Quadrupole moments determine the crystal structures of organic semiconductors

Abstract

This study demonstrates that a two-dimensional plot of in-plane quadrupole moments Q in organic semiconductors provides a phase diagram of their crystal structures. When the long- and short-axis Q_xx and Q_yy are positive, the crystal has a herringbone (HB) structure. Introduction of electron-deficient rings makes Q_yy negative, leading to a stacking structure. Terminal electron-deficient moieties make Q_xx negative, which also results in a stacking structure. By contrast, electron-rich groups do not change the HB structure. All these aspects are explained by the various quadrupole moments and the largely different intermolecular distances. The crystal structures of many materials reveal a transition region between the HB and stacking structures, consisting of a HB-like θ-structure with a large dihedral angle of 130°. In alkyl thienoacenes, the alkyl chain extends parallel to the stacking direction in HB crystals, but vertically in θ-structures, though both have the same alkyl orientation within a molecule. The alkyl chain increases Q_xx but decreases Q_yy, which sometimes changes the HB structure to a θ-structure. Side methylthio groups make Q_yy negative, resulting in a stacking structure. Naphthalene tetracarboxylic diimide derivatives have a brickwork structure due to |Q_xx| < |Q_yy|, whereas the perylene analogues have an ordinary stacking structure owing to |Q_xx| > |Q_yy|.