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 p ) of the lowest triplet excited state (T 1 ) and the nonradiative rate based on intramolecular vibrations at room temperature [ k nr (RT)] from T 1 for heavy atom-free conjugated structures are determined by considering the triplet yield and quenching rate from T 1 . 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 T 1 to the ground state (S 0 ). 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 T 1 together with the large transition dipole moments of the S m –S 0 transitions. Knowledge of this mechanism is important for developing future efficient persistent room-temperature phosphorescence from doped aromatic materials and aromatic crystals.