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 ...

Heterocoronene, a new redox-active core fragment, is utilized for the synthesis of room-temperature columnar discotic liquid crystals (DLCs). Three wedge-shaped side chains having different lengths of alkyl tails are introduced at the periphery of the heterocoronene core to prepare three kinds of discotic molecules, 1 (R = C 10 H 21 ), 2 (R = C 12 H 25 ), and 3 (R = C 14 H 29 ). X-ray diffraction (XRD) analysis confirmed the packing variation in the columnar lattices regulated by alkyl chains of discrete length and steric bulk. When used in space charge limited current devices, compound 1 exhibits a high hole mobility value of 8.84 cm 2 /V s at ambient temperature, whereas compounds 2 and 3 show efficient ambipolar charge transport behavior with maximum hole (μ h ) and electron (μ e ) mobilities of 0.70 and 3.59 cm 2 /V s, respectively, for compound 3. The mobility values (μ h = 8.84 cm 2 /V s for 1 and μ e = 3.59 cm 2 /V s for 3) are remarkable and the highest ever disclosed for any DLC-based organic semiconductor, promising to deliver a good balance between mobility and processability in devices. The grazing incidence small-and wide-angle X-ray scattering experiments are employed to quantify the extent of alignment in the film state, which correlates with the observed trend of mobility values.

We report the synthesis, mesomorphic behavior, and mobility values of a series of highly ordered N-substituted triindole-based columnar liquid crystals. Shortening the length of N-alkylic substituents from N-dodecyl to N-methyl chains results in a drastic approach of the disks within the columns and in an impressive increase in charge carrier mobility. An study of aggregation in solution provide insights into the intermolecular forces responsible of the reduction of the intrastack distance as the size of the N-alkyl chains is decreased and offer evidence of stabilization of the columns by the contribution of cooperative CH−π interactions. The materials presented here exhibit mobility values, even in totally misaligned columnar phases, that may compete with those of the best polycrystalline organic semiconductors, without the need of costly vacuum evaporation processes.

The design, synthesis and characterization of five diphenylhydrazone end capped molecules as donors for Bulk Heterojunction (BHJ) solar cells are described. The use of the hydrazone donor instead of the more common amine one is advantageous in terms of synthetic access and overall stability. We show that the variation of both the nature and the substitution pattern of the electron deficient conjugated bridge enables the tuning of the optical energy gap as well as of the HOMO and LUMO. In particular, our data show that the low energy shift of the HOMO level along the series of hydrazone compounds, one of the critical parameters affecting the performance of BHJ cells, is as high as 0.42 eV. We tested all derivatives as donors in a BHJ configuration, using the fullerene derivative [6,6]-phenyl-C 71 -butyric acid methyl ester (PC 71 BM) as the acceptor. We found that the trend we observed in the progressive stabilization of the chromophore HOMO level upon increasing the electron accepting capabilities of the conjugated bridge is reflected in the progressive increase of the cell open circuit voltage (V oc ). Our preliminary results show that it is possible to obtain a V oc as high as 0.74 V, along with a power conversion efficiency of 1.2%.