MECHANISM AND SOLVENT DEPENDENCE OF THE SOLVENT-CATALYZED PSEUDO-INTRAMOLECULAR PROTON-TRANSFER OF 7-HYDROXYQUINOLINE IN THE 1ST ELECTRONICALLY EXCITED SINGLET-STATE AND IN THE GROUND-STATE OF ITS TAUTOMER
MECHANISM AND SOLVENT DEPENDENCE OF THE SOLVENT-CATALYZED PSEUDO-INTRAMOLECULAR PROTON-TRANSFER OF 7-HYDROXYQUINOLINE IN THE 1ST ELECTRONICALLY EXCITED SINGLET-STATE AND IN THE GROUND-STATE OF ITS TAUTOMER
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DOI：
10.1039/f29898500039
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发表时间：
1989-01-01
期刊：
影响因子：
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通讯作者：
VARMA, CAGO
中科院分区：
文献类型：
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作者：
KONIJNENBERG, J;EKELMANS, GB;VARMA, CAGO
7-羟基喹啉在纯醇溶液中的皮秒时间分辨荧光显示了两种不同的激发态互变异构过程。有一个快速的过程，这似乎是唯一的情况下，在环己烷中的溶液含有少量的酒精。这种快速过程发生在电子激发的1：2溶质-醇复合物中。较慢的过程与激发的溶质-醇复合物中的互变异构有关，所述复合物至少在最初含有两个以上的溶剂分子。醇分子的大小决定了这两个过程在光互变异构反应中的相对重要性，用瞬态吸收光谱观察到互变异构体基态的反向双质子转移过程。基态互变异构体以单指数衰减，其速率常数取决于醇的性质及其在溶液中的浓度。在低醇浓度的限制下，速率常数与醇的质子供体强度相关。反应速率常数与温度有关，并具有与温度无关的氘同位素效应。前面提出的两步过程的解释也适用于这里。第一步涉及热活化溶剂重组以实现溶质-醇复合物的正确构象。第二步被认为是在具有适当构象的络合物中的热无辅助质子隧穿过程。
Picosecond time-resolved fluorescence of solutions of 7-hydroxyquinoline in neat alcohols has revealed two different excited-state tautomerization processes. There is a fast process which appears as the only one in the case of solutions in cyclohexane containing a small amount of alcohol. This fast process occurs within electronically excited 1:2 solute–alcohol complexes. The slower process is associated with tautomerization in excited solute–alcohol complexes which contain, at least initially, more than two solvent molecules. The size of the alcohol molecule determines the relative importance of the two processes in the phototautomerization.The reverse double proton transfer process of the tautomer in its ground state has been observed by transient absorption spectroscopy. The groundstate tautomer decays monoexponentially with a rate constant depending both on the nature of the alcohol and on its concentration in the solution. In the limit of low alcohol concentrations, the rate constant correlates with the proton donor strength of the alcohol. The rate constant is temperature-dependent, and has a deuterium isotope effect which is temperature-independent. A previously suggested explanation in terms of a two-step process also applies here. The first step involves thermally activated solvent reorganization to achieve a proper conformation of a solute–alcohol complex. The second step is considered to be a thermally unassisted proton-tunnelling process in the complex with the proper conformation.