STUDIES ON SELF-REPAIRING PHOTOINDUCED ELECTRON TRANSFER PROCESS FROM THIOKETONES

硫代酮(吡啶硫酮等)的稳态光照射在甲基紫精存在下，阳离子通过自修复的光诱导电子转移过程持续产生自由基阳离子。由于该反应体系不使用牺牲剂，这种新型的光致电子转移过程在光能转换系统中将是有用的。在本项目中，我们利用稳态和激光-闪光光解方法研究了这种新型的光诱导电子转移过程的机理和优化。本循环的第一步是电子从三重态激发的吡啶硫酮转移到电子受体，如甲基紫精。吡啶硫酮自由基阳离子的去质子化和二聚化生成二硫化物，由于吸热反电子转移过程，二硫化物对甲基紫精自由基阳离子的反应较小。因此，生成的甲基紫精自由基阳离子在几百秒内缓慢衰减。反应体系在不同pH条件下的产物分析和动力学分析支持了所提出的反应机理。这种自修复的光致电子转移过程也可以应用于光催化氢原子破坏体系。4-吡啶硫酮和甲基紫精在乙醇存在下的稳态光解生成紫精自由基阳离子。紫精的自由基阳离子即使在黑暗中也没有完全衰变，这表明乙醇被最初的吡啶硫酮生成的硫代自由基去质子化。这一发现表明，乙醇的催化脱氢是通过自修复光致电子转移过程进行的。

Steady-state light irradiation to thioketones (pyridinethiones etc.) in the presence of methylviologen dication generates radical cation persistently by self-repairing photoinduced electron transfer process. This novel photoinduced electron transfer process will be useful for light energy conversion systems, since the reaction system does not use sacrificial reagent. In the present project, we investigated the mechanism and optimization of the novel photoinduced electron transfer process by means of steady-state and laser-flash photolysis methods. The initial step of the present cycle is electron transfer from the triplet excited pyridinethiones to electron acceptors such as methylviologen. Deprotonation and dimerization of the radical cation of pyridinethiones generate disulfide which is less reactive to methylviologen radical cation due to the endothermic back electron transfer process. Thus, generated methylviologen radical cation decayed slowly over several hundred seconds. The proposed reaction mechanisms are supported by the products analysis and kinetic analysis of the reaction systems under the various pH conditions. The self-repairing photoinduced electron transfer process can be also applied to photocatalytic H-atom abstruction system. Steady-state photolysis of 4-pyridinethione and methylviologen in the presence of alcohol generated viologen radical cation. The radical cation of viologen did not decay completely even in the dark, indicating deprotonation of alcohol by the thio radical which was generated from initial pyridinethione. This finding indicates catalytic dehydrogenation of alcohol by the self-repairing photoinduced electron transfer process.