Hessz Dóra, Hégely Bence, Kállay Mihály, Vidóczy Tamás, Kubinyi Miklós
Institute of Materials and Environmental Chemistry, Research Centre for Natural Sciences, Hungarian Academy of Sciences , P.O. Box 286, 1519 Budapest, Hungary.
J Phys Chem A. 2014 Jul 17;118(28):5238-47. doi: 10.1021/jp504496k. Epub 2014 Jul 2.
The ground- and excited-state protonation of Coumarin 102 (C102), a fluorescent probe applied frequently in heterogeneous systems with an aqueous phase, has been studied in aqueous solutions by spectroscopic experiments and theoretical calculations. For the dissociation constant of the protonated form in the ground state, pKa = 1.61 was obtained from the absorption spectra; for the excited-state dissociation constant, pKa* = 2.19 was obtained from the fluorescence spectra. These values were closely reproduced by theoretical calculations via a thermodynamic cycle (the value of pKa* also by calculations via the Förster cycle) using an implicit–explicit solvation model (polarized continuum model + addition of a solvent molecule). The theoretical calculations indicated that (i) in the ground state, C102 occurs primarily as a hydrogen-bonded water complex, with the oxo group as the binding site, (ii) this hydrogen bond becomes stronger upon excitation, and (iii) in the ground state, the amino nitrogen atom is the protonation site, and in the excited state, the carboxy oxygen atom is the protonation site. A comprehensive analysis of fluorescence decay data yielded the values kpr = 3.27 × 10(10) M(–1) s(–1) for the rate constant of the excited-state protonation and kdpr = 2.78 × 10(8) s(–1) for the rate constant of the reverse process (kpr and kdpr were treated as independent parameters). This, considering the relatively long fluorescence lifetimes of neutral C102 (6.02 ns) and its protonated form (3.06 ns) in aqueous media, means that a quasi-equilibrium state of excited-state proton transfer is reached in strongly acidic solutions.
香豆素102（C102）是一种常用于含水性相的非均相体系中的荧光探针，通过光谱实验和理论计算研究了其在水溶液中的基态和激发态质子化。对于基态质子化形式的解离常数，从吸收光谱中得到pKa = 1.61；对于激发态解离常数，从荧光光谱中得到pKa* = 2.19。使用隐式-显式溶剂化模型（极化连续介质模型+添加一个溶剂分子）通过热力学循环（pKa*的值也通过福斯特循环计算）进行的理论计算能够很好地重现这些值。理论计算表明：（i）在基态下，C102主要以氢键水络合物的形式存在，以羰基作为结合位点；（ii）激发后这种氢键变强；（iii）在基态下，氨基氮原子是质子化位点，在激发态下，羧基氧原子是质子化位点。对荧光衰减数据的综合分析得出激发态质子化速率常数kpr = 3.27×10(10) M(–1) s(–1)，反向过程速率常数kdpr = 2.78×10(8) s(–1)（kpr和kdpr被视为独立参数）。考虑到中性C102及其质子化形式在水性介质中相对较长的荧光寿命（分别为6.02 ns和3.06 ns），这意味着在强酸性溶液中达到了激发态质子转移的准平衡状态。
