Acid–base regulated inclusion complexes of β-cyclodextrin with 1-[2-(4-fluorophenyl)-2-oxoethyl]-4,4′-bipyridinium dichloride displaying multistimuli-responsive chromic behaviors and photomodulable fluorescence

Abstract

Multistimuli-responsive materials are highly desirable due to their convenient controllability and potential applications in smart window, sensor and bionic manufacturing. Herein, we designed and synthesized a guest molecule 1-[2-(4-fluorophenyl)-2-oxoethyl]-4,4′-bipyridinium dichloride (FOV·Cl₂), whose ketone–enol tautomerism (FOV²⁺ ↔ FHV⁺) in aqueous solution is governed by pH values. The solid-state inclusion complex formed β-cyclodextrin (β-CD) and FOV²⁺ (in the ketone form) demonstrated multiple chromic behaviors under the external stimuli of light, heat and vapors of ammonia and some organic amines, which were characterized by UV-vis and ESR spectroscopy. For comparison, the chromic behavior of the solid-state inclusion complex of β-CD with FHV⁺ (in the enol form) was also investigated. In addition, we found that the fluorescence emission of the inclusion complex FOV²⁺@β-CD can be reversibly modulated by the photochromic process. The chromic and fluorescent properties of the FOV²⁺@β-CD endow it with practical utility in inkless printing, ammonia sensing and multiple anti-counterfeiting. This work focused on demonstrating the multistimuli-responsive chromic behaviors of the inclusion complexes and exploring their promising applications.