IR ABSORPTION ANISOTROPY IN PERYLENE-3,4,9,10-TETRACARBOXYLIC ACID DIANHYDRIDE B. G. Shulitski a and V. V. Filippov b* UDC 535.34:539.216 Vacuum-deposited perylene-3,4,9,10-tetracarboxylic acid dianhydride thin films are investigated by IR-spectro- polarimetry with variable incident angle of plane-polarized radiation. Anisotropy of absorption bands in the region 700–900 cm –1 is detected. Absorption bands in the subregions 728–736 and 806–810 cm –1 are shown to be of triplet nature with orthogonally oriented dipole moments within the triplet. Keywords: thin film, perylene-3,4,9,10-tetracarboxylic acid dianhydride, microcrystallite, IR-spectropolarimetry. Introduction. Organic semiconducting materials are currently widely employed in the production of functional instruments and devices such as organic transistors and microcircuits, solar cells, light-emitting diodes and electrolumi- nescent displays, radio-frequency ID chips, electronic paper, etc. [1]. Semiconducting materials based on perylene de- rivatives, which are aromatic compounds, are some of the most common ones and are used as dyes in the textile industry. The optical and electric properties of these materials are comparable with those of inorganic broad-band semiconductors. Perylene-3,4,9,10-tetracarboxylic acid dianhydride (PTCADA) is a special base molecule among perylene de- rivatives. Like most perylene derivatives, materials based on PTCADA are typically anisotropic and, therefore, have anisotropic physicochemical properties. For example, the conductivity parallel and perpendicular to the orientation axis of molecular PTCADA crystallites can differ by six orders of magnitude [2]. The optical properties of PTCADA are also strongly anisotropic [3]. A study of the anisotropy of the structure and properties suggests ways of using it to fabricate functional devices based on perylene derivatives. Herein the specifics of IR absorption anisotropy in the re- gion 700–900 cm –1 for various incident angles of radiation relative to the substrate normal are studied. The isolated PTCADA molecule (Fig. 1a) has point-group symmetry D 2h and 46 IR-active modes of B u -sym- metry, including 18 B 1u (longitudinal axis x) and 18 B 2u (transverse axis y) vibrations in the molecular plane in addi- tion to 10 B 3u vibrations perpendicular to the molecular plane (z axis). The single-crystalline cell of PTCADA contains two molecules situated perpendicular to each other. PTCADA molecules interact in the solid state through weak dispersive forces of intermolecular interaction. This leads to polymorphism, i.e., a variety of single-crystallite structures based on them (clusters, whiskers). PTCADA molecules can be crystallized in two monoclinic phases with crystal symmetry P2 1 /c (C 2h 5 ) that are designated the α- and β-forms of PTCADA and have different cell constants (Fig. 1a). Crystalline α- and β-forms of PTCADA have cell constants a = 3.72 A ° , b = 11.96 A ° , c = 17.34 A ° and a = 3.78 A ° , b = 19.30 A ° , c = 10.77 A ° . Microcrystallites of the monoclinic system are in the optical sense a biaxial anisotropic medium. They have a so-called stacked layered structure with coplanar molecular planes. However, the x and y axes of separate needle-like microcrystallites in the film are randomly oriented. This enables films of oriented PTCADA microcrystallites to be viewed as having uniaxial anisotropy. The packing of such molecular single-crystal- lites in a film (texture) deposited on a substrate is determined by the substrate type (surface states) and the deposition conditions. Figure 1b shows as an example a photomicrograph of a PTCADA film 10 μm thick that formed needle- like vertically oriented PTCADA microcrystallites. Control of the anisotropy of the structure of such films (type of molecular packing in the single-crystallite, orientation and density of single-crystallite packing in the film) makes it a Belarusian State University of Informatics and Radioelectronics, Minsk; b B. I. Stepanov Institute of Physics, National Academy of Sciences of Belarus, 68 Nezavisimosti Ave., Minsk, 220072, Belarus; e-mail: v.filippov@ dragon.bas-net.by. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 76, No. 5, pp. 697–704, September–October, 2009. Original article submitted June 8, 2009. Journal of Applied Spectroscopy, Vol. 76, No. 5, 2009 0021-9037/09/7605-0660 ©2009 Springer Science+Business Media, Inc. 660 ∗ To whom correspondence should be addressed.