Expected and Unexpected Behavior of the Orientational Order and Dynamics Induced by Azobenzene Solutes in a Nematic

Abstract

We have explored the changes in the phase stability, orientational order, and dynamics of the nematic 4-cyano-4'-n-pentylbiphenyl (5CB) doped with either the trans or the cis form of different p-azobenzene derivatives using the ESR spin-probe technique. In particular, we have studied the effects induced by each of the seven nonmesogenic 4-R-phenylazobenzenes (R = H, F, Br, CH3, CF3, On-Bu, Ot-Bu) at 1% and 7% mole fraction on the order parameter <P2> and on the shift of the nematic-isotropic transition temperature (TNI), as reported by a nitroxide spin probe, and we have tried to relate them to the solute shape and charge distribution. In all the cases the presence of the azo-derivative causes a depression of T(NI), more pronounced for the cis isomers. The dependence of <P2> on the reduced temperature T* = T/T(NI) remains the same as that of pure 5CB in all trans-doped samples at 1% and 7% and decreases only slightly in the cis at 1%. However, we observe different and in some cases large variations (up to 25%) in <P2> for the cis at 7%, showing solute effects that go beyond the shift in T(NI). Surprisingly enough, even at the highest concentration, the probe dynamics appears to be essentially independent of the nature, the configuration, and the concentration of the different solutes and very similar to that observed in the pure 5CB.

Stochastic model for photoinduced anisotropy

“…Another condition that may greatly enhance the anisotropic response of the sample is the presence of mesogenic interactions between the chromophores. In such cases, a variety of technological applications have been devised 1–8 …”

Director configuration in the twist-bend nematic phase of CB11CB

“…The nitroxide spin probe, used for doping the CB11CB LC, was the 3β -doxyl-5α-cholestane free radical (Aldrich, hereafter referred to as CSL). This was employed in a number of previous studies 25,[31][32][33] where it proved to be a reliable probe to monitor the order and the dynamics of the LC system, due to its size, morphology and rigidity, which results in a strong orientation by the LC host. The CSL structure is shown in Figure 2 together with the chosen ordering (x,y,z, solid line) and magnetic (x',y',z', dashed line) molecular frames and the indication of its two main reorientational motions, tumbling and spinning, with the corresponding components of the rotational diffusion tensor: D ⊥ (reorientation of the molecular long axis) and D (rotation around the long axis), respectively.…”

EPR spectroscopy and molecular dynamics modelling: a combined approach to study liquid crystals

“…Over the years, Claudio Zannoni's group has produced eminent contributions to the study of various LC phenomena by EPR. Examples of applications include the study of polyliquid crystalline states [4], determination of the helix pitch and handedness of induced cholesteric mesophases, CONTACT Vasily S. Oganesyan v.oganesyan@uea.ac.uk including the ones induced by a chiral probe [5,6], analysis of the effects of dispersed hydrophobic aerosol particles and solutes such as azobenzene on the order and dynamics of nematic LCs [7,8] and determination of director configuration in the twist-bend nematic phase [9] as well as applications to lyotropic LCs [10,11]. Figure 1(a) shows two nitroxide spin probes commonly used in LC studies, namely 3-DOXYL-5-cholestane (CSL) and 5-DOXYL stearic acid (5DSA) alongside two nematic 4-cyano-4-n-pentylbiphenyl (nCB) LC mesogens.…”