High-Pressure Kinetics of Azo Dyes in Nematic Liquid Crystals

Abstract

The introduction of organic photochromes into nematic liquid crystals (LCs) enables the modulation of their macroscopic properties and gives rise to unique applications. In particular, azo compounds are the dyes of common choice for this task because of their clean and reversible isomerization. In this context, gaining a good insight into how the LC medium influences the kinetics of the isomerization process is essential to develop novel liquid-crystalline materials with specific capabilities. Herein, we report the measure for the first time of the pressure activation parameters for the cis-to-trans thermal isomerization reaction of azo dyes in this self-organized state of condensed matter. For this purpose, a novel setup has been developed, which can also be utilized for highly lightabsorbing solutions. Initially, the mechanism of the thermal back reaction in the isotropic phase of the LC has been confirmed and compared with that observed in standard solvents (ΔV ⧧ ≈ 0). Further increase in pressure or decrease in temperature leads to the measure of the pressure dependence of such cis-to-trans thermal isomerization process in the nematic phase of the LC. This dependence, showing an increase in the rate with pressure, clearly does not directly relate to the activation volume of the motion of the dye but to the increase in the order of the LC.

Iron complexes of bridging azo ligands in aqueous solution: changes in the thermal switching mechanism on coordination and oxidation state of metal centres

“…Alternatively, the spontaneous cis-to-trans thermal conversion of the solutions of all these compounds, after photochemical excitation, was studied kinetically as a function of solvent, temperature and pressure (Table S1 †). 8,[11][12][13] Fig. 1a shows some selected standard changes observed in the UV-Vis spectra after irradiation, and Fig.…”

“…8 In all cases, the comprehensive solvent, temperature and pressure dependence of the isomerisation kinetics, including the determination of the thermal (ΔH ╪ and ΔS ╪ ) and pressure (ΔV ╪ ) activation parameters, 9,10 has been found to be an ideal tool for this purpose. [11][12][13] The simplest coordination of these compounds to metal centres usually involves the nitrogen donors of the diazene moiety to the metals. 14 These reactions normally produce photochemically inactive mono-and dinuclear-cyclometalated derivatives.…”

“…8 In all cases, the comprehensive solvent, temperature and pressure dependence of the isomerisation kinetics, including the determination of the thermal (Δ H ╪ and Δ S ╪ ) and pressure (Δ V ╪ ) activation parameters, 9,10 has been found to be an ideal tool for this purpose. 11–13…”