Evidence for excited-state intramolecular proton transfer in 4-chlorosalicylic acid from combined experimental and computational studies: Quantum chemical treatment of the intramolecular hydrogen bonding interaction

The photophysical study of a pharmaceutically important chlorine substituted derivative of salicylic acid viz., 4-chlorosalicylic acid (4ClSA) has been carried out by steady-state absorption, emission and time-resolved emission spectroscopy. A large Stokes shifted emission band with negligible solvent polarity dependence marks the spectroscopic signature of excited-state intramolecular proton transfer (ESIPT) reaction in 4ClSA. Theoretical calculation by ab initio and Density Functional Theory methods yields results consistent with experimental findings. Theoretical potential energy surfaces predict the occurrence of proton transfer in S1-state. Geometrical and energetic criteria, Atoms-In-Molecule topological parameters, Natural Bond Orbital population analysis have been exploited to evaluate the intramolecular hydrogen bond (IMHB) interaction and to explore its directional nature. The inter-correlation between aromaticity and resonance assisted H-bond is also discussed in this context. Our results unveil that the quantum chemical treatment is a more accurate tool to assess hydrogen bonding interaction in comparison to geometrical criteria.

Graphical abstractDownload full-size imageHighlights► Experimental and computational studies on the photophysics of 4-chlorosalicylic acid. ► Spectroscopically established ESIPT reaction substantiated by theoretical calculation. ► Quantum chemical treatment of IMHB unveils strength, nature and directional nature. ► Superiority of quantum chemical treatment of H-bond over geometric criteria. ► Role of H-bond as a modulator of aromaticity.