Effect of molecular structure and packing density of an azo self-assembled monolayer on liquid crystal alignment

• Published in: Journal of colloid and interface science Vol. 407; pp. 310 - 317
• Main Authors: Vengatesan, M.R., Lee, Seung-Ho, Son, Jong-Ho, Lim, Jeong-Ku, Song, Jang Kun
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier Inc 01-10-2013; Elsevier
• Subjects: Alignment; Alignment layer; Azo; chemical structure; Chemistry; Dipping; Exact sciences and technology; General and physical chemistry; glass; Liquid crystals; Molecular structure; Monomers; Packing density; Photo isomerization; Self-assembled monolayer (SAM); Self-assembled monolayers; silicon; Surface physical chemistry; Toluene; Self-assembled monolayer (SAM); Photo isomerization; Alignment layer; Liquid crystals; Azo; Self assembly; Monolayer; Molecular packing; Molecular structure; Isomerization; Density; Azo compound; Molecular assembly; Self-assembled monolayer
• ISSN: 0021-9797; 1095-7103
• DOI: 10.1016/j.jcis.2013.06.014

•Photo-switchable liquid crystal alignment using azo-SAM is demonstrated.•Azo-SAM with long octyloxy chain does not show photo-switchable LC alignment.•Azo-SAM with short methyl group exhibits photo-switchable LC alignment.•High packing density of azo-SAM suppresses the photo-switchable LC alignment.
We studied the alignment of liquid crystals (LCs) on a photo-switchable azo-containing self-assembled monolayer (azo-SAM) with different packing densities and molecular structures. The packing density of the azo-SAM substrates was varied by changing the dipping time of the substrate in azosilane monomers solution (2mM in toluene). The thickness of the monolayer on the silicon substrate increased as the dipping time was increased. The relative surface packing density on the glass substrates was estimated from the surface energies of the azo-SAM. The photo-induced dynamics of liquid crystal alignment on the azo-SAM significantly varied according to the packing density of the azo-SAM and the structure of the azo-SAM molecules. The azo-SAM from long octyloxy chain-terminated azosilane (azo-S1) possessed stable homeotropic alignment even after photobuffing, while the azo-SAM from short methyl group-terminated azosilane monomer (azo-S2) showed photo-switchable homeotropic and planar alignments. However, when the packing density was increased to an excessive degree, even the azo-SAM from azo-S2 exhibited stable homeotropic alignment regardless of photobuffing.