Vibronic effects accelerate the intersystem crossing processes of the through-space charge transfer states in the triptycene bridged acridine-triazine donor-acceptor molecule TpAT-tFFO.

Quantum chemical studies employing combined density functional and multireference configuration interaction methods suggest five excited electronic states to be involved in the prompt and delayed fluorescence emission of TpAT-tFFO. Three of them, a pair of singlet and triplet charge transfer (CT) states (S and T) and a locally excited (LE) triplet state (T), can be associated with the (Me → N) conformer, the other two CT-type states (S and T) form the lowest excited singlet and triplet states of the (Me → Ph) conformer. The two conformers, which differ in essence by the shearing angle of the face-to-face aligned donor and acceptor moieties, are easily interconverted in the electronic ground state whereas the reorganization energy is substantial in the excited singlet state, thus explaining the two experimentally observed time constants of prompt fluorescence emission. Forward and reverse intersystem crossing between the singlet and triplet CT states is mediated by vibronic spin-orbit interactions involving the LE T state. Low-frequency vibrational modes altering the distance and alignment of the donor and acceptor π-systems tune the S and T states (likewise S and T) into and out of resonance. The enhancement of intersystem crossing due to the interplay of vibronic and spin-orbit coupling is considered a general feature of organic through-space charge-transfer thermally activated delayed fluorescence emitters.