光致噻吨酮与胺类、酚类、醇类的电子转移和氢转移反应

利用激光闪光光解方法研究了一系列胺类、酚类、醇类在脱氧乙腈中猝灭噻吨酮(TX)三重态的反应,得到了相应的瞬态吸收光谱和猝灭速率常数(kq).通过对光谱演变特性的分析,推断出三重态噻吨酮与不含有活泼氢的胺发生了电子转移反应,与含有活泼氢的胺发生了电子-质子转移反应.三重态噻吨酮与酚类、醇类反应中观察到噻吨酮加氢自由基的生成,据此推断出三重态噻吨酮与酚类、醇类发生了氢转移反应.胺类的猝灭速率常数随着反应自由能变(ΔG)的增大而减小,说明电子转移影响了噻吨酮三重态的猝灭.酚类的猝灭速率常数先随ΔG增大而减小,后随酚阳离子的酸性增强逐渐增大,可能是猝灭过程中电子转移影响减弱的同时氢转移影响逐渐增强.醇类的猝灭速率常数随着醇的α-C—H键能的增大而减小,说明α-C—H键能是影响噻吨酮三重态猝灭的关键因素.比较以前研究的胺类、酚类、醇类与三重态呫吨酮(XT)、芴酮(FL)反应的结果可知,由于分子结构差异性的影响,相关的猝灭速率常数按照呫吨酮、噻吨酮、芴酮的顺序逐渐减小.

Photoinduced Electron and Hydrogen Transfer Reactions of Thioxanthone with Amines, Phenols and Alcohols

The quenching reactions of triplet thioxanthone (TX) by a series of amines, phenols, and alcohols were investigated by laser flash photolysis in deoxygenated acetonitrile. We obtained corresponding transient absorption spectra and quenching rate constants (k_q). Fromchanges in the transient absorption spectra, we determine that the electron transfer reactions occur between triplet TX and amines without an active hydrogen while electron/proton transfer reactions occur between triplet TX and amines with an active hydrogen. The appearance of hydrogenated radicals can be regarded as evidence for hydrogen transfer reactions in the TX/phenol and TX/alcohol systems. In the TX/amine systems, the quenching rate constants decreased with an increase in the free energy change (ΔG). This indicates that electron transfer reactions influence the quenching of triplet TX. In the TX/phenol systems, the quenching rate constants decreased with an increase in ΔG firstly, then increased with an increase in phenol cation acidity. This can be explained by considering that charge transfer and hydrogen transfer may play separate but important roles. In the TX/ alcohol system, the quenching rate constants decreased with an increase in the α-C—H bonding energy of alcohols, and this indicates that the α-C—H bonding energy is a key factor during triplet TX quenching. By comparison with previous studies about the quenching reactions of triplet xanthone (XT) and fluorenone (FL) by a series of amines, phenols, and alcohols, it is established that because of a discrepancy in molecular configurations the quenching rate constants decrease according to the following order: XT, TX, and FL.