Controlling the photoswitching of 2-(4′-diethylamino-2′-hydroxyphenyl)-1H-imidazo-[4,5-b]pyridine by pH

Abstract

2-(4′-Diethylamino-2′-hydroxyphenyl)-1H-imidazo-[4,5-b]pyridine (DHP) has two mutually independent reaction paths, excited state intramolecular proton transfer (ESIPT) and twisted intramolecular charge transfer (TICT). To uncover the effect of acidity and basicity on the reaction coordinate, the molecule was studied in the complete range, from higher acidity H0 (Hammett’s acidity scale) – 10 to high basicity H_ (Yagil’s basicity scale) 16.4 using UV–visible and fluorescence techniques, along with quantum mechanical calculations. The molecule possesses complicated prototropic equibrium with multiple cations and anions in addition to the neutral species. The emission maximum changes from 375 nm to 560 nm depending on the acidity and basicity of the solution. All the prototropic species were identified experimentally and comprehensively entrenched with the help of computational calculations. The methoxy derivative was studied to substantiate the assignment. The neutral species predominantly emits the tautomer emission. Upon deprotonation one red shifted emission and one blue shifted emission are observed due to formation of two different monoanions. Even at higher basicity H_ 16.2, the dianion formation was not completed. Three monocations are formed, one emits normal emission, one emits tautomer emission and the third one emits highly Stokes shifted ICT emission. Among the dications, one dication undergoes biprotonic phototautomersism to produce tautomer of other dication in the excited state. Unlike dianion, the trication formation is completed at higher acidity H0 − 10.