Self-organizing liquid crystal perylene diimide thin films: Spectroscopy, crystallinity, and molecular orientation

Three different liquid crystal (LC) perylene diimides were investigated with respect to the optical and physical characteristics of their thin films. Films were prepared by spin-coating, vacuum evaporation, and Langmuir-Blodgett (LB) techniques on substrates such as microscope glass, indium-tin oxide-coated glass and highly oriented pyrolytic graphite. Films were characterized by polarized optical microscopy, absorption and fluorescence emission spectroscopy, and X-ray diffraction. The self-organizing ability of the LC perylene diimides allows them to rapidly reach a stable, low-energy configuration, unlike many thin film materials, and reveals that they are driven to organize and orient in a highly specific fashion, independent of substrate or deposition method. The molecules tend to form a slipped stack arrangement that maximizes attractive π-π electronic interactions, with the π-π stacking axis oriented parallel to the substrate. Relative to the substrate plane, the LC 1 perylene cores are tilted ∼47° along the stacking axis and ∼58° perpendicular to this direction. The two other LCs have similar structures. An analysis of the intermolecular electronic and steric interactions, and of the interactions between the molecules and the substrates, is proposed to explain why this is such a strongly preferred orientation. The implications for the potential use of such molecules in electronic and photovoltaic applications is discussed.