Hydrogen bonding can be used to significantly enforce the intra‐columnar stacking order in discotic mesogens. The ordered hexagonal columnar mesophase of a HAT‐CONHR derivative is characterized by the smallest inter‐disk distance ever found in columnar liquid crystals (3.18–3.20 Å). This additional attractive interaction between the disks in the column results in a regular disc stacking and thus in a high charge‐carrier mobility over the whole investigated temperature range (from room temperature up to 200 °C).

The synthesis and characterization of two families of ionic liquid crystal dendrimers consisting of the ammonium salts of the commercially available poly(amidoamine) (G = 0-5) and poly(propylene imine) (G = 1-5) dendrimers and three long-chain carboxylic acids are reported. The liquid crystalline behavior was investigated by means of differential scanning calorimetry, polarizing light optical microscopy, and X-ray diffractometry. The thermal stability of the ionic materials was further studied by NMR. Most of the dendrimers show lamellar mesomorphism, and two of them exhibit columnar mesomorphism. On the basis of the experimental results, we propose models both at the molecular level and in the mesophase for all the materials.

The results of the study presented here show a new example of the use of liquid crystals and the interactions involved in the mesomorphic state to build up complex molecular organizations. We have pursued a design strategy in which hydrogen bonding allows the combination of the pi-stacking tendency of melamine and the lateral interaction capability of V-shaped molecules, which has been extensively demonstrated through nematic, smectic, or columnar mesophases. This combination addresses the formation of columnar arrangements with inherent helical organization. In this work, nonmesomorphic as well as mesomorphic V-shaped acids, with a structure similar to banana liquid crystals, have been complexed to a 2,4,6-triamino-1,3,5-triazine derivative in a proportion 3 to 1, respectively. Hydrogen-bonded supramolecules whose formation and stability in solution have been proven by infrared and NMR techniques have been thus obtained. DOSY experiments have allowed us to assess in solution the presence of the complexes and their tetrameric composition. All the complexes display mesogenic ability, and their mesomorphic organization has been studied by X-ray diffraction and CD spectroscopy. Results allow us to propose a helical columnar model for the mesophase originated from a propeller-like conformation of the supramolecular complexes.

A concept for highly ordered solid-state structures with bright fluorescence is proposed:  liquid crystals based on tetraethynylpyrene chromophores, where the rigid core is functionalized with flexible, promesogenic alkoxy chains. The synthesis of this novel material is presented. The thermotropic properties are studied by means of differential scanning calorimetry (DSC), cross-polarized optical microscopy (POM), and X-ray diffraction. The mesogen possesses an enantiotropic Colh phase over a large temperature range before clearing. The material is highly fluorescent in solution and, most remarkably, in the condensed state, with a broad, strongly red shifted emission. Fluorescence quantum yields (ΦF) have been determined to be 70% in dichloromethane solution and 62% in the solid state. Concentration- and temperature-dependent absorption and emission studies as well as quantum-chemical calculations on isolated molecules and dimers are used to clarify the type of intermolecular interactions present as well as their influence on the fluorescence quantum yield and spectral properties of the material. The high luminescence efficiency in the solid state is ascribed to rotated chromophores, leading to an optically allowed lowest optical transition.

The synthesis, chemical characterization and liquid crystalline behavior of a series of seven poly(amidoamine) (PAMAM) codendrimers are described. These compounds were obtained by grafting two types of terminal promesogenic units, that carry either one or two decyloxy chains, in various proportions onto the third generation of PAMAM dendrimer. The average number of promesogenic units was determined by using an original interpretation of the NMR spectra. X-ray diffraction studies show that these compounds exhibit lamellar and/or columnar mesophases. The type of mesophase is determined by the number of each kind of promesogenic units present in a given codendrimer and by the temperature. The evolution of the supramolecular organization of the different molecules is explained as a function of the number of alkoxy chains present around the dendritic core and compared to the models developed for the homodendrimers totally functionalized by promesogenic units with either one or two decyloxy chains.

The synthesis and the liquid crystalline behavior of a novel series of poly(amidoamine) (PAMAM)-dendrimer derivatives is described. The series consists of five new compounds that contain 4, 8, 16, 32, and 64 peripheral mesogenic ester units attached to the 0-, 1-, 2-, 3-, and 4-generation of poly(amidoamine) (PAMAM), respectively. These five compounds exhibit a smectic A mesophase, for which a structural model is proposed that is valid for several generations of dendrimers. This model is based on a structural characterization which mainly consists of X-ray diffraction studies. The molecules of these compounds exhibit a high degree of plasticity and tend to adopt the most convenient shape to afford liquid crystalline behavior. The mesogenic units form parallel aggregations that give rise to the smectic structure.

IntroductionFullerodendrimers, 1 which combine the outstanding electrochemical 2 and photophysical 3 properties of C 60 with the unique structural features of dendrimers, 4 generated fascinating studies in supramolecular chemistry and materials science. 5 Dendrimers play two major roles depending upon their architecture and functionalities: they increase the solubility of C 60 in organic solvents 6 or in water 7 (solubilizing effect), and they isolate C 60 from the external environment such as oxygen and solvent molecules (protection effect). 8 Both effects can be adjusted to specific experimental conditions by synthetic chemistry at the dendrimer level. Dendrimers prevent also the formation of aggregates resulting from strong interactions between C 60 units; highly ordered Langmuir and Langmuir-Blodgett films were so obtained. 9 With the view to construct supramolecular fullerene materials, whose properties could be of interest in nanotechnology (e.g., molecular switches, solar cells), we became interested in fullerene-containing thermotropic liquid crystals. We developed two concepts to design liquid-crystalline fullerenes: 10 in the first concept, C 60 was functionalized with liquid-crystalline malonates by applying the Bingel reaction 11 (leading to mesomorphic methanofullerenes 12 ), and in the second one, C 60 was functionalized with liquid-crystalline aldehydes and N-methylglycine or an amino acid derivative by applying the 1,3-dipolar addition reaction 13 (leading to mesomorphic fulleropyrrolidines 14 ). A great variety of liquid crystals was obtained, such as fullerene-ferrocene dyads (smectic A phases), 12a,b,d,14b fullerene-OPV conjugates (smectic A phases), 14a fullerene-TTF dyads (smectic A and B phases), 12g a hexa-adduct of C 60 (smectic A phase), 12c a chiral C 60 derivative (cholesteric phase), 12f and dendritic liquid-crystalline methanofullerenes (smectic A phases; an additional short-range nematic phase was observed for the second-generation dendrimer). 12e A bis(methano)fullerene was also reported (mesophase not identified). 15 Two other approaches were described: in the first one, C 60 was complexed by mesomorphic cyclotriveratrylene (CTV) derivatives (nematic and cubic phases), 16 and in the second one, five aromatic groups were attached around one pentagon of C 60 , yielding conical molecules (columnar phases). 17 Whereas methanofullerenes undergo retro-Bingel reaction upon chemical 18 and electrochemical 19 reduction, fulleropyrrolidines lead to stable reduced species. 13 Dendritic liquid-crystalline fulleropyrrolidines would represent an interesting family of electroactive macromolecules, as they would combine the electrochemical behavior 2 of C 60 with the rich mesomorphism of dendrimers. 20 We describe, herein, the synthesis, characterization, liquid-crystalline properties, and supramolecular organization of the dendritic liquid-crystalline fulleropyrrolidines 1-7 (Charts 1 and 2). By means of cyclic voltammetry experiments carried out on the secondgeneration dendrimer 2, it is de...

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 ...