Reflectance Spectroscopic Studies of the Cation Radical and the Triplet of Pyrene on Alumina

Article abstract of DOI:10.1021/j100171a049

The triplet state and the cation radical of pyrene are produced on excitation of pyrene adsorbed on γ-alumina.The characteristic absorption spectra of these two alumina bound transients are similar to those in solution and exhibit absorption bands at 415 and 450 nm for the triplet and the cation, respectively.The triplet state decays exponentially with a lifetime of 5.1 ms, while the cation decays in a more complicated fashion which can be described by a Gaussian distribution of the logarithm of the rate constant at various adsorption sites.Movement of ferrocene to the triplet state on the surface quenches the triplet via energy transfer.The bimolecular quenching rate constants of pyrene triplet by ferrocene are 3.31 x 10¹¹, 2.63 x 10¹¹, and 2.20 x 10¹¹ m² mol^-1 s^-1 for alumina at pretreatment temperatures of 130, 250, and 350 deg C, respectively.Pretreatment of the alumina surface at high temperatures decreases the number of surface hydroxyl groups.These bind adsorbed molecules less strongly than the acidic sites formed at the high pretreatment temperature.Hydration of the surface by small amounts of coadsorbed water increases the number of surface hydroxyl groups and enhances the degree of dynamic quenching.The formation of the cation radical of pyrene on alumina is a two-photon process, and the cation radical is quenched dynamically by ferrocene.The studies clearly distinguish the role played by heat activation of alumina on the diffusion and rates of photoinduced process on the surfaces.A comparison with similar results on silica shows that adsorbed molecules are less mobile on alumina.