Liquid crystalline perylene diimides: Architecture and charge carrier mobilities

The phase behavior of three N-alkyl-substituted perylene diimide derivative s is examined by differential scanning calorimetry and polarized optical mi croscopy. The occurrence of multiple phase transitions indicates several cr ystalline and several liquid crystalline phases. X-ray diffraction measurem ents show that the liquid crystalline phases display high structural orderi ng in all three dimensions: smectic layers are formed, and within these sme ctic layers an additional ordering in columns is observed. Molecular modeli ng confirms this result and substantiates smectic ordering with interdigita ting alkyl chains that determine the distance between the smectic layers. T he ordering in columns is favored by pi-pi interactions between the cofacia lly oriented perylene molecules and by the elliptic shape of the molecule. Finally, intermolecular dipole-dipole interactions between the carbonyl gro ups of the imide moieties cause the perylene molecules to orient on average with a slight rotation between neighboring molecules within a columnar sta ck. Following the determination of the electronic transition dipole moment, this orientation, which still involves substantial pi-pi interactions, cou ld be confirmed by UV/vis spectroscopy of perylene aggregates. To gauge the potential of these materials as organic semiconductors, the charge carrier mobility of one of the perylene derivatives has been measured by pulse-rad iolysis time-resolved microwave conductivity. A value in excess of 0.1 cm(2 ) V-1 s(-1) is found in the liquid crystalline phase, and a value in excess of 0.2 cm(2) V-1 s(-1) is found for the crystalline phase. These values ar e comparable with the highest values previously found for other discotic ma terials.