Crystal structures of organic semiconductors

Abstract Crystal structures of organic semiconductors are investigated from the viewpoint of the quadrupole moments Q. Acenes and thienoacenes have positive in-plane long- and short-axis Qxx and Qyy, leading to a herringbone (HB) structure because the out-of-plane Qzz is dominant due to the traceless character of Q, which prefers the vertical molecular arrangement. Compounds containing electron-deficient rings such as quinone and pyrazine have negative Qyy, and the resulting Qxx dominance brings about a stacking structure. Accordingly, a 2-dimensional plot of Qxx and Qyy for various compounds provides a “phase diagram” of the crystal structures, which is also verified by a simple unit Q model; the border between the HB and stacking structures is not exactly Qyy = 0 but depends on the molecular width. On this border, a HB-like θ-structure with a large dihedral angle θ of 130° appears. Naphthalene diimides with large molecular width have a brickwork structure owing to |Qxx| < |Qyy|, but the perylene analogs have the stacking structure due to |Qxx| > |Qyy|. Organic donors such as tetrathiafulvalene have negative Qyy, and organic acceptors such as tetracyanoquinodimethane have negative Qxx, in which the neutral crystals have stacking structures. The mixed-stack charge-transfer complexes usually have a ring-over-bond type overlap with a finite offset because the face-to-face steric interaction is still repulsive, but a few complexes between diamines and fluoranil have an eclipsed overlap due to the electrostatic attraction.