Singlet fission in a hexacene dimer: energetics dictate dynamics.

Samuel N Sanders,Elango Kumarasamy,Luis M Campos,Matthew Y Sfeir,Kealan J Fallon

doi:10.1039/c9sc05066c

Samuel N Sanders, Elango Kumarasamy + Show 3 more

Open Access

https://doi.org/10.1039/c9sc05066c

Copy DOI

Abstract

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 lower triplet energy of hexacene results in an increase in the rate of triplet pair recombination, following the energy gap law for radiationless transitions. These results point to design rules for singlet fission chromophores: the energy gap between singlet and triplet pair should be minimal, and the gap between triplet pair and ground state should be large.

Highlights

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
Dimers serve as model systems to study singlet ssion. They represent the fundamental smallest number of chromophores required for Singlet fission (SF) and varying the connectivity between the chromophores can lead to insightful structure–property relationships of the constrained excitons, from the generation,[19,20,21,22] separation,[23,24,25,26] and recombination[27,28,29,30] of triplet states, to the elucidation of the bound triplet pair state.[31,32,33,34]
For incoherent triplet pair formation, the signature of vibrational mediation has been the dependence of the singlet ssion rate constant on the energetic driving force DES-TT. This driving force increases with n, the number of rings in the oligoacene chromophore, such that tetracene (Tc, n 1⁄4 4) < pentacene (Pc, n 1⁄4 5) < hexacene (Hc, n 1⁄4 6)

Summary

Introduction

The potential to exploit exciton multiplication in a variety of applications has sparked interest to develop materials to understand intrinsic fundamental details of excited state dynamics.[1,2,3,4,5,6,7] Singlet ssion, where one photon produces two excitons, can occur in organic chromophores with energetically low-lying triplet states.[8,9] This process requires electronic interaction between two or more chromophores, and so most research has focused on molecular crystals, polymers, or dimer assemblies in solution.[10,11,12,13,14,15,16,17,18] Dimers serve as model systems to study singlet ssion. Singlet ssion time constants on the order of 10 ns have been reported.[47,48] Recent calculations have suggested that molecular vibrations are essential to bring the energy of the singlet and triplet pair into resonance, enabling fast SF.[17] As such, we would expect a similar dependence of the singlet ssion rate constant as a function of driving force, i.e., as the energy difference between the singlet and triplet pair increases, the probability of

Results

Conclusion