A Self‐locked β‐Cyclodextrin‐rhodamine B Spirolactam with Photoswitching Properties

Supramolecular organisation and self-assembly are the pillars of functionality of many nanosystems. The covalent conjugate (6-spirolactam rhodamine B-6-monodeoxy)-β-cyclodextrin ( Rho- β CD ) is assembled as a self-included, rigid nanostructure, identical in the crystal and in aqueous solution, as revealed by detailed X-ray and NMR analyses. Rho- β CD self-assembly is the result of an interesting reaction pathway which partially de-aggregates Rho and disturbs the zwitterion↔spirolactone equilibrium. Rho- β CD is stable at pH 4.6 but displays controllable photoswitching between the coloured, fluorescent, zwitterionic and the colourless, non-fluorescent closed structures, during several iterative cycles. After an initial drop in absorbance, the on-off process continues without further changes under our irradiation conditions, a consequence of the specific self-locked arrangement of Rho in the cavity. Rho-βCD exemplifies a water soluble photoresponsive nanosystem with improved photostability suggesting promising applications in super resolution bioimaging.