Showing papers by "Prashant V. Kamat published in 1989"

Dynamic Burstein-Moss shift in semiconductor colloids

Abstract: Photoinduced blue shifts in CdS colloids have been time resolved by picosecond pump-probe measurements. The blue shift appears within the time domain of the pump pulse (18 ps) and is found to increase with increased pump light intensity. Calculations of the predicted blue shift from a dynamic Burstein-Moss shift agree with the experimental results at the lower laser intensities, but the predicted shifts are greater than observed values at the higher light intensities.

Photochemistry on surfaces. 4. Influence of support material on the photochemistry of an adsorbed dye

Abstract: The absorption and emission properties of rose bengal have been characterized on the surfaces of SiO{sub 2}, Al{sub 2}O{sub 3}, and TiO{sub 2} particles. The intrinsic properties of these oxide supports play an important role in effecting and controlling the photochemistry of the adsorbed dye. On the Al{sub 2}O surface, a biphotonic photoionization process leads to the production of semioxidized dye. However, a charge injection from the excited dye into the conduction band dominates on the TiO{sub 2} particles. Steady-state photolysis indicates a rapid photodegradation of the dye only on the surface of TiO{sub 2}. The diffuse reflectance laser flash photolysis experiments which elucidate the mechanism of photochemical processes on these oxide supports are described.

Photoelectrochemistry in semiconductor particulate systems. 13. Surface modification of cadmium sulfide semiconductor colloids with diethyldithiocarbamate

Abstract: Etude de l'interaction entre le diethyldithiocarbamate et une electrode constituee par une suspension de particules colloidales de CdS

Photophysics and photochemistry of phenosafranin dye in aqueous and acetonitrile solutions

Abstract: The singlet and triplet excited state properties of phenosafranin dye were characterized in aqueous and acetonitrile solutions. The singlet excited state exhibited an absorption maximum at ∼ 430 nm and a lifetime of 930 ps in aqueous medium. The transient absorption spectra of the protonated and unprotonated forms of excited triplet were investigated by the laser flash photolysis technique. The protonated phenosafranin triplet had absorption maxima at 380 nm (ϵ = 11 000 M −1 cm −1 ), 615 nm (ϵ = 9700 M −1 cm −1 ) and 665 nm (ϵ = 10 800 M −1 ) and the unprotonated phenosafranin triplet had absorption maxima at 410 nm (ϵ = 8800 M −1 cm −1 ), 700 nm (ϵ = 6900 M −1 cm −1 ) and 790 nm (ϵ = 11 600 M −1 cm −1 ) in aqueous medium. The p K a of the protonated triplet phenosafranin was found to be 7.25. Diffusion-controlled quenching of the excited phenosafranin triplet by amines led to the reduction of the dye. The absorption spectra of the semireduced phenosafranin, which was characterized in acetonitrile, exhibited absorption maxima at 440 nm (ϵ = 29 000 M −1 cm −1 ) and 870 nm (ϵ = 19 000 M −1 cm −1 ).

Photochemistry on surfaces. 2. Intermolecular electron transfer on colloidal alumina-coated silica particles

Abstract: Reductive quenching of two photoexcited ruthenium(II) complexes by an anionic electron donor, 2,2{prime}-azinobis(3-ethyl-benzothiazoline-6-sulfonate) (ABTS{sup 2{minus}}), in aqueous solution was examined by laser flash photolysis before and after adding positively charged colloidal (250-{angstrom} diameter) alumina-coated silica particles. The kinetics and quantum yields of electron transfer with an anionic sensitizer, RuL{sub 3}{sup 4{minus}} (L = bathophenanthroline disulfonate), and a cationic one, Ru(bpy){sub 3}{sup 2+} (bpy = 2,2{prime}-bipyridine), were compared. Coadsorption of ABTS{sup 2{minus}} and RuL{sub 3}{sup 4{minus}} by the particles greatly enhanced the rate of quenching such that only the reaction occurring on the surfaces of the particles was observed. Electron transfer from ABTS{sup 2{minus}} to RuL{sub 3}{sup 4-*} occurred by a static (nondiffusional) process, and the quenching efficiency was maximal when there was close to monolayer coverage of the reactants on the particles. Ru(bpy){sub 3}{sup 2+} was not adsorbed by the particles and served as a luminescent probe for the determination of the binding capacity of the particles for ABTS{sup 2{minus}}.

Photochemistry on surfaces. 3. Spectral and photophysical properties of monomeric and dimeric anthracenesulfonates adsorbed by colloidal alumina-coated silica particles

Abstract: Anthracene-2 sulfonate (2AS) molecules dimerized upon adsorption from aqueous solution onto positively charged alumina-coated silica particles (25-nm diameter). This ground-state dimerization was accompanied by an intensification and red-shift (from 370 to 401 nm) of the O-O absorption band and a large decrease in fluorescence quantum yield. Anthracene-1,5-disulfonate (1,5AS) and anthracene-1-sulfonate molecules also adsorbed to the particles but did not dimerize. Triplet states of the adsorbed molecules were characterized by laser flash photolysis. Direct photoexcitation of the 2AS dimer failed to produce an observable triplet state, but photosensitization yielded a triplet state of the dimer resembling that of the monomer. The triplet state of the adsorbed 1,5AS molecule was obtained by either direct or sensitized excitation. Binding capacities of the particles for 2AS and 1,5AS indicated that at maximal coverage the 2AS molecules were tilted with respect to the surface while the 1,5AS molecules were coplanar. Dimerization of 2AS occurred even when fewer than 1% of available adsorption sites were occupied.

Electron Transfer Reactions. Reaction of Sydnones with Potassium.

Abstract: On propose des mecanismes mettant en jeu la formation initiale d'intermediaires sous forme de radicaux anions