Photoswitchable Association of an Azobenzne–Bipyridinium Diad to Eosin: Photostimulated “On‐Off” Guest Binding

The reversible photoinduced cis–trans isomerization of the azobenzene unit in 1, together with the distinctly different association constants of the cis and trans isomers for the binding of 1 to eosin Y (2, K = 38 000 and 3000 M⁻¹ 1, respectively) can be utilized to construct a molecular switch for the formation and dissociation of the complex between 1 and 2 that is light‐controlled. Such photo‐switchable units are important in information storage systems and signal processing on the molecular level. (Figure Presented.)