8367Acknowledgment. We thank the National Science Foundation into the effect of surface oxygen protonation on the photodynamic processes of semiconductor metal oxides. All the data reveal that the electronic and structural perturbations of the polyoxometalate complex induced by solvent, including the presence or absence of electron donor-acceptor interactions, salt effects, ion pairing effects, and other medium effects are substantially less important on the photoredox chemistry than those induced by protonation.(Grant CHE-9022317) for support of this work. Supplementary Material Available:Twelve figures addressing kinetics, quantum yield, electrochemical, and spectral (IE3W NMR and UV-visible) measurements (1 3 pages). Ordering information is given on any current masthead page.Polyene 21Ag and llBu States and the Photochemistry of Previtamin D3 Abstract: The quantum yields of the photoproducts from previtamin D3 were measured at different wavelengths with monochromatic irradiation. While the quantum yield for the cis-trans isomerization decreases with increasing wavelength near 303 nm, the ones for formation of the ring-closure products increase dramatically. This increase in photocyclization yield with decreasing photon energy is attributed to the involvement of both the 1B and 2A excited states of previtamin D3. This hypothesis is supported by measurements of the previtamin D3 fluorescence spectrum, fluorescence lifetime, wavelength dependence of the fluorescence quantum yield, and temperature dependence of fluorescence intensity. All of these data are integrated into a potential energy surface diagram that is consistent with both the photochemical and spectroscopic behavior. 'University of California, Berkeley. *Recipient of a Feodor-Lynen Fellowship of the Alexander von Humboldt i University of California, Riverside. Foundation, 1986-1987. Scheme I. Photoreactions of Previtamin D,In an earlier study at Berkeley: the calculated quantum yields for the formation of pro D3 and lumisterol3 (lumi,) from pre D3 (1) General reviews of the photochemistry related to the previtamin D3 system: (a) Dauben, W. G.; Mclnnis, D. M. Yamachuchi, H.; Ogata, Y.; Kunii, T.; Kagei, K.; Katsui, G.; Toyoshima, s.; Yasumura, M.; Kobayashi, T. J. Nut?. Sci. Vitaminol. 1980,26, 545. (d) Barton, D. H. R.; Hesse, R. H.; Pechet, M. M.; Rizzardo, E.

. Can. J. Chem. 65, 245 (1987). The fluorescence properties of various N-carbonyl-substituted indoles (compounds 1-7) are examined. The N-benzoyl-indole derivatives 1 and 3-5 are shown to fluoresce weakly and the effect of solvent polarity upon the energy of the emitting state and upon the quantum yield of fluorescence is described. It is concluded that the initially formed singlet excited state is non-emissive and can relax to an intramolecular charge transfer state which is weakly fluorescent. The solvent effects are consistent with a general type of solvent interaction, and the solvent-induced shifts in the wavelength of the fluorescence emission correlate well with the Lippert equation relating the stabilisation of the excited state to a function of solvent dielectric and refractive index. A poor correlation with the empirical ET(30) solvent polarity parameter is observed. [Traduit par la revue]

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The mechanism of the photochemical cycloaddition reaction between N-benzoylindole, 1, and cyclopentene to give cyclobutane adducts 2 and 3 has been examined. The triplet excited state lifetime and quantum yield of intersystem crossing were determined for 1 as (2.8 ? 0.3) X 10-'s and 0.39 * 0.01, respectively, using the triplet counting procedure. In addition, the dependence of the quantum yield of cycloadduct formation upon the concentration of cyclopentene and upon the concentration of excited state quenchers has been determined. The results are used to propose a mechanistic model in which the triplet excited state of 1 reacts with cyclopentene to give a triplet 1,4-biradical intermediate. Following spin inversion the biradical intermediate reverts to the ground state starting materials or proceeds to the products 2 and 3; this partitioning, along with the quantum yield of intersystem crossing, gives rise to a limiting quantum yield of cycloaddition at infinite alkene concentration of 0.061. It is calculated that 84% of the biradical intermediates revert to the starting materials and 16% proceed to cycloadducts. The quantum yield data are also used to calculate two independent values of the rate constant for reaction of the triplet excited 1 with alkene; the values are (1.8 I+-0.1) x 1 0 '~-I s -' and (4.0 2 0.8) X 106M-' S -I . Some evidence for self quenching of the triplet excited state of 1 by ground state 1 was also observed. The quantum yield of intersystem crossing and the triplet excited state lifetime of 1 were found to vary with the solvent used; this is discussed in terms of the possible existence of a charge transfer triplet excited state.Key words: indole, photocycloaddition, mechanism.BIMSARA W. DISANAYAKA et ALAN C. WEEDON. Can. J. Chem. 68, 1685 (1990).On a CtudiC le mtcanisme de la reaction de cycloaddition photochimique entre le N-benzoylindole, 1, et le cyclopentkne qui conduit aux adduits cyclobutaniques 2 et 3. Utilisant la technique de comptage des ttats triplets, on a Ctabli que le temps de vie de 1'Ctat triplet excite et le rendement quantique du croisement intersystkme du compose 1 sont respectivement de (2,3 5 0,3) X lo-' s et 0,39 * 0,Ol. De plus, on a dttermink la relation qui existe entre le rendement quantique pour la formation du cycloadduit et, d'une part, la concentration de cyclopenttne et, d'autre part, la concentration des pikges des Ctats excites. On a utilisC les rCsultats obtenus pour proposer un modkle mkcanistique dans lequel l'ttat triplet excitt du produit 1 reagit avec le cyclopentkne pour donner un intermediaire triplet 1,4-biradicalaire. Aprks une inversion de spin, llintermCdiaire biradicalaire retourne B I'Ctat fondamental des produits reactionnels ou il se transforme pour donner les produits 2 et 3; cette ripartition prise de concert avec le rendement quantique du croisement intersystkme placent i 0,061, la limite du rendement quantique be la cycloaddition a une concentration infinie d'alckne. On a calcul& que 84% des intermediaires biradicalaires retournent aux ...