In the present study, we have reinvestigated photochemistry of 2,5‐dihydroxy benzoic acid (DHBA) in a series of organic solvents of different polarities and the aqueous solution of varying pH from 1 to 14. The investigation was carried out using steady‐state and nanosecond (ns) time‐domain fluorescence measurement techniques at room temperature. Some hitherto not explored spectral features with new insight on different forms of DHBA are reported. It is found that DHBA exists as monoanionic and neutral forms in various polar protic, aprotic, and nonpolar solvents. In aqueous solution (pH 1 to 14), neutral, monoanionic, and dianion forms are found to be responsible for the observed optical spectral profiles. The observed significant Stokes shift in emission is attributed to species undergoing excited‐state intramolecular proton transfer (ESIPT) in different solvents. The analysis presented in this work elucidates and validate the presence of various species and conformers of DHBA.

中文翻译：

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回顾光化学2,5-二羟基苯甲酸（龙胆酸）：溶剂和pH值的影响

在本研究中，我们对2,5-二羟基苯甲酸（DHBA）在一系列不同极性的有机溶剂和pH值为1至14的水溶液中的光化学进行了重新研究。室温下的纳秒（ns）时域荧光测量技术。报道了一些迄今为止尚未探索的光谱特征，并对不同形式的DHBA有了新的见解。发现DHBA以单阴离子和中性形式存在于各种极性质子，非质子和非极性溶剂中。在水溶液（pH 1至14）中，发现中性，单阴离子和双阴离子形式是观察到的光谱特征的原因。观察到的明显的斯托克斯发射位移归因于在不同溶剂中经历了激发态分子内质子转移（ESIPT）的物种。这项工作中提出的分析阐明并验证了DHBA各种物种和构象体的存在。