High-Sensitivity Aminoazobenzene Chemisorbed Monolayers for Photoalignment of Liquid Crystals

Abstract

We describe a new type of optically controlled liquid crystal alignment layer that demonstrates unprecedented performance. It consists of an aminoazobenzene-type material with a very simple molecular structure, which is derived from methyl red by a one-step synthesis. We have devised a method of forming covalently attached monolayers of this material on glass by an amine-assisted condensation reaction involving the triethoxysilane end of the molecule. A nematic liquid crystal (LC) cell made with the monolayer and a rubbed polymer layer was switched from a uniform state to a twisted state with a polarized 450 nm control beam having a dose of 5.5 mJ/cm(2). This is equivalent to an average of only one absorbed photon per azobenzene group. Through atomic force microscopy, absorption spectroscopy, spectroscopic ellipsometry, and second harmonic generation experiments, we have confirmed that layers of this type are smooth and uniform with a surface coverage consistent with a monolayer and that the azobenzene groups are tilted, on average, 55 degrees with respect to the surface normal. These characteristics lead to a large interaction energy density between the layer and LC. The monolayer's rapid response in developing anisotropy in this property can be attributed to a large absorption cross section, as well as the favorable tilt angle, which allows for sufficient photoisomerization free volume in a dense layer.

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

“…Importantly, we assume that after an isomerization event, the orientation of the molecule is chosen at random. 32,33 As anticipated in Sec. II, because of the high inplane packing density of the SAM, molecules must overlap during the relaxation process in order to become randomly oriented.…”

“…Under illumination with linearly polarized light (writing process), molecules preferentially aligned along the light polarization vector more likely switch toward the cis configuration, while the remaining trans molecules form nematic order perpendicular to the light polarization (holeburning). [32][33][34] The monolayer thus exhibits some birefringence Q(t). Since light illumination also induces the reverse, cisto-trans transition, in steady state a mixture of both isomers exists at a relative concentration that mostly depends on the light intensity and the monolayer density.…”

Athermal photofluidization of glasses

“…The azo system studied (dMR) is the derivative of the dye methyl red 19 shown in Fig. 1a, synthesized and covalently bonded onto glass substrates to form dense, photoactive, SAMs, as sketched in Fig.…”

“…1a 19,22 , and permits reorientation and B1-nm translation of the core. Figure 1b sketches the geometry of a locally oriented state with the molecules represented by rods having azimuthal orientation j and a length-to-width ratio corresponding to the footprint of the van der Waals (VDW) shape of the core in Fig.…”

“…2a inset shows that SAM birefringence depends only on absorbed fluence (absorbed energy/area), F A , and that the initial growth of Dn(t) is rather rapid, with the order of SAM being substantially developed at an absorbed fluence of F A B1 photon absorbed per molecule (1 pa/mol), as calculated from the measured optical absorbance of 514 nm light incident on an isotropic SAM. Note that F A ¼ 1 pa/mol at F ¼ 20 mJ cm À 2 (Supplementary Note 1) 19 .…”