Hierarchical Self-Assembly of Edge-On Nanocolumnar Superstructures of Large Disc-Like Molecules (original) (raw)

Large disk-like aromatic molecules bearing long peripheral aliphatic side-chains can form columnar liquid crystals, that is large oriented domains with good one-dimensional charge transport properties. [1] In most cases, these planar molecules adopt a natural face-on arrangement on the substrate and grow as vertical columnar stacks; they can find applications in sandwich devices such as solar cells and optical displays. [2] Besides, a number of techniques have recently been developed to grow columnar stacks lying parallel to the substrate surface, that is, with molecules standing edge-on, an orientation that could be useful for devices having coplanar electrodes such as field-effect transistors. For example, porphyrins, phthalocyanines, triphenylenes, [5] and hexa-peri-hexabenzocoronenes (HBCs) [6] can form uniaxial alignments lying horizontal on different types of substrates by using appropriate techniques. These techniques include zone casting, [6c] frictiontransferred PTFE templates, [1c,7] Langmuir-Blodgett films, stationary nozzles onto a moving substrate, and more recently magnetic field-induced, field-force alignment, and circularly polarized infrared irradiation. However, the lateral resolution of the uniaxial alignments in these studies is limited to a few nanometers and the growth of the columns is induced either by a field (magnetic or electrical), a mechanical constraint, or a dewetting process. Finally, most of the above cited techniques always end up with monolayers and do not permit to control the formation of multilayered columnar stacks.