Abstract
The phase behavior of three N-alkyl-substituted perylene diimide derivatives is examined by differential scanning calorimetry (DSC) and Polarized Optical Microscopy (POM). The occurrence of multiple phase transitions indicates several crystalline and several liquid crystalline phases. The liquid crystalline phases display high structural ordering in all three dimensions: smectic layers are formed and within these smectic layers an additional ordering in columns is observed. Molecular modeling confirms this result and substantiates smectic ordering with interdigitating alkyl chains that determine the distance between the smectic layers. The ordering in columns is favored by pi-pi interactions between the cofacially oriented perylene molecules and by the elliptic shape of the molecule. Finally, intermolecular dipole-dipole interactions between the carbonyl groups of the imide moieties cause the perylene molecules to orient on average with a slight rotation between neighboring molecules within a columnar stack. Following the determination of the electronic transition dipole moment, this orientation, which still involves substantial pi-pi. interactions, could be confirmed by UV/vis spectroscopy of perylene aggregates. In combination with the high electron mobility - at least 0.1 cm(2)V(-1)s(-1) in the liquid crystalline phases, and greater than or equal to 0.2 cm(2)V(-1)s(-1) for the crystalline phases - these materials are very promising materials for use as electron conductors in e.g., organic solar cells.