ISSN 0030-400X, Optics and Spectroscopy, 2014, Vol. 116, No. 1, pp. 33–46. © Pleiades Publishing, Ltd., 2014. Original Russian Text © N.N. Karaush, B.F. Minaev, G.V. Baryshnikov, V.A. Minaeva, 2014, published in Optika i Spektroskopiya, 2014, Vol. 116, No. 1, pp. 37–51. 33 INTRODUCTION Highly symmetric carbon framework compounds, such as fullerenes, graphenes, and nanotubes, have opened up a new era both in materials science and in quantum theory of many-electron systems [1]. A vari- ety of unusual properties of these materials are con- stantly throwing down the gauntlet to theoretical physics. For example, the properties of graphenes strongly depend on terminal structures at the ends of these polycyclic monolayers [1]. At the same time, simpler models of new types of cyclic compounds have been scarcely studied, although the investigation of specific features of their electronic structure could be informative for the modern theory of many-electron systems that are characteristic of carbon nanoassoci- ates with sp 2 -hybridization. Combinations of six-membered and five-mem- bered rings, which are typical of fullerenes and some nanotubes, also occur rather frequently in conven- tional organic molecules with a highly symmetric closed ring structure and small rings containing oxy- gen or sulfur atoms. Examples of such compounds are tetraoxa[8]circulene (TOC) and octathio[8]circulene (OTC) (Fig. 1). These compounds have highly sym- metric molecules with an unusual internal framework formed by an eight-membered carbon ring. The struc- ture of the simplest TOC compound is shown in Fig. 1a. This compound was synthesized a half-cen- tury ago [2]; later, its formula was proved using X-ray diffraction and nuclear magnetic resonance (NMR) spectroscopy [3–7]. The OTC compound was synthe- sized much later in 2006 [8], and this synthesis became a revolutionary event in the synthetic chemistry. Actu- ally, the OTC compound, also referred to as the sul- flower (consonant with sunflower), is a new form of carbon sulfide C 2 S [8]. Moreover, the TOC and OTC compounds are sufficiently chemically inert and ther- mally stable, which has made it possible to use their semiconducting properties in the design and fabrica- tion of organic field-effect transistors and light-emit- ting diodes [9–11]. In the organic chemistry, there are not many well- known highly symmetric ring compounds (benzene, coronene, corannulene, fullerene), and their physical and chemical properties have been thoroughly investi- gated both experimentally and theoretically by quan- tum-chemical methods. Until recently, the theory of electronic properties of the TOC compound and sulflowers was neither gen- eralized nor considered in detail [2–10]. There are only few published studies in which calculations of the electronic spectra [11] and infrared (IR) absorption SPECTROSCOPY OF ATOMS AND MOLECULES A Comparative Study of the Electronic Structure and Spectra of Tetraoxa[8]circulene and Octathio[8]circulene N. N. Karaush, B. F. Minaev*, G. V. Baryshnikov, and V. A. Minaeva Bogdan Khmelnitsky National University, Cherkasy, 18031 Ukraine * e-mail: bfmin@rambler.ru Received May 14, 2013 Abstract—The electronic structure and spectra of two highly symmetric molecules of circulenes, namely, tet- raoxa[8]circulene and octathio[8]circulene, which belong to the symmetry point groups D 4h and D 8h , respec- tively, have been investigated by the density functional theory (DFT) method using the hybrid functional B3LYP in the 6-31G(d) basis set. The infrared (IR) spectra of these molecules in the ground and excited trip- let states have been compared. The comparison of the electronic absorption spectra of both molecules has revealed that the first electronic transition is forbidden and determined by the electronic–vibrational inter- action due to the degenerate e u modes. The ability of the studied circulenes to fluoresce and phosphoresce has been analyzed, because these compounds are of interest as promising materials for organic light-emitting diodes. DOI: 10.1134/S0030400X13120084 S S S S S S S S X Y O O O O I II III IV Y X α α' β β' (а) (b) Fig. 1. Molecular structures of (a) TOC and (b) OTC.