Theoretical Investigation of Singlet Fission in Molecular Dimers: The Role of Charge Transfer States and Quantum Interference

Singlet fission (SF) is a spin-allowed process by which a singlet excited state splits into a pair of triplet states. This process can potentially increase the efficiency of organic solar cells by a factor of 1.5. In this article, we study the dynamics of SF in different molecular aggregates of perylenediimide (PDI) derivatives, pentacene, and 1,3-diphenylisobenzofuran (DPB). To compute the SF rate, we have adopted a Markovian density matrix propagation approach to model SF in a molecular dimer. This approach allows accounting for both the coherent and incoherent processes that mediate the triplet formation. Our calculations show that SF can be much faster in PDI derivatives than in pentacene and DPB. Our analysis also indicates that SF is principally mediated by a superexchange mechanism that involves charge transfer states as virtual intermediates. In addition, because of the existence of different pathways for the formation of the triplet states, signatures of quantum interference are clearly observed.