A critical approach toward resonance-assistance in the intramolecular hydrogen bond interaction of 3,5-diiodosalicylic acid: a spectroscopic and computational investigation.

The photophysics of a prospective drug molecule, 3,5-diiodosalicylic acid (3,5-DISA), having a wide spectrum of biological and medicinal applications, have been investigated using spectroscopic techniques and computational analyses. The remarkably large Stokes' shifts in various solvents from 3,5-DISA has been intertwined with the occurrence of an excited-state intramolecular proton transfer (ESIPT) reaction. Concurrently, the emergence of an intriguing dual emission feature in less interacting solvents is also reported and the spectral response of 3,5-DISA toward the variation of medium acidity/basicity has been exploited to decipher the nature of various species present in different solvents. Our experimental results, unveiling the occurrence of an ESIPT reaction in 3,5-DISA, have been aptly substantiated from computational studies in which the operation of ESIPT has been explored from structural as well as energetics (analysis of potential energy surface (PES)) perspectives. A major focus of the present study is on the evaluation of the intramolecular hydrogen bond (IMHB) interaction in 3,5-DISA, including the application of various methodologies to estimate the IMHB energy and subsequently, an in-depth analysis of the IMHB interaction reveals its partially covalent nature through the application of advanced quantum chemical tools, e.g., the natural bond orbital (NBO) method. In this context, the interplay between the aromaticity of the benzene nucleus and the IMHB energy has been rigorously explored, showing indications for the occurrence of resonance-assisted hydrogen bonding (RAHB) in 3,5-DISA. To this end, the geometric as well as magnetic criteria of aromaticity have been analyzed.