Mesoporous silica thin films were shown to be an appropriate matrix for immobilization of discrete electroactive moieties, yielding uniform transparent thin film electrodes with defined texture and enhanced electrochemical activity. The mesoporous silica films prepared on conducting FTO-coated glass substrate were postsynthetically functionalized. Alkoxysilanes were used as precursors for subsequent grafting via ionic or covalent bonds of representative electroactive species, such as polyoxometalate PMo12O(40)3-, hexacyanoferrate(III), and ferrocene. The electrochemically active concentration within the silica-based composite electrodes achieves 90, 260, and 60 micromol cm(-3) for polyoxometalate, hexacyanoferrate(III), and ferrocene, respectively. The amount of molecules involved in the charge-transfer sequence is proportional to the film thickness and comparable to the total amount of embedded guests. Thus, eventually the whole bulk volume of the modified silica films is electrochemically accessible. Immobilization in the chemically modified silica matrix alters the redox potential of the electroactive molecules. Electron exchange between the adjacent redox centers (electron hopping) is proposed as a possible charge propagation pathway through the insulating silica matrix, which is supported by the fact that the high charge uptake is observed also for the hybrid electrodes with the covalently anchored redox guests.
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 (y75%) 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 SiO 2 or Al 2 O 3 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.
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