Transient intermediates in the photolysis of iodonium cations

Abstract

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.

Laser flash photolysis studies of the formation and reactivities of phenyl(naphthyl)methyl carbocations generated from phosphonium salt precursors

“…For example, in polar solvents such as acetonitrile and alcohols these salts give exclusively cation formation, whereas in less polar solvents such as dioxane there are substantial yields of radical. These results are in line with those obtained for irradiation of similar iodonium and sulfonium salts for which heterolytic and homolytic cleavage reactions have been reported (35)(36)(37). One advantage of the phosphonium salts is their stability in aqueous and alcohol solvents under conditions where other typically used cation precursors, such as halides, undergo rapid ground state solvolysis.…”

Electron transfer sensitized photolysis of 'onium salts

“…280 nm. A photoinduced electron transfer mechanism is usually proposed for this sensitization (1)(2)(3), although recent results (4) indicate that a triplet energy transfer pathway is also feasible. In this study we concentrate on the electron transfer pathway, in which decomposition of the 'onium salts proceeds from the corresponding heteroatomcentered free radicals ( I ) , vis-a-vis direct photolysis, in which reaction occurs from an excited state of the cation, thought to be the lowest triplet, at least in the case of diphenyliodonium cation (4).…”

Photo-Induced Micellization of Block Copolymers

“…These signals were attributed to iodobenzene based on the photodecomposition mechanism of diaryliodonium salts [56,64,69]. The iodobenzene, coupled with biphenyl have been observed in the photodecomposition of DPI [65,70]. This replacement of the signals did not occur when the irradiation of a solution containing only DPI was carried out for 3.5 h. Furthermore, the dark reaction of the copolymer solution in the presence of DPI promoted no micellization.…”