Excited state dynamics of bay and peri benzothienyl perylene to understand the excimer formation and its dissociation

Abstract

Many planar acenes such as pyrene, perylene etc. are known to form dimeric complex of their ground and excited state (excimer formation). Relaxation of excimer, carrier mobility & exciton diffusion in the solid state, and even singlet fission depends on the interaction between ground and excited state molecules in excimer. The role of excimer in triplet formation can be understood by: S0↑↓+S1↑↓⇄{S1S01↑↓↑↓}⇄{T1T11↑↑↓↓}⇄T1↑↑+T1↓↓. Intermediate excimer (T1T1)1 formation i.e., (SnS0)1 → (T1T1)1, and its decay to triplets have been studied by several groups, however, reports on the dissociation of excimer are very few. In this work, we aimed to study the dynamics of excimer formation and its further dissociation using perylene derivatives. We synthesized two positional isomers of benzothienyl substituted perylene i.e., peri-BT and bay-BT, and studied their photophysical properties in solution, nanoaggregates, and thermally evaporated thin films. Excited state dynamics in thin films suggest the dissociation of 1(excimer) to a monomeric singlet excited state in a microsecond timescale. Transient absorption spectral characteristics suggest the formation of a charge transfer state within picoseconds, followed by decay to a long-lived (few microseconds) excimer state. The formation of CT is promoted by strong intermolecular interaction in the thin film.