Liquid-Crystalline Solvents as Mechanistic Probes. 21. Control of Norrish II 1,4-Biradical Reactivity by the Phase and Molecular Dimensions of an Ordered Solvent

Article abstract of DOI:10.1021/ja00277a034

The fates of four triplet 1,4-biradicals (produced as Norrish II intermediates during the irradiations of n-decanophenone, 2-cyclohexyl-1-(4-ethylphenyl)-1-ethanone, 4-cyclohexyl-1-phenyl-1-butanone, and 5-cyclohexyl-1-phenyl-1-pentanone) have been explored in crystalline-, smectic B-like, nematic-, and isotropic-phase solutions of trans,trans-4'-n-butyl<1,1'-bicyclohexyl>-4-carbonitrile (BCCN).The experimental probe is the ratio of elimination to cyclization products.The results indicate that the ordered phases of BCCN behave as a molecular ruler with regard to solute conformations; the micromorphology of each phase imposes a different set of constraints upon the shapes of the 1,4-biradicals.A rationalization for the seemingly bizarre phase-dependent changes in product ratios, based upon the shapes and labilities of the 1,4-biradicals, is presented.