Deciphering the photophysics of 5-chlorosalicylic acid: evidence for excited-state intramolecular proton transfer

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.