Reprinted from the Journal of Physical Chemistry, 96, 9666-9673 (1992).

©American Chemical Society, 1992.

Photochemical Probes of Conformational Mobility in Liquid Crystals. Substituent Effects on the Photochemistry of 4-Alkoxy-beta-arylpropiophenones in the Smectic B Phases of CCH-nOm and CCH-n Liquid Crystals

Mark S. Workentin and William J. Leigh*

Department of Chemistry, McMaster University, Hamilton, Ontario, Canada L8S 4M1

Abstract: The effects of solute structure on the ability of thermotropic liquid crystals to control solute conformational mobility have been investigated using nanosecond laser flash photolysis techniques. The conformation-dependent triplet lifetimes (tau) of 4-methoxy-beta-phenylpropiophenone (1) and a series of benzoyl and beta-aryl-substituted derivatives (2-7) have been measured as a function of temperature in the nematic and smectic liquid crystalline and isotropic phases of three trans,trans-4-alkoxy-4'alkyl-1,1'-bicyclohexyl (CCH-nOm) liquid crystals. Solubility limits of each of the ketones in the smectic B phase of CCH-5O2 as a function of temperature have been determined by deuterium nmr spectroscopy, using a series of selectively deuterated analogs. All of the ketones studied were found to be homogeneously solubilized throughout the majority of the smectic phase temperature range at bulk concentrations of 0.5 mol% or less. The triplet lifetime of 1 shows only slight sensitivity to solvent order in this liquid crystal. While minor variations in the structure of the alkoxy substituent reduce this sensitivity further, substitution at the para position of the beta-phenyl ring magnifies the effect of solvent order on tau. In the latter case, tau increases by a factor of between 3.0 and 8.5 in the smectic phase compared to the isotropic phase at the same temperature, as a result of para substitution with methyl-, isopropyl-, n-hexyl, or cyclohexyl substituents on the beta-phenyl ring. The magnitude of the effect correlates with the size of the beta-aryl substituent. Similar studies were carried out with a few of the compounds in the nematic and crystal B phases of CCH-n liquid crystals. The Cr-B phase of CCH-4 is more highly ordered than the Sm-B phase of CCH-5O2, and the effects on tau are correspondingly magnified. It is concluded that the above effects result mainly from alteration of the equilibrium constant for trans/gauche interconversion in smectic liquid crystals compared to the value in isotropic solvents and not specificially due to a slowing down of the conformational motions required to achieve the excited-state quenching geometry.