Theoretical Approach for the Development of Organic Semiconductors on the Basis of the MO Symmetry: Thienoacene as an Ex
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Theoretical Approach for the Development of Organic Semiconductors on the Basis of the MO Symmetry: Thienoacene as an Example Hirotaka Kojima and Takehiko Mori Department of Organic and Polymeric Materials, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8552, Japan.
ABSTRACT We have explored materials for organic field-effect transistors (OFETs) from the viewpoint of theoretical calculations. The herringbone structure, which realizes two-dimensional conduction, is investigated in detail. Transfer integrals (t) are calculated systematically as a function of the dihedral angle between the molecular planes ( ) and the displacement along the molecular long axis (D). Acenes, oligothiophenes, thienoacenes and tetrathiafulvalenes are investigated, and are discussed from the molecular orbital (MO) symmetry. Thienoacenes (nTAs) are particularly examined as a candidate of OFET materials from the calculations of transfer integrals and reorganization energies () based on the energy levels and the MO symmetry. LUMO of nTAs have MO symmetry suitable for conduction, but these orbitals are usually not related to the conduction. We have investigated the electronic properties of the derivatives with dicarboximide moiety. nTA-tetracarboxydiimide is expected to show the herringbone structure and exhibit n-type transport from the properties of LUMO. INTRODUCTION Theoretical approaches provide a convenient way for developing advanced OFET materials [1]. Transfer integrals are informative to predict the charge mobility. We have calculated transfer integrals of herringbone compounds. The herringbone structure (figure 1(b)) is a representative packing motif in which molecules incline alternately with a dihedral angle of , and is advantageous to achieve two-dimensional (2D) conduction. For example, pentacene has 53° [2], and other many compounds have of 50–60°. On the other hands, nTAs have relatively large ; for example, 5TA has 130°. Gavezzotti and Desiraju have classified these structures to two types: the authentic herringbone structure with = 40–70°, and the -structure with = 90–110°, where the usual stacking structure with > 150° is called the -structure [3,4]. Here, we have investigated the relation between and the carrier transport. The molecules after the structural optimization are placed in a herringbone geometry, and rotated by varying . In addition, we have also considered the displacement along the molecular long axis (D). It has been known that transfer integrals oscillate with respect to D in acenes owing to the MO phases [5,6]. We have calculated transfer integrals of pentacene, sexithiophene (6T), pentathienoacene (5TA) and dibenzothiafulvalene (DBTTF) for HOMO (hole conduction) and LUMO (electron conduction) [7]. In addition, reorganization energy () is computed for several compounds. represents the energy change accompanied by the structural deformation in the charge transfer, so small leads to high mobility. In general, large -systems are advantageous from the viewpoint of
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