Modulation of the order and the nematic-isotropic transition of 5CB liquid crystal doped with azobenzene solutes: an ESR study

Photoresponsive liquid crystal systems, formed by a nematic liquid crystal (LC) doped with azobenzene derivatives, are of great interest in view of their application as optical-switching and image-storing materials [1] and as model systems for more complex photoresponsive polymers [2]. The azo-derivative trans-cis reversible photo-isomerization can produce two types of photomodulation of the LC mesophase [3]: changes in the LC director orientation (“order-order” change) and shift of the phase transitions (“order-disorder” change). Qualitatively, the working principle of the photochemical phase transition is based on the change of molecular shape of the photochromic guest molecule: the trans form of the azobenzene derivative has a rodlike shape similar to that of the host LC, while the cis isomer destabilizes the LC phase because of its bent molecular shape. However, a detailed investigation of the relation between the azo-derivative molecular features and the changes in trans-host and respectively cis-host solute-solvent system is, to our knowledge, still lacking. Here we present the results of an ESR spin-probe study of the effects produced by both the trans and the cis forms of different p-azobenzene derivatives dissolved in 5CB on the order and dynamics of a nitroxide probe. In particular, we study the shift of the N-I phase transition temperature (TNI) and the changes in the temperature dependence of the order and their relationship with the composition of the mixture. Eight closely related non-mesogenic p-azobenzene derivatives with molecular formula 4-X-azobenzene (X= H, Br, F, CH3, CF3, OCH3, O-nBu, O-tBu) were used (at a mole fraction of 0.01 and 0.08). Even at the lower mole fraction studied the presence of the azo-compound caused, in all the cases, a depression of the TNI. The cis isomer slightly reduced the order of the 5CB nematic phase and, as expected, induced a larger TNI shift with respect to the trans isomer. At the higher molar fraction, we observed an analogous behavior with enhanced effects. In this case, we also detected with microscope investigations a biphasic region, near the TNI. The dynamics was essentially unaffected by the presence of the azo-compound in all the cases studied. These findings are currently examined to relate them to molecular properties, in particular changes of dipole moments or steric effects, but it seems that more refined theoretical interpretations and atomistic simulations will be required for a satisfactory rationalization. [1] K. Ichimura, Chem. Rev., 100, 1847 (2000). [2] H. Finkelmann, E. Nishikawa, G.G. Pereira and M. Warner, Phys. Rev. Lett., 87, 015501 (2001). [3] O. Tsutsumi, T. Shiono, T. Ikeda and G. Galli, J. Phys. Chem. B, 101, 1332 (1997); T. Ikeda, J. Mater. Chem., 13, 2037 (2003).