Conductivity columnar phases

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The electric conductivity was also measured for complexes of taper-shaped mesogens with oligo(ethylene oxide) central groups. The DC conductivity is in a range of 10 9 to 10 6 S cm-1 and shows a step-like increase at the crystal-columnar phase transition [86]. It was also shown that taper-shaped molecules adjacent to different endo-receptors such as crown ethers or oligo(ethylene oxide) chains were miscible with a poly(methacrylate) matrix and formed isomorphic phases [87]. Applications as columnar reaction media for polymerizations were foreseen. Comprehensive summaries of Percec s taper-shaped molecules can be found in the literature [88, 89]. 

Terminal crown ethers with taper-shaped substituents often possess columnar phases. Sometimes, the columnar order is only observed after the complexation of specific salts. The uptake of salts often results in the crown ethers assembling side by side forming supramolecular cylinders with the crown moieties on the inside and the alkyl chains on the outside leading to possible ion-conducting arrangements within an insulating jacket. Indeed, it was shown that matrix-fixed tubular arrangements are ion conductors. 

In summary, phthalocyanines modified with crown ethers are interesting synthetic targets as they are prone to form columnar phases. Their electron conductivity and complexation properties make them interesting candidates for the design of sensor materials or supramolecular switches. 

Extensive studies have been conducted to investigate the formation of chiral columns or helical superstructures in chiral and nonchiral disk- [53], star- [54, 55], and board-shaped [56] molecules. However, spontaneous deracemization has never been unambiguously demonstrated in discotic columnar phases consisting of nonchiral or racemic molecules. We recently observed clear evidence showing chiral resolution in a disk-like molecules with a dibenzo[g,p]chrysene core [57]. 

Only very few conductivity measurements have been performed on metal-lomesogens other than metallophthalocyanine complexes. Godquin-Giroud et al. demonstrated a conductivity of the order of 10 S cm for the copper(II) complex of the /3-diketone bis(3,4-nonyloxybenzoyl)methane in a hexagonal columnar phase. ... 

In general, for side chain liquid-crystalline polymers, macroscopic molecular alignment is not easy and therefore clear evidence of electronic charge carrier transport was confirmed first in liquid crystals with low molecular weight. In the 1990s, fast electronic conduction was verified in discotic columnar phases of triphenylene derivatives [79,80] and hexabenzocoronene derivatives [81,82] as well as smectic phases of 2-phenylbenzothiazole [83, 84] and 2-phenylnaphthalene derivatives [85], as shown in Fig. 14. Carrier... 

The modest pressures of 5 kbar (0.5 GPa) necessary to cause transitions to columnar phases in polymers make the design of apparatus using large specimens relatively straightforward and it is easy to accommodate associated characterization techniques such as thermal conductivity or nuclear magnetic resonance (NMR) probes. Much of the author s early work was with a piston-cylinder apparatus with a working length of 5 cm and a diameter of... 

Interest in the ether derivatives arises from the fact that they are photoconductive and that, when doped with a few mol per cent of an oxidant such as aluminium trichloride, they become semiconductors [59], Oriented samples of the phase can be produced, and it is found that the preferred direction of conduction is parallel to the director the system behaves rather like a molecular wire, with the conducting stack of aromatic cores being surrounded by an insulating sheath of alkyl chains. The hexa-hexylthioether derivative gives a helical ordered columnar phase at low temperatures [58, 60], which shows remarkably high charge-carrier mobility in experiments in... 

The columnar phases of metallomesogens have the electrical properties of molecular semiconductors [77-84]. Figure 22 presents the Arrhenius plots of the a.c. conductivity (unaligned samples of copper phthalocyanines (13) determined by Van der Pol et al. [83]. The conductivity increases with increasing temperature. The activation energy is approximately 0.5-0.6 eV at 175 °C for the compounds 13 with R=0C8H 7 and OC12H25. 

Figure 11-7. The structure of homeotropically aligned columnar phase (left) an ideal transient photocurrent in a TOF measurement in a trap-free sample (right). The fast mode is electronic conduction including electron and hole mobilities the slow mode is ionic charge transport...

According to the accumulated results for charge carrier transport in mesophases, this hardly depends on the mesophase, except for the mobility, see later. Hereafter, the mobility indicates the mobility within a conduction channel, i.e., in the layer for a smectic liquid crystal and in a column for a columnar phase, unless specified otherwise. 

Fig. 2.13 Intra-columnar mobility in the hexagonal columnar phases (Db) of various discotic materials. The n indicates the number of carbon atoms in a discotic core of the materials. The mobility was determined by pulsed radiolysis transient microwave conductivity (PR-TRMC) measurements...

Fig. 3.1 Molecular structure and schematic of the conducting columnar Colj, phase of a HATn triphenylene-based discogen 1 (n is the number of carbons in each side-chain. Hence in HATS there are six pentyloxy side-chains)...

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