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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.

p-Terphenyls carrying two or four fluorine atoms in the central ring were synthesized via Suzuki-Miyaura cross-coupling reaction using phenylboronic acids and fluorinated dibromobenzenes. By calorimetric and X-ray diffraction analyses it was found that the p-terphenyls carrying two fluorine atoms develop nematic and/or smectic (A and C) phases. On the contrary, the p-terphenyls substituted with four fluorine atoms are not liquid crystals. Differences found in the thermal behavior are explained in terms π – πF interactions arising from the electron withdrawing ability of the fluorine atom. Such π – πF interactions let the p-terphenyls stack in a staggered configuration that explains the stability of the smectic C phase. On the other hand, the semi-fluorinated p-terphenyls absorb and emit in the blue region of the electromagnetic spectrum; a measured large Stokes shift indicates that these compounds are nearly transparent to their own emitted light. This optical characteristic, combined with liquid crystals properties of low order, makes these new p-terphenyls potential candidates for linearly polarized blue-emitting layers in electroluminescent devices.

Two methacrylate monomers, carrying a p -terphenyl laterally substituted with one or two cyano groups, were synthesized and studied from the liquid crystal and optical points of view. The synthetic route involved several reactions, including the Suzuki reaction that allows the coupling of aryl groups. The chemical structure of these new monomers, as well as that of their corresponding precursors, was confirmed by 1 H and 13 C NMR spectroscopies. The mesomorphic behavior of both monomers and precursors was studied by differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X-ray diffraction (XRD) analyses. Molecules bearing one cyano group melt at lower temperature and display a mesomorphic behavior over larger temperature intervals as compared to those bearing two cyano groups. The four compounds (two monomers and two precursors) showed only non-ordered smectic phases. By X-ray diffraction it was determined that molecules stack in single layers within the smectic phase, except for one of the monomers that stacks in double layers. The electrochemical characterization revels that these molecules behaves as p type materials, and that their electronic nature (band gap) can be modulated by introducing CN groups. The molecules absorb and emit in the blue spectral range, a region of high energy light. The large Stokes' shifts indicate that the geometry of the molecules changes from aromatic in the electronic ground state to a more coplanar quinoid structure in the first excited state. The fluorescence quantum yield is high (70–80%) suggesting that these new monomers can be used as precursors for the synthesis of fluorescent liquid crystal polymers, which is a work in progress.

This work deals with the synthesis and thermotropic behavior of a hexyloxy-substituted pyridyl-ethynylene-azobenzene and its halogen-bonded complex (XB complex) with tetrafluoroiodophenyl decanoate. The XB complex here reported is a highly anisotropic photoactive mesogen. It displays only smectic-type order where two different lamellar stacks seem to co-exist. The smectic period (determined by XRD) shorter than the theoretical length of the XB-complex suggests a single layer stacking of molecules, which may be arranged head-to-tail to satisfy the electrostatic interactions between fluorinated and non-fluorinated rings. Finally, photo-isomerization and NMR experiments seem indicate that the halogen-bonded complex is stable in solution.

Four functional methacrylate monomers, two of them carrying a biphenyleneazobenzene group, and the other two an azotolane group, were synthesized. The synthetic route involved several reactions, among which are the Suzuki-Miyaura and Sonogashira cross-coupling reactions that allowed us to prepare long rigid rodlike azobenzene cores. The chemical structure of these new azo-monomers was confirmed by proton nuclear magnetic resonance (1H NMR) spectroscopy, and the thermotropic liquid-crystalline behavior was determined from differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X-ray diffraction (XRD) analyses. The four monomers showed a mesomorphic behavior that extends over a broad temperature range. The phase transitions of the monomers carrying a biphenyleneazobenzene core were much broader and weaker than those of monomers carrying an azotolane group. Also, the former showed no crystallization in cooling to room temperature. The four studied monomers displayed liquid crystal phases with lamellar order (SmA and SmC), but only those carrying an azotolane core developed an additional nematic phase. The lamellar stacking of monomers is discussed in terms of lateral interactions between the rigid rodlike cores. Both types of monomers showed a trans to cis photo-conversion of around 90% when irradiated with UV–light. These new monomers are precursors of photo-responsive liquid crystal polymers.

Poly(4-vinylpyridine) was partially (20% and 50%) and fully (100%) quaternized with a bromo-terminated 4,4′-dialkyloxy-substituted cyanophenyleneazobenzene group (Azo-Br). The resulting liquid crystal azopolymers were studied by differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and X-ray diffraction (XRD) to determine their thermotropic behavior. They showed only a smectic phase that extends over a broad temperature range. The smectic period was found to correspond to a double layer stacking where the pendant groups (mesogens) are partially interpenetrated. On the other hand, the azopolymers were irradiated (UV-vis wavelengths) to induce trans-to-cis conversion (photoisomerization), molecular alignment (birefringence), and mass migration (surface relief gratings). Results showed the lowest trans-to-cis photo conversion for the polymer with the highest content (100%) of chromophore units, and it was simply attributed to constrains arising from their close packing. On the contrary, this polymer showed the highest birefringence value suggesting a coordinated motion of the tightly held chromophore units at the smectic domains, as it was indeed reported for other photo-active liquid crystal azopolymers. Finally, surface relief gratings, having a sinusoidal profile with regular periodicity, were registered. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44819.