Columnar discotic carboxylic esters of 2-hydroxy-3,6,7,10,11-penta(1-pentyloxy) triphenylene, characterized by a variety of functional groups, were prepared with the purpose of studying the effects of the substituents on the mesophase behaviour. Bulky substituents and polar substituents were found to destabilize the columnar mesophase. The clearing temperature is lowered and the melting point of the crystalline phase, as well as the tendency to crystallize from the mesophase. is increased. Specific substituents cause the formation of novel discotic mesophases characterized by a superstructure.

The hole transport properties of ester substituted hexaalkyloxytriphenylenes are investigated and compared with the materials without ester substituents. The high hole mobilities of the recently investigated discotic liquid crystals of the hexaalkyloxytriphenylene-type are restricted to the very small temperature range of their mesophase. It is extended substantially by substitution of one ester group, as this hinders crystallization and results in glass formation. It is found that the substitution of an ester group alters the temperature and field dependence of the mobility completely. In the ester substituted compounds the mobility µ is not independent of temperature but follows a ln µ∝1/T2 law. We attribute this to the dipole moment of the ester group which causes random fluctuations in the local electric field and leads to disorder dominated charge carrier hopping as the prevailing transport mechanism.

Columnar discotic materials are considered for applications in the area of photoconductivity and light-emitting diodes. A major requirement is their stability at elevated temperatures and in the presence of oxygen. The thermal and thermo-oxidative behaviour of discotic triphenylene derivatives was investigated by us using various methods, in particular by chemiluminescence (CL), UV-vis absorption spectroscopy and in situ thermogravimetry–mass spectroscopy (TG–MS). Various degradation processes are described for increasing temperature, and their influences on functional properties are discussed. Copyright © 1999 John Wiley & Sons, Ltd.

The suitability fo columnar discotics for applications in light emitting diodes (LEDs) has been investigated following the demonstration of their unusually large charge carrier mobilities. Results of X‐ray diffraction, computer simulations, and electroluminescence measurements are presented and discussed for discotic systems such as that shown in the Figure. Their use in LEDs is expected to the advantageous. magnified image

The glass transition of highly ordered columnar discotics within the framework of the concept of fragility is discussed. The discussion is based on experimental findings from dielectric relaxation spectroscopy, calorimetric, and X-ray scattering measurements. The findings are that discotics in their three-dimensionally ordered columnar plastic state are strong glass formers and those in the two-dimensionally ordered columnar hexagonal phase are fragile. We were unable to find correlations between the fragility and the degree of non-exponentiality of the relaxation function and correlations between fragility and the magnitudes of the stepwise change of the specific heat. The change of the thermal expansion of the lattice parameters at the glass transition, however, seems to depend on the phase type and is due to the anisotropy of the discotic structures.

X-ray reflectivity investigations were performed on organic light emitting diodes (LEDs) with the purpose of characterizing those properties which control the performance of the devices. These are the thickness of the electrodes and of the active organic films, the roughness of the layers and of the substrate, as well as the internal structure of the organic films. Based on the experimental results and corresponding simulations on multilayer polymer/metal structures, it has become obvious that X-ray reflectometry constitutes a non-destructive technique for characterizing light emitting diodes with a complex multilayer architecture.

The formation of a re-entrant columnar discotic hexagonal ordered phase (re-entrant Col(ho) phase) is reported for the asymmetrically substituted triphenylene 3,6,7,10,11-pentapentyloxytriphenylene-2-yl pivaloate (pivaloate) and its structural and dynamic properties described. The re-entrant phase behaviour is strongly modified by doping the pivaloate with the electron acceptor 2,4,7-trinitrofluorenone (TNF).

Charge‐carrier transport in organic materials is the fundamental physical process behind devices such as laser printers. Charge‐carrier mobility data are presented for hexabutyloxytriphenylene (H4T), which exhibits an unusually high charge‐carrier mobility, which can be traced to the formation of a plastic discotic phase. The Figure shows the normal discotic hexagonal texture with six‐fold symmetry observed for H4T at 144°C; this symmetry disappears at lower temperatures (see also the cover). magnified image

A plastic columnar discotic phase is reported for an asymmetrically substituted triphenylene. It is characterized by a three-dimensional crystal-like registry of ordered columns in a hexagonal lattice while the disc molecules within the columns are able to rotate. At the phase transition from the normal discotic hexagonal phase to the new phase only very minute changes in structure and dynamics occur.

In conjunction with the treatment of acute urinary tract calculous disease the spasmolytic effect on ureteral peristalsis was studied clinically and in the experimental animal. Spasmolytic action was demonstrated using direct myogenic-acting pharmaceuticals (Papavarin®), β-adrenergics, and by analgesics and sedatives with ventral nervous effect.