R. Beavington

Control of charge transport and intermolecular interaction in organic light-emitting diodes by dendrimer generation

A novel family of conjugated dendrimers is used as model compounds to explore the effect of intermolecular interactions on photophysical and transport properties. The Figure shows the third generation of the dendrimers. The dendrimer generation controls the degree of chromophore interaction, which leads to a unique correlation between the chemical structure of the molecules and the macroscopic device properties (see also inside front cover).

Control of Electrophosphorescence in Conjugated Dendrimer Light‐Emitting Diodes

We present a novel platinum porphyrin based phosphorescent dendrimer for use as a triplet harvesting dopant in organic light-emitting diodes. Two types of dendritic host materials are used. Through the choice of a common branching architecture around the emissive chromophore unit of both guest and host materials, we are able to achieve excellent miscibility. The relative contribution of guest to host emission is found to depend strongly on the energy level offsets of the two blend materials, indicating strong trapping processes. Under pulsed operation, we observe a striking dependence of the emission spectrum on pulse period, independent of the host material used. This spectral modification is attributed to the quenching of triplet excitations at high excitation densities. We find excellent agreement between our measured data and a model based on bimolecular recombination.

Investigations of excitation energy transfer and intramolecular interactions in a nitrogen corded distrylbenzene dendrimer system

The photophysics of an amino-styrylbenzene dendrimer (A-DSB) system is probed by time-resolved and steady state luminescence spectroscopy. For two different generations of this dendrimer, steady state absorption, emission, and photoluminescence excitation spectra are reported and show that the efficiency of energy transfer from the dendrons to the core is very close to 100%. Ultrafast time-resolved fluorescence measurements at a range of excitation and detection wavelengths suggest rapid (and hence efficient) energy transfer from the dendron to the core. Ultrafast fluorescence anisotropy decay for different dendrimer generations is described in order to probe the energy migration processes. A femtosecond time-scale fluorescence depolarization was observed with the zero and second generation dendrimers. Energy transfer process from the dendrons to the core can be described by a Forster mechanism (hopping dynamics) while the interbranch interaction in A-DSB core was found to be very strong indicating the crossover to exciton dynamics.

The optoelectronic properties of electroluminescent dendrimers

Dendrimers can be used as the emissive layer in organic light-emitting diodes. We have synthesised conjugated dendrimers containing meta-linked stilbene dendrons and luminescent porphyrin or triarylamine cores. The HOMO energy levels of the amine-cored dendrimers were studied by cyclic voltammetry and modelling of device characteristics. Both techniques showed that charge was injected directly into the core and importantly that the energy levels did not change with dendrimer generation.

Structure-property relationships in conjugated molecules

Conjugated dendrimers are ideal materials to study the structure-property relationships in conjugated molecules. We have developed a family of dendrimers that contain t-butyl surface groups, stilbene dendrons, and luminescent cores. For porphyrin and tris(distyrylbenzenyl) amine cored dendrimers cyclic voltammetry (CV) showed that the redox processes occurred at the core. Combining the results from the CV experiments with the device characteristics it was determined that for the dendrimers the cores were held further away from each other with increasing generation. For the amine cored dendrimers it was found that the hole mobility decreased with generation, which is consistent with the cores being further apart. We also found for the amine cored dendrimers that the decrease in hole mobility was matched with increase in device efficiency

Improvement of luminescence efficiency by electrical annealing in single-layer organic light-emitting diodes based on a conjugated dendrimer

In this paper, we study the effects of electrical annealing at different voltages on the performance of organic light-emitting diodes. The light-emitting diodes studied here are single-layer devices based on a conjugated dendrimer doped with 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole as the emissive layer. We find that these devices can be annealed electrically by applying a voltage. This process reduces the turn-on voltage and enhances the brightness and efficiency. We obtained an external electroluminescence quantum efficiency of 0.07% photon/electron and a brightness of 2900 cd m(-2) after 12.4 V electrical annealing, which are about 6 times and 9 times higher than un-annealing devices, respectively. The improved luminance and efficiency are attributed to the presence of a space charge field near the electrodes caused by charging of traps.

Nanoengineering of organic semiconductors for light-emitting diodes: control of charge transport

The effect of intermolecular interactions on the properties of organic semiconductors is investigated using a family of conjugated dendrimers as model systems. Increasing the amount of branching, or generation number, of these molecules reduces the degree of interaction between the chromophores. The effect of this on both photophysical and charge transporting properties is reported. It is found that an increase in generation gives rise to a reduction in the red tail emission of the dendrimer. Time of flight measurements show a slowing of charge transport with increasing generation, which is found to be related to the films becoming more insulating. The results show that dendrimer generation provides an elegant way of controlling intermolecular interactions.

Charge transport in conjugated dendrimers for light-emitting diodes

We report a study on a novel family of conjugated dendrimers suitable for organic electroluminescence applications. By increasing the branching of the dendrimer, the separation between adjacent cores in dendrimer films is increased, which in turn results in a slowing of charge transport. We investigate this using the time of flight technique and observe non-Gaussian transport in thin films of dendrimers.

Bis-porphyrin arrays. Part 2. The synthesis of asymmetrically substituted bis-porphyrins

A strategy for the synthesis of asymmetrically substituted tetraazaanthracene linked bis-porphyrins in which the two porphyrin rings contain differences in their peripheral substituents has been developed. The method is illustrated by the preparation of bis-porphyrins with a single meso-halophenyl and seven meso-3,5-di-tert-butylphenyl substituents. The bis-porphyrins were prepared by condensation of a porphyrin-α-dione with benzene-1,2,4,5-tetraamine to form a porphyrin diaminoquinoxaline intermediate which was subsequently condensed with a second different porphyrin-α-dione. The key issue in the synthesis was the separation of the desired asymmetrically substituted bis-porphyrin from the symmetric bis-porphyrin by-products of similar polarities. Enhanced separation of the bis-porphyrin products was achieved by chelation of a metal into one of the porphyrin rings, the metal being introduced at the porphyrin-α-dione stage. Copper was successfully used when chelated into the less polar porphyrin-α-dione but the use of zinc in the more polar porphyrin-α-dione to enhance bis-porphyrin separation was unsuccessful as the pyridinium hydrochloride produced in the reaction was found to demetallate the porphyrins.