Role of Carbonyl Distortions Facilitating Persistent Room-Temperature Phosphorescence
Shuzo Hirata, Takuya Kamatsuki
Abstract
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 (T 1 ) 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 T 1 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 ( k p ) and the rate constant of the nonradiative transition ( k nr ) from T 1, reveal that k p and k nr 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*– n π* mixing between the high-order singlet excited state and the ground state, mainly enhancing k p compared with k nr 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.