Intra-Molecular Singlet Fission in Quinoidal Dihydrothiophene - The

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Intra-Molecular Singlet Fission in Quinoidal Dihydrothiophene Kalishankar Bhattacharyya, Dayasindhu Dey, and Ayan Datta J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00230 • Publication Date (Web): 08 Feb 2019 Downloaded from http://pubs.acs.org on February 8, 2019

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The Journal of Physical Chemistry

Intra-Molecular Singlet Fission in Quinoidal Dihydrothiophene Kalishankar Bhattacharyya,1 Dayasindhu Dey,2 and Ayan Datta1* 1School

2S.

of Chemical Sciences, Indian Association for the Cultivation of Science, 2A and 2B Raja S. C. Mullick Road, Jadavpur – 700032, Kolkata, West Bengal, India

N. Bose National Centre for Basic Sciences, Block - JD, Sector - III, Salt Lake, Kolkata 700098, India

Abstract: Singlet Fission provides a promising mechanistic pathway to overcome the ShockleyQueisser limit of solar cell efficiencies. There are hardly handful molecules which are known to exhibit intra-molecular singlet fission (iSF). Most of the investigated iSF systems based on the donor-acceptor architectures have low lying triplet state energies which cause serious limitation for efficient opto-elelctronic devices. Herein, we demonstrate that quinoidal bis(diarylmethylene) dihydrothiophene (QDT) acts as promising candidate for intramolecular singlet fission with the triplet state arising near ~1.0 eV. Based on ab-initio quantum chemical calculations, ground and excited states of QDT are thoroughly investigated using TD-DFT and CASSCF which corroborate the criteria for iSF. Potential energy scan (PES) along the normal modes indicates the terminal C=C bond is responsible for plausible bright to dark state transition. Spin-flip RAS-SF calculations reveal the nature of multiexciton states arising in the higher singlet excited states and efficient iSF pathway in QDT. Attachment-detachment density analyses reveal that charge transfer states play a pivotal role during the formation of triplet pair from initially singlet state. Non-adiabatic coupling (NAC) between the lowest dark and bright single exciton states demonstrate favourable formation of coupled triplet pairs and significant rate for the formation of the independent triplet states that promotes efficient iSF in QDT.

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Introduction: Singlet Fission (SF), a photo-induced excited state phenomenon, downconverts the excited singlet state by splitting into two coupled triplet excitons. This spin allowed SF process has attracted much attention of the photovoltaics community to generate a new pathway to increases the theoretical solar cell efficiencies.1,2 In organic semiconductor, it has been shown that multiple exciton generations (MEG) from a single photon can enhance the ShockleyQueisser limit of the efficiency from 32% to 44.4%.3 Since the SF occurs in the ultrafast regime (sub-ps time scale), the two triplet pair eventually get separated away and can be harvested as a free charge carrier into the donor/acceptor (D/A) interfaces.4 Indeed, complete separation of triplet exciton in D/A interfaces could enhance the external quantum efficiencies to the limit of ~200%.5 A set of the general rules have been proposed by Michl to characterize the ground and excited state of SF active chromophore.6 In the visible region, the energy separation between first triplet state (T1) and ground state (S0) has to be half or less than the energy separation of the first excited singlet state (S1) and ground state (S0), i.e. E(S1-S0) > 2E(T1-S0). This implies that the exothermicity of SF proceeds with favourable separation of two triplet exciton with high quantum yields. The second condition of SF is that the second excited triplet state (T2) needs to be higher than twice of first triplet states (T1), i.e. E(T2-S0) > 2E(T1-S0) in order to compete with the detrimental triplet-triplet annihilation. For SF, the bright excited singlet state (S2a) non-adiabatically switches to the dark multiexciton singlet states (S1a) which are electronically classified as correlated triplet pair state 1[T1T1]. Since SF is a spin-allowed process, the transition from S2a state to S1a occurs in ultra-fast pico-second (ps) timescale.1 However, SF dynamics including charge-transfer states (CT) via super-exchange or mediated


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pathways are much faster compared to the direct SF dynamics. Till date, SF has been reported in organic crystals particularly large acene derivative (tetracene, pentacene, and hexacene),7 1,3-diphenylisobenzofuran,8 conjugated polymers (perylene derivative),9 carotenoids,10 and in biradicaloid molecules.11 It is noteworthy that Baldo and co-worker reported that efficient SF occurs in a pentacene-fullerene hybrid solar cell with ~200% quantum triplet yield.12 Based on the captodative strategy, Zeng et al reported that substitution of C atom by B and N in azaborine derivative makes it efficient small molecule for SF.11,13 Recently, we have designed Si substitution anthracene derivative based on the first principle calculation.14 Furthermore, the singlet/triplet energy levels could be tuned by CN substitution in 9-sila-anthracene. Morphology of SF capable chromophore extensively controls coupling between the S0S1 and 1ME states. Michl has reported that slipped parallel orientation of monomers would enhance singlet fission coupling between the S0S1 and 1ME state and thus maximize singlet fission quantum yield in intermolecular singlet fission.15 We have demonstrated intermolecular singlet fission in TIPS-Anthracene polymorphs (P-I and PII) where even small variation in packing orientation strongly modulated SF efficiencies.16 Till date, most of the studies demonstrated intermolecular SF process in polycrystalline thin films, whereas a few studies have been reported intra-molecular singlet fission (iSF).17-19 Wasielewski et al., demonstrated the occurrence of iSF in a covalent dimer of terrylenediimide derivative (TDI) with a controlled π-π arrangement.20 After generation of singlet exciton, covalent linker in TDI dimer facilitated rapid formation of the triplet-triplet pair through virtual charge-transfer states in pico-second timescale. Sanders et al., observed the generation of two triplets in a molecule through excimer formation in solution of the TIPS-pentacene covalent dimer.21 SF mechanism in such dimer is referred as an intramolecular singlet fission (iSF). However, SF quantum yield of covalent dimer is significantly low which is attributed to the absence of efficient intermolecular coupling


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between the nearest pair. In fact, the molecular orientation in the dimer is highly sensitive towards the fine adjustment in the crystalline thin film. For the first time Varnavski et al., have shown that conjugated donor-acceptor co-polymer i.e. isolated tetracyano quinoidal bithiophene (QBT) acts as efficient intramolecular singlet fission candidate with excellent quantum yield ~180%.18 Such copolymers are attractive for iSF since it avoids the nearest packing order between the neighbours in the crystalline motif. Indeed, linked copolymers are highly desirable for localization of multi-exciton states after direct photoexcitation facilitating the electron separation to the bound electron acceptor entities. Herein, we demonstrate that the recently synthesized conjugated copolymer quinoidal dihydrothiophene (QDT) shows efficient intramolecular singlet fission based on the quantum chemical calculations.22 Characterization of electronic states involved in the iSF was thoroughly investigated by the high-level computational methods. SA-CASSCF calculations elucidate the possible electronic coupling occurs between bright states and dark states based on the normal mode analysis. The possibility of excited state relaxation was thoroughly investigated by RAS-2SF PES scan around the terminal C=C bond rotation. Separation of triplet pairs and attachment-detachment density analyses bestow the involvement of charge transfer states in iSF mechanism in QDT.

Computational Details: The geometry optimization of closed cell singlet and first triplet states of QDT were optimized at B3LYP/6-31+G(D,P) level of theory.23,24 Harmonic frequencies were computed in both geometries to check the absence of imaginary mode in the optimized geometries. Excited state energies of the QDT were obtained by using the time-dependent density functional perturbation (TDDFT) method to calculate the vertical S0-S1 and S0-T1 gaps. All the geometries optimization and TDDFT calculation were done using the Gaussian G09


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program package.25 We have employed the state average complete active space selfconsistent field method (SA-CASSCF) to calculate the excited state energy level incorporating the higher correlation effect. For the SA-CASSCF calculation, we have chosen an active space of 6 electrons in 6 orbitals from the HF run on the S0 geometries. In order to include the dynamical correlation, we further performed SA-CASSCF/NEVPT2 calculation using the (6,6) active space. All SA-CASSCF/NEVPT2 calculations were performed with ORCA program package.26 We have carried out constrained HF optimization using the 631G(D) basis set to visualize the separated triplet pairs in QDT based on the method developed by Zimmerman et al.27 Based on the ROHF quintet reference, RAS-2SF were performed using 4 electrons in 4 orbital active space. Finally, norms of the one-particle density matrix (||γ||2),28 relevant electronic energy gaps and rates were evaluated based on the RAS-2SF wave functions.29,30 Constrained Hartree-Fock optimizations and RAS-SF calculations were performed with the Q-Chem software.31

Results and Discussions:

Figure1. Schemetic Representation of the structure of Quinodal dihydrothiophene (QDT) alongwith atom numbering.


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The structure of QDT (see Figure1) was optimized at B3LYP/6-31+G(d,p) level of theory followed by frequency calculation to ensure the absence of vibrational instabilities. We examined the structure-properties relationship of QDT to explore how the geometric and electronic properties affect the optical properties and singlet fission in this system. In QDT, ground state (S0) forms a quinoidal structure as evident from the bond length of C17-C18=144 pm and C18-C20/C15-C17= 136 pm respectively, exhibiting the alternation of single and double bond character. Also the C20-C21 and C21-C22 bond connecting to the bis(diphenylmethylene) analog lies in the range of single/double bond length which is corroborated with experimental single-crystal X-ray crystallographic analyses.22 (See Supporting Info file) In optimized T1 structure, C18-C20/C15-C17 and C21-C22 bond increase approximately ~4 pm in contrast to the S0 structure. Such molecular structural change supports the presence of resonance structure between quinoidal and diradical character. In fact, the slight diradical character (y0 = 0.21, see Supp. Info. File for definition) appearing in QDT might enhance the presence of low lying triplet states upon optical excitation. In order to explore the excited state energy level ordering for singlet fission, we have performed TD-DFT and SA-CASSCF calculation are shown in Table1. Based on the TDDFT calculation, the lowest lying singlet states (S1) arises at 2.49 eV in the Frack-Condon (FC) region which is consistent with the experimental absorption maxima at 2.60 eV.22 The first excited state S1 is an optically allowed exciton state (11B) where the transition occurs mostly from the HOMO → LUMO excitation. The first triplet excited (T1) state lies at 0.98 eV resulting from HOMO → LUMO transition. The next higher lying singlet excited state (S2) arises at 3.26 eV based on the transition from HOMO-1→LUMO and HOMO→LUMO+1 orbital. The energetic criteria of SF i.e. ESF = S1-2T1 is therefore, successfully satisfied. The energy level position of S1 and T1 suggests that the two triplet states are energetically accessible from the bright S1 state. It is also found that the second triplet state (T2) is 0.18 eV


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higher than the S1 state. Hence, the possibility of triplet-triplet annihilation or up-conversion to compete with SF can be safely excluded. However, single reference excitation methods like TDDFT method are incapable to locate a multi-exciton state, therefore, S1a is not shown in Table1. Table1. Computed excited state energies (in eV) of QDT using TD-DFT, SA-CASSCF (6,6), NEVPT2(6,6) and RAS-2SF(4,4) method in the gas phase geometries. Character of the excited state are analysed based on the SA-CASSCF orbitals. ME = multi-exciton state, LE=locally excited states. Method

S1a

S2a

T1

T2

2.49

0.98

2.66

TD-B3LYP/ 6-31+G(D,P) CASSCF (6,6) NEVPT2 (6,6)

4.10 (ME) 2.45

4.96 (LE) 3.44

1.81 1.57

3.68 3.15

RAS-2SF (4,4)

4.32

4.54

2.26

3.45

It has been reported that strongly correlated multireference method such as SA-CASSCF is an excellent method for determining the excited energy level and in principle can successfully capture the double excitonic states in SF.32 In order to calculate the excitation energy level, we first employed SA-CASSCF method. As can be seen from the Table1, computed vertical excitation energy shows a strong optically allowed 1B state lies at 4.96 eV. This single electron excitation mainly arises solely from HOMO to LUMO orbital. In the Franck-Condon region, the dark state 21A lies 0.86 eV lower than the 11B. Clearly, this dipole forbidden electronic transition (11A→21A) mainly arises from double electron excitation resulting in a multi-exciton characteristic in 21A state. We have also computed the lowest lying triplet excited states at SA-CASSCF/6-31G(d) level and found that the first excited triplet states T1 (13B) arises at 1.80 eV above the S0 (11A) state. Notably, all singlet and triplet excitation energies are overestimated in SA-CASSCF/6-31G(d) level which is


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attributed to the lack of dynamical correlation in the above-mentioned method. Previously, Varnavski et al. have reported that dynamical correlation included second order perturbation theory accurately estimated the excited state energy level ordering in similar type quinoidal chromophore.18 Indeed, the CASSCF method fails to produce accurate energy gap between bright and dark states, though it can capture the double electronic excitation of highly electron correlated dark states in SF. In fact, dark multi-exciton state character of S1a is higher than twice of the first triplet state energy (i.e. 21A→2× 1.81 = 3.62 eV) demonstrating high exoergicity of SF pathway based on the SA-CASSCF level. We further computed the excited state energy level using second-order multireference perturbation method. Based on the NEVPT2/6-31G(d) calculation, the excitation energies of the bright S2a and S1a states are 3.44 and 2.45 eV, respectively. According the NEVPT2 results for QDT, the dark state is 0.99 eV more stable than the S2a state which is excellent consistent with the previous CASPT2 studies on the quinoidal biothiophene series. It was found that S2a has locally-excited character (LE) while multi-exciton character (ME) arises in S1a. Similar observation is also found from RAS-SF calculation. Therefore in adiabatic framework, we considered that S1a has a multiexciton state character throughout the manuscript. It is important to note that, computed vertical excitation energies are overestimated at CASSCF and RAS-SF methods with respect to NEVPT2 values which corroborates that strongly correlated multireference methods are required to accurately capture the SF process. We have further performed a constrained Hartree -Fock geometry optimizations to get insight the decoupled triplet structure geometries. As shown in Figure2, AA triplets were constrained to one half and BB triplets to another half of the QDT. Orbital localization in these geometries clearly demonstrated independent triplets could be generated from the S1a states. The position of decoupled triplet states towards the terminal end of diphenylmethylene groups can be set as a marker for possible triplet pair separation from dark states.


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LULO

HOLO

HOLO-1

LULO-1

Figure2. Localized Frontier orbital involved in the independent triplet pair (HOLO-Highest Occupied Localized Orbital, LULO-Lowest Occupied Localized orbital).

We have been carried out normal mode analyses to qualitatively probe the transition pathway between bright and dark exciton states at SA-CASSCF/6-31G(d) level. Based on the normal mode calculations, we identified all the modes associated with thiophene ring and double bonds connecting the aryl moieties are found them to run parallel except the two important normal modes which varies the bond length between the thiophene rings. We study the potential energy surface scan (PES) of these two normal modes namely, symmetric and antisymmetric mode which are responsible for the interaction between the electronic states after photo excitation. As shown in Figure3, symmetric modes preferably drive strong interaction between S2a and S1a states with negligible energy difference (0.05 eV). Upon distortion along the symmetric mode towards the positive x-direction resulted in an increment of bond length in each thiophene chromophores. Therefore, both the states come close to each other and there is a possibility of non-adiabatical switching from bright state to dark state. In contrast, when we go in the negative x-direction along the symmetric mode, a large splitting occurs between S1a and S2a states.


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Figure3. Variation of S2a and S1a state energies with respect to displacement along (a) symmetric normal mode and (b) asymmetric normal mode. The transition vectors of the displacement of the corresponding modes are shown below in each plot. This can be qualitatively explained on the basis of bond length decrease of thiophene during displacement of normal mode along the vibrational coordinate. Along the antisymmetric mode of displacement, no crossing point founds between the S2a and S1a states. As shown in Figure3, antisymmetric mode causes large energy splitting between these states attributed by the simultaneous bond length increment on one thiophene and bond length decrease on another thiophene ring. Inspecting the orbitals involved in this vibrational mode, we found that the delocalized orbital becomes localized on one half of the thiophene ring which triggers the stabilization of triplet-triplet states. It is important to note that the energy required to reach these crossing point is higher (~0.62 eV) from S1a electronic states but belongs in the


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vibrational region of C-H stretching mode. Indeed, the above computations depicts a qualitative picture of transition from bright to dark states might possible. Previously it has been reported that multi-exciton states in polyenes consist of spin-flip excitation states.18,27 We also investigated the formation of double triplets states by means of restricted active space spin flip (RAS-SF) quantum chemical calculations. Although RAS-SF method is unable to incorporate the dynamical correlation, however, it qualitatively reproduces the excited state energy level ordering of different spin states. The RAS-2SF calculations were carried out using (4,4) active spaces from the quintet references. Based on the RAS-2SF calculation, the bright and double triplet states arise at 4.54 eV and 4.32 eV which are highly overestimated compared to the TDDFT based excitation energies (See Table1). Nevertheless, these energy ordering again validates the SF criteria and bright singlet exciton states might be directly converted to the double triplet states. In order to characterize charge-transfer properties in the optically active S2a state, we have performed the attachmentdetachment density analysis. It is reported that the charge transfer state plays an important role in mediating SF process in intermolecular singlet fission.33 Here, we also find that the weak charge transfer character in S2a states expedites the formation of coupled triplet states from the bright exciton states.34,35 As shown in Figure 4, the electron density is localized on the left and right units while hole density predominantly localized on the donor moieties of QDT. Presence of such charge transfer states should facilitate iSF to a significant in harmony with previous experimental results.17


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Figure4. Hole wavefunction ( detachment density) and (b) electron wavefunction (attachment density) plot for the S2a states of QDT. In order to gain more insight into the low-lying energy states, we have performed spin-flip RAS(4,4) scan from 0°-90° angle of one of the C=C terminal bonds. Interestingly, it is found that S2a states become destabilized whereas S1a state gets stabilized indicating that diradical character progressively increase in the system. As shown in Figure5, S2a and S1a states come closer to each other and cross at 20° angle with no energy penalty with respect to the undistorted structure. It is important to note that, S2a→ S1a transition can be described as S1→ 1ME

transition based on the nonadiabatic picture, however it does not represent the S1→ 1ME

transition after the crossing point. Indeed, constrained HF optimization also reveals that the triplet states are independently localized on the terminal sites again giving confidence to the iSF pathway.

Figure5. RAS-2SF/6-31+G(d) potential energy surface (PES) of QDT during the rotation of terminal C=C bond (ΦC12-C13-C14-C15). LE and ME represent the adiabatic S2a and S1a states respectively.


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Finally, electronic couplings between the two adiabatic lowest singlet states (S2a and S1a) and SF rates were calculated using a method developed by Krylov and co-workers based on the many-electron correlated wave functions (RAS-SF).29 Based on RAS-SF computation, S2a and S1a reveal the bright singlet exciton and multi-exciton character respectively, thus we demonstrated that S2a and S1a are S1 and 1ME state respectively. Then the S1→ 1ME process can be well approximated by the S2a→ S1a process involving the nonadiabatic coupling. RASSF computation consider all configurations with equal footing, hence interacting between them give the adiabatic states with different character.29 The rate of formation of multiexciton states and separation of two triplets from the correlated triplet pairs were calculated based on the Fermi Golden rule and linear free energy method using the equations (1) and (2) ||  || 2 (  ESF ) r1  r  S1  1ME  : ( ) e E

(1)

r2  r  1ME  1   1  : e b

(2)

where ΔE is the energy difference between S1 and 1ME states, ESF is the energy difference between S1 and 2T1 states which will determine the formation of multi-exciton states. Eb denotes the difference between 5ME and 1ME. Based on the RAS-2SF wave function method, we computed the norm of one-particle transition density matrices, ||γ||, between the S1 and 1ME

states as shown in Table2. To investigate SF mechanism in adiabatic wavefunction

framework, ||γ||, which is proxy to the non-adiabatic coupling term (NAC), acts as an extremely useful parameter for calculating the degree of coupling between the two triplets pair spatially localized into the terminal end of QDT in the multi-exciton state as shown in Figure 2. As shown in Table2, ESF of QDT is 0.02 eV which again validates the exoergicity of the SF. Favourable energetic condition and computed NAC, (||γ||2) between S1 and 1ME = 0.71 demonstrates the possibilities of formation two correlated triplet pair in S1a from the


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bright exciton states. Indeed, the rate for the first step determining the formation of correlated triplet pair (1ME) is ~25% faster than generation of decoupled triplets. The computed rate for separation of 1ME into two independent triplets is also in excellent agreement with the previous reports.31 Table2. Computed Electronic Energies (in eV), NAC Values and Singlet Fission rates of the QDT (α = 0.5 in the equations 1 and 2). ΔE

ESF

Eb

||γ||2

log(r1)

log(r2)

0.22

0.02

0.08

0.71

0.85

-0.68

Conlcusion: In summary, we have shown that the quinoidal dihydrothiophene (QDT) acts as a promising molecule for intra-molecular singlet fission molecule. SA-CASSCF and RAS-SF calculations confirm the presence of multi-exciton states through which facile separation of independent triplet pairs occurs in presence of charge transfer mediated pathways. Indeed, Constrained Hartree Fock optimizations also reveal that the correlated triplets are spatially localized on each half of the QDT. CAS-SCF PES scan has shown that symmetric vibrational mode is responsible for the couplings between S2a and S1a states while antisymmetric mode drives the localization of the triplet pairs. The formation of coupled triplet pair rate is significantly faster compared to the independent triplet pair generation which might reduce the overall singlet fission yield. We hope, that our prediction can certainly boost to find small organic chromophores with quinoidal character towards intra-molecular singlet fission based photovoltaic devices.


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ASSOCIATED CONTENT Supporting Information Cartesian coordinate for the structures reported, additional calculations. This information is available free of charge via the Internet. AUTHOR INFORMATION Corresponding Author Corresponding Author: [email protected]. Phone: +91-33-24734971. Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT KB thanks IACS for Research Fellowship. AD thanks DST, BRNS and INSA for partial funding. We thank CRAY supercomputer and IBM P7 cluster for computational facilities. We thank Dr. David Casanova for valuable insights and fruitful discussions. We thank Dr. Manoranjan Kumar for discussions.

REFERENCES (1) Smith, M. B.; Michl, J. Singlet fission. Chem. Rev. 2010, 110, 6891-6936. (2) Monahan, N.; Zhu, X.-Y. Charge transfer–mediated singlet fission. Annu. Rev. Phys. Chem. 2015, 66, 601-618. (3) Congreve, D. N.; Lee, J.; Thompson, N. J.; Hontz, E.; Yost, S. R.; Reusswig, P. D.; Bahlke, M. E.; Reineke, S.; Van Voorhis, T.; Baldo, M. A. External quantum efficiency above 100% in a singlet-exciton-fission–based organic photovoltaic cell. Science 2013, 340, 334-337. (4) Rao, A.; Wilson, M. W.; Hodgkiss, J. M.; Albert-Seifried, S.; Bassler, H.; Friend, R. H. Exciton fission and charge generation via triplet excitons in pentacene/C60 bilayers. J. Am. Chem. Soc. 2010, 132, 12698-12703.


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(5) 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. (6) Smith, M. B.; Michl, J. Recent advances in singlet fission. Annu. Rev. Phys. Chem. 2013, 64, 361-386. (7) Akdag, A.; Havlas, Z. k.; Michl, J. Search for a small chromophore with efficient singlet fission: biradicaloid heterocycles. J. Am. Chem. Soc. 2012, 134, 1462414631. (8) Schwerin, A. F.; Johnson, J. C.; Smith, M. B.; Sreearunothai, P.; Popovic, D.; Černý, J. i.; Havlas, Z.; Paci, I.; Akdag, A.; MacLeod, M. K. Toward designed singlet fission: electronic states and photophysics of 1, 3-diphenylisobenzofuran. J. Phys. Chem. A 2009, 114, 1457-1473. (9) Renaud, N.; Sherratt, P. A.; Ratner, M. A. Mapping the relation between stacking geometries and singlet fission yield in a class of organic crystals. J. Phys. Chem. Lett. 2013, 4, 1065-1069. (10) Wang, C.; Tauber, M. J. High-yield singlet fission in a zeaxanthin aggregate observed by picosecond resonance Raman spectroscopy. J. Am. Chem. Soc. 2010, 132, 13988-13991. (11) Zeng, T.; Ananth, N.; Hoffmann, R. Seeking small molecules for singlet fission: a heteroatom substitution strategy. J. Am. Chem. Soc. 2014, 136, 12638-12647. (12) Yost, S. R.; Lee, J.; Wilson, M. W.; Wu, T.; McMahon, D. P.; Parkhurst, R. R.; Thompson, N. J.; Congreve, D. N.; Rao, A.; Johnson, K. A transferable model for singletfission kinetics. Nat. Chem. 2014, 6, 492. (13) 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. (14) Bhattacharyya, K.; Pratik, S. M.; Datta, A. Small organic molecules for efficient singlet fission: role of silicon substitution. J. Phys. Chem. C 2015, 119, 2569625702. (15) Buchanan, E. A.; Michl, J. Packing Guidelines for Optimizing Singlet Fission Matrix Elements in Noncovalent Dimers. J. Am. Chem. Soc. 2017, 139, 15572-15575. (16) Bhattacharyya, K.; Datta, A. Polymorphism controlled singlet fission in tipsanthracene: role of stacking orientation. J. Phys. Chem. C 2017, 121, 1412-1420. (17) Busby, E.; Xia, J.; Wu, Q.; Low, J. Z.; Song, R.; Miller, J. R.; Zhu, X.; Campos, L. M.; Sfeir, M. Y. A design strategy for intramolecular singlet fission mediated by charge-transfer states in donor–acceptor organic materials. Nat. Mater. 2015, 14, 426. (18) Varnavski, O.; Abeyasinghe, N.; Aragó, J.; Serrano-Pérez, J. J.; Ortí, E.; Lopez Navarrete, J. T.; Takimiya, K.; Casanova, D.; Casado, J.; Goodson III, T. High yield ultrafast intramolecular singlet exciton fission in a quinoidal bithiophene. J. Phys. Chem. lett. 2015, 6, 1375-1384. (19) Fu, H.; Wu, Y.; Wang, Y.; Chen, J.; Zhang, G.; Yao, J.; Zhang, D. Intramolecular Singlet Fission in Antiaromatic Polycyclic Hydrocarbon. Angew. Chem. Int Ed. 2017. (20) Margulies, E. A.; Miller, C. E.; Wu, Y.; Ma, L.; Schatz, G. C.; Young, R. M.; Wasielewski, M. R. Enabling singlet fission by controlling intramolecular charge transfer in π-stacked covalent terrylenediimide dimers. Nat. Chem. 2016, 8, 1120. (21) Sanders, S. N.; Kumarasamy, E.; Pun, A. B.; Trinh, M. T.; Choi, B.; Xia, J.; Taffet, E. J.; Low, J. Z.; Miller, J. R.; Roy, X. Quantitative intramolecular singlet fission in bipentacenes. J. Am. Chem. Soc. 2015, 137, 8965-8972.


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Page 17 of 18 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(22) Takeda, T.; Akutagawa, T. Preparation, Structure, and Redox Behavior of Bis (diarylmethylene) dihydrothiophene and Its π-Extended Analogues. J. Org. Chem. 2015, 80, 2455-2461. (23) Becke, A. D. Density‐functional thermochemistry. I. The effect of the exchange‐only gradient correction. J. Chem. Phys. 1992, 96, 2155-2160. (24) Hariharan, P. C.; Pople, J. A. The influence of polarization functions on molecular orbital hydrogenation energies. Theo. chim. Acta. 1973, 28, 213-222. (25) Frisch, M.; Trucks, G.; Schlegel, H. B.; Scuseria, G.; Robb, M.; Cheeseman, J.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. Gaussian 09, revision a. 02, gaussian. Inc., Wallingford, CT 2009, 200. (26) Neese, F.; Wenmohs, F. ORCA 3.0. 1, Max-Planck-Institute for Bioinorganic Chemistry, Mülheim ad Ruhr, Germany, 2012, With contributions from U. Becker, D. Bykov, D. Ganyushin, AR Hansen, R. Izsak, DG Liakos, C. Kollmar, S. Kossmann, DA Pantazis, T. Petrenko, C. Reimann, C. Riplinger, M. Roemelt, B. Sandhöfer, I. Schapiro, K. Sivalingam, B. Wezisla, M. Kallay, S. Grimme and E. Valeev. (27) Chien, A. D.; Molina, A. R.; Abeyasinghe, N.; Varnavski, O. P.; Goodson III, T.; Zimmerman, P. M. Structure and dynamics of the 1 (TT) state in a quinoidal bithiophene: Characterizing a promising intramolecular singlet fission candidate. J. Phys. Chem. C 2015, 119, 28258-28268. (28) Matsika, S.; Feng, X.; Luzanov, A. V.; Krylov, A. I. What we can learn from the norms of one-particle density matrices, and what we can’t: some results for interstate properties in model singlet fission systems. J. Phys. Chem. A 2014, 118, 11943-11955. (29) Feng, X.; Luzanov, A. V.; Krylov, A. I. Fission of entangled spins: An electronic structure perspective. J. Phys. Chem. Lett. 2013, 4, 3845-3852. (30) Feng, X.; Krylov, A. I. On couplings and excimers: lessons from studies of singlet fission in covalently linked tetracene dimers. Phys. Chem. Chem. Phys. 2016, 18, 7751-7761. (31) Shao, Y.; Gan, Z.; Epifanovsky, E.; Gilbert, A. T.; Wormit, M.; Kussmann, J.; Lange, A. W.; Behn, A.; Deng, J.; Feng, X. Advances in molecular quantum chemistry contained in the Q-Chem 4 program package. Mol. Phys. 2015, 113, 184-215. (32) Momeni, M. R. Intramolecular Singlet Fission in Quinoidal Bi-and Tetrathiophenes: A Comparative Study of Low-Lying Excited Electronic States and Potential Energy Surfaces. J. Chem. Theo. Comput. 2016, 12, 5067-5075. (33) Lukman, S.; Chen, K.; Hodgkiss, J. M.; Turban, D. H.; Hine, N. D.; Dong, S.; Wu, J.; Greenham, N. C.; Musser, A. J. Tuning the role of charge-transfer states in intramolecular singlet exciton fission through side-group engineering. Nat. Commun. 2016, 7, 13622. (34) Basel, B. S; Zirzlmeier, J; Hetzer, C; Reddy, S. R; Phelan, B. T; Krzyaniak, M. D; Volland, M. K; Coto, P. B; Young, R. M; Clark, T; Thoss, M.; Tykwinski, R; Wasielewski, M. R; Guldi, D. M. Eidence for charge-transfer mediation in the primary events of singlet fission in a weakly coupled pentacene dimer. Chem. 2018, 4, 1092-1111. (35) Hu, J; Xu, K; Shen, L; Wu, Q; He, G; Wang J. Y; Pei, J; Xia, J; Sfeir, M. Y. New insights into the design of conjugated polymers for intramolecular singlet fission. Nat. Commun. 2018, 9, 2999.


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