New Development in Nonradiative Transition by a Thermal Lensing Method at High Pressure

利用时间分辨热透镜测定三重态形成的量子产率，研究了从第一激发单重态非辐射跃迁的机理。研究的第一组分子是蒽羰基化合物。除醇溶剂外，9-乙酰蒽的三重态产率基本一致，表明非辐射失活仅通过系间交叉进行，不存在内部转化过程。而在甲基环己烷中，9-苯甲酰镧的三重态产率为0.2，表明内转化是主要的非辐射过程，且三重态产率与溶剂有很强的依赖性，在极性溶剂中，内转化的贡献较小。9-溴-10-苯甲酰镧、9-氰基-10-苯甲酰镧和9-萘酰镧等蒽羰基化合物也发生了内转化。搭建了高压热透镜仪，在高压4000 atm下测量了甲基环己烷中9-乙酰镧和9-苯甲酰镧的热透镜信号。9-乙酰蒽的三重态产率随压力的增加而增加，这一结果不能从理论上解释，因为在1atm时三重态产率已经是统一的。必须检查是否存在高压下测量所固有的问题。9-苯并酰蒽的三重态产率随压力的增加而显著增加，表明压力降低了内转化的贡献。在常压下对其他羰基芳香分子如乙酰萘、苯甲酰萘、乙酰菲和乙酰芘的测量中，证实了内转化的作用。此外，还发现9,10-二甲氧基蒽在极性溶剂中发生内转化。

The mechanism for the nonradiative transition from the first excited singlet state was investigated by determining the quantum yield of triplet state formation with a time-resolved thermal lensing. The first group of molecules studied was anthracene carbonyl compounds. The triplet yield for 9-acetylanthracene was approximately unity in solvents besides alcoholic solvents, indicating that the nonradiative deactivation proceeds only through intersystem crossing and the internal conversion process is absent. In contrast, the triplet yield for 9-benzoylanthracene was 0.2 in methylcyclohexane, indicating that the internal conversion is a main nonradiative process, and the triplet yield was strongly dependent on the solvent, showing a tendency that the contribution of internal conversion is smaller in polar solvents. The occurrence of internal conversion was also found out for anthracene carbonyl compounds such as 9-bromo-10-benzoylanthracene, 9-cyano-10-benzoylanthracene and 9-naphthoylanthracene.The instrument for thermal lensing at high pressure was constructed, and thermal lens signals were measured for 9-acetylanthracene and 9-benzoylanthracene in methylcyclohexane at pressures up to 4000 atm. The triplet yield for 9-acetylanthracene increased with pressure, and this result cannot be theoretically explained because the triplet yield is already unity at 1 atm. It must be examined whether the problem inherent to the measurement at high pressures is present. The triplet yield for 9-benzoylanthracene increased remarkably with pressure, indicating that the contribution of internal conversion is decreased by pressure.In measurements at a normal pressure for other carbonyl aromatic molecules such as acetylnaphthalene, benzoylnaphthalene, acetylphenanthrane and acetylpyrene, the contribution of internal conversion was confirmed. Further, the occurrence of internal conversion for 9,10-dimethoxyanthracene in polar solvents was found out.