H-Bond stabilized columnar discotic liquid crystals

Since 1977, more than 2300 publications on discotic (disk-like) liquid crystalline materials have appeared. Discotic liquid crystals, which usually consist of polyaromatic molecules surrounded by long peripheral alkyl tails, can form liquid crystalline mesophases in a wide temperature range. Within these mesophases, the molecules self-assemble in large columnar stacks viap-stacking interactions between the aromatic cores. These assemblies can provide an efficient charge transport pathway, as indicated by their informal designation as " molecular-wires ". As a result, they nowadays attract significant interest for optoelectronic applications. Several possible applications where discotic liquid crystals could be used are field effect transistors (FETs), light emitting diodes (LEDs) and photovoltaic solar cells.In all the discotic materials described so far, the molecules inside the columns, can still rotate around the columnar axis, slide out of the columns or oscillate within the columnar stack. This represents a limitation of their applicability as " molecular-wires ". Therefore, in this research, an additional stabilization of columnar discotic mesophases was envisaged in order to increase their organization, without the frequently concomitant loss of processability that would result from extensions of the aromatic core. The research described in this thesis focuses on the stabilization of columnar mesophases by highly tunable (i.e. controlled) H-bonding interactions, without enlarging the aromatic cores and thus maintaining their processability. Functional H-bonding groups, such as urea, amide, thiourea or 1,3,5-benzenetrisamide, have been used in this work, in order to create a H-bonding network parallel to the columnar axis, alongside the existing π-π stacking interactions between the disk-like molecules.In Chapter 1 an overview of the different types of liquid crystalline phases is presented, with emphasis on the organization of columnar discotic mesophases. Since this work focuses on the hydrogen bond stabilization of the triphenylene-based discotic liquid crystals, the most important synthetic approaches towards these materials are discussed. Several of these methods were applied to prepare the materials investigated in Chapters 2 to 5. Besides the synthetic aspect, several characterization techniques, which are normally used to investigate the properties of the liquid crystal materials, are shortly discussed.Chapter 2deals withthe synthesis and thermotropic properties ofa series of hexaalkoxytriphenylenes that contain one amide, urea or thiourea group in one of their alkoxy tails, as H-bond forming group. The biphenyl route turns out to be the best with respect to yields and versatility, as compared to other methods. The optical polarization microscopy, differential scanning calorimetry and X-ray studies show that the intermolecular hydrogen bonding has a negative influence on the formation and stability of the columnar liquid crystalline phases: The stronger the hydrogen bonding, the more the liquid crystallinity is suppressed. This is probably due to disturbance of thep-pstacking of the triphenylene disks and a lower flexibility of the alkyl tails. So far, the urea and amide containing triphenylene derivatives do not exhibit liquid crystalline properties, probably because an H-bond stabilization of the crystalline state is obtained. However, several thiourea derivatives show columnar hexagonal (Col _h ) mesophases, because in these compounds thep-pinteractions are more important than the relatively weak thiourea hydrogen bonding.Because in the columnar mesophases the motions of the aromatic core and the alkyl tails are strongly correlated, the length of the alkyl chains, which surround the disk-like aromatic cores, is an important factor that determines the stability of the columns. Since in the previous chapter long substituents carrying an urea, amide or thiourea group have been used, a complementary study, with focus on the synthesis and phase behavior of unsymmetrically substituted hexaalkoxytriphenylenes, is described in Chapter3. Inthis study, one of the hexyloxy chains in hexahexyloxytriphenylene (HAT6) is replaced by either a shorter or longer chain. In the series HAT-(OC ₆ H ₁₃ ) ₅ -(OC_n H _{2 n +1} ), with n ranging from 2 to 18, the compounds with n³13 are not liquid crystalline anymore. For all compounds with n£12, Col _h phases are found. Furthermore, X-ray investigations show that the intercolumnar distance gradually increases from 20.19 Å to 22.03 Å, with increasing n , while a small odd-even effect on the increase of the intercolumn distance with n is observed. This odd-even effect is also found in the change of ΔH of isotropization with n . The interdisk distance remains constant (3.6 Å).Since the stabilization of the columnar mesophase by H-bonding interactions still remains an unsolved issue, Chapter 4 describes a 1,3,5-benzenetrisamide with three pendant hexaalkoxytriphenylene groups, as a new approach for the intermolecular H-bond-stabilization of columnar discotic liquid crystalline materials. This compound forms a columnar hexagonal plastic (Col _hp ) discotic phase with a clearing point at 208°C. Surprisingly, the material does not crystallize on cooling from the isotropic phase, even after annealing for a few days at room temperature, but goes into a glassy state due to the enforced H-bonding network formed between the benzenetrisamide units. Modeling studies show that the central 1,3,5-benzenetrisamide cores are rotated 60°with respect to each other, which makes the adjacent triphenylene moieties to stack with a small 15°rotation. This small rotation observed between the adjacent triphenylene units plays a key role, since for such a rotation ~85 % of the maximum charge transfer integral is achieved. This result is correlated to the very high charge carrier mobility of 0.12 cm ² V ^-1 s ^-1 at 180°C, found for this particular compound. This value is the second highest ever reported for liquid crystalline triphenylene systems. This is a first proof for the intermolecular H-bonding stabilization of the columnar organization of triphenylene moieties, without loosing the ease of processing provided by the liquid crystalline phase.Since the modeling discussed in the previous chapter suggests that the conformation of the butyl spacer, which is used for that particular compound, dictates thep-poverlap of the triphenylene units within the liquid crystal phase, a new series of 1,3,5-benzenetrisamide derivativeswith threehexaalkoxytriphenylene pendant groups is prepared and their properties discussed in Chapter5. Inthis case, the length of the spacer, as well as the size of the ortho -substituent at the triphenylene core is varied. Taking into account these modifications, all these materials show liquid crystalline behavior with high isotropization points (170-200°C). Different columnar hexagonal phases have been identified going from a columnar hexagonal plastic (Col _hp ) phase, when using a butyl spacer, with a shorter butyl ortho -substituent of the triphenylene core, to a columnar hexagonal disordered (Col _hd ) phase, when a pentyl spacer is used and a hexyl ortho -substituent is present on the triphenylene groups.Achiral 1,3,5-benzenetrisamide was shown to form columnar stacks with a single helical organization, both in an apolar solvent and in a film, as found by circular dichroism studies. By using pulse-radiolysis time-resolved microwave conductivity, charge carrier mobilities as high as 0.25 cm ² V ^-1 s ^-1 are found for the liquid crystalline phase (Col _h ) of this chiral derivative. This mobility is twice the highest value ever reported for triphenylene-based liquid crystalline materials, approaching mobilities found for hexabenzocoronene-based liquid crystals, with a much larger aromatic core. This is a substantial improvement regarding the stabilization of disk-like molecules within their liquid crystal phase, without expanding the aromatic cores, but by tuning the H-bonding and the length of the spacer.