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

Abstract

The radiative rate ( k_p) of the lowest triplet excited state (T₁) and the nonradiative rate based on intramolecular vibrations at room temperature [ k_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 k_nr(RT) but greatly enhanced k_p. The k_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 k_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.