Liquid crystal and photo-induced properties of polymers carrying pyridylazobenzene groups and iodopentafluorobenzene rings self-assembled through halogen bond

Abstract

In this work, we report on the synthesis, liquid crystal behavior and photo-induced optical properties of a polymer and a copolymer, both carrying lateral pyridylazobenzene groups and iodopentafluorobenzene (IPFB) rings self-assembled through halogen bond. The formation of the halogen-bonded complexes was confirmed by Fourier transform infrared (FTIR) spectroscopy; the mesomorphic behavior was determined by differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X-ray diffraction (XRD); and the optical properties were photo-induced with a 405 nm laser beam. The pyridylazobenzene-functionalized polymer showed a smectic-type order (smectic A and crystE phases) whereas the copolymer and the halogen-bonded complexes with IPFB rings formed aggregates whose molecular order in the z-coordinates is suppressed (no layers). The photo-isomerization (in solution) of the polymer and copolymer led to a trans-to-cis isomer conversion (%cis) of 82, and 95, respectively. In film, the %cis values resulted much inferior but still high enough to induce optical properties, as for instance, in films of the copolymer (%cis = 60) and its halogen-bonded complex (%cis = 46) that responded quite well in periodical light-induced writing/erasure experiments. Only the halogen-bonded copolymer complex allowed us to register regular surface relief gratings (SRGs) suggesting that it would be a good candidate for optical data storage applications.