Department of Chemistry, University of Calcutta, 92 A. P. C. Road, Calcutta 700009, India.
Photochem Photobiol Sci. 2010 Jan;9(1):57-67. doi: 10.1039/b9pp00094a. Epub 2009 Dec 1.
A detailed photophysical study of a pharmaceutically important chlorine-substituted derivative of salicylic acid viz., 5-chlorosalicylic acid (5ClSA), has been carried out by steady-state absorption, emission and time-resolved fluorescence spectroscopy and compared with its parent molecule salicylic acid (SA). A large Stokes-shifted emission band with negligible solvent polarity dependency indicates the spectroscopic signature of excited-state intramolecular proton transfer reaction. Presence of various ground-state species has been confirmed by a thorough investigation in different solvents and by pH variation experiments. Quantum chemical calculation by ab initio Hartree-Fock (HF) and Density Functional Theory (DFT) methods yields results consistent with experimental findings. Particularly, evaluation of potential energy surfaces for the S(0) and S(1) states across the proton transfer coordinate reveals distinct theoretical support for the inoperativeness of the GSIPT reaction as well as the occurrence of ESIPT process. The less favourable ESIPT reaction in 5ClSA is due to reduction of ground-state intramolecular hydrogen bond strength and excited-state negative charge density at the acceptor (-COOH) oxygen atom and increase of the excited-state barrier with respect to SA.
已通过稳态吸收、发射和时间分辨荧光光谱法对具有重要药用价值的水杨酸氯取代衍生物（即 5-氯水杨酸（5ClSA））进行了详细的光物理研究，并将其与母体分子水杨酸（SA）进行了比较。大斯托克斯位移的发射带几乎不依赖于溶剂极性，表明存在激发态分子内质子转移反应的光谱特征。通过在不同溶剂中的彻底研究和 pH 值变化实验，证实了各种基态物种的存在。通过从头算 Hartree-Fock（HF）和密度泛函理论（DFT）方法进行量子化学计算，得到的结果与实验结果一致。特别是，对质子转移坐标上 S(0)和 S(1)态的势能面进行评估，为 GSIPT 反应的不可操作性以及 ESIPT 过程的发生提供了明确的理论支持。在 5ClSA 中，ESIPT 反应的不利情况是由于降低了基态分子内氢键强度以及受电子（-COOH）氧原子上的激发态负电荷密度，并且相对于 SA 增加了激发态势垒。
