Hydroxyl groups tune the pH sensitivity of Imidazo[4,5-b]pyridine acrylonitrile derivatives as revealed by spectroscopy and DFT calculations

Abstract We investigated the potential of imidazo[4,5- b ]pyridine acrylonitrile derivatives as pH sensors, focusing on their spectroscopic responses to varying pH conditions. By incorporating different numbers of hydroxyl groups and evaluating the role of the unsubstituted NH group on the heterocyclic core, we characterized their UV–Vis absorption properties in solvents of variable polarity and buffer solutions (pH 1–13). The results demonstrate significant solvatochromic behavior, with hydroxyl group substitution and NH proton availability driving bathochromic and hypsochromic shifts. UV–Vis pH titrations revealed enhanced sensitivity in compounds with multiple hydroxyl groups, particularly in acidic and basic conditions. Computational DFT analysis identified the pyridine nitrogen as the primary protonation site and confirmed the role of phenolic hydroxyl groups in increasing acidity. NMR analysis validated these findings, showing chemical shift changes consistent with pyridine nitrogen protonation. Importantly, the combined experimental-computational approach enabled deconvolution of overlapping protonation equilibria and assignment of individual spectral features to specific protonation states. These compounds show promise as versatile pH sensors for optical sensing, optoelectronic purposes, and organo-electronic materials due to their robust spectroscopic properties. Notably, compound 8 emerged as an encouraging candidate for monitoring transitions from neutral to higher pH values, owing to its significant red shift in UV–Vis spectra near neutral pH, making it particularly suitable for detecting subtle alkalinity changes in environmental and biomedical applications.