Tautomerism in 8-(phenyldiazenyl)quinolin-5-ol: An attempt for pH activated rotary switch

Abstract

The tautomerism of 8-(phenyldiazenyl)quinolin-5-ol has been studied by a combination of theoretical (DFT calculations) and experimental (UV–Vis and NMR) methods. The detailed study of neutral molecules has shown that the hydrazone tautomeric form, stabilized by an intramolecular N–H⋯N hydrogen bond, is solely present in most of the solvents. In strong proton acceptor solvents, besides the dominant hydrazone form, the deprotonated form also appears. Solvent effects on the absorption maxima of the hydrazone form are interpreted by the linear solvation energy relationship concept, using Kamlet-Taft and Catalán models. Upon deprotonation, a substantial structural transformation is observed in the studied compound leading to a slow rotation around the Cquin-N bond. The process, as monitored by 1H NMR, is strongly solvent assisted and facilitated by proton acceptor solvents. Consequently, the investigated dye could be considered as a base-activated rotary switch.