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 mu s) 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 1(1)B(u) state to the dark 2(1)A(g) surface. Characterization of QOT2s 2(1)A(g) 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 energy cost of 1.76 eV. Consequently, the hypothesis that the long-lived excited state corresponds to 2(1)A(g), a spin singlet, strongly interacting double triplet, was explored. Transition moment calculations to assign excited-state absorption signals and investigations into internal conversion and intersystem crossing decay pathways indicate that a long-lived 2(1)A(g) state with (1)(TT) character is consistent with the available experimental data.