Structure and Dynamics of the 1(TT) State in a Quinoidal Bithiophene: Characterizing a Promising Intramolecular Singlet Fission Candidate

Tetracyanoquinodimethane bithiophene (QOT2) has a long-lived (57 μs) photoinduced excited state that may correspond to triplets resulting from intramolecular singlet fission (SF). Since SF usually occurs through intermolecular processes, a detailed description of the excited states involved and their evolution is needed to verify this hypothesis. The photoresponse of QOT2 is investigated using high-level electronic structure methods and quantum dynamics simulations, which show ultrafast passage through a conical intersection from the bright 11Bu state to the dark 21Ag surface. Characterization of QOT2’s 21Ag wave function found it to be composed of two strongly coupled triplets, leading to the first detailed electronic structure description of an intramolecular 1(TT) state. The population of such a state upon excitation of QOT2 raises the possibility of SF through conformational changes that decouple the triplets. However, reaching an appropriate geometry for decoupled triplets appears unlikely given the ...