nchem.1518

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    Organic photochemistry Exciting excited-state aromaticity

    Ottosson, Henrik

    Nature Chemistry
    (2012),
    4
    (12),
    969-971CODEN:
    NCAHBB;
    ISSN: 1755-4330.

    (Nature Publishing Group)

    In 1972, Baird published rules describing aromaticity and antiaromaticity in the lowest triplet excited states of annulenes. The fortieth anniversary of Baird’s rules – which are the reverse of Hueckel’s rules for aromaticity and antiaromaticity in the ground state – ought to be celebrated before 2012 comes to an end.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhslagtrrI&md5=a9873efae25716d69e581059a2402995

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    Rosenberg, M.; Dahlstrand, C.; Kilså, K.; Ottosson, H. Excited State Aromaticity and Antiaromaticity: Opportunities for Photophysical and Photochemical Rationalizations. Chem. Rev. 2014, 114, 5379– 5425, DOI: 10.1021/cr300471v

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    Excited State Aromaticity and Antiaromaticity: Opportunities for Photophysical and Photochemical Rationalizations

    Rosenberg, Martin; Dahlstrand, Christian; Kilsaa, Kristine; Ottosson, Henrik

    Chemical Reviews (Washington, DC, United States)
    (2014),
    114
    (10),
    5379-5425CODEN:
    CHREAY;
    ISSN: 0009-2665.

    (American Chemical Society)

    In this review the authors described the development of the excited state aromaticity concept over time, a development which has progressed slowly over several decades and in different branches of chem. As a result, the concept has not been as visible to the chem. community as desirable. This review provides accumulative collection of studies that strongly support the existence of aromaticity and antiaromaticity effects in the lowest excited states of cyclic π-conjugated mols. The excited state (anti)aromaticity effects could be equally useful for understanding of excited state properties and processes as ground state aromaticity is useful for rationalization of ground state properties and processes.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXlsl2ks7s%253D&md5=4f1023c0fa4c093032d1bc71d30885db

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    Oh, J.; Sung, Y. M.; Hong, Y.; Kim, D. Spectroscopic Diagnosis of Excited-State Aromaticity: Capturing Electronic Structures and Conformations upon Aromaticity Reversal. Acc. Chem. Res. 2018, 51, 1349– 1358, DOI: 10.1021/acs.accounts.7b00629

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    36

    Spectroscopic Diagnosis of Excited-State Aromaticity: Capturing Electronic Structures and Conformations upon Aromaticity Reversal

    Oh, Juwon; Sung, Young Mo; Hong, Yongseok; Kim, Dongho

    Accounts of Chemical Research
    (2018),
    51
    (6),
    1349-1358CODEN:
    ACHRE4;
    ISSN: 0001-4842.

    (American Chemical Society)

    A review. Aromaticity, the special energetic stability derived from cyclic [4n + 2]π-conjugated electronic structures, has been the topic of intense interest in chem. because it plays a crit. role in rationalizing mol. stability, reactivity, and phys./chem. properties. Recently, the pioneering work by Colin Baird on aromaticity reversal, postulating that arom. (antiarom.) character in the ground state reverses to antiarom. (arom.) character in the lowest excited triplet state, has attracted much scientific attention. The completely reversed aromaticity in the excited state provides direct insight into understanding the photophys./chem. properties of photoactive materials. In turn, the application of arom. mols. to photoactive materials has led to numerous studies revealing this aromaticity reversal. However, most studies of excited-state aromaticity have been based on the theor. point of view. The exptl. evaluation of aromaticity in the excited state is still challenging and strenuous because the assessment of (anti)aromaticity with conventional magnetic, energetic, and geometric indexes is difficult in the excited state, which practically restricts the extension and application of the concept of excited-state aromaticity. Time-resolved optical spectroscopies can provide a new and alternative avenue to evaluate excited-state aromaticity exptl. while observing changes in the mol. features in the excited states. Time-resolved optical spectroscopies take advantage of ultrafast laser pulses to achieve high time resoln., making them suitable for monitoring ultrafast changes in the excited states of mol. systems. This can provide valuable information for understanding the aromaticity reversal.This Account presents recent breakthroughs in the exptl. assessment of excited-state aromaticity and the verification of aromaticity reversal with time-resolved optical spectroscopic measurements. To scrutinize this intriguing and challenging scientific issue, expanded porphyrins have been utilized as the ideal testing platform for investigating aromaticity because they show distinct arom. and antiarom. characters with aromaticity-specific spectroscopic features. Expanded porphyrins exhibit perfect arom. and antiarom. congener pairs having the same mol. framework but different nos. of π electrons, which facilitates the study of the pure effect of aromaticity by comparative analyses. On the basis of the characteristics of expanded porphyrins, time-resolved electronic and vibrational absorption spectroscopies capture the changes in electronic structure and mol. conformations driven by the change in aromaticity and provide clear evidence for aromaticity reversal in the excited states. The approaches described in this Account pave the way for the development of new and alternative exptl. indexes for the evaluation of excited-state aromaticity, which will enable overarching and fundamental comprehension of the role of (anti)aromaticity in the stability, dynamics, and reactivity in the excited states with possible implications for practical applications.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXjvVSksLc%253D&md5=8e24d0c31644703531f08b314da63df7

37. 37

    Fallon, K. J.; Budden, P.; Salvadori, E.; Ganose, A. M.; Savory, C. N.; Eyre, L.; Dowland, S.; Ai, Q.; Goodlett, S.; Risko, C.; Scanlon, D. O.; Kay, C. W. M.; Rao, A.; Friend, R. H.; Musser, A. J.; Bronstein, H. Exploiting Excited-State Aromaticity to Design Highly Stable Singlet Fission Materials. J. Am. Chem. Soc. 2019, 141, 13867– 13876, DOI: 10.1021/jacs.9b06346

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    Exploiting excited-state aromaticity to design highly stable singlet fission materials

    Fallon, Kealan J.; Budden, Peter; Salvadori, Enrico; Ganose, Alex M.; Savory, Christopher N.; Eyre, Lissa; Dowland, Simon; Ai, Qianxiang; Goodlett, Stephen; Risko, Chad; Scanlon, David O.; Kay, Christopher W. M.; Rao, Akshay; Friend, Richard H.; Musser, Andrew J.; Bronstein, Hugo

    Journal of the American Chemical Society
    (2019),
    141
    (35),
    13867-13876CODEN:
    JACSAT;
    ISSN: 0002-7863.

    (American Chemical Society)

    Singlet fission, the process of forming two triplet excitons from one singlet exciton, is a characteristic reserved for only a handful of org. mols. due to the atypical energetic requirement for low energy excited triplet states. The predominant strategy for achieving such a trait is by increasing ground state diradical character; however, this greatly reduces ambient stability. Herein, we exploit Baird’s rule of excited state aromaticity to manipulate the singlet-triplet energy gap and create novel singlet fission candidates. We achieve this through the inclusion of a [4n] 5-membered heterocycle, whose electronic resonance promotes aromaticity in the triplet state, stabilizing its energy relative to the singlet excited state. Using this theory, we design a family of derivs. of indolonaphthyridine thiophene (INDT) with highly tunable excited state energies. Not only do we access novel singlet fission materials, they also exhibit excellent ambient stability, imparted due to the delocalized nature of the triplet excited state. Spin-coated films retained up to 85% activity after several weeks of exposure to oxygen and light, while analogous films of TIPS-pentacene showed full degrdn. after 4 days, showcasing the excellent stability of this class of singlet fission scaffold. Extension of our theor. anal. to almost ten thousand candidates reveals an unprecedented degree of tunability and several thousand potential fission-capable candidates, while clearly demonstrating the relationship between triplet aromaticity and singlet-triplet energy gap, confirming this novel strategy for manipulating the exchange energy in org. materials.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXhsFSku73L&md5=0a0a7bc43984992df2b7883fde556f2f

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    Shukla, D.; Wan, P. Evidence for a Planar Cyclically Conjugated 8π System in the Excited State: Large Stokes Shift Observed for Dibenz[b,f]oxepin Fluorescence. J. Am. Chem. Soc. 1993, 115, 2990– 2991, DOI: 10.1021/ja00060a063

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    38

    Evidence for a planar cyclically conjugated 8π system in the excited state: large Stokes shift observed for dibenz[b,f]oxepin fluorescence

    Shukla, Deepak; Wan, Peter

    Journal of the American Chemical Society
    (1993),
    115
    (7),
    2990-1CODEN:
    JACSAT;
    ISSN: 0002-7863.

    The fluorescence emissions of dibenz[b,f]oxepin (I, X=O) and dibenzo[b,f]thiepin (I, X=S) exhibit unexpectedly large Stokes shifts (≈4720 cm-1). The equil. geometries of I on the excited-state surface are planar and are the fluorescent states. The driving force for torsional twisting to a planar geometry is believed to be the attainment of a conjugated 8π cyclic array in the central ring, which would be highly unfavorable (antiarom.) in the ground state. The vibrational fine structure (progression of ≈1500 cm-1) and relatively long lifetime of the emission band obsd. for I (X=O), along with corroborating reactivity data, indicate that these compds. exhibit the characteristics of a planar and arom. mol. in S1.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK3sXitlegt78%253D&md5=dacbf0e58476739b6f9ba604f61a1482

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    Toldo, J.; El Bakouri, O.; Solà, M.; Norrby, P.-O.; Ottosson, H. Is Excited-State Aromaticity a Driving Force for Planarization of Dibenzannelated 8π-Electron Heterocycles?. ChemPlusChem 2019, 84, 712– 721, DOI: 10.1002/cplu.201900066

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    Is Excited-State Aromaticity a Driving Force for Planarization of Dibenzannelated 8π-Electron Heterocycles?

    Toldo, Josene; El Bakouri, Ouissam; Sola, Miquel; Norrby, Per-Ola; Ottosson, Henrik

    ChemPlusChem
    (2019),
    84
    (6),
    712-721CODEN:
    CHEMM5;
    ISSN: 2192-6506.

    (Wiley-VCH Verlag GmbH & Co. KGaA)

    Compds. with dibenzannelated heterocycles with eight π-electrons are found in a range of applications. These mols. often adopt a bent structure in the ground state (S0) but can become planar in the first excited states (S1 and T1) because of the cyclically conjugated 4nπ central ring, which fulfils the requirements for excited state aromaticity. We report on a quantum chem. investigation of the arom. character in the S1 and T1 states of dibenzannelated seven- and six-membered heterocycles with one, two, or three heteroatoms in the 8π-electron ring. These states could have ππor nπcharacter. We find that compds. with one or two heteroatoms in the central ring have ππstates as their S1 and T1 states. They are to a significant degree influenced by excited state aromaticity, and their optimal structures are planar or nearly planar. Among the heteroatoms, nitrogen provides for the strongest excited state aromaticity whereas oxygen provides for the weakest, following the established trend of the S0 state. Yet, dibenzannelated seven-membered-ring compds. with N=N bonds have non-arom. nπstates with strongly puckered structures as their S1 and T1 states.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXotl2rsbw%253D&md5=1e0da0b30c73875f80df7c775be455ab

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    Ryerson, J. L.; Zaykov, A.; Aguilar
    Suarez, L. E.; Havenith, R. W. A.; Stepp, B. R.; Dron, P. I.; Kaleta, J.; Akdag, A.; Teat, S. J.; Magnera, T. F.; Miller, J. R.; Havlas, Z.; Broer, R.; Faraji, S.; Michl, J.; Johnson, J. C. Structure and Photophysics of Indigoids for Singlet Fission: Cibalackrot. J. Chem. Phys. 2019, 151, 184903, DOI: 10.1063/1.5121863

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    Structure and photophysics of indigoids for singlet fission: Cibalackrot

    Ryerson, Joseph L.; Zaykov, Alexandr; Aguilar Suarez, Luis E.; Havenith, Remco W. A.; Stepp, Brian R.; Dron, Paul I.; Kaleta, Jiri; Akdag, Akin; Teat, Simon J.; Magnera, Thomas F.; Miller, John R.; Havlas, Zdenek; Broer, Ria; Faraji, Shirin; Michl, Josef; Johnson, Justin C.

    Journal of Chemical Physics
    (2019),
    151
    (18),
    184903/1-184903/16CODEN:
    JCPSA6;
    ISSN: 0021-9606.

    (American Institute of Physics)

    We report an investigation of structure and photophysics of thin layers of cibalackrot, a sturdy dye derived from indigo by double annulation at the central double bond. Evapd. layers contain up to three phases, two cryst. and one amorphous. Relative amts. of all three have been detd. by a combination of X-ray diffraction and FT-IR reflectance spectroscopy. Initially, excited singlet state rapidly produces a high yield of a transient intermediate whose spectral properties are compatible with charge-transfer nature. This intermediate more slowly converts to a significant yield of triplet, which, however, does not exceed 100% and may well be produced by intersystem crossing rather than singlet fission. The yields were detd. by transient absorption spectroscopy and cor. for effects of partial sample alignment by a simple generally applicable procedure. Formation of excimers was also obsd. In order to obtain guidance for improving mol. packing by a minor structural modification, calcns. by a simplified frontier orbital method were used to find all local maxima of singlet fission rate as a function of geometry of a mol. pair. The method was tested at 48 maxima by comparison with the ab initio Frenkel-Davydov exciton model. (c) 2019 American Institute of Physics.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXitFOls77L&md5=baac68449b6fc5222a85530d568fd2cc

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    Ueda, M.; Jorner, K.; Sung, Y. M.; Mori, T.; Xiao, Q.; Kim, D.; Ottosson, H.; Aida, T.; Itoh, Y. Energetics of Baird Aromaticity Supported by Inversion of Photoexcited of Chiral [4n]Annulene Derivatives. Nat. Commun. 2017, 8, 346, DOI: 10.1038/s41467-017-00382-1

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    Energetics of Baird aromaticity supported by inversion of photoexcited chiral [4n]annulene derivatives

    Ueda Michihisa; Xiao Qi; Aida Takuzo; Itoh Yoshimitsu; Jorner Kjell; Ottosson Henrik; Sung Young Mo; Kim Dongho; Mori Tadashi; Aida Takuzo

    Nature communications
    (2017),
    8
    (1),
    346
    ISSN: .

    For the concept of aromaticity, energetic quantification is crucial. However, this has been elusive for excited-state (Baird) aromaticity. Here we report our serendipitous discovery of two nonplanar thiophene-fused chiral [4n]annulenes (Th4) COT Saddle and (Th6) CDH Screw , which by computational analysis turned out to be a pair of molecules suitable for energetic quantification of Baird aromaticity. Their enantiomers were separable chromatographically but racemized thermally, enabling investigation of the ring inversion kinetics. In contrast to (Th6) CDH Screw , which inverts through a nonplanar transition state, the inversion of (Th4) COT Saddle , progressing through a planar transition state, was remarkably accelerated upon photoexcitation. As predicted by Baird’s theory, the planar conformation of (Th4) COT Saddle is stabilized in the photoexcited state, thereby enabling lower activation enthalpy than that in the ground state. The lowering of the activation enthalpy, i.e., the energetic impact of excited-state aromaticity, was quantified experimentally to be as high as 21-22 kcal mol(-1).Baird’s rule applies to cyclic π-conjugated molecules in their excited state, yet a quantification of the involved energetics is elusive. Here, the authors show the ring inversion kinetics of two nonplanar and chiral [4n]annulenes to support Baird’s rule from an energetic point of view.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A280%3ADC%252BC1cbhsVKgtQ%253D%253D&md5=825b23cf4008a0a4fb10e38653618bb3

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    Eckert-Maksić, M.; Vazdar, M.; Barbatti, M.; Lischka, H.; Maksić, Z. B. Automerization Reaction of Cyclobutadiene and Its Barrier Height: An Ab Initio Benchmark Multi-reference Average-Quadratic Coupled Cluster Study. J. Chem. Phys. 2006, 125, 064310, DOI: 10.1063/1.2222366

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    Automerization reaction of cyclobutadiene and its barrier height: An ab initio benchmark multireference average-quadratic coupled cluster study

    Eckert-Maksic, Mirjana; Vazdar, Mario; Barbatti, Mario; Lischka, Hans; Maksic, Zvonimir B.

    Journal of Chemical Physics
    (2006),
    125
    (6),
    064310/1-064310/9CODEN:
    JCPSA6;
    ISSN: 0021-9606.

    (American Institute of Physics)

    The problem of the double bond flipping interconversion of the two equiv. ground state structures of cyclobutadiene (CBD) is addressed at the multireference av.-quadratic coupled cluster level of theory, which is capable of optimizing the structural parameters of the ground, transition, and excited states on an equal footing. The barrier height involving both the electronic and zero-point vibrational energy contributions is 6.3 kcal mol-1, which is higher than the best earlier theor. est. of 4.0 kcal mol-1. This result is confirmed by including into the ref. space the orbitals of the CC σ bonds beyond the std. π orbital space. It places the present value into the middle of the range of the measured data (1.6-10 kcal mol-1). An adiabatic singlet-triplet energy gap of 7.4 kcal mol-1 between the transition state 1Btg and the first triplet 3A2g state is obtained. A low barrier height for the CBD automerization and a small ΔE(3A2g,1B1g) gap bear some relevance on the highly pronounced reactivity of CBD, which is briefly discussed.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD28Xotl2itbw%253D&md5=7bd50762674678b87933cda1da6505e3

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    Bryce-Smith, D.; Gilbert, A. The Organic Photochemistry of Benzene – I. Tetrahedron 1976, 32, 1309– 1326, DOI: 10.1016/0040-4020(76)85002-8

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    The organic photochemistry of benzene. I

    Bryce-Smith, D.; Gilbert, A.

    Tetrahedron
    (1976),
    32
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    1309-26CODEN:
    TETRAB;
    ISSN: 0040-4020.

    A review with 113 refs.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaE2sXltVej&md5=4f5b65d8510cec636484fa2f5bb63355

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    Michl, J.; Thulstrup, E. W. Why is Azulene Blue and Anthracene White? A Simple MO Picture. Tetrahedron 1976, 32, 205– 209, DOI: 10.1016/0040-4020(76)87002-0

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    Why is azulene blue and anthracene white? A simple MO picture

    Michl, J.; Thulstrup, E. W.

    Tetrahedron
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    32
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    205-9CODEN:
    TETRAB;
    ISSN: 0040-4020.

    The longest wavelength singlet-singlet absorption band which dets. the color of the title compds. (I and II, resp.) is due to HOMO→LUMO excitation. The large difference between the lowest singlet-singlet excitation energies of I and II, which have almost identical ionization potentials and electron affinities, was interpreted by the explicit introduction of an electron repulsion term into the simple Hueckel picture in which excitation energies are expressed as orbital energy differences. The differences in the magnitude of the singlet-triplet splitting and the structural features responsible for differences between I and II are discussed.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaE28XhsFSjtLk%253D&md5=50db8207db1eed4acb0ee141c2795230

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    Ottosson, H.; Kilså, K.; Chajara, K.; Piqueras, M. C.; Crespo, R.; Kato, H.; Muthas, D. Scope and Limitations of Baird’s Theory on Triplet State Aromaticity: Application to the Tuning of Singlet-Triplet Energy Gaps in Fulvenes. Chem. – Eur. J. 2007, 13, 6998– 7005, DOI: 10.1002/chem.200700362

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    Scope and limitations of Baird’s theory on triplet state aromaticity: application to the tuning of singlet-triplet energy gaps in fulvenes

    Ottosson, Henrik; Kilsa, Kristine; Chajara, Khalil; Carmen Piqueras, Mari; Crespo, Rauel; Kato, Haruhisa; Muthas, Daniel

    Chemistry – A European Journal
    (2007),
    13
    (24),
    6998-7005, S6998/1-S6998/23CODEN:
    CEUJED;
    ISSN: 0947-6539.

    (Wiley-VCH Verlag GmbH & Co. KGaA)

    Utilizing Baird’s theory on triplet state aromaticity, we show that the singlet-triplet energy gaps (ΔEST) of pentafulvenes are easily varied through substitution by as much as 36 kcal mol-1. This exploits the fact that fulvenes act as arom. chameleons in which the dipoles reverse on going from the singlet ground state (S0) to the lowest ππtriplet state (T1); thus, their electron distributions are adapted so as to achieve some aromaticity in both states. The results are based on quantum chem. calcns. with the OLYP d. functional theory method and the CASPT2 ab initio method, as well as spectroscopic detn. of ΔEST by triplet sensitization. The findings can also be generalized to fulvenes other than the pentafulvenes, even though the effect is attenuated as the size of the fulvene increases. Our studies thus reveal that triplet-state aromaticity can greatly influence the properties of conjugated compds. in the T1 state.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2sXhtVWhu7nE&md5=f6aae8f28ca97d01bda50455b2e3056f

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    Rosenberg, M.; Ottosson, H.; Kilså, K. Influence of Excited State Aromaticity in the Lowest Excited Singlet States of Fulvene Derivatives. Phys. Chem. Chem. Phys. 2011, 13, 12912– 12919, DOI: 10.1039/c0cp02821e

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    Influence of excited state aromaticity in the lowest excited singlet states of fulvene derivatives

    Rosenberg, Martin; Ottosson, Henrik; Kilsa, Kristine

    Physical Chemistry Chemical Physics
    (2011),
    13
    (28),
    12912-12919CODEN:
    PPCPFQ;
    ISSN: 1463-9076.

    (Royal Society of Chemistry)

    The absorption spectra and excited state dipole moments of four differently substituted fulvenes have been investigated both exptl. and computationally. The results reveal that the excited state dipole moment of fulvenes reverses in the first excited singlet state when compared to the ground state. The oppositely polarized electron d. distributions, which dominate the ground state and the first excited singlet state of fulvenes, resp., reflect the reversed π-electron counting rules for aromaticity in the two states (4n + 2 vs. 4n, resp.). The results show that substituents indeed influence the polarity of fulvenes in the two states, however, cooperative interactions between the substituents and the fulvene moiety are most pronounced in the ground state.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXosVWju7w%253D&md5=a98bb2bea600d477e5aa074422c890ad

47. 47

    Jorner, K.; Emanuelsson, R.; Dahlstrand, C.; Tong, H.; Denisova, A. V.; Ottosson, H. Impact of Ground and Excited State Aromaticity on Silole and Cyclopentadiene Excitation Energies and Excited State Polarities. Chem. – Eur. J. 2014, 20, 9295– 9303, DOI: 10.1002/chem.201402577

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    47

    Impact of Ground- and Excited-State Aromaticity on Cyclopentadiene and Silole Excitation Energies and Excited-State Polarities

    Jorner, Kjell; Emanuelsson, Rikard; Dahlstrand, Christian; Tong, Hui; Denisova, Aleksandra V.; Ottosson, Henrik

    Chemistry – A European Journal
    (2014),
    20
    (30),
    9295-9303CODEN:
    CEUJED;
    ISSN: 0947-6539.

    (Wiley-VCH Verlag GmbH & Co. KGaA)

    A new qual. model for estg. the properties of substituted cyclopentadienes and siloles in their lowest ππexcited states is introduced and confirmed through quantum chem. calcns., and then applied to explain earlier reported exptl. excitation energies. According to model, which is based on excited-state aromaticity and antiaromaticity, siloles and cyclopentadienes are cross-hyperconjugated arom. chameleons that adapt their electronic structures to conform to the various aromaticity rules in different electronic states (Hueckel’s rule in the π2 electronic ground state (S0) and Baird’s rule in the lowest ππexcited singlet and triplet states (S1 and T1)). By using pen-and-paper arguments, one can explain polarity changes upon excitation of substituted cyclopentadienes and siloles, and one can tune their lowest excitation energies by combined considerations of ground- and excited-state aromaticity/antiaromaticity effects. Finally, the arom. chameleon model can be extended to other monocyclic compd. classes of potential use in org. electronics, thereby providing a unified view of the S0, T1, and S1 states of a range of different cyclic cross-π-conjugated and cross-hyperconjugated compd. classes.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhtFWns7%252FP&md5=dbf6a578035ae514213747fe5e5745fb

48. 48

    Yu, D.; Rong, C.; Lu, T.; De Proft, F.; Liu, S. Baird’s Rule in Substituted Fulvene Derivatives: An Information-Theoretic Study on Triplet-State Aromaticity and Antiaromaticity. ACS Omega 2018, 3, 18370– 18379, DOI: 10.1021/acsomega.8b02881

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    48

    Baird’s Rule in Substituted Fulvene Derivatives: An Information-Theoretic Study on Triplet-State Aromaticity and Antiaromaticity

    Yu, Donghai; Rong, Chunying; Lu, Tian; De Proft, Frank; Liu, Shubin

    ACS Omega
    (2018),
    3
    (12),
    18370-18379CODEN:
    ACSODF;
    ISSN: 2470-1343.

    (American Chemical Society)

    Originated from the cyclic delocalization of electrons resulting in extra stability and instability, aromaticity and antiaromaticity are important chem. concepts whose appreciation and quantification are still much of recent interest in the literature. Employing information-theoretic quantities can provide us with more insights and better understanding about them, as we have previously demonstrated. In this work, we examine the triplet-state aromaticity and antiaromaticity, which are governed by Baird’s 4n rule, instead of H.ovrddot.uckel’s 4n + 2 rule for the singlet state. To this end, we have made use of 4 different aromaticity indexes and 8 information-theoretic quantities, examd. a total of 22 substituted fulvene derivs., and compared the results both in singlet and triplet states. It is found that cross-correlations of these two categories of mol. property descriptors enable us to better understand the nature and propensity of aromaticity and antiaromaticity for the triplet state. Our results have not only demonstrated the existence and validity of Baird’s rule but also shown that H.ovrddot.uckel’s rule and Baird’s rule indeed share the same theor. foundation because with these cross-correlation patterns we are able to distinguish them from each other simultaneously in both singlet and triplet states. Our results should provide new insights into the nature of aromaticity and antiaromaticity in the triplet state and pave the road toward new ways to quantify this pair of important chem. concepts.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXisF2gurfP&md5=513803bf6d3f6f51b1197bd2eeabe5dc

49. 49

    Yadav, S.; El Bakouri, O.; Jorner, K.; Tong, H.; Dahlstrand, C.; Solà, M.; Ottosson, H. Exploiting the Aromatic Chameleon Character of Fulvenes for Computational Design of Baird-Aromatic Triplet Ground State Compounds. Chem. – Asian J. 2019, 14, 1870– 1878, DOI: 10.1002/asia.201801821

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    49

    Exploiting the Aromatic Chameleon Character of Fulvenes for Computational Design of Baird-Aromatic Triplet Ground State Compounds

    Yadav, Sangeeta; El Bakouri, Ouissam; Jorner, Kjell; Tong, Hui; Dahlstrand, Christian; Sola, Miquel; Ottosson, Henrik

    Chemistry – An Asian Journal
    (2019),
    14
    (10),
    1870-1878CODEN:
    CAAJBI;
    ISSN: 1861-4728.

    (Wiley-VCH Verlag GmbH & Co. KGaA)

    Due to the reversal in electron counts for aromaticity and antiaromaticity in the closed-shell singlet state (normally ground state, S0) and lowest ππtriplet state (T1 or T0), as given by Hueckel’s and Baird’s rules, resp., fulvenes are influenced by their substituents in the opposite manner in the T1 and S0 states. This effect is caused by a reversal in the dipole moment when going from S0 to T1 as fulvenes adapt to the difference in electron counts for aromaticity in various states; they are arom. chameleons. Thus, a substituent pattern that enhances (reduces) fulvene aromaticity in S0 reduces (enhances) aromaticity in T1, allowing for rationalizations of the triplet state energies (ET) of substituted fulvenes. Through quantum chem. calcns., we now assess which substituents and which positions on the pentafulvene core are the most powerful for designing compds. with low or inverted ET. As a means to increase the π-electron withdrawing capacity of cyano groups, we found that protonation at the cyano N atoms of 6,6-dicyanopentafulvenes can be a route to on-demand formation of a fulvenium dication with a triplet ground state (T0). The five-membered ring of this species is markedly Baird-arom., although less than the cyclopentadienyl cation known to have a Baird-arom. T0 state.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXislCitLg%253D&md5=56b200076c5a89ac4cb327de5d0f6fc7

50. 50

    Möllerstedt, H.; Crespo, R.; Piqueras, M. C.; Ottosson, H. Fulvenes, Fulvalenes, and Azulene: Are They Aromatic Chameleons?. J. Am. Chem. Soc. 2004, 126, 13938– 13939, DOI: 10.1021/ja045729c

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    50

    Fulvenes, fulvalenes, and azulene: are they aromatic chameleons?

    Mollerstedt Helene; Piqueras Mari Carmen; Crespo Raul; Ottosson Henrik

    Journal of the American Chemical Society
    (2004),
    126
    (43),
    13938-9
    ISSN: 0002-7863.

    On the basis of the theory of Baird on reversal of Huckel’s rule for aromaticity and antiaromaticity of annulenes when going from the electronic ground state (S0) to the lowest pipitriplet state (T1) (J. Am. Chem. Soc. 1972, 94, 4941), we argue that fulvenes, fulvalenes, and azulene are “aromatic chameleons”. The dipole moments of fulvenes in T1 should be of comparable magnitude to those of S0, but due to the reversal of Huckel’s aromaticity rule in T1, their dipole should be in the opposite direction. Thereby, they are capable of adopting some aromaticity in both the T1 and S0 states as they adapt their dipolar resonance structures. The same applies to fulvalenes and azulene in their lowest quintet states (Q1) when compared to S0. Our hypothesis on chameleon behavior is supported by quantum chemical OLYP, CASSCF, and CASPT2 calculations of dipole moments, pi-orbital populations, and energies.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A280%3ADC%252BD2crmtlSnsw%253D%253D&md5=6ada97f5aed47cd51272b76fd46cec4f

51. 51

    Solel, E.; Kozuch, S. Tuning the Spin, Aromaticity, and Quantum Tunneling in Computationally Designed Fulvalenes. J. Org. Chem. 2018, 83, 10826– 10834, DOI: 10.1021/acs.joc.8b01541

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    Tuning the Spin, Aromaticity, and Quantum Tunneling in Computationally Designed Fulvalenes

    Solel, Ephrath; Kozuch, Sebastian

    Journal of Organic Chemistry
    (2018),
    83
    (18),
    10826-10834CODEN:
    JOCEAH;
    ISSN: 0022-3263.

    (American Chemical Society)

    Pentafulvalene is a sym. unsatd. hydrocarbon built from two five-membered rings connected by an exocyclic double bond, where each ring is one electron short of being a 6π-electron arom. system. Here, we show computationally that by selectively introducing electron withdrawing and donating substituents, we can design pentafulvalene derivs. that exhibit tunable aromaticity properties. Pentafulvalene can be shaped into a species with connected arom.-antiarom. rings, which can also achieve π-bond shifting by carbon tunneling. We propose an NMR technique that can exptl. prove such tunneling mechanism. In addn., we devised a doubly arom. fulvalene involving both H.ovrddot.uckel and Baird aromaticities. These results can open possibilities to create novel mols. in terms of spin state, aromaticity, and reactivity by quantum tunneling.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXhsFejsL3L&md5=befaf110854ae53c7119a7a3d89f16b3

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    Breslow, R.; Chang, H. W.; Hill, R.; Wasserman, E. Stable Triplet States of Some Cyclopentadienyl Cations. J. Am. Chem. Soc. 1967, 89, 1112– 1119, DOI: 10.1021/ja00981a015

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    Stable triplet states of some cyclopentadienyl cations

    Breslow, Ronald; Chang, Hai Won; Hill, Roger; Wasserman, Edel

    Journal of the American Chemical Society
    (1967),
    89
    (5),
    1112-19CODEN:
    JACSAT;
    ISSN: 0002-7863.

    A no. of cyclopentadienyl cations have been prepd. in soln., and for all of them a triplet state has been detected. This is the ground state for the pentachloro cation (I), a low-lying excited state for the pentaphenyl cation (II, R=Ph), and a somewhat higher state for less sym. cations (II, R=p-MeOC6H4, p-ClC6H4 Me, β-naphthyl, p-MeC6H4). The methods used for detection include E.S.R. spectra of frozen solns., magnetic susceptibility detns., and an N.M.R. method. 20 references.

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    Breslow, R. Antiaromaticity. Acc. Chem. Res. 1973, 6, 393– 398, DOI: 10.1021/ar50072a001

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    Antiaromaticity

    Breslow, Ronald

    Accounts of Chemical Research
    (1973),
    6
    (12),
    393CODEN:
    ACHRE4;
    ISSN: 0001-4842.

    A review with 38 refs.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaE2cXis1eqsQ%253D%253D&md5=090a552d977463ce6175e9c2823840b1

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    Wörner, H. J.; Merkt, F. Photoelectron Spectroscopic Study of the First Singlet and Triplet States of the Cyclopentadienyl Cation. Angew. Chem., Int. Ed. 2006, 45, 293– 296, DOI: 10.1002/anie.200503032

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    Photoelectron spectroscopic study of the first singlet and triplet states of the cyclopentadienyl cation

    Worner, Hans Jakob; Merkt, Frederic

    Angewandte Chemie, International Edition
    (2006),
    45
    (2),
    293-296CODEN:
    ACIEF5;
    ISSN: 1433-7851.

    (Wiley-VCH Verlag GmbH & Co. KGaA)

    The pulsed-field ionization zero-kinetic-energy photoelectron spectrum of the C5H5 radical is recorded after direct excitation from the ~X 2E”1 ground state and two-photon excitation via the ~A 2A”2 electronically excited state. The intensity distributions of the spectra enables the characterization of the energetics, symmetry, and structure of the lowest two electronic states of the cation.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD28Xlslyksg%253D%253D&md5=ab491b0b14470afb559019afeae7b4ef

55. 55

    Conrad-Burton, F. S.; Liu, T.; Geyer, F.; Costantini, R.; Schlaus, A. P.; Spencer, M. S.; Wang, J.; Sanchez, R. H.; Zhang, B.; Xu, Q.; Steigerwald, M. L.; Xiao, S.; Li, H.; Nuckolls, C. P.; Zhu, X. Controlling Singlet Fission by Molecular Contortion. J. Am. Chem. Soc. 2019, 141, 13143– 13147, DOI: 10.1021/jacs.9b05357

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    Controlling singlet fission by molecular contortion

    Conrad-Burton, Felisa S.; Liu, Taifeng; Geyer, Florian; Costantini, Roberto; Schlaus, Andrew P.; Spencer, Michael S.; Wang, Jue; Sanchez, Raul Hernandez; Zhang, Boyuan; Xu, Qizhi; Steigerwald, Michael L.; Xiao, Shengxiong; Li, Hexing; Nuckolls, Colin P.; Zhu, Xiaoyang

    Journal of the American Chemical Society
    (2019),
    141
    (33),
    13143-13147CODEN:
    JACSAT;
    ISSN: 0002-7863.

    (American Chemical Society)

    Singlet fission, the generation of two triplet excited states from the absorption of a single photon, may potentially increase solar energy conversion efficiency. A major roadblock in realizing this potential is the limited no. of mols. available with high singlet fission yields and sufficient chem. stability. Here, we demonstrate a strategy for developing singlet fission materials in which we start with a stable mol. platform and use strain to tune the singlet and triplet energies. Using perylene diimide as a model system, we tune the singlet fission energetics from endoergic to exoergic or iso-energetic by straining the mol. backbone. The result is an increase in the singlet fission rate by 2 orders of magnitude. This demonstration opens a door to greatly expanding the mol. toolbox for singlet fission.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXhsVGkur7O&md5=3ed7aa9b50809e99e7e51647d061fd5b

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    Clar, E. The Aromatic Sextet; Wiley: New York, 1972.

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    Solà, M. Forty Years of Clar’s Aromatic π-Sextet Rule. Front. Chem. 2013, 1, 22, DOI: 10.3389/fchem.2013.00022

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    Forty years of Clar’s aromatic π-sextet rule

    Sola Miquel

    Frontiers in chemistry
    (2013),
    1
    (),
    22
    ISSN: 2296-2646.

    In 1972 Erich Clar formulated his aromatic π-sextet rule that allows discussing qualitatively the aromatic character of benzenoid species. Now, 40 years later, Clar’s aromatic π-sextet rule is still a source of inspiration for many chemists. This simple rule has been validated both experimentally and theoretically. In this review, we select some particular examples to highlight the achievement of Clar’s aromatic π-sextet rule in many situations and we discuss two recent successful cases of its application.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A280%3ADC%252BC2cflvVGmuw%253D%253D&md5=9fe5e8101921ef264034c40046a70b9d

58. 58

    El
    Bakouri, O.; Poater, J.; Feixas, F.; Solà, M. Exploring the Validity of the Glidewell–Lloyd Extension of Clar’s π-Sextet Rule: Assessment from Polycyclic Conjugated Hydrocarbons. Theor. Chem. Acc. 2016, 135, 205, DOI: 10.1007/s00214-016-1970-1

59. 59

    Ayub, R.; Bakouri, O. E.; Jorner, K.; Solà, M.; Ottosson, H. Can Baird’s and Clar’s Rules Combined Explain Triplet State Energies of Polycyclic Conjugated Hydrocarbons with Fused 4nπ- and (4n+2)π-Rings?. J. Org. Chem. 2017, 82, 6327– 6340, DOI: 10.1021/acs.joc.7b00906

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    59

    Can Baird’s and Clar’s Rules Combined Explain Triplet State Energies of Polycyclic Conjugated Hydrocarbons with Fused 4nπ- and (4n + 2)π-Rings?

    Ayub, Rabia; El Bakouri, Ouissam; Jorner, Kjell; Sola, Miquel; Ottosson, Henrik

    Journal of Organic Chemistry
    (2017),
    82
    (12),
    6327-6340CODEN:
    JOCEAH;
    ISSN: 0022-3263.

    (American Chemical Society)

    Compds. that can be labeled as “arom. chameleons” are π-conjugated compds. that are able to adjust their π-electron distributions so as to comply with the different rules of aromaticity in different electronic states. We used quantum chem. calcns. to explore how the fusion of benzene rings onto arom. chameleonic units represented by biphenylene, dibenzocyclooctatetraene, and dibenzo[a,e]pentalene modifies the first triplet excited states (T1) of the compds. Decreases in T1 energies are obsd. when going from isomers with linear connectivity of the fused benzene rings to those with cis- or trans-bent connectivities. The T1 energies decreased down to those of the parent (isolated) 4nπ-electron units. Simultaneously, we observe an increased influence of triplet state aromaticity of the central 4n ring as given by Baird’s rule and evidenced by geometric, magnetic, and electron d. based aromaticity indexes (HOMA, NICS-XY, ACID, and FLU). Because of an influence of triplet state aromaticity in the central 4nπ-electron units, the most stabilized compds. retain the triplet excitation in Baird π-quartets or octets, enabling the outer benzene rings to adapt closed-shell singlet Clar π-sextet character. Interestingly, the T1 energies go down as the total no. of arom. cycles within a mol. in the T1 state increases.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXot1yntr4%253D&md5=cb8d23972464cea417558c657e9332ae

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    Paci, I.; Johnson, J. C.; Chen, X.; Rana, G.; Popović, D.; David, D. E.; Nozik, A. J.; Ratner, M. A.; Michl, J. Singlet Fission for Dye-Sensitized Solar Cells: Can a Suitable Sensitizer Be Found?. J. Am. Chem. Soc. 2006, 128, 16546– 16553, DOI: 10.1021/ja063980h

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    60

    Singlet Fission for Dye-Sensitized Solar Cells: Can a Suitable Sensitizer Be Found?

    Paci, Irina; Johnson, Justin C.; Chen, Xudong; Rana, Geeta; Popovic, Duska; David, Donald E.; Nozik, Arthur J.; Ratner, Mark A.; Michl, Josef

    Journal of the American Chemical Society
    (2006),
    128
    (51),
    16546-16553CODEN:
    JACSAT;
    ISSN: 0002-7863.

    (American Chemical Society)

    Improvements in the efficiency of dye-sensitized photovoltaic cells are possible by using dyes capable of singlet fission into 2 triplets, thus producing 2 electron-hole pairs from a single photon. In addn. to derivs. of large alternant hydrocarbons, those of biradicals are also candidates for a favorable ordering of excited-state energy levels, E(T2), E(S1)> 2E(T1). A large no. of favorable structures was examd. by the semiempirical PPP method and some also by the time-dependent DFT method. Several candidates were identified for exptl. examn.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD28XhtlWmur3E&md5=1209f57bd46e51e21e69c84b2c350bfb

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    Akdag, A.; Havlas, Z.; Michl, J. Search for a Small Chromophore with Efficient Singlet Fission: Biradicaloid Heterocycles. J. Am. Chem. Soc. 2012, 134, 14624– 14631, DOI: 10.1021/ja3063327

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    Search for a Small Chromophore with Efficient Singlet Fission: Biradicaloid Heterocycles

    Akdag, Akin; Havlas, Zdenek; Michl, Josef

    Journal of the American Chemical Society
    (2012),
    134
    (35),
    14624-14631CODEN:
    JACSAT;
    ISSN: 0002-7863.

    (American Chemical Society)

    Of the five small biradicaloid heterocycles whose S1, S2, T1, and T2 adiabatic excitation energies were examd. by the CASPT2/ANO-L-VTZP method, two have been found to meet the state energy criterion for efficient singlet fission and are recommended to the attention of synthetic chemists and photophysicists.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhtFOisr7L&md5=41828188421bf220751c941b25e216bf

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    Minami, T.; Nakano, M. Diradical Character View of Singlet Fission. J. Phys. Chem. Lett. 2012, 3, 145– 150, DOI: 10.1021/jz2015346

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    Diradical Character View of Singlet Fission

    Minami, Takuya; Nakano, Masayoshi

    Journal of Physical Chemistry Letters
    (2012),
    3
    (2),
    145-150CODEN:
    JPCLCD;
    ISSN: 1948-7185.

    (American Chemical Society)

    The feasibility conditions of singlet fission on the excitation energy differences are revealed as functions of the multiple diradical characters yi [defined by the occupation nos. of the LUNO (=Lowest Unoccupied Natural Orbital) + i (i=0, 1, …), where 0 ≤ yi ≤ 1 and yi ≥ yj (i> j)] using the linear H4 full CI model. The diradical characters suited for singlet fission are found to lie in the region with y0> 0.10 except for y0 ∼ y1, though its energy efficiency is better in case of smaller y0, to which diradical and multiradical compds. with low/intermediate diradical characters such as open-shell singlet polycyclic arom. hydrocarbons belong. These findings indicate that the multiple diradical character is an effective indicator for exploring mol. systems for efficient singlet fission.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXhs1Ojur3L&md5=76b624a9c479e109deb646a65856640e

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    Ito, S.; Nagami, T.; Nakano, M. Diradical Character-Based Design for Singlet Fission of Bisanthene Derivatives: Aromatic-Ring Attachment and π-Plane Twisting. J. Phys. Chem. Lett. 2016, 7, 3925– 390, DOI: 10.1021/acs.jpclett.6b01885

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    Diradical Character-Based Design for Singlet Fission of Bisanthene Derivatives: Aromatic-Ring Attachment and π-Plane Twisting

    Ito, Soichi; Nagami, Takanori; Nakano, Masayoshi

    Journal of Physical Chemistry Letters
    (2016),
    7
    (19),
    3925-3930CODEN:
    JPCLCD;
    ISSN: 1948-7185.

    (American Chemical Society)

    We demonstrate a diradical character-based mol. design for singlet fission using polycyclic arom. hydrocarbons, bisanthene derivs. Two types of chem. modifications-arom.-ring attachment and π-plane twisting-are examd. in order to satisfy the energy level matching condition for singlet fission. Detailed anal. of the electronic structures of the model mols. using nucleus-independent chem. shift, MOs, and their energies has demonstrated the usefulness of the relationship between the resonance structure and aromaticity and that between nonplanarity of π-conjugated systems and redn. of orbital overlap for tuning the diradical character. This result provides a novel design guideline for polycyclic arom. hydrocarbons toward efficient singlet fission.

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    Wirz, J. Electronic Structure and Photophysical Properties of Planar Conjugated Hydrocarbons with a 4n-Membered Ring, Part II. Jerusalem Symposia on Quantum Chemistry and Biochemistry 1977, 10, 283– 294, DOI: 10.1007/978-94-010-1273-7_24

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    Electronic structure and photophysical properties of planar conjugated hydrocarbons with a 4n-membered ring

    Wirz, Jakob

    Jerusalem Symposia on Quantum Chemistry and Biochemistry
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    10
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    283-94CODEN:
    JSQCA7;
    ISSN: 0075-3696.

    The UV of I through VI, which exhibit systematic features which differ radically from the pattern obsd. with benzenoid hydrocarbons, are assigned using a PDP-SCF-CI calcn. The assignments are corroborated by polarization measurements on III through V. The triplet rate absorption spectra and energies of III, V, and VI are detd. by flash photolysis and energy transfer expts. The fluorescence and triplet yield trends are an example of the photophys. consequences of an avoided hypersurface crossing.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaE1cXhsFWrsbo%253D&md5=b3c9bf58088a0af108b358c4c37c31d4

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    Zhao, Y.; Truhlar, D. G. The M06 Suite of Density Functionals for Main Group Thermochemistry, Thermochemical Kinetics, Noncovalent Interactions, Excited States, and Transition Elements: Two New Functionals and Systematic Testing of Four M06-class Functionals and 12 Other Function. Theor. Chem. Acc. 2008, 120, 215– 241, DOI: 10.1007/s00214-007-0310-x

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    The M06 suite of density functionals for main group thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and transition elements: two new functionals and systematic testing of four M06-class functionals and 12 other functionals

    Zhao, Yan; Truhlar, Donald G.

    Theoretical Chemistry Accounts
    (2008),
    120
    (1-3),
    215-241CODEN:
    TCACFW;
    ISSN: 1432-881X.

    (Springer GmbH)

    We present two new hybrid meta exchange-correlation functionals, called M06 and M06-2X. The M06 functional is parametrized including both transition metals and nonmetals, whereas the M06-2X functional is a high-nonlocality functional with double the amt. of nonlocal exchange (2X), and it is parametrized only for nonmetals. The functionals, along with the previously published M06-L local functional and the M06-HF full-Hartree-Fock functionals, constitute the M06 suite of complementary functionals. We assess these four functionals by comparing their performance to that of 12 other functionals and Hartree-Fock theory for 403 energetic data in 29 diverse databases, including ten databases for thermochem., four databases for kinetics, eight databases for noncovalent interactions, three databases for transition metal bonding, one database for metal atom excitation energies, and three databases for mol. excitation energies. We also illustrate the performance of these 17 methods for three databases contg. 40 bond lengths and for databases contg. 38 vibrational frequencies and 15 vibrational zero point energies. We recommend the M06-2X functional for applications involving main-group thermochem., kinetics, noncovalent interactions, and electronic excitation energies to valence and Rydberg states. We recommend the M06 functional for application in organometallic and inorganometallic chem. and for noncovalent interactions.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1cXltFyltbY%253D&md5=c31d6f319d7c7a45aa9b716220e4a422

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    Andersson, K.; Malmqvist, P.-Å.; Roos, B. O. Second-Order Perturbation Theory with a Complete Active Space Self-Consistent Field Reference Function. J. Chem. Phys. 1992, 96, 1218– 1226, DOI: 10.1063/1.462209

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    Second-order perturbation theory with a complete active space self-consistent field reference function

    Andersson, Kerstin; Malmqvist, Per Aake; Roos, Bjoern O.

    Journal of Chemical Physics
    (1992),
    96
    (2),
    1218-26CODEN:
    JCPSA6;
    ISSN: 0021-9606.

    The recently implemented second-order perturbation theory based on a complete active space SCF ref. function has been extended by allowing the Fock-type one-electron operator, which defines the zeroth-order Hamiltonian to have nonzero elements also in nondiagonal matrix blocks. The computer implementation is now less straightforward and more computer time will be needed in obtaining the second-order energy. The method is illustrated in a series of calcns. on N2, NO, O2, CH3, CH2, and F-.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK38XptFKhsw%253D%253D&md5=9b11f4ec21a64ab3f5cfd44355156b1a

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    Zeng, T.; Hoffmann, R.; Ananth, N. The Low-Lying Electronic States of Pentacene and Their Roles in Singlet Fission. J. Am. Chem. Soc. 2014, 136, 5755– 5764, DOI: 10.1021/ja500887a

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    The Low-Lying Electronic States of Pentacene and Their Roles in Singlet Fission

    Zeng, Tao; Hoffmann, Roald; Ananth, Nandini

    Journal of the American Chemical Society
    (2014),
    136
    (15),
    5755-5764CODEN:
    JACSAT;
    ISSN: 0002-7863.

    (American Chemical Society)

    We present a detailed study of pentacene monomer and dimer that serves to reconcile extant views of its singlet fission. We obtain the correct ordering of singlet excited-state energy levels in a pentacene mol. (E (S1)

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXktlens7o%253D&md5=f7a7f04cad176c35966c447e4ae956c9

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    Grotjahn, R.; Maier, T. M.; Michl, J.; Kaupp, M. Development of a TDDFT-Based Protocol with Local Hybrid Functionals for the Screening of Potential Singlet Fission Chromophores. J. Chem. Theory Comput. 2017, 13, 4984– 4996, DOI: 10.1021/acs.jctc.7b00699

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    Development of a TDDFT-Based Protocol with Local Hybrid Functionals for the Screening of Potential Singlet Fission Chromophores

    Grotjahn, Robin; Maier, Toni M.; Michl, Josef; Kaupp, Martin

    Journal of Chemical Theory and Computation
    (2017),
    13
    (10),
    4984-4996CODEN:
    JCTCCE;
    ISSN: 1549-9618.

    (American Chemical Society)

    Chromophores suitable for singlet fission need to meet specific requirements regarding the relative energies of their S0, S1, and T1 (and T2) electronic states. Accurate quantum-chem. computations of the corresponding energy differences are highly desirable for materials design. Methods based on d. functional theory (DFT) have the advantage of being applicable to larger, often more relevant systems compared to more sophisticated post-Hartree-Fock methods. However, most exchange-correlation functionals do not provide the needed accuracy, in particular, due to an insufficient description of the T1 state. Here the authors use a recent singlet fission chromophore test set to evaluate a wide range of DFT-based methods, with an emphasis on local hybrid functionals with a position-dependent exact-exchange admixt. New ref. vertical CC2/CBS benchmark excitation energies for the test set were generated, which exhibit somewhat more uniform accuracy than the previous CASPT2-based data. These CC2 ref. data were used to evaluate a wide range of functionals, comparing full linear-response TDDFT, the Tamm-Dancoff approxn. (TDA), and ΔSCF calcns. Two simple 2-parameter local hybrid functionals and the more empirical M06-2X global meta-GGA hybrid provide the overall best accuracy. Due to its lower empiricism and wide applicability, the Lh12ct-SsifPW92 local hybrid is suggested as the main ingredient of an efficient computational protocol for prediction of the relevant excitation energies in singlet fission chromophores. Full TDDFT for the S1, S2, and T2 excitations is combined with ΔSCF for the T1 excitations. Making use also of some error compensation with suitable DFT-optimized structures, even the most crit. T1 excitations can be brought close to the target accuracy of 0.20 eV, while the other excitation energies are obtained even more accurately. This fully DFT-based protocol should become a useful tool in the field of singlet fission.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXhsVelu7vJ&md5=6c0e0f9703d2e210438c1029ea0900c7

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    Schleyer, P. v. R.; Maerker, C.; Dransfeld, A.; Jiao, H.; van Eikema
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    Nucleus-independent chemical shifts: a simple and efficient aromaticity probe

    Schleyer, Paul v.R.; Maerker, Christoph; Dransfeld, Alk; Jiao, Haijun; van Eikema Hommes, Nicolaas J. R.

    Journal of the American Chemical Society
    (1996),
    118
    (26),
    6317-6318CODEN:
    JACSAT;
    ISSN: 0002-7863.

    (American Chemical Society)

    Nucleus-independent chem. shifts (NICS), the neg. of the abs. magnetic shieldings (in ppm) computed at the ab initio GIAO-HF/6-31 + Glevel at ring centers (non-weighted means of the heavy atom coordinates), are proposed as a simple and efficient magnetic probe for characterizing aromaticity and antiaromaticity. For a series of five membered heterocycles, NICS correlate with arom. stabilization energies, magnetic susceptibility exaltations, and geometric criteria of aromaticity. Arom. compds. have neg. NICS (e.g., -9.7 for benzene and -15.1 for pyrrole), whereas antiarom. systems, in contrast, exhibit pos. NICS values (18.1 for pentalene and 27.6 for cyclobutadiene). In addn., NICS can characterize the individual rings in polycyclic arom. (e.g., -19.7 and -7.0 for the five- and seven-membered rings in azulene) and antiarom. (e.g., -2.5 and 22.5 for the six- and four-membered rings in benzocyclobutadiene) systems as well as the spherical aromaticity of cage compds., e.g., closo-B12H122- (-34.4) and the 1,3-dehydro-5,7-adamantanediyl dication (-50.1). The C60 NICS confirm that the 5-rings are paramagnetic and the 6-rings are diamagnetic, but the magnitudes are not large.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK28XjsFCis7Y%253D&md5=fd205be78733a8f593307d4863afb340

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    Fallah-Bagher-Shaidaei, H.; Wannere, C. S.; Corminboeuf, C.; Puchta, R.; Schleyer, P. v. R. Which NICS Aromaticity Index for Planar π Rings Is Best?. Org. Lett. 2006, 8, 863– 866, DOI: 10.1021/ol0529546

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    Which NICS Aromaticity Index for Planar π Rings Is Best?

    Fallah-Bagher-Shaidaei, Hossein; Wannere, Chaitanya S.; Corminboeuf, Clemence; Puchta, Ralph; Schleyer, Paul v. R.

    Organic Letters
    (2006),
    8
    (5),
    863-866CODEN:
    ORLEF7;
    ISSN: 1523-7060.

    (American Chemical Society)

    Five increasingly sophisticated aromaticity indexes, based on nucleus-independent chem. shifts (NICS), were evaluated against a uniform set of arom. stabilization energies (ASE) for 75 mono- and polyheterocyclic five-membered rings. While acceptable statistical correlations were given by all of the NICS methods, the most fundamentally grounded index, NICS(0)πzz (based on the π contribution to the out-of-plane zz tensor component), performed best statistically (cc=0.980) and in practice. The easily computable NICS(1)zz index is a useful alternative (cc=0.968).

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD28XpsVKgtQ%253D%253D&md5=c83cb9ff9c976543f65e5c7f05ec92d6

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    Wolinski, K.; Hinton, J. F.; Pulay, P. Efficient Implementation of the Gauge-Independent Atomic Orbital Method for NMR Chemical Shift Calculations. J. Am. Chem. Soc. 1990, 112, 8251– 8260, DOI: 10.1021/ja00179a005

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    Efficient implementation of the gauge-independent atomic orbital method for NMR chemical shift calculations

    Wolinski, Krzysztof; Hinton, James F.; Pulay, Peter

    Journal of the American Chemical Society
    (1990),
    112
    (23),
    8251-60CODEN:
    JACSAT;
    ISSN: 0002-7863.

    An implementation of the gauge independent at. orbital (GIAO) method for the ab initio SCF calcn. of NMR chem. shifts is described. By using modern techniques borrowed from anal. deriv. methods, the efficiency of the GIAO method was significantly improved. Results with several basis sets, some of them large, are presented for methane, Me fluoride, cyclopropene, cyclopropane, oxirane, benzene, carbon disulfide, the sulfate and thiosulfate anions, di-Me sulfide, DMSO, and di-Me sulfone. Computer timings for energy and chem. shielding calcns. are given for a few large org. mols. Comparisons are made with the individual gauge for localized orbitals method of Schindler and Kutzelnigg, and with the localized orbital/local origin method of Hansen and Bouman. The GIAO method appears to converge faster than the localized techniques, i.e., it provides the same accuracy with a smaller basis, particularly for the individual tensor components. The computational effort for the ab initio calcn. of the NMR chem. shifts is only ∼2.5 times that of the energy calcn.

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    Bultinck, P.; Rafat, M.; Ponec, R.; Van Gheluwe, B.; Carbó-Dorca, R.; Popelier, P. Electron Delocalization and Aromaticity in Linear Polyacenes: Atoms in Molecules Multicenter Delocalization Index. J. Phys. Chem. A 2006, 110, 7642– 7648, DOI: 10.1021/jp0609176

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    Electron Delocalization and Aromaticity in Linear Polyacenes: Atoms in Molecules Multicenter Delocalization Index

    Bultinck, Patrick; Rafat, Michel; Ponec, Robert; Van Gheluwe, Bart; Carbo-Dorca, Ramon; Popelier, Paul

    Journal of Physical Chemistry A
    (2006),
    110
    (24),
    7642-7648CODEN:
    JPCAFH;
    ISSN: 1089-5639.

    (American Chemical Society)

    Mol. aromaticity in the linear polyacenes is studied using an atoms in mols. based six center index (SCI-AIM) which measures the electron delocalization. SCI-AIM values for the linear polyacenes indicate decreasing aromaticity going from outer to inner rings in the polyacene series. The SCI-AIM approach is compared to a Mulliken-like approach, and a crit. comparison to the PDI index is made.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD28XkvFGltrY%253D&md5=8cefc19e4f1e2ddea2ae2e1d65e5d072

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    Krygowski, T. M. Crystallographic Studies of Inter- and Intramolecular Interactions Reflected in Aromatic Character of π-Electron Systems. J. Chem. Inf. Model. 1993, 33, 70– 78, DOI: 10.1021/ci00011a011

74. 74

    Krygowski, T. M.; Cyranski, M. K. Structural Aspects of Aromaticity. Chem. Rev. 2001, 101, 1385– 1419, DOI: 10.1021/cr990326u

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    Structural Aspects of Aromaticity

    Krygowski, Tadeusz Marek; Cyranski, Michal Ksawery

    Chemical Reviews (Washington, D. C.)
    (2001),
    101
    (5),
    1385-1419CODEN:
    CHREAY;
    ISSN: 0009-2665.

    (American Chemical Society)

    A review, with 240 refs. in which the importance of a notion in the chem. vocabulary and an outline of the criteria of aromaticity is discussed.

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    Bearpark, M. J.; Bernardi, F.; Olivucci, M.; Robb, M. A.; Smith, B. R. Can Fulvene S₁ Decay Be Controlled? A CASSCF Study with MMVB Dynamics. J. Am. Chem. Soc. 1996, 118, 5254– 5260, DOI: 10.1021/ja9542799

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    Can Fulvene S1 Decay Be Controlled? A CASSCF Study with MMVB Dynamics

    Bearpark, Michael J.; Bernardi, Fernando; Olivucci, Massimo; Robb, Michael A.; Smith, Barry R.

    Journal of the American Chemical Society
    (1996),
    118
    (22),
    5254-5260CODEN:
    JACSAT;
    ISSN: 0002-7863.

    (American Chemical Society)

    CASSCF and CASMP2 calcns. show that the min. on the fulvene S1 potential energy surface is an S0/S1 conical intersection with a 90° twisted methylene group. We have also located a distinct planar azulene-like crossing point at higher energy, where the methylene is free to twist. The fulvene intersection – which exists for all twist angles – leads to efficient, irreversible radiationless decay and explains the lack of obsd. S1 fluorescence. We have modeled the femtosecond excited state motion leading to ultrafast decay that would be initiated by exciting the 0-0 and higher vibrational levels using semiclassical mol. dynamics with a hybrid quantum-mech./force-field potential (MMVB). Our simulation suggests that, with increased vibrational energy, decay occurs in the planar crossing region before relaxation by twisting can take place, and that isomerization might only be seen by pumping the 0-0 transition in laser studies.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK28XivFyntb0%253D&md5=6b2b454acfba778a7c8b5928cd9c738c

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    Mendive-Tapia, D.; Lasorne, B.; Worth, G. A.; Bearpark, M. J.; Robb, M. A. Controlling the Mechanism of Fulvene S₁/S₀ Decay: Switching Off the Stepwise Population Transfer. Phys. Chem. Chem. Phys. 2010, 12, 15725– 15733, DOI: 10.1039/c0cp01757d

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    Controlling the mechanism of fulvene S1/S0 decay: switching off the stepwise population transfer

    Mendive-Tapia, David; Lasorne, Benjamin; Worth, Graham A.; Bearpark, Michael J.; Robb, Michael A.

    Physical Chemistry Chemical Physics
    (2010),
    12
    (48),
    15725-15733CODEN:
    PPCPFQ;
    ISSN: 1463-9076.

    (Royal Society of Chemistry)

    Direct quantum dynamics simulations were performed to model the radiationless decay of the first excited state S1 of fulvene. The full space of thirty normal mode nuclear coordinates was explicitly considered. By default, ultrafast internal conversion takes place centered on the higher-energy planar region of the S1/S0 conical intersection seam, giving the stepwise population transfer characteristic of a sloped surface crossing, and leading back to the ground state reactant. Two possible schemes for controlling whether stepwise population transfer occurs or not-either altering the initial geometry distribution or the initial momentum compn. of the photo-excited wavepacket-were explored. In both cases, decay was successfully induced to occur in the lower-energy twisted/peaked region of the crossing seam, switching off the stepwise population transfer. This absence of re-crossing is a direct consequence of the change in the position on the intersection at which decay occurs (our target for control), and its consequences should provide an exptl. observable fingerprint of this system.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXhsVyrt7jF&md5=ea4a9b217937390b6bd4f72f6f814b5b

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    Gogonea, V.; Schleyer, P. v. R.; Schreiner, P. R. Consequences of Triplet Aromaticity in 4nπ-Electron Annulenes: Calculation of Magnetic Shieldings for Open-Shell Species. Angew. Chem., Int. Ed. 1998, 37, 1945– 1948, DOI: 10.1002/(SICI)1521-3773(19980803)37: 13/143.0.CO;2-E

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    Consequences of triplet aromaticity in 4nπ-electron annulenes: calculation of magnetic shieldings for open-shell species

    Gogonea, Valentin; Schleyer, Paul von Rague; Schreiner, Peter R.

    Angewandte Chemie, International Edition
    (1998),
    37
    (13/14),
    1945-1948CODEN:
    ACIEF5;
    ISSN: 1433-7851.

    (Wiley-VCH Verlag GmbH)

    This paper presents evidence that triplet states of 4nπ-electron annulenes are arom. The first comprehensive ab initio [B3LYP and CCSD(T) levels] calcns. of adiabatic singlet-triplet sepns., arom. stabilization energies, and the magnetic properties of six neutral or charged 4nπ-electron annulenes are reported. Chem. shifts, magnetic susceptibilities, and magnetic susceptibility exaltations for singlet and triplet species were calcd. at the GIAO-SCF level.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK1cXlsFWgsbY%253D&md5=2a97a2b3c22e35a0acf48aef37565c94

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    Villaume, S.; Fogarty, H. A.; Ottosson, H. Triplet-State Aromaticity of 4n -Electron Monocycles: Analysis of Bifurcation in the π Contribution to the Electron Localization Function. ChemPhysChem 2008, 9, 257– 264, DOI: 10.1002/cphc.200700540

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    Triplet-state aromaticity of 4nπ-electron monocycles: analysis of bifurcation in the π contribution to the electron localization function

    Villaume, Sebastien; Fogarty, Heather A.; Ottosson, Henrik

    ChemPhysChem
    (2008),
    9
    (2),
    257-264CODEN:
    CPCHFT;
    ISSN: 1439-4235.

    (Wiley-VCH Verlag GmbH & Co. KGaA)

    The π contribution to the electron localization function (ELF) is used to compare 4nπ- and (4n + 2)π-electron annulenes, with particular focus on the aromaticity of 4nπ-electron annulenes in their lowest triplet state. The anal. is performed on the electron d. obtained at the level of OLYP d. functional theory, as well as at the CCSD and CASSCF ab initio levels. Two criteria for aromaticity of all-carbon annulenes are set up: the span in the bifurcation values ΔBV(ELFπ) should be small, ideally zero, and the bifurcation value for ring closure of the π basin RCBV(ELFπ) should be high (≥ 0.7). From these criteria, nearly all 4nπ-electron annulenes are arom. in their lowest triplet states, similar to (4n + 2)π-electron annulenes in their singlet ground states. For singlet biradical cyclobutadiene and cyclooctatetraene constrained to D4h and D8h symmetry, resp., the RCBV(ELFπ) at the CASSCF level is lower (0.531 and 0.745) than for benzene (0.853), even though they have equal proportions of α- and β-electrons.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1cXltVKqsbk%253D&md5=d1ec85d8d39aed6e2f4b99aa836c630e

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    Gao, X.; Bai, S.; Fazzi, D.; Niehaus, T.; Barbatti, M.; Thiel, W. Evaluation of Spin-Orbit Couplings with Linear-Response Time-Dependent Density Functional Methods. J. Chem. Theory Comput. 2017, 13, 515– 524, DOI: 10.1021/acs.jctc.6b00915

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    Evaluation of Spin-Orbit Couplings with Linear-Response Time-Dependent Density Functional Methods

    Gao, Xing; Bai, Shuming; Fazzi, Daniele; Niehaus, Thomas; Barbatti, Mario; Thiel, Walter

    Journal of Chemical Theory and Computation
    (2017),
    13
    (2),
    515-524CODEN:
    JCTCCE;
    ISSN: 1549-9618.

    (American Chemical Society)

    A new versatile code based on Python scripts was developed to calc. spin-orbit coupling (SOC) elements between singlet and triplet states. The code, named PySOC, is interfaced to third-party quantum chem. packages, such as Gaussian 09 and DFTB+. SOCs are evaluated using linear-response (LR) methods based on time-dependent d. functional theory (TDDFT), the Tamm-Dancoff approxn. (TDA), and time-dependent d. functional tight binding (TD-DFTB). The evaluation employs Casida-type wave functions and the Breit-Pauli (BP) spin-orbit Hamiltonian with an effective charge approxn. For validation purposes, SOCs calcd. with PySOC are benchmarked for several org. mols., with SOC values spanning several orders of magnitudes. The computed SOCs show little variation with the basis set, but are sensitive to the chosen d. functional. The benchmark results are in good agreement with ref. data obtained using higher-level spin-orbit Hamiltonians and electronic structure methods, such as CASPT2 and DFT/MRCI. PySOC can be easily interfaced to other third-party codes and other methods yielding CI-type wave functions.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XitVKqtbrK&md5=875811e61c508533165946a658d2e29a

80. 80

    Samanta, P. K.; Kim, D.; Coropceanu, V.; Brédas, J. L. Up-Conversion Intersystem Crossing Rates in Organic Emitters for Thermally Activated Delayed Fluorescence: Impact of the Nature of Singlet vs Triplet Excited States. J. Am. Chem. Soc. 2017, 139, 4042– 4051, DOI: 10.1021/jacs.6b12124

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    Up-Conversion Intersystem Crossing Rates in Organic Emitters for Thermally Activated Delayed Fluorescence: Impact of the Nature of Singlet vs. Triplet Excited States

    Samanta, Pralok K.; Kim, Dongwook; Coropceanu, Veaceslav; Bredas, Jean-Luc

    Journal of the American Chemical Society
    (2017),
    139
    (11),
    4042-4051CODEN:
    JACSAT;
    ISSN: 0002-7863.

    (American Chemical Society)

    The rates for up-conversion intersystem crossing (UISC) from the T1 state to the S1 state are calcd. for a series of org. emitters with an emphasis on thermally activated delayed fluorescence (TADF) materials. Both the spin-orbit coupling and the energy difference between the S1 and T1 states (ΔEST) are evaluated, at the d. functional theory (DFT) and time-dependent DFT levels. The calcd. UISC rates and ΔEST values are in good agreement with available exptl. data. The authors’ results underline that small ΔEST values and sizable spin-orbit coupling matrix elements have to be simultaneously realized to facilitate UISC and ultimately TADF. Importantly, the spatial sepn. of the highest occupied and lowest unoccupied MOs of the emitter, a widely accepted strategy for the design of TADF mols., does not necessarily lead to a sufficient redn. in ΔEST; in fact, either a significant charge-transfer (CT) contribution to the T1 state or a minimal energy difference between the local-excitation and charge-transfer triplet states is required to achieve a small ΔEST. Also, having S1 and T1 states of a different nature is found to strongly enhance spin-orbit coupling, which is consistent with the El-Sayed rule for ISC rates. Overall, the authors’ results indicate that having either similar energies for the local-excitation and charge-transfer triplet states or the right balance between a substantial CT contribution to T1 and somewhat different natures of the S1 and T1 states, paves the way toward UISC enhancement and thus TADF efficiency improvement.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXjsFaltbc%253D&md5=178d3a35f1c218c9a3e2d69ef9b8a97c

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    Marian, C. M. Spin-Orbit Coupling and Intersystem Crossing in Molecules. Wiley Interdiscip. Rev. Comput. Mol. Sci. 2012, 2, 187– 203, DOI: 10.1002/wcms.83

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    Spin-orbit coupling and intersystem crossing in molecules

    Marian, Christel M.

    Wiley Interdisciplinary Reviews: Computational Molecular Science
    (2012),
    2
    (2),
    187-203CODEN:
    WIRCAH;
    ISSN: 1759-0884.

    (Wiley-Blackwell)

    A review. Many light-induced mol. processes involve a change in spin state and are formally forbidden in non-relativistic quantum theory. To make them happen, spin-orbit coupling (SOC) has to be invoked. Intersystem crossing (ISC), the nonradiative transition between two electronic states of different multiplicity, plays a key role in photochem. and photophysics with a broad range of applications including mol. photonics, biol. photosensors, photodynamic therapy, and materials science. Quantum chem. has become a valuable tool for gaining detailed insight into the mechanisms of ISC. After a short introduction highlighting the importance of ISC and a brief description of the relativistic origins of SOC, this article focusses on approx. SOC operators for practical use in mol. applications and reviews state-of-the-art theor. methods for evaluating ISC rates. Finally, a few sample applications are discussed that underline the necessity of studying the mechanisms of ISC processes beyond qual. rules such as the El-Sayed rules and the energy gap law.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38Xjs1Kltrk%253D&md5=86c7360557a3d0c769f984b41edc6406

82. 82

    Finke, A. D.; Jahn, B. O.; Saithalavi, A.; Dahlstrand, C.; Nauroozi, D.; Haberland, S.; Gisselbrecht, J.-P.; Boudon, C.; Mijangos, E.; Schweizer, W. B.; Ott, S.; Ottosson, H.; Diederich, F. The 6,6-Dicyanopentafulvene Core: A Template for the Design of Electron-Acceptor Compounds. Chem. – Eur. J. 2015, 21, 8168– 8176, DOI: 10.1002/chem.201500379

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    The 6,6-Dicyanopentafulvene Core: A Template for the Design of Electron-Acceptor Compounds

    Finke, Aaron D.; Jahn, Burkhard O.; Saithalavi, Anas; Dahlstrand, Christian; Nauroozi, Djawed; Haberland, Sophie; Gisselbrecht, Jean-Paul; Boudon, Corinne; Mijangos, Edgar; Schweizer, W. Bernd; Ott, Sascha; Ottosson, Henrik; Diederich, Francois

    Chemistry – A European Journal
    (2015),
    21
    (22),
    8168-8176CODEN:
    CEUJED;
    ISSN: 0947-6539.

    (Wiley-VCH Verlag GmbH & Co. KGaA)

    The electron-accepting ability of 6,6-dicyanopentafulvenes (DCFs) can be varied extensively through substitution on the five-membered ring. The redn. potentials for a set of 2,3,4,5-tetraphenyl-substituted DCFs, with varying substituents at the para-position of the Ph rings, strongly correlate with their Hammett σp-parameters. By combining cyclic voltammetry with DFT calcns. ((U)B3LYP/6-311+G(d)), using the conductor-like polarizable continuum model (CPCM) for implicit solvation, the abs. redn. potentials of a set of twenty DCFs were reproduced with a mean abs. deviation of 0.10 eV and a max. deviation of 0.19 eV. The exptl. studied DCFs have redn. potentials within 3.67-4.41 eV, however, the computations reveal that DCFs with exptl. redn. potentials ≤5.3 eV could be achieved, higher than that of F4-TCNQ (5.02 eV). Thus, the DCF core is a template that allows variation in the redn. potentials by ∼1.6 eV.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXnt1Wjurw%253D&md5=d9b28a1b3a701cf5b914c1229bc96f82

83. 83

    Sekiguchi, A.; Tanaka, M.; Matsuo, T.; Watanabe, H. From a Cyclobutadiene Dianion to a Cyclobutadiene: Synthesis and Structural Characterization of Tetrasilyl-Substituted Cyclobutadiene. Angew. Chem., Int. Ed. 2001, 40, 1675– 1677, DOI: 10.1002/1521-3773(20010504)40:93.0.CO;2-G

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    From a cyclobutadiene dianion to a cyclobutadiene: synthesis and structural characterization of tetrasilyl-substituted cyclobutadiene

    Sekiguchi, Akira; Tanaka, Masanobu; Matsuo, Tsukasa; Watanabe, Hidetoshi

    Angewandte Chemie, International Edition
    (2001),
    40
    (9),
    1675-1677CODEN:
    ACIEF5;
    ISSN: 1433-7851.

    (Wiley-VCH Verlag GmbH)

    Lithiation of tetrasilyl cyclobutadiene cobalt complex I with lithium in THF followed by reaction with BrCH2CH2Br gave cyclobutadiene deriv. II which on photolysis gave cyclododecadiyne III. The crystal structure of II was detd.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD3MXjslChurY%253D&md5=135e594aa9c206992860456c9e19a014

84. 84

    Maier, G.; Neudert, J.; Wolf, O. Tetrakis(Trimethylsilyl)Cyclobutadiene and Tetrakis(Trimethylsilyl)Tetrahedrane. Angew. Chem., Int. Ed. 2001, 40, 1674– 1675, DOI: 10.1002/1521-3773(20010504)40:93.0.CO;2-I

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    Tetrakis(trimethylsilyl)cyclobutadiene and tetrakis(trimethylsilyl)tetrahedrane

    Maier, Gunther; Neudert, Jorg; Wolf, Oliver

    Angewandte Chemie, International Edition
    (2001),
    40
    (9),
    1674-1675CODEN:
    ACIEF5;
    ISSN: 1433-7851.

    (Wiley-VCH Verlag GmbH)

    Coupling reaction of tris(trimethylsilyl)cyclopropenylium hexachloroantimonate with lithiated (trimethylsilyl)diazomethane gave 11% trimethylsilyl[1,2,3-tris(trimethylsilyl)-2-cyclopropen-1-yl]diazomethane which on thermolysis at 50° gave tetrakis(trimethylsilyl)cyclobutadiene (8) along-with Me3SiN:C:C(SiMe3)C(SiMe3):C:NSiMe3. Irradn. of a mixt. of 8 gave title tetrakis(trimethylsilyl)tetrahedrane.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD3MXjslChurg%253D&md5=1e142ee8dd1c2b5ad6bc00ced811cae2

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    Hafner, K.; Süss, H. U. 1,3,5-Tri- Tert-Butylpentalene. A Stabilized Planar 8π-Electron System. Angew. Chem., Int. Ed. Engl. 1973, 12, 575– 577, DOI: 10.1002/anie.197305751

86. 86

    Levi, Z. U.; Tilley, T. D. Versatile Synthesis of Pentalene Derivatives via the Pd-Catalyzed Homocoupling of Haloenynes. J. Am. Chem. Soc. 2009, 131, 2796– 2797, DOI: 10.1021/ja809930f

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    Versatile Synthesis of Pentalene Derivatives via the Pd-Catalyzed Homocoupling of Haloenynes

    Levi, Zerubba U.; Tilley, T. Don

    Journal of the American Chemical Society
    (2009),
    131
    (8),
    2796-2797CODEN:
    JACSAT;
    ISSN: 0002-7863.

    (American Chemical Society)

    A palladium-catalyzed, reductive homocoupling of haloenynes, e.g. ArC≡CPh (Ar=2-BrC6H4, 2-Br-3-thienyl), yields a diverse range of polysubstituted pentalene derivs., including annulated dibenzopentalenes, e.g. I, and dithienylpentalenes, e.g. II.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXhsFKhur0%253D&md5=a07efe544c93fe3d1dafc743fcb840db

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    Miyamoto, T.; Odaira, Y. The Reaction of Phenanthro[ζ]cyclobutadiene. Tetrahedron Lett. 1973, 14, 43– 46, DOI: 10.1016/S0040-4039(01)95572-8

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    Wu, Y.; Wang, Y.; Chen, J.; Zhang, G.; Yao, J.; Zhang, D.; Fu, H. Intramolecular Singlet Fission in an Antiaromatic Polycyclic Hydrocarbon. Angew. Chem., Int. Ed. 2017, 56, 9400– 9404, DOI: 10.1002/anie.201704668

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    Intramolecular singlet fission in an antiaromatic polycyclic hydrocarbon

    Wu, Yishi; Wang, Yuancheng; Chen, Jianwei; Zhang, Guanxin; Yao, Jiannian; Zhang, Deqing; Fu, Hongbing

    Angewandte Chemie, International Edition
    (2017),
    56
    (32),
    9400-9404CODEN:
    ACIEF5;
    ISSN: 1433-7851.

    (Wiley-VCH Verlag GmbH & Co. KGaA)

    Singlet fission (SF), in which one singlet exciton (S1) splits into two triplets (T1) on adjacent mols. through a correlated triplet-pair 1(TT) state, requires precise but difficult tuning of exciton energetics and intermol. electronic couplings in the solid state. Antiarom. 4nπ dibenzopentalenes (DPs) are demonstrated as a new class of single-chromophore-based intramol. SF materials that exhibit an optically allowed S2 state with E(S2)> 2 × E(T1) and an optically forbidden S1 state. Ultrafast population transfer from a high-lying S2 state to a 1(TT) state was obsd. in monomeric soln. of styryl-substituted DP (SDP) on a sub-picosecond timescale. There is evidence of exciton diffusion (ED) of the 1(TT) state to yield two individual long-lived triplets in SDP thin film. The overall triplet yield via intramol. SF and subsequent triplet-pair diffusion can be as high as 142 ± 10 % in thin film.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXhtFensr3J&md5=eadbf6ec7c2d393250af8570cee6fb44

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    Hudson, B. S.; Kohler, B. E. Polyene Spectroscopy: The Lowest Energy Excited Singlet State of Diphenyloctatetraene and Other Linear Polyenes. J. Chem. Phys. 1973, 59, 4984– 5002, DOI: 10.1063/1.1680717

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    Polyene spectroscopy. Lowest energy excited singlet state of diphenyloctatetraene and other linear polyenes

    Hudson, Bruce S.; Kohler, Bryan E.

    Journal of Chemical Physics
    (1973),
    59
    (9),
    4984-5002CODEN:
    JCPSA6;
    ISSN: 0021-9606.

    Optical absorption and emission spectra at liq. He temps. of solns. of all-trans-1,8-diphenyloctatetraene in single crystals of bibenzyl and polycryst. n-paraffin matrixes are presented which show an excited singlet state at a lower energy than the 1Bu state responsible for the strong absorption of all linear polyenes. The transition from the ground state to this level has an oscillator strength of ∼0.05. The location of this low-energy weak transition removes a no. of discrepancies between the expected and obsd. fluorescence properties of diphenyloctatetraene. The obsd. vibronic pattern is consistent with the Raman spectrum which is also reported. Substantial indirect evidence is presented which indicates that many, and perhaps all, linear polyenes have a similar ordering of excited states. A review of the available information on diphenylpolyenes is presented.

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    Holtom, G. R.; McClain, W. M. Two-photon excitation spectra of the low energy excited states of diphenylhexatriene and diphenyloctatetraene. Chem. Phys. Lett. 1976, 44, 436– 439, DOI: 10.1016/0009-2614(76)80699-9

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    Two-photon excitation spectra of the low energy excited states of diphenylhexatriene and diphenyloctatetraene

    Holtom, G. R.; McClain, W. M.

    Chemical Physics Letters
    (1976),
    44
    (3),
    436-9CODEN:
    CHPLBC;
    ISSN: 0009-2614.

    All-trans-diphenylhexatriene and diphenyloctatetraene in soln. show a very strong 2-photon absorption beginning at about the same energy as the one-photon spectrum. This supports the postulated presence of a low-lying 1Ag electronic state.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaE2sXmtlSisg%253D%253D&md5=d4c0ade0dc5e7a0a6955f4e9f4047d33

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    Fiedor, L.; Dudkowiak, A.; Pilch, M. The Origin of the Dark S₁ State in Carotenoids: A Comprehensive Model. J. R. Soc., Interface 2019, 16, 20190191, DOI: 10.1098/rsif.2019.0191

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    The origin of the dark S1 state in carotenoids: a comprehensive model

    Fiedor, Leszek; Dudkowiak, Alina; Pilch, Mariusz

    Journal of the Royal Society, Interface
    (2019),
    16
    (158),
    20190191/1-20190191/11CODEN:
    JRSICU;
    ISSN: 1742-5662.

    (Royal Society)

    A review. In carotenoids, by analogy to polyenes, the symmetry of the π-electron system is often invoked to explain their peculiar electronic features, in particular the inactivity of the S0 → S1 transition in one-photon excitation. In this review, we verify whether the mol. symmetry of carotenoids and symmetry of their π-electron system are supported in exptl. and computational studies. We focus on spectroscopic techniques which are sensitive to the electron d. distribution, including the X-ray crystallog., electronic absorption, two-photon techniques, CD, NMR, Stark and vibrational spectroscopies, and on this basis we seek for the origin of inactivity of the S1 state. We come across no exptl. and computational evidence for the symmetry effects and the existence of symmetry restrictions on the electronic states of carotenoids. They do not possess an inversion center and the C2h symmetry approxn. of carotenoid structure is by no means justified. In effect, the application of symmetry rules (and notification) to the electronic states of carotenoids in this symmetry group may lead to a wrong interpretation of exptl. data. This conclusion together with the results summarized in the review allows us to advance a consistent model that explains the inactivity of the S0 → S1 transition. Within this model, S1 is never accessible from S0 due to the neg. synergy of (i) the contributions of double excitations of very low probability, which elevate S1 energy, and (ii) a non-verticality of the S0 → S1 transition, due to the breaking of Born-Oppenheimer approxn. Certainly, our simple model requires a further exptl. and theor. verification.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXms1yjtQ%253D%253D&md5=9b6ee2db80fe0577ed2d5282481f95bc

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    Baker, W.; McOmie, J. F. W.; Parfitt, S. D.; Watkins, D. A. M. 799. Attempts to Prepare New Aromatic Systems. Part VI. 1:2–5:6-Dibenzopentalene and Derivatives. J. Chem. Soc. 1957, 0, 4026– 4037, DOI: 10.1039/JR9570004026

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    92

    Attempts to prepare new aromatic systems. VI. 1,2,5,6-Dibenzopentalene and derivatives

    Baker, Wilson; McOmie, J. F. W.; Parfitt, S. D.; Watkins, D. A. M.

    Journal of the Chemical Society
    (1957),
    
    (),
    4026-37CODEN:
    JCSOA9;
    ISSN: 0368-1769.

    2-Benzylideneindan-1,3-dione was added to PhMgBr (from 8.3 g. PhBr) in Et2O, boiled 1 hr., poured into dil. HCl, and on elution from Al2O3 with alc. gave 2-diphenylmethyl-3-phenylinden-1-one (I), m. 154° [differs from Kohler’s (C.A. 1, 1849) conclusion that 3,4-dihydro-3-oxo-4-phenyl-1,2,5,6-dibenzopentalene (II) was formed]; 2,4-dinitrophenylhydrazone, m. 249°; oxime, m. 159-62°. 2-(Diphenylmethyl)indan-1,3-dione (III), m. 128-9°, was similarly prepd. by using increased amts. of reagents. III and PhMgBr gave I. I with Cr2O3 in HOAc gave Ph2CO and 2-PhCOC6H4CO2H while III gave Ph2CO, Ph2CHCO2H, and phthalic acid. Ph2CHCH(CO2H)2 (2 g.) was heated with 20 g. polyphosphoric acid at 120° 1 hr., poured into H2O, and crystd. from C6H6 to give 3,4,7,8-tetrahydro-3,4-dioxo-1,2,5,6-dibenzopentalene (IV), m. 259°, sol. in hot but not cold aq. NaOH, gives a CHCl3 sol. green Cu deriv., gives no color with alc. FeCl3, and remains unchanged by Na in hot ethylene glycol; mono-2,4-dinitrophenylhydrazone, m. 297° (decompn.); mono-4-toluenesulfonylhydrazone, m. 116°. 3-Phenylindan-1-one (V) (2,4-dinitrophenylhydrazone, m. 209-10°) and Ph2CHCH2CO2H were prepd. from cinnamic acid and AlCl3. V (21 g.) in 50 cc. (EtO)2CO (VI) was added slowly to 150 cc. VI in which 2.3 g. Na was dissolved and heated at 150° to give trans-Et 1-oxo-3-phenylindan-2-carboxylate (VII), m. 103-4°; 2,4-dinitrophenylhydrazone, m. 179°. The ester prepd. by Yost (C.A. 45, 2928i) was probably the cis isomer. VII (2 g.) was heated at 160° 3 min. with excess polyphosphoric acid and poured into H2O to give IV. V (10 g.), 9 g. (CO2Et)2, and 40 cc. EtOH was added with stirring to 100 cc. warm EtOH contg. 10 g. Na and poured into dil. HCl giving 1-oxo-3-phenyl-2-indanylglyoxylic acid (VIII), m. 213°, and probably 3-phenyl-2-(3-phenyl-1-indenylidene)indan-1-one, m. 185°; Me ester of VIII, m. 148°. Cyclization of VIII with polyphosphoric acid gave IV. IV (1.0 g.), 6 g. Zn-Hg, 20 cc. H2O, 50 cc. concd. HCl, 1 cc. HOAc, and 5 cc. MePh was refluxed 40 hrs. giving 3,4,7,8-tetrahydro-1,2,5,6-dibenzopentalene (IX), m. 95°. IX (0.2 g. and 0.3 g. chloranil was boiled in 10 cc. C6H6 14 hrs., poured into dil. NaOH, and extd. with Et2O to give 3(or 7)-(2,3,5,6-tetrachloro-4-hydroxyphenoxy)-1,2,5,6-dibenzopentalene (X), m. 210°. Sublimation of X at 230°/12 mm. gave IX. IV (0.5 g.) and 5 g. PCl5 was heated at 100° 5 min. to give 3,3,4,4,7,8-hexachloro-3,4,7,8-tetrahydro-1,2,5,6-dibenzopentalene, m. 207°, and 8(or 7)-chloro-3,4,7,8-tetrahydro-3,4-dioxo-1,2,5,6-dibenzopentalene, m. 172°. IV with LiAlH4 gave 2 stereoisomers of 3,4,7,8-tetrahydro-3,4-dihydroxy-1,2,5,6-dibenzopentalene: isomer A, m. 262° (di-Ac deriv., m. 109-10°; Bz deriv., m. 169°); isomer B, m. 200° (di-Ac deriv., m. 158°). IV (1 g.) in Et2O was boiled with MeMgI and 3,4,7,8-tetrahydro-3-methylene-4-oxo-1,2,5,6-dibenzopentalene (XI), m. 156°; sepd. XI gives a red soln. with concd. H2SO4. 2-Benzylidene-3-phenylindan-1-one (XIa) (5 g.), 200 cc. C6H6, and 50 g. AlCl3 was refluxed 6 hrs. and poured into H2O giving 3,4,7,8-tetrahydro-3-oxo-4-phenyl-1,2,5,6-dibenzopentalene (XII), m. 132°; 2,4-dinitrophenylhydrazone, m. 271°. To 2.5 g. XII in a warm soln. of 1 g. Na in 50 cc. ethylene glycol was added pure N2H4, the mixt. refluxed 20 hrs., poured into H2O, and extd. with Et2O to give 3,4,7,8-tetrahydro-3-phenyl-1,2,5,6-dibenzopentalene (XIII), m. 112°. XII (10 g.) and 10 g. PCl5 in 100 cc. C6H6 was boiled 10 hrs., distd. to dryness in vacuo, and crystd. from light petr. giving 3,3-dichloro-3,4,7,8-tetrahydro-4-phenyl-1,2,5,6-dibenzopentalene (XIV), m. 151°, which hydrolyzes in damp air or alc. KOH or reacts with Ag2O in anhyd. C6H6 to give XII. XIV with Zn-HCl gave XIII. When XIV is melted under reduced pressure or boiled in pyridine 45 min., 3,3,4(or 7)-trichloro-3,4,7,8-tetrahydro-4-phenyl-1,2,5,6-dibenzopentalene, m. 214° (decompn.), is obtained. The 4-toluenesulfonylhydrazone of XII, m. 204°, reacts with Na in ethylene glycol to give 3,4,7,8-tetrahydro-3-(2-hydroxyethoxy)-4-phenyl-1,2,5,6-dibenzopentalene, m. 147°, and similarly replacing glycol with cyclohexanol as solvent XII gave 3-cyclohexyloxy-3,4,7,8-tetrahydro-4-phenyl-1,2,5,6-dibenzopentalene, m. 133°. XII with LiAlH4 gave 3,4,7,8-tetrahydro-3-hydroxy-4-phenyl-1,2,5,6-dibenzopentalene (XV), m. 176-8° (Ac deriv., m. 151°), while reduction with Al(OCHMe2)3 gave a 2nd isomer (XVI) of XV, m. 148°; Ac deriv., m. 145°. The Ac derivs. of XV and XVI do not decomp. on heating alone or with anhyd. K2CO3. XV (1 g.) and 1 g. anhyd. CuSO4 was boiled 4 hrs. in 30 cc. xylene, then filtered, and the residue washed with Et2O. Evapn. of the filtrate and washings gave 4,7(?)-dihydro-4-phenyl-1,2,5,6-dibenzopentalene (XVII), m. 178-80°, and an unknown compd. (XVIII), m. 158-9°. XVII and XVIII were also prepd. by heating XV with anhyd. CuSO4 7 hrs., but after 10 hrs. only XVIII was isolated. A similar dehydration of XVI gave XVII after 1.5 hrs. and XVIII after 3 hrs. boiling. When either XV or XVI was heated with P2O5 in C6H6 a substance was formed which melted at about 60°, solidified at about 80°, and then m. 154°, ν 745 and 702 cm.-1 Dry air contg. Br and CHCl3 passed through a CHCl3 soln. of XII yielded 3,4-dihydro-3-oxo-4-phenyl-1,2,5,6-dibenzopentalene (XIX), m. 266-9° (decompn.). IV and PhMgBr refluxed 1 hr. gave 3,4,7,8-tetrahydro-3-hydroxy-4-oxo-3-phenyl-1,2,5,6-dibenzopentalene (XX), m. 120°. Similarly, IV and excess PhMgBr or PhLi gave XIX and XX. The epoxide of XIa (XXI), m. 164°, was prepd. by the reaction of H2O2 with XIa in the presence of NaOH. 1,3-Dihydroxy-2,4-diphenylnaphthalene, n. 163-5°, was prepd. by the reaction of XXI with polyphosphoric acid at 160° and 192°, by heating with excess concd. HCl 3 hrs., or by reaction with BF3. XII and PhMgBr in Et2O boiled 5 hrs. gave 3,4,7,8-tetrahydro-3-hydroxy-3,4-diphenyl-1,2,5,6-dibenzopentalene (XXII), m. 160°. Dehydration of XXII with anhyd. CuSO4 gave 4,7-dihydro-3,4-diphenyl-1,2,5,6-dibenzopentalene (XXIII), m. 202-4°. XXIII in CCl4 with oxides of N prepd. from fuming HNO3, H2SO4, and As2O3 gave a cryst. addn. compd., C28H20N2O4, m. 242°.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaG1cXisFCnuw%253D%253D&md5=4d4659ba869a3fbe16dfcf2bd812856d

93. 93

    Oshima, H.; Fukazawa, A.; Yamaguchi, S. Facile Synthesis of Polycyclic Pentalenes with Enhanced Hückel Antiaromaticity. Angew. Chem., Int. Ed. 2017, 56, 3270– 3274, DOI: 10.1002/anie.201611344

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    93

    Facile Synthesis of Polycyclic Pentalenes with Enhanced Hueckel Antiaromaticity

    Oshima, Hiroya; Fukazawa, Aiko; Yamaguchi, Shigehiro

    Angewandte Chemie, International Edition
    (2017),
    56
    (12),
    3270-3274CODEN:
    ACIEF5;
    ISSN: 1433-7851.

    (Wiley-VCH Verlag GmbH & Co. KGaA)

    Pentalenes represent highly reactive Hueckel antiaroms. with 8π electrons. Usually, pentalenes are stabilized by incorporation of two benzene rings in a fused fashion. In dibenzo[a,e]pentalenes, however, the high aromaticity of the fused benzene rings compromises the inherent antiaromaticity of the pentalene core. Herein, the authors disclose that this forfeited antiaromaticity can be restored by fusing four addnl. arom. rings onto the peripheral positions of dibenzo[a,e]pentalenes. Such polycyclic pentalenes were prepd. by successive transannular cyclizations via in situ-generated tetrakisdehydro[16]annulenes. The thus obtained compds. showed intriguing properties, for example, characteristic absorptions in the visible-to-near-IR (NIR) region and low redn. potentials. These results hence afford a design principle to produce highly antiarom. yet stable pentalenes. The antiaromaticity of the pentalene core can be widely tuned via the degree of aromaticity of the peripherally fused rings.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXlsVyltg%253D%253D&md5=896814635a6672ca1309b0416ded7118

94. 94

    Konishi, A.; Okada, Y.; Kishi, R.; Nakano, M.; Yasuda, M. Enhancement of Antiaromatic Character via Additional Benzoannulation into Dibenzo[a,f]Pentalene: Syntheses and Properties of Benzo[a]Naphtho[2,1-f]Pentalene and Dinaphtho[2,1-a,f]Pentalene. J. Am. Chem. Soc. 2019, 141, 560– 571, DOI: 10.1021/jacs.8b11530

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    Enhancement of Antiaromatic Character via Additional Benzoannulation into Dibenzo[a,f]pentalene: Syntheses and Properties of Benzo[a]naphtho[2,1-f]pentalene and Dinaphtho[2,1-a,f]pentalene

    Konishi, Akihito; Okada, Yui; Kishi, Ryohei; Nakano, Masayoshi; Yasuda, Makoto

    Journal of the American Chemical Society
    (2019),
    141
    (1),
    560-571CODEN:
    JACSAT;
    ISSN: 0002-7863.

    (American Chemical Society)

    Understanding the structure-property relationships in antiarom. mols. is crucial for controlling their electronic properties and designing new org. optoelectronic materials. Dibenzo[a,f]pentalene, a structural isomer of dibenzopentalene, displays open-shell and antiarom. character harmonization, which is not shared by the well-known isomer, dibenzo[a,e]pentalene. The next questions of interest concern the topol. effects of the π-extension on the harmonization of the open-shell and antiarom. character in the dibenzo[a,f]pentalene π-system. Herein, we describe the synthesis and characterization of the π-extended (bis)annulated analogs, benzo[a]naphtho[2,1-f]pentalene 4 and dinaphtho[2,1-a,f]pentalene 5. The solid-state structures and the magnetic and optoelectronic properties characterized these π-extended analogs as closed-shell antiarom. mols., in sharp contrast with dibenzo[a,f]pentalene 2. In these π-extended analogs, the open-shell character was annihilated whereas the antiarom. character was retained. The fusion of addnl. hexagons into 2 shifted the main 4nπ-conjugated circuit from a global to a local system. Further investigations into magnetic ring currents using gauge-including magnetically induced current (GIMIC) calcns. suggested that an enhanced local paratropic ring current appeared in the pentalene core of 5. The preservation of the benzenoid character in the addnl. fused hexagons confined the paratropicity to the pentalene subunit, and the inherent presence of an o-quinoidal structure highlighted the 4nπ-electron delocalization on the pentalene unit. The antiaromaticity of 4 and 5 was characterized by their small HOMO-LUMO energy gap. Both exptl. and computational results demonstrated that the [a,f]-type ring fusion of the pentalene core effectively enhanced the antiarom. character compared with the [a,e]-type ring fusion in the reported bisannulated[a,e]pentalenes. The findings of this study could potentially be used for the rational design of optoelectronic devices based on novel antiarom. mols.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXisVKrsbzO&md5=a1ce6ec35e7e45013c274dbfc1442461

95. 95

    Ehrmaier, J.; Rabe, E. J.; Pristash, S. R.; Corp, K. L.; Schlenker, C. W.; Sobolewski, A. L.; Domcke, W. Singlet-Triplet Inversion in Heptazine and in Polymeric Carbon Nitrides. J. Phys. Chem. A 2019, 123, 8099– 8108, DOI: 10.1021/acs.jpca.9b06215

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    95

    Singlet-Triplet Inversion in Heptazine and in Polymeric Carbon Nitrides

    Ehrmaier, Johannes; Rabe, Emily J.; Pristash, Sarah R.; Corp, Kathryn L.; Schlenker, Cody W.; Sobolewski, Andrzej L.; Domcke, Wolfgang

    Journal of Physical Chemistry A
    (2019),
    123
    (38),
    8099-8108CODEN:
    JPCAFH;
    ISSN: 1089-5639.

    (American Chemical Society)

    According to Hund’s rule, the lowest triplet state (T1) is lower in energy than the lowest excited singlet state (S1) in closed-shell mols. The exchange integral lowers the energy of the triplet state and raises the energy of the singlet state of the same orbital character, leading to a pos. singlet-triplet energy gap (ΔST). Exceptions are known for biradicals and charge-transfer excited states of large mols. in which the HOMO and the LUMO are spatially sepd., resulting in a small exchange integral. In the present work, we discovered with ADC(2), CC2, EOM-CCSD, and CASPT2 calcns. that heptazine (1,3,4,6,7,9,9b-heptaazaphenalene or tri-s-triazine) exhibits an inverted S1/T1 energy gap (ΔST ≈ -0.25 eV). This appears to be the first example of a stable closed-shell org. mol. exhibiting S1/T1 inversion at its equil. geometry. The origins of this phenomenon are the nearly pure HOMO-LUMO excitation character of the S1 and T1 states and the lack of spatial overlap of HOMO and LUMO due to a unique structure of these orbitals of heptazine. The S1/T1 inversion is found to be extremely robust, being affected neither by substitution of heptazine nor by oligomerization of heptazine units. Using time-resolved photoluminescence and transient absorption spectroscopy, we investigated the excited-state dynamics of 2,5,8-tris(4-methoxyphenyl)-1,3,4,6,7,9,9b-heptaazaphenalene (TAHz), a chem. stable heptazine deriv., in the presence of external heavy atom sources as well as triplet-quenching oxygen. These spectroscopic data are consistent with TAHz singlet excited state decay in the absence of a low-energy triplet loss channel. The absence of intersystem crossing and an exceptionally low radiative rate result in unusually long S1 lifetimes (of the order of hundreds of nanoseconds in nonaq. solvents). These features of the heptazine chromophore have profound implications for org. optoelectronics as well as for water-splitting photocatalysis with heptazine-based polymers (e.g., graphitic carbon nitride) which have yet to be systematically explored and exploited.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXhs12jsbbK&md5=09a33ee027b0f9ba2631e3cc0df26355

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    Corey, J. Y. Siloles. Part 1: Synthesis, Characterization, and Applications. Adv. Organomet. Chem. 2011, 59, 1– 180, DOI: 10.1016/B978-0-12-378649-4.00001-0

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    Siloles: part 1: synthesis, characterization, and applications

    Corey, Joyce Y.

    Advances in Organometallic Chemistry
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    59
    (),
    1-180CODEN:
    AOMCAU;
    ISSN: 0065-3055.

    (Academic Press)

    A review. This review focuses on the synthesis, characterization, and uses of siloles.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhsVKhs7s%253D&md5=f59a264b150986369fb3b54095afc0d3

97. 97

    Corey, J. Y. Siloles. Part 2: Silaindenes (Benzosiloles) and Silafluorenes Dibenzosiloles): Synthesis, Characterization, and Applications. Adv. Organomet. Chem. 2011, 59, 1– 180, DOI: 10.1016/B978-0-12-378649-4.00001-0

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    Siloles: part 1: synthesis, characterization, and applications

    Corey, Joyce Y.

    Advances in Organometallic Chemistry
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    (),
    1-180CODEN:
    AOMCAU;
    ISSN: 0065-3055.

    (Academic Press)

    A review. This review focuses on the synthesis, characterization, and uses of siloles.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhsVKhs7s%253D&md5=f59a264b150986369fb3b54095afc0d3

98. 98

    Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. Self-Consistent Molecular Orbital Methods. XX. A Basis Set for Correlated Wave Functions. J. Chem. Phys. 1980, 72, 650– 654, DOI: 10.1063/1.438955

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    Self-consistent molecular orbital methods. XX. A basis set for correlated wave functions

    Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A.

    Journal of Chemical Physics
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    (1),
    650-4CODEN:
    JCPSA6;
    ISSN: 0021-9606.

    A contracted Gaussian basis set (6-311G is developed by optimizing exponents and coeffs. at the Moller-Plesset (MP) second-order level for the ground states of first-row atoms. This has a triple split in the valence s and p shells together with a single set of uncontracted polarization functions on each atom. The basis is tested by computing structures and energies for some simple mols. at various levels of MP theory and comparing with expt.

    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaL3cXpvFyitA%253D%253D&md5=eb331bad0222adcbe7cad51527273725

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    Widmark, P.-O.; Persson, B. J.; Roos, B. O. Density Matrix Averaged Atomic Natural Orbital (ANO) Basis Sets for Correlated Molecular Wave Functions. Theor. Chim. Acta 1991, 79, 419– 432, DOI: 10.1007/BF01112569

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    Density matrix averaged atomic natural orbital (ANO) basis set for correlated molecular wave functions II. Second row atoms

    Widmark, Per Olof; Persson, B. Joakim; Roos, Bjoern O.

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    419-32CODEN:
    TCHAAM;
    ISSN: 0040-5744.

    Generally contracted basis sets for second row atoms have been constructed using the At. Natural Orbital (ANO) approach, with modifications for allowing symmetry breaking and state averaging. The ANOs are constructed by averaging over several at. states, pos. and neg. ions, and atoms in an external elec. field. The contracted basis sets give virtually identical results as the corresponding uncontracted sets for the at. properties, which they have been designed to reproduce. The design objective has been to describe the ionization potential, the electron affinity, and the polarizability as accurately as possible. The result is a set of well balanced basis sets for mol. calcns. The starting primitive sets are 17s12p5d4f for the second row atoms Na-Ar. Corresponding ANO basis sets for first row atoms have recently been published.

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     Andersson, K.; Malmqvist, P.-Å.; Roos, B. O. Second-Order Perturbation Theory with a Complete Active Space Self-Consistent Field Reference Function. J. Chem. Phys. 1992, 96, 1218– 1226, DOI: 10.1063/1.462209

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     Second-order perturbation theory with a complete active space self-consistent field reference function

     Andersson, Kerstin; Malmqvist, Per Aake; Roos, Bjoern O.

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     96
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     1218-26CODEN:
     JCPSA6;
     ISSN: 0021-9606.

     The recently implemented second-order perturbation theory based on a complete active space SCF ref. function has been extended by allowing the Fock-type one-electron operator, which defines the zeroth-order Hamiltonian to have nonzero elements also in nondiagonal matrix blocks. The computer implementation is now less straightforward and more computer time will be needed in obtaining the second-order energy. The method is illustrated in a series of calcns. on N2, NO, O2, CH3, CH2, and F-.

     https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK38XptFKhsw%253D%253D&md5=9b11f4ec21a64ab3f5cfd44355156b1a

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     Aquilante, F.; Autschbach, J.; Carlson, R. K.; Chibotaru, L. F.; Delcey, M. G.; De Vico, L.; Fdez Galván, I.; Ferré, N.; Frutos, L. M.; Gagliardi, L.; Garavelli, M.; Giussani, A.; Hoyer, C. E.; Li Manni, G.; Lischka, H.; Ma, D.; Malmqvist, P. Å.; Müller, T.; Nenov, A.; Olivucci, M.; Pedersen, T. B.; Peng, D.; Plasser, F.; Pritchard, B.; Reiher, M.; Rivalta, I.; Schapiro, I.; Segarra-Martí, J.; Stenrup, M.; Truhlar, D. G.; Ungur, L.; Valentini, A.; Vancoillie, S.; Veryazov, V.; Vysotskiy, V. P.; Weingart, O.; Zapata, F.; Lindh, R. Molcas 8: New Capabilities for Multiconfigurational Quantum Chemical Calculations across the Periodic Table. J. Comput. Chem. 2016, 37, 506– 541, DOI: 10.1002/jcc.24221

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     Molcas 8: New capabilities for multiconfigurational quantum chemical calculations across the periodic table

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     Journal of Computational Chemistry
     (2016),
     37
     (5),
     506-541CODEN:
     JCCHDD;
     ISSN: 0192-8651.

     (John Wiley & Sons, Inc.)

     In this report, we summarize and describe the recent unique updates and addns. to the MOLCAS quantum chem. program suite as contained in release version 8. These updates include natural and spin orbitals for studies of magnetic properties, local and linear scaling methods for the Douglas-Kroll-Hess transformation, the generalized active space concept in MCSCF methods, a combination of multiconfigurational wave functions with d. functional theory in the MC-PDFT method, addnl. methods for computation of magnetic properties, methods for diabatization, anal. gradients of state av. complete active space SCF in assocn. with d. fitting, methods for constrained fragment optimization, large-scale parallel multireference CI including analytic gradients via the interface to the COLUMBUS package, and approxns. of the CASPT2 method to be used for computations of large systems. In addn., the report includes the description of a computational machinery for nonlinear optical spectroscopy through an interface to the QM/MM package COBRAMM. Further, a module to run mol. dynamics simulations is added and two surface hopping algorithms are included to enable nonadiabatic calcns. Finally, we report on the subject of improvements with respects to alternative file options and parallelization. © 2015 Wiley Periodicals, Inc.

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     Karton, A.; Daon, S.; Martin, J. M. L. W4–11: A High-Confidence Benchmark Dataset for Computational Thermochemistry Derived from First-Principles W4 Data. Chem. Phys. Lett. 2011, 510, 165– 178, DOI: 10.1016/j.cplett.2011.05.007

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     W4-11: A high-confidence benchmark dataset for computational thermochemistry derived from first-principles W4 data

     Karton, Amir; Daon, Shauli; Martin, Jan M. L.

     Chemical Physics Letters
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     510
     (4-6),
     165-178CODEN:
     CHPLBC;
     ISSN: 0009-2614.

     (Elsevier B.V.)

     We show that the purely first-principles Weizmann-4 (W4) computational thermochem. method developed in our group can reproduce available Active Thermochem. Tables atomization energies for 35 mols. with a 3σ uncertainty of under 1 kJ/mol. We then employ this method to generate the W4-11 dataset of 140 total atomization energies of small first-and second-row mols. and radicals. These cover a broad spectrum of bonding situations and multireference character, and as such are an excellent, quasi-automated benchmark (available electronically as Supporting Information) for parametrization and validation of more approx. methods (such as DFT functionals and composite methods). Secondary contributions such as relativity can be included or omitted at will, unlike with exptl. data. A broad variety of more approx. methods is assessed against the W4-11 benchmark and recommendations are made.

     https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXnvF2qsLs%253D&md5=41b53b92e3d9c4f450bc55a5e5df4eb7

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     Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 16; 2016.

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     (a) Herges, R.; Geuenich, D. Delocalization of Electrons in Molecules. J. Phys. Chem. A 2001, 105, 3214– 3220, DOI: 10.1021/jp0034426

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     Delocalization of Electrons in Molecules

     Herges, Rainer; Geuenich, Daniel

     Journal of Physical Chemistry A
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     3214-3220CODEN:
     JPCAFH;
     ISSN: 1089-5639.

     (American Chemical Society)

     We report a general method for the investigation and quantification of delocalization in mols. The method is based on the anisotropy of the current-induced d. (ACID). Compared to the c.d., which has been frequently used to investigate delocalization, the ACID approach has several advantages: it is a scalar field which is invariant with respect to the relative orientation of the magnetic field and the mol., it is not a simple function of the overall electron d., it has the same symmetry as the wave function, and it can be plotted as an isosurface. Several selected examples demonstrate the predictive power and the general applicability of this method.

     https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD3MXhslalsLc%253D&md5=c4470f1868fbbb885d2d7a15a89334ed

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     (b) Geuenich, D.; Hess, K.; Köhler, F.; Herges, R. Anisotropy of the Induced Current Density (ACID), a General Method To Quantify and Visualize Electronic Delocalization. Chem. Rev. 2005, 105, 3758– 3772, DOI: 10.1021/cr0300901

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     Anisotropy of the Induced Current Density (ACID), a General Method To Quantify and Visualize Electronic Delocalization

     Geuenich, Daniel; Hess, Kirsten; Koehler, Felix; Herges, Rainer

     Chemical Reviews (Washington, DC, United States)
     (2005),
     105
     (10),
     3758-3772CODEN:
     CHREAY;
     ISSN: 0009-2665.

     (American Chemical Society)

     A review. The ACID method is an intuitive and generally applicable method for the investigation and visualization of electron delocalization and bond conjugation in ground, excited and transition states. In the prototype examples for different types of delocalization presented in this review, the ACID anal. is in agreement with previous alternative theor. studies.

     https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2MXhtVWhtrjL&md5=c9d12d8f520d8221e2b831e6ac773f01

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     Gershoni-Poranne, R.; Stanger, A. The NICS-XY-Scan: Identification of Local and Global Ring Currents in Multi-Ring Systems. Chem. – Eur. J. 2014, 20, 5673– 5688, DOI: 10.1002/chem.201304307

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     The NICS-XY-Scan: Identification of Local and Global Ring Currents in Multi-Ring Systems

     Gershoni-Poranne, Renana; Stanger, Amnon

     Chemistry – A European Journal
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     (19),
     5673-5688CODEN:
     CEUJED;
     ISSN: 0947-6539.

     (Wiley-VCH Verlag GmbH & Co. KGaA)

     Nucleus-independent chem. shift (NICS)-based methods are very popular for the detn. of the induced magnetic field under an external magnetic field. These methods are used mostly (but not only) for the detn. of the aromaticity and antiaromaticity of mols. and ions, both qual. and quant. The ghost atom that serves as the NICS probe senses the induced magnetic field and reports it in the form of an NMR chem. shift. However, the source of the field cannot be detd. by NICS. Thus, in a multi-ring system that may contain more than one induced current circuit (and therefore more than one source of the induced magnetic field) the NICS value may represent the sum of many induced magnetic fields. This may lead to wrong assignments of the aromaticity (and antiaromaticity) of the systems under study. In this paper, we present a NICS-based method for the detn. of local and global ring currents in conjugated multi-ring systems. The method involves placing the NICS probes along the X axis, and if needed, along the Y axis, at a const. height above the system under study. Following the change in the induced field along these axes allows the identification of global and local induced currents. The best NICS type to use for these scans is NICSπZZ, but it is shown that at a height of 1.7 Å above the mol. plane, NICSZZ provides the same qual. picture. This method, namely the NICS-XY-scan, gives information equiv. to that obtained through c.d. anal. methods, and in some cases, provides even more details.

     https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXltFKltLo%253D&md5=e14576284fe51d6367c6f10dc68fcadf

106. 106

     Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. Ab Initio Calculation of Vibrational Absorption and Circular Dichroism Spectra Using Density Functional Force Fields. J. Phys. Chem. 1994, 98, 11623– 11627, DOI: 10.1021/j100096a001

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     Ab Initio Calculation of Vibrational Absorption and Circular Dichroism Spectra Using Density Functional Force Fields

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     Journal of Physical Chemistry
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     98
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     11623-7CODEN:
     JPCHAX;
     ISSN: 0022-3654.

     The unpolarized absorption and CD spectra of the fundamental vibrational transitions of the chiral mol. 4-methyl-2-oxetanone are calcd. ab initio. Harmonic force fields are obtained using d. functional theory (DFT), MP2 and SCF methodologies, and a [5s4p2d/3s2p] (TZ2P) basis set. DFT calcns. use the LSDA, BLYP, and Becke3LYP (B3LYP) d. functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with expt. The MP2 force field yields spectra in slightly worse agreement with expt. than the B3LYP force field. The SCF force field yields spectra in poor agreement with expt. The basis set dependence of B3LYP force fields is also explored: the 6-31Gand TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreement with expt.

     https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK2cXmvVSitbY%253D&md5=93486da1864d900b4527d020cf36171f

107. 107

     Yanai, T.; Tew, D. P.; Handy, N. C. A New Hybrid Exchange-Correlation Functional Using the Coulomb-Attenuating Method (CAM-B3LYP). Chem. Phys. Lett. 2004, 393, 51– 57, DOI: 10.1016/j.cplett.2004.06.011

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     A new hybrid exchange-correlation functional using the Coulomb-attenuating method (CAM-B3LYP)

     Yanai, Takeshi; Tew, David P.; Handy, Nicholas C.

     Chemical Physics Letters
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     393
     (1-3),
     51-57CODEN:
     CHPLBC;
     ISSN: 0009-2614.

     (Elsevier Science B.V.)

     A new hybrid exchange-correlation functional named CAM-B3LYP is proposed. It combines the hybrid qualities of B3LYP and the long-range correction presented by Tawada et al. [J. Chem. Phys., in press]. We demonstrate that CAM-B3LYP yields atomization energies of similar quality to those from B3LYP, while also performing well for charge transfer excitations in a dipeptide model, which B3LYP underestimates enormously. The CAM-B3LYP functional comprises of 0.19 Hartree-Fock (HF) plus 0.81 Becke 1988 (B88) exchange interaction at short-range, and 0.65 HF plus 0.35 B88 at long-range. The intermediate region is smoothly described through the std. error function with parameter 0.33.

     https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2cXlsFKgtbs%253D&md5=75f311240ff8ebedb174757f3eedbf3e

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     Matito, E. ESI-3D: Electron Sharing Indexes Program for 3D Molecular Space Partitioning; Institute of Computational Chemistry and Catalysis: Girona, Catalonia, Spain, 2006. http://iqc.udg.es/~eduard/ESI.

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     Yamaguchi, K. Self-Consistent Field: Theory and Applications (Studies in Physical and Theoretical Chemistry); Carbo, R., Klobukoswski, M., Eds.; Elsevier Science: Amsterdam, The Netherlands, 1990; pp 727.