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

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.