A photochromic–acidochromic HCl fluorescent probe. An unexpected chloride-directed recognition

Abstract

Non-classical protomerism of Schiff bases offers several advantages; for example, specific interactions in the –CN– linkage can be controlled and differentiated because the interactions are not governed by keto–enol tautomerism. Herein, the pH sensing properties of a new protomeric Schiff base probe (S1) are reported. In particular, among several acids, the probe displays significant optical responses upon interaction with hydrochloric acid (HCl). X-ray structural analysis confirmed the existence of an intermolecular interaction with HCl through a –CN⋯H–Cl⋯O– linkage. Moreover, an optical response via a second channel is manifested as photochromic fluorescence behavior. The properties of S1 were investigated by UV-vis and fluorescence spectroscopy in a solution and the solid state. Its strong acidofluorochromic behavior was analyzed and its pK_a and values were determined, which revealed a photobasic character. Positive solvatochromism that resulted from specific interactions taking place in S1 was studied using four different solvent scales, namely, Lippert–Mataga, Kamlet–Taft, Catalán and the recently proposed scale of Laurence et al., which yielded consistent results. Finally, theoretical calculations were conducted to analyze the mechanism of the probe in terms of natural transition orbitals (NTOs) and the spatial extent of charge transfer excitations.