Visual understanding of vibronic coupling and quantitative rate expression for singlet fission in molecular aggregates

Singlet fission (SF) is an exciton dissociation process that generates two triplet excitons from one singlet exciton. Because the exciton dissociation process involves internal conversion between multiexcitonic states, it is necessary to calculate multiexcitonic vibronic couplings (VCs) and identify what kinds of vibrational modes activate internal conversion to fully understand the SF mechanism. In this study, we developed a method of visually understanding "interstate"vibronic couplings and "quantitatively"evaluating SF rates for dimers in molecular aggregates. We applied the method to tetracene dimers in crystals by computing and visually analyzing interstate VCs between the multiexcitonic states within a configuration interaction scheme. From the calculated VCs, a low-frequency mode (62 cm-1) was found to strongly promote the generation of correlated triplet pairs from a photoexcited singlet state. The spatial overlap between the wave functions of the correlated triplet pairs and singlet state has a large distribution, leading to the large VC of the low-frequency mode. From the calculated VCs, we estimated SF rates in the temperature range from 50 to 300 K. The calculated SF rate was quantitatively in good agreement with an experimental result, which validated our method of calculating VCs and SF rates for dimers in molecular aggregates.