Intersystem Crossing in Boron-Based Donor-Spiro-Acceptor Organic Chromophore: A Detailed Theoretical Study.

Abstract

Intersystem crossing and reverse intersystem crossing (rISC) processes were investigated in a boron-based donor-spiro-acceptor organic chromophore which shows thermally activated delayed fluorescence. Due to the perpendicular arrangement between donor and acceptor moieties, the HOMO and the LUMO are spatially separated, and the compound shows charge transfer (CT) transitions. We found both S1 and T1 excited states are CT in nature (i.e., electron and hole wave functions are localized on acceptor and donor units, respectively) and T2, which is higher in energy than S1 and T1, is locally excited in nature (i.e., both electron and hole wave functions are localized on an acceptor unit). Because of the same nature of excitation (i.e., CT here), the spin-orbit coupling matrix element between S1 and T1 is very low and insignificant exciton conversion occurs from the T1 state to the S1 state (and vice versa). Our combined time-dependent density functional theory and quantum dynamics simulation shows that the rISC process from the T1 state to the S1 state can be enhanced by the presence of a nearby local excited triplet state (i.e., T2 state here). A smaller gap between the T1 and T2 states efficiently establishes the rISC route.