Role of Carbonyl Distortions Facilitating Persistent Room-Temperature Phosphorescence

A decrease in the room-temperature phosphorescence (RTP) yield and RTP lifetime with increasing temperature is often explained by the increased nonradiative transition from the lowest triplet excited state (T1) caused by molecular vibrations of the chromophores. Here, we report a positive contribution of molecular vibrations and the distortion of vanillic acid (VA) dispersed in amorphous insulating polymer hosts to phosphorescence characteristics. We investigated triplet generation as well as photophysical processes from T1 of VA dispersed in poly(methyl methacrylate) (PMMA) and poly(vinyl alcohol) (PVA). Comparisons between optically measured data and calculation-based data, regarding the phosphoresce rate (kp) and the rate constant of the nonradiative transition (knr) from T1, reveal that kp and knr of the dispersed VA negligibly changed in PMMA or PVA, indicating that intermolecular processes between VA and PMMA are related to a large RTP quenching of VA in PMMA. Vibrational out-of-plane distortion of the carbonyl moiety of VA induced ππ*–nπ* mixing between the high-order singlet excited state and the ground state, mainly enhancing kp compared with knr of VA. Although vibrations are often reported to quench RTP, this report suggests that some distortions induced by vibrations of other chromophores contribute to RTP enhancement of molecular solid materials.