Chemical Science View Article Online EDGE ARTICLE View Journal | View Issue Singlet fission in a hexacene dimer: energetics dictate dynamics† Cite this: Chem. Sci., 2020, 11,1079 a a a All publication charges for this article Samuel N. Sanders,‡ Elango Kumarasamy,‡ Kealan J. Fallon, have been paid for by the Royal Society Matthew Y. Sfeir *bc and Luis M. Campos *a of Chemistry Singlet fission (SF) is an exciton multiplication process with the potential to raise the efficiency limit of single junction solar cells from 33% to up to 45%. Most chromophores generally undergo SF as solid-state crystals. However, when such molecules are covalently coupled, the dimers can be used as model systems to study fundamental photophysical dynamics where a singlet exciton splits into two triplet excitons within individual molecules. Here we report the synthesis and photophysical characterization of singlet fission of a hexacene dimer. Comparing the hexacene dimer to analogous tetracene and pentacene dimers reveals that excess exoergicity slows down singlet fission, similar to what is observed in molecular crystals. Conversely, the Received 8th October 2019 lower triplet energy of hexacene results in an increase in the rate of triplet pair recombination, following Accepted 6th December 2019 the energy gap law for radiationless transitions. These results point to design rules for singlet fission Creative Commons Attribution 3.0 Unported Licence. DOI: 10.1039/c9sc05066c chromophores: the energy gap between singlet and triplet pair should be minimal, and the gap between rsc.li/chemical-science triplet pair and ground state should be large. Introduction intramolecular singlet ssion (iSF) compounds.17,35–38 In crystals, it is now commonly accepted that both coherent and incoherent The potential to exploit exciton multiplication in a variety of formation of triplet pairs is possible, even within the same 35,39 applications has sparked interest to develop materials to system. Experimental signatures of vibrational coherences understand intrinsic fundamental details of excited state have been detected using ultrafast vibrational and 2D electronic This article is licensed under a dynamics.1–7 Singlet ssion, where one photon produces two spectroscopy, where both inter- and intramolecular vibrational 40–44 excitons, can occur in organic chromophores with energetically modes have been found to be important. For incoherent low-lying triplet states.8,9 This process requires electronic triplet pair formation, the signature of vibrational mediation has Open Access Article. Published on 09 December 2019. Downloaded 9/26/2021 8:15:28 AM. interaction between two or more chromophores, and so most been the dependence of the singlet ssionrateconstantonthe D research has focused on molecular crystals, polymers, or dimer energetic driving force ES-TT. This driving force increases with n, assemblies in solution.10–18 Dimers serve as model systems to the number of rings in the oligoacene chromophore, such that study singlet ssion. They represent the fundamental smallest tetracene (Tc, n ¼ 4) < pentacene (Pc, n ¼ 5) < hexacene (Hc, n ¼ 6). number of chromophores required for SF and varying the However, the singlet ssion rate constant is not monotonic with D connectivity between the chromophores can lead to insightful ES-TT, being maximized when the driving force approaches zero structure–property relationships of the constrained excitons, and decreasing for more exo- or endothermic conditions. The rate 19–22 23–26 27–30 D from the generation, separation, and recombination constant decreases considerably in hexacene, where ES-TT is on 36,45,46 of triplet states, to the elucidation of the bound triplet pair the order of several molecular vibrations. state.31–34 The important role of vibrations in iSF has also been 17 It has been established that molecular vibrations play a key explored. However, no coherent generation of triplet pairs has role in mediating singlet ssioninbotholigoacenemolecular been reported to date. Rather, singlet ssion has been shown to crystals (intermolecular singlet ssion, xSF) and in acene be a purely incoherent process, which can span any time scale that can kinetically compete with decay of the photoexcited singlet. In bridged molecular dimers, singlet ssion time a Department of Chemistry, Columbia University, New York, NY 10027, USA. E-mail: constants on the order of 10 ns have been reported.47,48 Recent [email protected] calculations have suggested that molecular vibrations are bPhotonics Initiative, Advanced Science Research Center, City University of New York, New York, NY 10031, USA. E-mail: [email protected] essential to bring the energy of the singlet and triplet pair into 17 cDepartment of Physics, Graduate Center, City University of New York, New York, NY resonance, enabling fast SF. As such, we would expect 10016, USA a similar dependence of the singlet ssion rate constant as † Electronic supplementary information (ESI) available. See DOI: a function of driving force, i.e., as the energy difference between 10.1039/c9sc05066c the singlet and triplet pair increases, the probability of ‡ SNS and EK contributed equally to this work. This journal is © The Royal Society of Chemistry 2020 Chem. Sci.,2020,11,1079–1084 | 1079 View Article Online Chemical Science Edge Article overcoming this energy difference by coupling to molecular vibrations should decrease. However, no iSF materials with a large energetic driving force have been reported to date that would allow us to test this hypothesis. While dimers of tetracene and pentacene have been exten- sively studied, the excited state dynamics of hexacene dimers (and longer oligoacenes, n > 6) remain unknown. This is not surprising given that the stability of the oligoacenes is compromised as their p-system is extended and it has been a major challenge to stabilize heptacene (n ¼ 7)49 as calculations predict the emergence of an open-shell diradical character of the singlet ground-state.50,51 Additionally, oligoacenes with low triplet energies are needed to understand triplet pair decay processes. A large range of triplet pair lifetimes has been observed in iSF compounds. Fig. 1 Increasing the exothermicity of singlet fission by extension of Coupled triplet pairs in contiguous molecular dimers have been the fused acene units. Singlet fission is isoergic in tetracene (n ¼ 4), shown to undergo a rapid non-radiative geminate decay exothermic in pentacene, and highly exothermic in hexacene. The T1 process, while coupled triplet pairs in bridged iSF compounds and S1 energies are marked by light/dark gray lines and the triple pair have been found to persist for 1 ms.52 While recently reported (TT) energy is given by solid bars (solubilizing/stabilizing groups omitted for clarity). molecular design schemes have shown ways to suppress fast recombination and permit quantitative generation of free trip- lets in individual molecules,23 the nature of this phenomena is excited state dynamics with our previously reported BP16 and Creative Commons Attribution 3.0 Unported Licence. still unexplained. Preliminary data has showed that concerted BT59,60 with similar connectivity and protecting groups. While triplet pair decay follows the energy gap law for radiationless Suzuki–Miyaura cross coupling conditions between bromi- transitions, suggesting that multi-vibration relaxation to the 29 nated and analogous borylated acene partners has been the singlet ground state is directly occurring. Nonetheless, a wider workhorse strategy for preparing acene dimers, it was crucial range of triplet pair energies are needed to establish this fact. to simplify the synthetic manipulation of the relatively Thus, in order to develop general guidelines for the design and unstable hexacene derivatives. Therefore, we adopted a mild synthesis of SF chromophores, it is imperative to understand and step-economical strategy to perform a homocoupling of 2- how the intrinsic energies of the materials impact the forma- bromo (TIBS)hexacene using bis(cyclooctadiene)nickel(0) at tion, dissociation, and recombination of multiple exciton states This article is licensed under a room temperature (Fig. 2), forming the hexacene dimer in in individual molecules. high yield. Here, we study the excited state dynamics of a hexacene The absorption spectra (Fig. 2) show only a modest bath- dimer (bihexacene, BH, n ¼ 6) – the most exothermic known SF ochromic shi, comparing the monomer, Br-HC, to BH, indi- chromophore of the oligoacene series. Moreover, we compare Open Access Article. Published on 09 December 2019. Downloaded 9/26/2021 8:15:28 AM. cating little excited state delocalization over the entire molecule the excited state dynamics of bitetracene (BT, n ¼ 4), bipenta- (ESI Fig. S3†), similar to what is observed in BT and BP deriv- cene (BP, n ¼ 5), and pentacene–tetracene and pentacene– atives. The energy of the singlet excited state was estimated hexacene heterodimers (PT, PH) that have similar core from the absorption onset to be 1.55 eV. Given that the energy connectivity and stabilizing/solubilizing groups. Within this of the triplet state in solution phase hexacene is estimated to be series, both the energetic driving force for singlet ssion and 0.55 eV,50,54 this gives rise to a highly exothermic driving force the total triplet energy changes signicantly (Fig. 1) using triplet 53,54 for ssion, approximately double what has been reported for energies taken from literature (summary in ESI†). These BP, the most exothermic previously reported SF