Photoionization on insulator surfaces: diffuse reflectance laser flash photolysis of distyrylbenzenes adsorbed on silica and alumina

Abstract

269mechanism of chain growth at the interface of the transition metal and of intrinsic fluorite-type oxide for the dissociation of CO on palladium and the formation of high alcohols in syngas conversion.Some points however remain unclear. Thus a high alcohol selectivity is found on the intrinsic fluorite-type oxide. The catalytic site has been assigned to a set formed by transition-metal atom (or adjacent atoms) and a couple of adjacent vacant anionic sites and anionic sites on the support (Scheme VII). Now we may expect from the intrinsic nature of these oxides an ordered distribution of surface anionic vacancies. Thus the specific nature of the catalytic site may induce some special alcohol selectivity rather than a Schulz-Flory distribution. To test this hypothesis, further work must be undertaken in order to prepare catalysts supported on fluorite-type oxides with a controlled and homogeneous distribution of anionic vacancies.Acknowledgment. Many thanks are due to Mr. Lavergne (Universitt de Paris VI), Mrs. Bacri, and Mr. Vennegues (Universitt de Strasbourg) for TEM, BET, and XRD experiments, respectively.The absorption and fluorescence of 0-, m-, and p-distyrylbenzenes on surfaces of polycrystalline silica and alumina were investigated in the adsorbed state. Time-resolved diffuse reflectance transient absorption spectra were recorded following pulsed nanosecond laser excitation at 354 nm. In each case, a long-lived transient was detected and assigned to radical-cation absorption, and in two cases a short-lived transient was observed and assigned to triplet-triplet absorption. The radicals were also detected by ESR spectroscopy after laser excitation. Experimental observations of the laser fluence dependence of the radical-ion formation efficiency are consistent with a mechanism involving the sequential absorption of two photons during the same laser pulse. It is shown that model calculations for one-and two-photon excitation processes give predictions that are in good agreement with the measurements of the triplet-state and radical-ion production, respectively. The radicals decay by electron-ion recombination. The experimentally observed decay curves are well described by a temperature-activated diffusion model. Decay kinetics are discussed and tested in relation to this theoretical treatment.

Fluorescence and absorption spectra of oligophenylenevinylenes: Vibronic coupling, band shapes, and solvatochromism

“…For these materials the asymmetry of the absorption and fluorescence spectra are ascribed to the enlarged torsional mobility of the molecules in the electronic ground state S 0 as compared to the first excited singlet state S 1 . [2][3][4][5]19,20,[23][24][25] The structures of the nPVs in the S 0 state were predicted by quantum chemical calculations at different levels of theory for nϭ1, 2,26 -35 and nϭ2 -4, 35 and they are in quite good accordance with x-ray diffraction studies of 1PV, 36,37 4PV, 38 and binary composites of 2PV. 39 In the vinylene unit, the structures exhibit a pronounced bond alteration, which leads to low torsional barriers for the phenyl rings around the C 1 -C 2 single bonds.…”

Photochemistry of lignin model compounds on solid supports

“…Accordingly, we have examined the behavior of several model ketones (1-7) on three solid supports (silica gel, Na-X zeolite, and cellulose) using a combination of diffuse reflectance laser flash photolysis and product studies. The diffuse reflectance flash photolysis techniques developed by Wilkinson and coworkers over the last decade have considerable potential for studying transients such as triplets, radicals, and radical ions in opaque solids (18)(19)(20)(21)(22)(23)(24)(25)(26). The present studies provide insight into the manner in which the initial photochemistry and the behavior of photogenerated radical pairs are modified for lignin model ketones in a constrained environment.…”

Bimolecular processes on silica gel surfaces: energetic factors in determining electron-transfer rates

“…Fig 4. Fitted kinetic decay of naphthalene radical cation at 680 nm using Lévy (--) and Albery (---) models; naphthalene concentration 1.0 µmol g Ϫ1 .…”