Wolfgang Paulus

Photopolymerization and transport properties of liquid crystalline triphenylenes

Abstract The synthesis of polymerizable triphenylene derivatives with acrylate groups is described. The thermal properties and the photoconductive properties of the discotic monomers as well as of the polymers is investigated. In the mesophase typically photoinduced hole conductivity with a charge carrier mobility exceeding a value of 10−4 cm2/Vs is observed with conventional time-of-flight (TOF) techniques. Different conditions for the photopolymerization of the acrylates in the mesophase are compared. The influence of the photoinitiators and the polymerization on the mesomorphic behavior and on the hole conductivity is described. Already the presence of small amounts of photoinitiator influences the packing within the mesophase. The polymerization further decreases the order and creates defects. This results in an at least one order of magnitude reduced charge carrier mobility at room temperature, compared to the monomer.

Mixed polymeric monolayers and Langmuir-Blodgett multilayers with functional low molecular weight guest compounds

Abstract Mixed monolayers and Langmuir-Blodgett multilayers of functional low molecular weight guest compounds, especially nonlinear optical (NLO) dyes, within the matrix of an amphotropic spacer polymer have been prepared. The polymer matrix enabled the transfer of guest compounds not capable of self-organizing at the air-water interface by themselves. The structure of the LB multilayers and the transfer process were studied by small angle X-ray scattering and UV-visible spectroscopy. Good NLO coefficients were found in the mixed films.

ELECTRONIC PROPERTIES OF DISCOTIC LC-MATERIALS

Abstract We have made extensive studies of the charge-carrier mobilities in the discotic hexagonal mesophases of triphenylene-based discotic liquid crystals. Using the time-of-flight technique, transient photocurrents were measured yielding charge-carrier mobilities for various electric fields and temperatures. Starting from promising results obtained with the monomeric discotic liquid crystalline model compound hexapentyloxytriphenylene (H5T), we synthesized a “discotic twin” with two triphenylene units linked together by a suitable spacer. Additionally, we synthesized a discotic liquid crystalline oligomer consisting of four triphenylene units which are bound to a flexible cyclosiloxane ring. In the dimer and in the oligomer the discotic mesophase can be maintained in a discotic glass. Our data show that in both H5T as well as in the dimer and in the oligomer hole mobilities on the order of magnitude of 10−3cm2/Vs can be reached. Since the mobilities obtained in the oligomeric compounds are comparable t...

Triphenylenes: a new class of hole transporting material in organic light-emitting diodes

Electroluminescent devices consisting of triphenylene derivatives as hole transport layer (HTL) and 8- hydroxyquinoline aluminium complex (Alq3) as emitting layer with ITO as anode and aluminium as cathode are presented. Triphenylene compounds were evaporated or spin coated from solution. In addition side chain polymers with triphenylenes are used. Finally we prepared a polymeric HTL by photopolymerization of an acrylate functionalized triphenylene monomer. Some of the devices, based on ITO/Triphenylene/Alq3/Al showed brightness values up to 1000 cd/m2 and low turn on voltages from 4-5 V.