Polyene photochemistry: solvent controlled state ordering

Abstract

Four nitrile retinoids were synthesized: 1,1-didemethyl- β-ionylidenacetonitrile; 5,6,7,8,9, 10,11,12-octahydroretinonitrile; 5,6,7,8-tetrahydroretinonitrile; and retinonitrile. These four compounds were dissolved in either hexane, ethanol, or acetonitrile and irradiated. Both initial photoproducts and photostationary states showed retinonitrile has solvent dependent photobehavior. 11-cis-Retinonitrile, like 11-cis-retinal, only appears after retinonitrile is irradiated in either acetonitrile or ethanol. The other three compounds, 1, 1-didemethyl- β-ionylidenacetonitrile, 5,6,7,8,9,10,11,12-octahydroretinonitrile, and 5,6,7,8-tetrahydroretinonitrile, showed no such solvent dependent photobehavior, all their photoproducts were present in all irradiation solvents. From the high energy of the nitrile ˡ(n,π*) transition, it follows solvent dependence must be a native polyene trait. Of the four compounds only the longest, retinonitrile, has solvent dependent photobehavior. These data suggest that a pair of ˡ(π , π*) states, a "covalent" 2ˡAgˉ state and an "ionic" 1ˡBu+ state, are responsible for this solvent effect and length requirement. The "ionic" 1ˡBu+ state is stabilized by polar solvents to become the lowest excited state and controls retinonitrile photochemis in ethanol and acetonitrile. This 1ˡBu+ state is not stabilized by non-polar solvents, and the "covalent" 2ˡAg- state remains the lowest excited state and controls retinonitrile photochemistry in hexane. The length requirement comes from the 2ˡAg- state not being stabilized by conjugation enough to be significantly below the 1ˡBu+ state until a "hexaene", retinonitrile. These results may generalize to other retinoids/ polyenes.