Photodecomposition studies, flash

Photolysis of [Rh(tfacac)3] (tfacac is the unsymmetrically substituted 1,1,1-trifluoromethyl-acac) reveals the existence of two photoinduced reaction paths the relative efficiency of the two paths is dramatically solvent dependent.1140 In cyclohexane, mer- cis isomerization is the only observed photoreaction, but if ethanol or 2-propanol is added to the solvent, the photoisomerization efficiency decreases, and photodecomposition occurs. The nature of the photodecomposition products is not specified, but the enhanced photoreactivity in the presence of tri(n-butyl)stannane, a hydrogen atom donor, and flash and continuous photolysis studies in mixed-solvent systems strongly implicate hydrogen atom abstraction from the solvent as a key step in the photodecomposition of wer-[Rh(tfacac)3] and suggests that the photo reactive states have considerable radical character .1140 Analysis of quantum efficiencies implies that at least two distinct photoproduced excited states must be involved. 

In a recent study of the kinetics of some reactions of C( S), Husain and Kirsch (125) have observed that as CO is added during the flash photolysis of C3O2, the C( S) yield first decreases then increases. This latter increase is attributed to the three-body recombination of C( P) and CO followed by photodecomposition of C2O with the production of C( S). 

The photodecomposition mechanism of BrO has been elucidated by flash photolytic studies in aqueous solutions containing NaOH and also from e.s.r. and optical absorption measurements in aqueous NaOH glasses after photolysis at 77 K. The sole primary reaction was decomposition to 0( D) and BrOj, and is therefore unlike the additive primary processes for the other halogen oxyanions. The radiolytic reduction of KBr04 in aerated and deaerated aqueous solutions has been investigated. In acidic medium the only reaction is to give H (or HO2) and Br04, with G(—BrO ) = 3.5. In neutral solution BrO is reduced by the... 

Braun and co-workers [32] at the National Bureau of Standards have recently developed a powerful new method of studying H atom reactions. The H atoms are generated from the flash photolysis of an olefin with <1% of photodecomposition and the H atom concentration monitored from the resonance fluorescence by atomic H excited by absorption of Lyman-a 1216 A radiation. So far only one transfer reaction has been studied over a range of temperatures by this method but others are likely to follow. The technique has also been extended to measurements of H atom concentrations in fast flow systems [42]. 

Oishi, S., Watanabe, Y., and Kuriyama, Y., Photodecomposition Mechanism of O-Diazonaphtho-quinones Studied by Laser Flash Photolysis with Infrared Detection, Chem. Lett., 2187,1994. Vleggaar, J.J.M., Huizer, A.H., Kraakman, P.A., Nijssen, W.P.M., Visser, R.J., and Varma, C.A.G.O., Photoinduced Wolff-Rearrangement of 2-Diazo-l-Naphthoquinones Evidence for the Participation of a Carbene Intermediate,/. Am. Chem. Soc., 116, 11754,1994. 

In the studies of Land and Porter (1963) flash photolysis was carried out on 18 different alkyl-radical-substituted phenols with R = CH3, C2H5, tert-C4H9, and/or CeHs in various nonpolar and polar solvents, and the absorption spectra of the corresponding phenoxy radical was observed in all cases. Grabner et al. (1990) made an extensive study of the photochemistry of the methylated phenols in hydrocarbon solvents at 293 K and with A,exdt = 275 nm they determined the quantum yields of fluorescence, triplet molecule formation, and photodecomposition. The quantum yields of photodecomposition of phenol and its 2-methyl-, 3-methyl-, 4-methyl-, 2,6-dimethyl-, 3,5-dimethyl-, 2,4,6-trimethyl-, and 2,3,5-trimethyl-derivatives, as measured through the absorption of the phenoxy radicals formed were 0.13, 0.18, 0.14, 0.08, 0.21, 0.14, 0.20, and 0.18, respectively. Evidence suggested that this photodecomposition occmred from a vibrationally excited triplet state. 

Another study, which used nanosecond laser flash photolysis and steady-state photolysis experiments, showed that the decarboxylation of the tolmetin is markedly reduced upon inclusion [17]. The rate constants of the decay of the intermediate transients involved in its photodecomposition were slowed down due to the effect of the hydrophobic CD nanocavity. Similar work carried out on the NSAID ketoprofen [42] and suprofen [43] has shown that inclusion in the j -CD cavity leads to a significant decrease in decarboxylation efficiency and to the opening of new photoreactive channels. Moreover, a modification of the distribution of the photoproducts and a considerable reduction of the quantum yield of singlet oxygen sensitized production either by the starting compound or... 

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