Intrinsic Analysis of Radiative and Room-Temperature Nonradiative Processes Based on Triplet State Intramolecular Vibrations of Heavy Atom-Free Conjugated Molecules toward Efficient Persistent Room-Temperature Phosphorescence.

The radiative rate ( k) of the lowest triplet excited state (T) and the nonradiative rate based on intramolecular vibrations at room temperature [ k(RT)] from T for heavy atom-free conjugated structures are determined by considering the triplet yield and quenching rate from T. Donor substitution did not strongly influence k(RT) but greatly enhanced k. The k(RT) values were comparable between donor-substituted molecules and nonsubstituted molecules, which we explain by similar vibrational spin-orbit coupling (SOC) related to the transition from T to the ground state (S). We attribute the enhancement of k induced by donor substitution to the appearance of a large SOC between high-order singlet excited states (S) and T together with the large transition dipole moments of the S-S transitions. Knowledge of this mechanism is important for developing future efficient persistent room-temperature phosphorescence from doped aromatic materials and aromatic crystals.