Photochromism, Thermochromism, and Electrochromism in Solid-State Host–Guest Inclusion Complexes of β‑Cyclodextrin with Dialkylcarboxyl-Substituted Viologens

Multi-stimuli-responsive chromic materials have immense potential for utilization. Herein, two supramolecular inclusion complexes were prepared by self-assembly of β-cyclodextrin (β-CD) with dialkylcarboxyl-substituted viologens, N,N′-di­(3-carboxy-propyl)-4,4′-bipyridinium dichloride (CPV·Cl₂) and N,N′-di­(6-carboxy-hexyl)-4,4′-bipyridinium dibromide (CHV·Br₂). The self-assembled inclusion complexes CPV²⁺@β-CD and CHV²⁺@β-CD₂ in the solid-state exhibited naked-eye photochromism, thermochromism, and electrochromism in response to multiple external stimuli including light, temperature, and electric field, respectively. Solid-state UV–vis diffuse reflectance and electron spin resonance (ESR) spectroscopy revealed that the observed photochromism, thermochromism and electrochromism are attributed to the formation of viologen free radicals induced by electron transfer under external stimuli. The excellent stimuli-response chromic properties of the title inclusion complexes support their practical utility in visual display, multiple anticounterfeiting, and multilevel information encryption.