Solubilization and photoreactivity of E-ethyl cinnamate in smectic B mesophases VII. Photoreactions in liquid crystals

Abstract

The relative quantum yields of (2 + 2) photocycloaddition for E-ethyl 4-methoxycinnamate 1 have been estimated in smectic and nematic phases as well as in isotropic melts of trans, trans-4′-n-butyl- and trans, trans-4′-ethylbicyclohexyl-4-carbonitrile (CCH-4 and CCH-2). The quantum yield is significantly higher in smectic B phases compared to the values in the isotropic and nematic range. The existence of a second smectic phase, S₂, of CCH-4 has been shown. The photoreaction does not occur in this mesophase. The solubilization behaviour of deuteriated E-ethyl 4-methoxycinnamate has been studied by means of ²H, ¹H, and ¹³CNMR as well as DSC and thermal microscopy. The cinnamate has a very low solubility in S_B phases (< 0·5 wt%) in contrast to the S₂ and nematic phases. The results indicate that phase separation occurs to cinnamate-rich nematic or isotropic regions below the bulk smectic B—nematic transition temperature. Thus, the high quantum yields are due to compartmentalization of the cinnamate in nematic or isotropic spheres of the biphasic smectic B—nematic or smectic B—isotropic systems.