LIQUID-CRYSTALLINE SOLVENTS AS MECHANISTIC PROBES .43. NORRISH-II REACTIONS OF NEAT LIQUID-CRYSTALLINE KETONES - COMPARISON BETWEEN NEMATIC AND SOLID-PHASE ORDER AND CONTROL OF PHOTOPRODUCT DISTRIBUTIONS

The Norrish II photoreactions of trans-1-(4-pentanoylphenyl)-4-Pentylcyclohexane (1a) and trans-1-heptyl-4-(4-pentanoylphenyl)cyclohexane (1b) have been examined in their solid, nematic, and isotropic phases. The influence of phase on the product distributions is found to be negligible in the nematic but significant in the solid phases. Phase order, probed by deuterium NMR spectroscopy of the compounds deuteriated at the methylene-alpha to the carbonyl group and by differential scanning calorimetry, has been correlated with the photochemical results. The lack of influence of the nematic phase on the Norrish II reactions of neat 1 is consistent with previously reported results obtained from irradiations of ketones doped in inert nematic matrices. The influence of solid phase order on the photoproduct selectivity of 1 is greater than that observed in some studies on analogous guest ketones in other solid phase hosts, but smaller than in others. A discussion of some of the factors leading to phase control over molecular motions along a reaction coordinate is presented.