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A key parameter in the performance of organic electronics devices is the mobility of charges. On the macroscopic level, it has been demonstrated that the highest charge carrier mobilities are obtained in highly ordered single-crystalline materials.[1] However, the inherent fragility of single crystals poses serious technological problems, thus considerably limiting their practical applications. An interesting alternative is offered by discotic mesogens, which are typically composed of a central aromatic core substituted with flexible alkylic chains.[2] Cores tend to form columnar stacks, maximizing porbital overlap between adjacent molecules and thus favoring a one-dimensional migration of charge carriers.[3] Furthermore, the inherent fluidity of liquid crystals induces advantageous properties, such as the ability to self-heal structural defects and easier alignment and processing from the isotropic phase. However, fluidity is also associated with intrastack dynamism of the functional units that reduces carrier mobility in the bulk. Therefore, the preferred strategy for improving mobility in discotic mesophases has been the enhancement of the intermolecular order within the stacks. To achieve this goal, different approaches have been explored: 1) linking the cores to peripheral alkylic chains through bulky moieties; [4] 2) introducing functional groups providing directional interactions [5] (e.g. hydrogen bonds); or even 3) inducing helical columnar arrangements that provide a higher degree of order. [4a, 6, 7] Herein, we present an experimental study on new triindole mesogens, one of them exhibiting very high hole mobility (m % 1.4 cm 2 V À1 s À1 ). We show how carrier mobility in such compounds does not depend only on the degree of intracolumnar order along the columns by itself, but also, as in most p-conjugated organic semiconductors, on the stacking distance between molecules, which usually decreases with increasing order.[8] Moreover, results show how intracolumnar molecular distance can be controlled by a suitable choice of the spacers between the aromatic core and the peripheral chains, underlining the promising role of ethynyl moieties as linkers in high-mobility columnar phases.Heptacyclic 10,15-dihydro-5H-diindolo[3,2-a:3',2'-c]carbazole (triindole) was recently introduced as a new core for discotic mesogens.[9] Attachment of six decyl chains (compound 1 in Scheme 1) resulted in columnar hexagonal mesophases, although no stacking periodicity was observed. Despite the intracolumnar disorder, 1 has a high hole mobility m = 0.02 cm 2 V À1 s À1 in the mesophase. In an attempt to raise the mobility values by increasing intracolumnar order, we have investigated the effect of sterically demanding phenyl (compound 2) and rigid alkyne (compound 3) spacers between the peripheral alkyl chains and the central triindole core.The synthesis of 2 and 3 is described in the Supporting Information. Both compounds are obtained as crystalline solids. On heating, they show mesomorphic behavior between room temperature and ...

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A novel approach to ambipolar semiconductors based on hydrogen-bonded complexes between a star-shaped tris(triazolyl)triazine and triphenylene-containing benzoic acids is described. The formation of 1:3 supramolecular complexes was evidenced by different techniques. Mesogenic driving forces played a decisive role in the formation of the hydrogen-bonded complexes in the bulk. All of the complexes formed by nonmesogenic components gave rise to hexagonal columnar (Colh) liquid crystal phases, which are stable at room temperature. In all cases, X-ray diffraction experiments supported by electron density distribution maps confirmed triphenylene/tris(triazolyl)triazine segregation into hexagonal sublattices and lattices, respectively, as well as remarkable intracolumnar order. These highly ordered nanostructures, obtained by the combined supramolecular H-bond/columnar liquid crystal approach, yielded donor/acceptor coaxial organization that is promising for the formation of ambipolar organic semiconductors with high mobilities, as indicated by charge transport measurements.

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Discotic (disk-shaped) molecules or molecular aggregates may form, within a certain temperature range, partially ordered phases, known as discotic liquid crystals, which have been extensively studied in the recent past. On the one hand, this interest was prompted by the fact that they represent models for testing energy and charge transport theories in organic materials. However, their long-range self-assembling properties, potential low cost, ease of processability with a variety of solvents and the relative ease of tailoring their properties via chemical synthesis, drove the attention of researchers also towards the exploitation of their semiconducting properties in organic electronic devices. This review covers recent research on the charge transport properties of discotic mesophases, starting with an introduction to their phase structure, followed by an overview of the models used to describe charge mobility in organic substances in general and in these systems in particular, and by the description of the techniques most commonly used to measure their charge mobility. The reader already familiar or not interested in such details can easily skip these sections and refer to the core section of this work, focusing on the most recent and significant results regarding charge mobility in discotic liquid crystals.

We report observation of the photorefractive effect in functionalized polymer-dispersed liquid crystals. The photoconductive properties are provided by the polymer matrix, and the field-dependent refractive-index changes are generated by the dispersed nematic liquid-crystal droplets. A high diffraction efficiency (8%) and a high refractive-index modulation amplitude (Dn = 2 x 10(-3)) are obtained in 53- microm-thick samples with an applied field of 22 V/microm.