Viologen-based host–guest supramolecule with tunable intramolecular/intermolecular electron transfer chromism and dynamic fluorescence

Abstract

Viologen, as a type of strong electron acceptor, is prone to undergo electron transfer (ET) and change color under external stimuli. However, due to the easy aggregation of viologen molecules, they usually suffer from poor fluorescence emission in the condensed phase. Herein, a new viologen derivative of VioCl₂·2Cl (1²⁺·2Cl) was designed and synthesized, in which the fluorescence was enhanced by introducing Me-β-CD to weaken the interactions between viologen molecules. Then a viologen-based host–guest supramolecule of 1²⁺@Me-β-CD was obtained by electrostatic interactions. Photo-/chemo-responded guest 1²⁺ supplies 1²⁺@Me-β-CD, a green and dark purple caused by intramolecular and intermolecular ET. Furthermore, 1²⁺@Me-β-CD displays an additional thermal responsive purple color. The triple chromic behaviors all exhibit excellent reversibility and cycling stability. As expected, 1²⁺@Me-β-CD exhibits strong photoluminescence (PL) in solid–liquid dual states, presenting an improved quantum yield (Φ) from 1²⁺ (Φ_s = 0.37%, Φ_l = 16.74%) to 1²⁺@Me-β-CD (Φ_s = 10.45%, Φ_l = 25.86%), and the fluorescence intensity can be dynamically modulated by light, heat, and acid/base vapors. The multi-responsive chromism and tunable fluorescence of 1²⁺@Me-β-CD allow for potential applications in information security and smart windows.