Dihedral Angle Dependence of Transfer Integrals in Organic Semiconductors with Herringbone Structures

The herringbone structure is a representative molecular packing in organic semiconductors, but there are some modifications with largely different dihedral angles ranging from 40 to 130 degrees. Dihedral angle theta dependence of the transfer integrals is systematically investigated on the basis of the molecular orbital calculations for various organic semiconductors including pentacene, picene, oligothiophene, fused thiophene, tetrathiafulvalenes (TTF), and benzothienobenzothiophene (BTBT). In the conventional organic semiconductors such as pentacene and oligothiophene, the transfer integral is a monotonously decreasing function of theta starting from the stacking geometry, and smaller theta provides more two-dimensional bands. When the molecules are slipped along the molecular long axis D, these compounds show characteristic oscillating structure of the transfer integrals depending on D, which is called D-modulation. Contrarily, TTF derivatives show oscillating structure of the transfer integrals depending on theta(theta-modulation) due to interaction at the side position. BTBT exhibits a pattern much different from the conventional organic semiconductors, and a considerable magnitude of transfer integrals is expected in the large theta region. These systematic investigations demonstrate that the theta-dependence of the charge transport largely depends on the original molecular orbital symmetry.