Photochromism of spiropyran in molecular sieve voids: effects of host-guest interaction on isomer status, switching stability and reversibility

Spiropyran or its preferred configurational isomers, respectively, are incorporated in faujasite (NaY, HY and DAY) cages by in situ synthesis, and in Si-MCM-41 channels by wetness impregnation. Luminescence spectra of the colored isomers indicate the non-aggregated incorporation of merocyanine forms. High quantum yields for the photochromism (similar to 75%) demonstrate the ready access of the chromophores for the photons of the laser pulses, used for the photoinduced switching between the different configurational isomers. The strong retardation of the thermal relaxation rate from the photoproduct cis-merocyanine to the thermodynamically preferred trans-merocyanine in the faujasite hosts (HY, DAY) in comparison to spiropyrans in SiO2 or Al2O3 is attributed to an increase of the rotation barriers by the imposed spatial restrictions. Among the different faujasites the dealuminated Y-zeolite (DAY) exhibits the lowest relaxation rates of the photoproduct and the highest switching reversibilities between the trans- and the cis-merocyanine.