Journal of Applied Spectroscopy, Vol. 83, No. 1, March, 2016 (Russian Original Vol. 83, No. 1, January–February, 2016) INFLUENCE OF METHYL SUBSTITUENTS ON AZO-DYE PHOTOALIGNMENT IN THIN FILMS V. S. Mikulich, * An. A. Murawski, UDC 539.216.2 Al. A. Muravsky, and V. E. Agabekov Photoalignment of azo dyes derived from salicylic acid in thin films (80–200 nm) was studied upon irradiation with polarized light (λ max = 457 nm). It is shown that different trends of molecular reorientation, i.e., in the layer plane or orthogonal to it, are observed depending on the position of the methyl substituent in the dye structure. A new distribution parameter Z that allows the portion of molecules reoriented in the layer plane during exposure to be determined is introduced. The novel azo dye potassium 3,7-bis[1-(4-hydroxy-3-carboxylate)phenylazo]-5,5′- dioxodibenzothiophene was synthesized. Its molecules are photoaligned in the layer plane upon irradiation with polarized light. Keywords: azo dye photoalignment, thin films, π–π interaction, structural isomerism. Introduction. Materials that align liquid crystals (LC) and form an anisotropic structure in thin films after irradiation by polarized light are highly interesting [1–5]. LC are more highly aligned and devoid of the defects generated by polishing if such films are used. Photoalignment can produce various LC orientations to create multi-domain structures that improve the angular characteristics of LC displays [2]. Azo dyes, thin films of which have high binding energies to LC and are thermally and photolytically stable occupy a special place among photoalignment materials [6]. They can be used not only in information displays (LC displays) but also as switches in waveguide systems, optically rewriteable paper [5, 6], and phase rasters for producing 3D images [7]. The chemical structure and intermolecular interactions in thin films that determine the material properties, e.g., resistance to crystallization, photosensitivity, and uniform deposition on the substrate, must be considered in developing new azo dyes for photoalignment [8]. The goal of the present work was to study the influence of a methyl substituent in the molecular structure of azo dyes on their photoalignment in thin films. The azo dyes were AbA-2 {potassium 4,4′-bis[1-(4-hydroxy-3- carboxylate)phenylazo]diphenyl}, FbF-2 [potassium 4,4′-bis[1-(4-hydroxy-3-carbyxolate-6-methyl)phenylzao]diphenyl}, GbG-2 {potassium 4,4′-bis[1-(4-hydroxy-3-carboxylate-5-methyl)phenylazo]diphenyl}, AzA-2 {potassium 4,4′-bis[1- (4-hydroxy-3-carboxylate)phenylazo]-3,3′-dimethyldiphenyl}, AtA-2 {potassium 3,7-bis[1-(4-hydroxy-3-carboxylate) phenylazo]-5,5′-dioxodibenzothiophene} with a central 5,5′-dioxodibenzothiophene core and salicylic-acid side groups, and the homolog GzG-2 {potassium 4,4′-bis[1-(4-hydroxy-3-carboxylate-5-methyl)phenylazo]-3,3′-dimethyldiphenyl}. (See diagram.) Experimental. The azo dyes were synthesized and purified as before [8]. Glass substrates (alkaline float glass, 2-mm thick, 50 × 25 mm) was cleaned in an ultrasonic bath successively in distilled H 2 O and i-PrOH and dried at 110 o C for 5 min. Substrates were also cleaned using UV in a Photo Surface Processor Model PL 16-110D (SEN Lights Corp., Japan). Thin films were deposited on glass substrates and excesses were removed with a bar (rod coating) using dye solutions (2%) in DMF at linear deposition rate 1 cm/s and 55 o C [9]. The resulting samples were dried at 100 o C for 5 min. Absorption spectra were recorded on a UV/Vis spectrometer (Maya2000 Pro, Ocean Optics, USA). Photoalignment of azo dyes was studied on a computerized apparatus [8, 9] that allowed the sample to be exposed (polarized radiation power 15 mW/cm 2 , λ max = 457 nm) for a given time (0–2550 s) and polarized absorption spectra to be 0021-9037/16/8301-0115 ©2016 Springer Science+Business Media New York 115 Institute of the Chemistry of New Materials, National Academy of Sciences of Belarus, 36 F. Skorina Str., Minsk, 220141, Belarus; e-mail: mikulich.vadim@gmail.com. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 83, No. 1, pp. 131–137, January–February, 2016. Original article submitted August 17, 2015. _____________________ * To whom correspondence should be addressed. DOI 10.1007/s10812-016-0252-y