Molecular Engineering of Mesomorphic Fluorene-Bridged Triphenylene Triads: Thermotropic Nematic/Columnar Mesophases, and p-Type Semiconducting Behavior

Discotic polyaromatic liquid crystals exhibiting 2D columnar mesophases are actively investigated for their potential utilization as one-dimensional organic semiconductors, due essentially to their long-range order self-organization and high charge carrier transport ability. The much less reported nematic-containing discotic materials have however been recently found to be of industrial importance too. In this context, highly conjugated fluorene-bridged triphenylene triads have been designed with the aim of inducing room temperature nematic phases. In our approach, the length of the alkyl chains bore by the central fluorene unit was changed in order to constrain the space-filling, to modulate the intermolecular pi-pi interactions, and to control the tendency of the molecular triads to stack into columns, and the subsequent aggregation of the columns in specific ways. This strategy was successful, and in addition to the naturally expected two-dimensional columnar mesophases, most triads exhibit the more elusive columnar nematic phase, as only the orientational-type long-range order is preserved. Photoluminescence, measured in both solution and thin films, revealed strong blue emission, characteristic of the fluorene moiety, with impressively high quantum efficiency. Charge carrier mobility measurements revealed novel p-type molecular semiconductors with hole transport mobility in the range of 0.2-2 x 10(-3)cm(2) V(-1 )s(-1) in the mesophases (Col(rec), Col(hex), and N-col) and up to 5 X 10(-4) cm(2 )V(-1)s(-1) in the isotropic phase. Reasons for the occurrence of these mesophases and their supramolecular structures are discussed along with the effects on the semiconducting properties.