Zhibao Lu

The creation of long-lasting glassy columnar discotic liquid crystals using ‘dimeric’ discogens

‘Dimeric’ discogens have been synthesised which have two triphenylene nuclei, each of which bears five β-OC 6 H 13 substituents, which are linked through the remaining β positions by a flexible O(CH 2 ) n O polymethylene chain (n=3-16). Calculations show that the minimum chain length for bridging between columns in the Col h phase of these compounds is O(CH 2 ) 7 O. It was found that only those dimers for which n>7 formed a Col h phase. This mesophase can be supercooled into a metastable glass which is relatively hard and in which the columnar order and orientation is retained. These glasses revert to the crystalline state within a few hours but we have shown that longer-lasting Col h glasses can be engineered through introducing an extra degree of disorder into the glass by making the ‘dimers’ subtly unsymmetrical. These ‘dimers’ either have differently substituted triphenylene rings at the two ends of the linking chain or an amide group within the chain itself. They give Col h aligned glasses which last for months at room temperature.

Helical geometry and liquid crystalline properties of 2,3,6,7,10,11-hexaalkoxy-1-nitrotriphenylenes

The single-crystal X-ray structure of 2,3,6,7,10,11-hexaethoxy-1-nitrotriphenylene confirms earlier calculations of the molecular geometry and shows that the α-nitro substituent imparts a helical twist to the triphenylene nucleus. Within the crystal, the molecules are arranged in tilted columns on an oblique lattice, and along the columns the molecular dipoles are arranged antiferroelectrically, with a molecule–molecule dipole–dipole interaction of ca. −5.5 kJ mol−1. It is suggested that this interaction may be important in stabilising the mesophase. Eleven other 2,3,6,7,10,11-hexaalkoxytriphenylenes and their mononitro derivatives (with side chains ranging from –OC2H5 to –OC16H33) have been prepared and characterised. Most of the α-nitrated compounds give enantiotropic columnar mesophases that show wider mesophase ranges than the precursor aryl ethers. For the systems with side chains from –OC4H9 to –OC11H23, α-nitration increases the mesophase range (relative to the simple aryl ether), and for the systems with –OC12H25, –OC14H29, and –OC16H33, α-nitration induces liquid crystal behaviour (the simple aryl ethers analogues being non-mesogenic). Although it had previously been claimed that 2,3,6,7,10,11-hexapropoxytriphenylene is not mesogenic, we found that it exhibits a plastic columnar phase between 100 and 175 °C.

Cyano substituted triphenylene-based discotic mesogens

Cyano groups have been introduced into alkoxytriphenylene-based mesogens by bromination followed by treatment with copper(I) cyanide. The introduction of cyano substitutents enhances the mesophases ranges.

2,3,7,8,12,13-Hexakis[2-(2-methoxyethoxy)ethoxy]tricycloquinazoline: a discogen which allows enhanced levels of n-doping

A synthesis has been developed for 2,3,7,8,12,13-hexakis[2-(2-methoxyethoxy)ethoxy]-tricycloquinazoline (TCQ6EO2M) in which the ethylenoxy side chains are introduced before elaboration of the heterocylic core. This discogen gives a hexagonal columnar phase (Colh) between 77 and 233°C. n-Doping using potassium metal is facilitated firstly by the electron poor/π-deficient nature of the core and secondly by the polyethylenoxy side chains which complex the potassium K+ counter-ions. The conductivity of the Colh phase of TCQ6EO2M doped with 10 mol % potassium (σ ∥ = 1.1 x 10-3 S m-1) is substantially higher than that previously reported for 2,3,7,8,12,13-hexa(hexylthio)tricycloquinazoline doped with 6 mol % potassium (σ ∥ = 2.9 x 10-5 S m-1). Photoconductivity studies of TCQ6EO2M using a time of flight sample configuration show transient photocurrents for both holes and electrons. From these an upper limit on the mobility for the electrons is estimated as ~10-4 cm2 V-1 s-1 at 150°C which is of the same magnitude as that for hole mobilities in other columnar discotic liquid crystals.

CPI induction of liquid crystal behaviour in triphenylenes with a mixture of hydrophobic and hydrophilic side chains

2,3,6,7,10,11-Hexasubstituted triphenylenes have been synthesized that contain a mixture of hydrophobic (C6H13O) and hydrophilic (CH3OCH2CH2OCH2CH2O) side chains. At one extreme HAT6 (1a) (six hydrophobic chains) shows thermotropic behaviour and at the other TP6EO2M (1e) (six hydrophilic chains) shows lyotropic behaviour. Of the triphenylenes with a mixture of hydrophobic and hydrophilic side chains, only the triphenylene with one hydrophilic side chain and five hydrophobic side chains (1b) gives a thermotropic columnar phase. None of the others show liquid crystal behaviour. However, all of these triphenylenes form binary 1:1 compounds when mixed with PDQ9 (2a) and with PTP9 (2b). These CPI (complimentary polytopic interaction) stabilized compounds give thermotropic hexagonal columnar phases over wide temperature ranges.

An unusual abrupt thermal spin-state transition in [FeL2][BF4]2 [L = 2,6-di(pyrazol-1-yl)pyridine]

The complex [Fe(L1H)2][BF4]2 exhibits an abrupt thermal spin-crossover which may be mediated by an order–disorder transition of the BF4− anions.

New Functionalised Derivatives of Hexaalkoxytriphenylene

We report our latest results on the development of functional triphenylene derivatives. Functionality can be introduced directly into the nucleus by, for example, nitration. Analysis of our recently determined single crystal x-ray structure of 1-nitro-2,3,6,7,10,11-hexaethoxytriphenylene (HAT2-NO2) suggests that the introduced dipole contributes significantly to the stability of the columnar structure. Synthesis of selectively hydroxylated alkoxytriphenylenes allows the functionalisation of one or more side-chains and we show examples of this in which the functional side-chain is: ethyleneoxy or ester or the side chain carries moieties such as: TTF, anthracene, ferrocene, fullerene or crown-ether. The final section details new findings on crown-ether terminated hexaalkoxytriphenylene derivatives, which form lyotropic lamellar phases.