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.
