Photo-Reversible Liquid Crystal Alignment using Azobenzene-Based Self-Assembled Monolayers: Comparison of the Bare Monolayer and Liquid Crystal Reorientation Dynamics

Abstract

Photosensitive surfaces treated to have in-plane structural anisotropy by illumination with polarized light can be used to orient liquid crystals (LCs). Here we report a detailed study of the dynamic behavior of this process at both short and long times, comparing the ordering induced in the bare active surface with that of the LC in contact with the surface using a high-sensitivity polarimeter that enables detailed characterization of the anisotropy of the active surface. The experiments were carried out using self-assembled monolayers (SAMs) made from dimethylaminoazobenzene covalently bonded to a glass surface through a triethoxysilane terminus. This surface gives planar alignment of the liquid crystal director with an azimuthal orientation that can be controlled by the polarization of actinic light. We find a remarkable long-term collective interaction between the orientationally ordered SAM and the director field of the LC: while an azobenzene based SAM in contact with an isotropic gas or liquid relaxes to an azimuthally isotropic state in the absence of light due to thermal fluctuations, an orientationally written SAM in contact with LC in the absence of light can maintain the LC director twist permanently, that is, the SAM is capable of providing azimuthal anchoring to the LC even in the presence of a torque about the surface normal. We find that the short-time, transient LC reorientation is limited by the weak azimuthal anchoring strength of the SAM and by the LC viscosity.

Equilibrium structures of anisometric, quadrupolar particles confined to a monolayer

“…59,60 Some of these systems show a remarkable variety of different structures, which can even be controlled via external parameters such as magnetic fields 61 or light with different polarization, 62 opening thereby the route to many interesting technological applications.…”

Synthesis and Characterization of Photo-Responsive Thermotropic Liquid Crystals Based on Azobenzene

“…Liquid crystals (LCs) containing an azobenzene moiety, either of low molar mass or polymeric in nature, have attracted tremendous attention as a result of their light-induced, photo-switchable, and elastic properties [4][5][6]. The introduction of reversible trans-cis photoisomerization of azobenzene can exert large effects on the properties of LCs [7][8][9][10].…”

Kinetic Monte Carlo simulations for birefringence relaxation of photo-switchable molecules on a surface

“…11,12 Among other things, 13 photoswitching molecules illuminated by light can reorient a nematic liquid crystal or directly control both the formation and relaxation of orientational order in a monolayer. 1,[14][15][16][17] Thus, many material properties such as mass transport, mobility, and viscosity can be efficiently tuned while producing hardly any heat. Although these phenomena have attracted much interest for many years, details of the microscopic dynamics still need to be clarified.…”