Journal of Organic Chemistry p. 3156 - 3161 (1983)
Update date:2022-08-11
Topics:
Zawadzki, Mary E.
Ellis, Arthur B.
The electronic spectra and photoreactivity of slurries of retinal isomers, prepared by adsorbing the isomers onto wet silica gel and suspending the support in cyclohexane, have been investigated.Adsorption of 9-cis-, 11-cis-, 13-cis-, and all-trans-retinal is accompanied by an ca. 3000-cm-1 red shift of their lowest energy absorption band maxima relative to their band positions in homogeneous cyclohexane solution.Irradiation of the slurries at 514.5 nm, a wavelength inefficiently absorbed in the absence of silica gel, leads to reasonably efficient photoisomerization of each of these isomers.Prolonged photolysis yields a mixture of the four isomers that is photostationary with respect to relative concentrations and richest in 11-cis-retinal, which constitutes ca. 35percent of this mixture.Although small quantities of other isomers are present, the photostationary composition of the heterogeneous photolysate can be predicted with reasonable accuracy from the relative absorptivities and primary photoprocesses of the four principal isomers comprising the photolysate.Comparisons with primary photoprocesses reported for retinal isomers in polar and nonpolar solvents reveal that adsorption onto silica gel can result in novel patterns of photoisomerization.Complementary comparisons are made with the electronic spectra and photoreactivity of adducts formed in hydrocarbon solution from retinal isomers and a lanthanide β-diketonate complex.The excited-state properties of these various retinal-based systems highlight the impotance of environment in controlling photoreactivity.Steric and electronic factors that may contribute to the observed features of silica gel mediated photoisomerization are discussed in this context.
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