Solid-State Supramolecular Inclusion Complexes of β-Cyclodextrin with Carboxyphenyl Viologens Showing Photochromic Properties

β-Cyclodextrins (β-CDs) are shown to form 1:1 and 2:1 host–guest inclusion complexes with two carboxyphenyl viologens, 1-(4-carboxyphenyl)-4,4′-bipyridinium chloride ( 1 + ·Cl – ) and 1,1′-bis(4-carboxyphenyl)-4,4′-bipyridinium dichloride ( 2 2 + ·Cl 2 – ), in an aqueous solution. The solid-state supramolecular host–guest inclusion complexes 1 + @β-CD and 2 2 + @β-CD 2 demonstrate excellent photochromic properties. The results of the electron-spin resonance (ESR) spectroscopy suggest that their photochromic behaviors are caused by the generation of free radicals (V •+ ). The energy-minimized structure and the calculated reorientation energy of the inclusion complex 1 + @β-CD support the electron transfer from the host β-CD to the guest 1 + . The practical applications of the inclusion complex 1 + @β-CD as erasable inkless printing media and an anticounterfeiting material were also studied. The present study provides a supramolecular method for the construction of photochromic materials.