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. Can. J. Chem. 66, 319 (1988). We have studied the anthracene-sensitized photolyses of both diphenyliodonium and triphenylsulphonium salts in solution using both steady-state and laser flash photolysis techniques. Photoproducts, namely, phenylated anthracenes along with iodobenzene or diphenylsulphide, respectively, are obtained from both salts with quantum efficiencies of ca. 0.1 at 375 nm. We infer the intermediacy of diphenyliodo and triphenylsulphur radicals formed by single electron transfer from the singlet-excited anthracene. We have developed a quantitative model of this chemistry, and identify the principal sources of inefficiency as back electron transfer, which occurs at nearly the theoretically limiting rate, intersystem crossing from the initially formed sensitizer-'onium salt encounter complex, and in-cage radical recombination. Faisant appel aux techniques de 1'Ctat stationnaire et de la photolyse laser eclair, on a CtudiC la ptiotolyse sensibiliske par l'anthrackne des sels de diphCnyliodonium et de triphCnylsulfonium en solution. Pour chacun des sels, a 375 nm, les efficacitCs quantiques en photoproduits, soit des anthracknes phtnylCs ainsi que respectivement de l'iodobenzkne et du sulfure de diphenyle, sont d'environ 0 , l . On en dCduit que les radicaux diphCnyliodo-et triphCnylsoufre se forment comme intermCdiaires par le transfert d'un Clectron de l'anthrackne excitC singulet. On a dCvelopp6 un modkle quantitatif de cette chimie et on a identifit que les principales causes de I'inefficacitC sont un transfert en retour d'tlectron qui se produit a une vitesse proche de la limite thkorique, un passage intersystkme se produisant a partir du complexe de rencontre sensibilisateur-sel onium qui se forme initialement et une recombinaison radicalaire se produisant dans la cage.[Traduit par la revue]

The photocatalytic deposition of silver from ethanol solution on TiO2 nanoparticles prepared with a chemisorbed surface alkoxide layer has been examined in real time by picosecond-resolved transient absorption spectroscopy. This photocatalyst formation of surface-trapped photoelectron states, hypothesized to be Ti(III), can be followed on the time scale of the experiment (≤10 ns). Loss of these electrons to recombination is inconsequential, presumably owing to sacrificial hole trapping by the surface alkoxide states. Silver deposition occurs on the same time scale, and the pseudo-first-order rate constant for growth of the silver(0) transient absorption is the same as for the disappearance of the Ti(III) states under these conditions. We infer that one-electron, inner sphere reduction of Ag(I) by Ti(III) is rate determining in the formation of the colloidal silver deposit. These particles must accordingly grow by a sequence of alternating electronic and ionic events analogous to those hypothesized to be involved in latent image formation in silver halide photography. The quantum yield for silver deposition under our conditions was estimated as ca. 0.8, much higher than that reported by other authors (refs , , and 6).

Can. J. Chem. 65, 2342(1987.Pulsed picosecond laser photolysis of the diphenyliodonium cation indicates that excitation of the allowed transition is followed by rapid (ps) deactivation to a homolytically dissociative state, presumably T I . Steady-state photolysis yields iodobenzeneand biphenyl from the primary radical products. Iodobiphenyl is also observed. We infer in-cage recombination of these radical products, with intervention of a carbocationic intermediate. Laser flash photolysis of diphenyliodonium iodide in its charge transfer band yields an initial transient with a lifetime of ca. 200 ps. A strongly absorbing secondary transient is observed on the nanosecond time scale. Iodobenzene is the exclusive reaction product. We infer that it is not formed directly from the primary photoproduct, but through in-cage recombination of phenyl cation with iodide anion, as in benzenediazonium iodide. R. J. DEVOE, M. R. V. SAHYUN, N. SERPONE et D. K. SHARMA. Can. J. Chem. 65, 2342 (1987) La photolyse laser pulste au picoseconde du cation diphtnyliodonium indique que l'excitation de la transition permise est suivie par une dksactivation rapide (ps) conduisant a un ttat dissocik homolytique, probablement T I . La photolyse A l'ttat dissocit homolytique, probablement T I . La photolyse 5 l'ktat stationnaire conduit a la formation d'iodobenzene et de biphtnyle a partir des produits radicalaires primaires. On observe aussi la formation d'iodobiphtnyle. On en conclut qu'il se produit une recombinaison en cage de ces produits radicalaires, avec une intervention d'un intermkdiare carbocationique. La photolyse laser flash de l'iodure de diphknyliodonium dans sa bande de transfert de charge conduit initialement la formation d'une espece transitoire dont le temps de demi-vie est d'environ 200 ps. On observe aussi la formation d'une espkce transitoire secondaire qui absorbe fortement et dont le temps de demi-vie est de l'ordre de la nanoseconde. L'iodobenzkne est le seul produit dq la rkaction.On en dtduit qu'il ne se forme pas directement a partir du photoproduit primaire, mais plutdt par une recombinaison en cage du cation phtnyle avec un anion iodure, comme dans le cas de l'iodure de benzenediazonium.[Traduit par la revue] Introduction Photolabile 'onium salts, particularly those of aryldiazonium, diaryliodonium, and triarylsulfonium cations, have found application in imaging media and in radiation curable coatings (1). The mechanism of photolysis of diphenyliodoniurn cations was probed by following the absorption spectroscopy of light absorbing transient intermediates on the nanosecond time scale by Klemm et al. (2) and by Pappas et a1. (3). These workers observed iodobenzene radical cation and, in the former instance, phenyl radical, both of which had been formed on a time scale fast compared to the resolution capabilities of their experiments. Accordingly, we decided to probe this chemistry with picosecond time-resolved absorption spectroscopy.

Fluorescence quantum yields and emission lifetimes have been measured at room temperature for a series of thiacarbocyanine dyes in various alcohol solvents. All radiationless deactivation events are accounted for by a process, e.g., torsional relaxation, whose rate is viscosity-dependent. The relaxation rate constant (k η) and the exponent of viscosity dependence (α) in the Wirtz−Gierer analysis decrease with increasing polymethine chain length (n) in the dye series. Two dyes were further examined by time-resolved picosecond laser flash spectroscopy. Recovery of ground state in both cases is biphasic. In both cases, the short recovery time corresponds to the fluorescence lifetime. We infer that torsional relaxation of the vertical S1 state is rate determining and does not cross between the S1 and So potential surfaces in the relaxed S1 conformation. The slower recovery process is identified with a pathway that leads to photoisomerization of the cyanine chromophores and implies involvement of a species intermediate between torsionally relaxed S1 and the ground-state photoisomer. This intermediate is suggested to be an isomerized triplet state.