Vázquez Sonia Ríos, Rodríguez M Carmen Ríos, Mosquera Manuel, Rodríguez-Prieto Flor
Departamento de Química Física, Facultade de Química, Universidade de Santiago de Compostela, E-15782 Santiago de Compostela, Spain.
J Phys Chem A. 2007 Mar 15;111(10):1814-26. doi: 10.1021/jp0653813. Epub 2007 Feb 20.
The influence of solvent, temperature, and viscosity on the phototautomerization processes of a series of o-hydroxyarylbenzazoles was studied by means of ultraviolet-visible (UV-vis) absorption spectroscopy and steady-state and time-resolved fluorescence spectroscopy. The compounds studied were 2-(2'-hydroxyphenyl)benzimidazole (HBI), 2-(2'-hydroxyphenyl)benzoxazole (HBO), 2-(2'-hydroxyphenyl)benzothiazole (HBT), 2-(3'-hydroxy-2'-pyridyl)benzimidazole (HPyBI), and the new derivative 2-(3'-hydroxy-2'-pyridyl)benzoxazole (HPyBO), this one studied in neutral and acid media. All of these compounds undergo an excited-state intramolecular proton transfer (ESIPT) from the hydroxyl group to the benzazole N3 to yield an excited tautomer in syn conformation. A temperature- and viscosity-dependent radiationless deactivation of the tautomer has been detected for all compounds except HBI and HPyBI. We show that this radiationless decay also takes place for 2-(3-methyl-1,3-benzothiazol-3-ium-2-yl)benzenolate (NMeOBT), the N-methylated analog of the tautomer, whose ground-state structure has anti conformation. In ethanol, the radiationless decay shows intrinsic activation energy for HPyBO and HBO; however, it is barrierless for HBT and NMeOBT and controlled instead by the solvent dynamics. The relative efficiency of the radiationless decay in the series of molecules studied supports the hypothesis that this transition is connected with a charge-transfer process taking place in the tautomer, its efficiency being related to the strength of the electron donor (dissociated phenol or pyridinol moiety) and electron acceptor (protonated benzazole). We propose that the charge transfer is associated with a large-amplitude conformational change of the tautomer, the process leading to a nonfluorescent charge-transfer intermediate. The previous ESIPT step generates the structure with the suitable redox pair to undergo the charge-transfer process; therefore, an excited-state intramolecular coupled proton and charge transfer takes place for these compounds.
通过紫外可见（UV-vis）吸收光谱、稳态荧光光谱和时间分辨荧光光谱研究了溶剂、温度和粘度对一系列邻羟基芳基苯并唑光互变异构过程的影响。所研究的化合物有2-（2'-羟基苯基）苯并咪唑（HBI）、2-（2'-羟基苯基）苯并恶唑（HBO）、2-（2'-羟基苯基）苯并噻唑（HBT）、2-（3'-羟基-2'-吡啶基）苯并咪唑（HPyBI）以及新衍生物2-（3'-羟基-2'-吡啶基）苯并恶唑（HPyBO），后者在中性和酸性介质中进行了研究。所有这些化合物都经历了从羟基到苯并唑N3的激发态分子内质子转移（ESIPT），以产生处于顺式构象的激发互变异构体。除HBI和HPyBI外，所有化合物都检测到了互变异构体的温度和粘度依赖性无辐射失活。我们表明，互变异构体的N-甲基化类似物2-（3-甲基-1,3-苯并噻唑-3-鎓-2-基）苯氧负离子（NMeOBT）的基态结构为反式构象，其也会发生这种无辐射衰变。在乙醇中，HPyBO和HBO的无辐射衰变显示出固有活化能；然而，HBT和NMeOBT的无辐射衰变没有势垒，而是由溶剂动力学控制。在所研究的一系列分子中，无辐射衰变的相对效率支持了这样一种假设，即这种跃迁与互变异构体中发生的电荷转移过程有关，其效率与电子供体（离解的苯酚或吡啶醇部分）和电子受体（质子化的苯并唑）的强度有关。我们提出电荷转移与互变异构体的大幅度构象变化有关，该过程导致非荧光电荷转移中间体的产生。先前的ESIPT步骤产生了具有合适氧化还原对的结构以进行电荷转移过程；因此，这些化合物发生了激发态分子内耦合质子和电荷转移。
