Holger Bengs

Induction of a nematic columnar phase in a discotic hexagonal ordered phase forming system

Abstract The induction of a nematic columnar phase in a discotic hexagonal ordered phase forming system is achieved by mixing hexakispentyloxytriphenylene 1 with a long chain derivative of trinitrofluorenone 3. The difference in chain length has a strong influence on the packing behaviour due to steric effects. The long hydrocarbon chains of the acceptor introduce a strong asymmetry into the electron donor acceptor complex. It could be shown by differential scanning calorimetry, optical microscopy and X-ray measurements that a nematic columnar phase is formed. In this mesophase the triphenylenes form columns but no hexagonal or orthorhombic lattice is built up. Each column behaves like a rod-like nematic mesogen. To prove that the long hexadecane alkyl chains of the acceptor are responsible for this induction, the acceptor 3 was mixed with the non-liquid-crystalline triphenylene derivative 2 containing six hexadecyloxy side groups. The long alkyl chains of the acceptor dissolve perfectly in the side chain...

Highly photoconductive discotic liquid crystals Structure–property relations in the homologous series of hexa-alkoxytriphenylenes

Abstract Four discotic liquid crystals (LCs) from the hexa-alkoxytriphenylene series (R = C n H2n + 1, n = 5, 7, 9, 11) classified as Dho phases were investigated with respect to their photoconducting properties. Under irradiation, all samples show photoconductivity within the mesophase, whereas in the isotropic phase the photocurrent drops to zero. All of the observed effects, the phase dependence of the photocurrent, the increasing values of the photocurrent with decreasing length of the side chains, and the higher photoconductivity during the cooling process, can be explained in terms of the transition temperatures, the intercolumnar distances and the orientation behaviour. Comparison of the photocurrents of the investigated homologues at a reduced temperature leads to an activation energy of 0·67 eV for the charge transport.

Discotic charge transfer twins Structure and mesophase behaviour of covalently linked triphenylenes and trinitrofluorenones

Abstract Discotic charge transfer twins, a novel class of discotic liquid-crystalline compounds were studied. These compounds consist of triphenylene units (as donors) which are chemically linked via flexible spacers of various lengths to trinitrofluorenone units (acting as acceptor). They display a liquid-crystalline phase over a wide temperature range extending up to 240–260°C. Based on X-ray analysis a structural model is proposed for the liquid-crystalline phase: the molecules are arranged in columns in such a way that mixed stacks occur, the intercolumnar packing possesses an orthorhombic symmetry. The neighbouring columns are connected along specific directions via flexible spacers which give rise to highly anisotropic structural properties of this columnar liquid-crystalline phase.

Charge-transfer absorption in doped columnar liquid crystals

The optical properties of doped columnar phases formed either by a 1 : 1 mixture of an electron donor [2,3,6,7,10,11-hexa-(n-pentyloxy)triphenylene] and an electron acceptor (2,4,7-trinitrofluoren-9-one) or by a dimer in which the donor (triphenylene moiety) and the acceptor (trifluorenone moiety) are covalently linked are reported and compared with those of the solutions. In concentrated solutions a charge transfer (CT) absorption is observed for both donor–aceptor pairs corresponding to 2.51 and 2.43 eV, respectively. For the 1 : 1 mixture, CT complex formation in solution is greatly favoured by the presence of molecular aggregates. The polarized absorption spectra obtained with oriented thin films reveal that the charge transfer occurs along the alternate donor–acceptor stacking.

Charge-transfer complexes of discogenic molecules : a time-resolved study based on Kerr ellipsometry

Charge-transfer complexes formed by a discogenic electron donor (2,3,6,7,10,11-hexa-n-pentyloxytriphenylene) and a non-discogenic electron acceptor (2,4,7-trinitrofluoren-9-one) in non-polar solvents are studied by picosecond time-resolved absorption spectroscopy based on Kerr ellipsometry. The transient spectra obtained following laser excitation either in the charge-transfer band or in the lower lying local acceptor band correspond to the oxidized donor and reduced acceptor. Recombination of the geminate ion pair occurs with a first order rate constant of 1.7 × 1010 s–1. It is shown that the polarization of the transient species absorption is orthogonal to the charge-transfer transition. It is also demonstrated that the relative orientation between the donor and the acceptor in the geminate ion pair is not random.

MOLECULAR-DYNAMICS OF DISCOTIC CHARGE-TRANSFER COMPLEXES, DIELECTRIC-SPECTROSCOPY AND H-2 NMR-STUDIES

Abstract Using a combination of solid state 2H NMR spectroscopy on selectively deuteriated samples and dielectric spectroscopy, the molecular dynamics of discotic charge-transfer (CT) complexes were investigated. These complexes show particular thermodynamic and flow properties. Considered were mixtures of low molar mass donors and acceptors, low molar mass donors with main chain acceptor polymers and covalently linked donor-acceptor twins with different lengths of the spacer. A main result is that correlated rotational motions of discotic molecules or groups about the columnar axis are observed in all systems except for the twin with the short spacer. This type of motion seems to be a general feature of columnar phases. The non-discotic acceptor which is incorporated in the columns participates in this motion. The twin possessing a long spacer displays at high temperatures an additional process: it performs a diffusion process between the columns. A further result is that broad biphasic regions exist in ...