甲基紫精与醇类间的光诱导电子转移及光催化反应的ESR研究

用ESR方法研究了甲基紫精(MV⁺⁺)与甲醇,乙醇及苄醇等之间的光诱导电子转移和光反应过程.结果发现,甲基紫精存在条件下,某些很稳定的醇类也可以发生光催化分解和光催化氧化的反应,甲基紫精在反应过程中起的是催化剂的作用在无氧条件下,MV⁺⁺/C₂H₅OH体系经UV光照10min后即可观察到很强的MV⁺离子基的信号,表明甲基紫精与乙醇分子之间发生了电子转移;当进行较长时间光辐照,则MV⁺离子基逐步消失而生成H原子和碳中心自由基。在通氧条件下进行光照时,则无MV⁺基信号而产生OOH基和碳中心自由基的信号.当体系中有一定量的水存在时,OOH基减弱而产生·OH基,且随着水量的增加,·OOH基的强度更为减弱直至消失,而同时羟基浓度则大大增加.另一个有趣而重要的现象是,对于MV⁺⁺/苄醇体系,在通氧条件下辐照时可给出很强的超氧阴离子基O₂^-的信号,而MV⁺⁺/甲醇体系可产生较弱的O₂^-信号.但是在无氧时甲基紫精的存在进行光辐照甲醇并不发生反应,而苄醇却可被甲基紫精光催化分解产生很强的苄氧基,羟甲基等自由基.本文详细地研究和讨论了甲基紫精与几种醇之间的光诱导电子转移和相应的均相光催化反应过程的机理.

ESR STUDIES ON THE PHOTOINDUCED ELECTRON TRANSFER AND PHOTOCATALYSIS PROCESSES OF METHYL-VIOLOGEN/ALCOHOL SYSTEMS

The photoinduced electron transfer and photoc.atalysis reaction processes of methyl viologen(MV ) with methyl alcohol, ethyl alcohol, propyl alcohol, iso-propyl alcohol, glycoland benzyl alcohol were studied by means of ESR method. Results indicated that some alcohols, which usually are very stable to UV rad.radiation, may be initiated to leading to photocatalytic decomposition and photo-oxidation in the presence of methyl viologen, and methylviologen played a key role of catalyst in photoreaction processes. As in the absence of oxygen, a very strong ESR signal of MV ion radical was observed in MV /C2H5OH systemirradiated for 10min, obviously, it implies that the photoinduced electron transfer processestook place between methyl viologen and ethyl alcohol; if radiation time was more long, the Hatoms and carbon-centered radicals might be detected, and MV radical would graduallydisappears. When the system was irradiated under the condition of bubbling O2, the OOHand carbon-centered radicals were observed and no signal of MV radical was given; the signal of OOH radical weakened and OH radical was formed in the presence of a fair amount ofwater, go a step further, with the increase of water amount, the signal intensity of OOHradical was further weakened and vanished, at the same time, the signal intensity of OHradical increased greatly. It is a very interesting fact that, as bubbing O2, MV /C6H5CH2OH system .could give a quite strong signal of superoxide anion radicals or2, andMV /CH3OH system might give a rather weak signal of O2. Furthermore, if no O2 existed, there would not be any observed reaction in MV-/CH3OH system irradiated by UV,however, benzyl alcohol could be photocatalytically decomposed, thus leading to the production of large amount of benzyl-oxyl radical C6H5CH2O and hydro-methyl radical CH2OH.The process and mechanism of photoinduced electron transfer and related photocatalytic reactions for methyl viologen with some alcohols were investigated and discussed in detail.