Photoreactive main chain liquid crystalline polyesters containing oxadiazole and bis(benzylidene) cycloalkanone units

Photoreactive main chain liquid crystalline polyesters containing oxadiazole and bis(benzylidene)cycloalkanone moieties were synthesized and characterized by structural, thermal, mesomorpbic, and optical measurements. The bis(benzylidene) cycloalkanone chromophores in the main chain can constitute both as a mesogen and photoreactive center, whereas 1,3,4-oxadiazole is a well-known fluorophore. The thermal properties of polymers were found to be inversely proportional not only to the spacer length but also to ring-size of cycloalkanones. Hot stage polarized optical microscopic investigations displayed enantiotropic nematic liquid crystalline phases and development of grainy to schlieren textures depends on the length of flexible spacer in the polymer backbone which was in accordance with DSC analysis. Both photoisomerization and photodimerization are observed from the absorption spectra and discussed. The fluorescence spectra in solution state at various concentrations showed that the polymers show blue-emission maxima and the Stokes shifts being 48-49 nm. The energy transfer occurred when increasing the concentration of the solution. The band gap energies calculated from the absorption spectra are in the range of 3.17-3.41 eV. (c) 2008 Wiley Periodicals, Inc.