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 (<i>k</i>_p) of the lowest triplet excited state (T₁) and the nonradiative rate based on intramolecular vibrations at room temperature [<i>k</i>_nr(RT)] from T₁ for heavy atom-free conjugated structures are determined by considering the triplet yield and quenching rate from T₁. Donor substitution did not strongly influence <i>k</i>_nr(RT) but greatly enhanced <i>k</i>_p. The <i>k</i>_nr(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 T₁ to the ground state (S₀). We attribute the enhancement of <i>k</i>_p induced by donor substitution to the appearance of a large SOC between high-order singlet excited states (S_m) and T₁ together with the large transition dipole moments of the S_m–S₀ transitions. Knowledge of this mechanism is important for developing future efficient persistent room-temperature phosphorescence from doped aromatic materials and aromatic crystals