Four generations of conjugated dendrimers that contain 1,3,5-tris(distyrylbenzenyl)benzene cores, stilbene dendrons, and t-butyl surface groups have been synthesized. The dendrimers were synthesized by coupling benzylphosphonate-focused dendrons with 1,3,5-tris(4-formylstilbenyl)benzene to give the dendrimers in yields in the range 60-82 %. We have probed the optoelectronic properties of the dendrimers by electrochemistry, photoluminescence, and in light-emitting device structures. we have found that the degree of aggregation is strongly generation dependent. We compared the properties of these benzene-centered dendrimers with an equivalent family of dendrimers that differs only in having a nitrogen atom as the central unit. We found that the aggregation of dendrimers was strongly dependent on the degree of delocalization across the central unit. the dendrimers with the benzene central unit, which have three localized distyrylbenzene chromophores, were found to aggregate more strongly in the solid state that those with nitrogen as the central unit. In the latter case the electroactive component is comprised of all three distyrylbenzene units and the nitrogen atom.

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.

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 Förster mechanism (hopping dynamics) while the interbranch interaction in A-DSB core was found to be very strong indicating the crossover to exciton dynamics.

An efficient strategy has been developed for the preparation of four generations of electroluminescent dendrimers that contain tris(distyrylbenzenyl)amine cores, stilbene dendrons, and tert-butyl surface groups. The synthesis involved coupling of benzylphosphonate focused dendrons with tris(4'-forinylstilbenyl)amine to give the dendrimers in yields ranging from 63 to 86%. The dendrimers were found to be monodisperse by gel-permeation chromatography. The zeroeth generation dendrimer underwent two chemically reversible oxidations while for the higher generations only one chemically reversible oxidation was observed. On reduction, the dendrimers were found to aggregate with the level of aggregation dependent on the switching potential. The four dendrimer generations were investigated by means of optical spectroscopy. Time-resolved luminescence of the dendrimers in solution showed that the excited state of each of the generations had a monoexponential decay with a lifetime of 1.8 ns. The photoluminescence quantum yield (PLQY) of the dendrimers in solution was independent of generation and was found to be in the region of 0.62. This suggests that the origin of the luminescence is the same for all dendrimer generations. In thin films, time-resolved luminescence of the zeroeth dendrimer generation revealed a long-lived luminescence component in the red part of the spectrum with a lifetime of 7.5 ns. This emission component could not be found in the first, second, and third generation dendrimers, where the long-lived luminescence had a lifetime of 1.5-3 ns at all detection wavelengths. Furthermore, the PLQY of the dendrimer films was found to be dependent on generation and significantly lower than the solution PLQYs. The dendrimer film PLQY increased with generation from 5% for the zereoth generation to 12% for the third generation. The differences observed in the time-resolved luminescence and PLQY of the dendrimers in the solid state arise from the fact that intermolecular interactions between the emissive cores of the dendrimers are considerably stronger in the zeroeth generation than in higher generations. The intermolecular interactions result in an aggregate, which we ascribe to an excited-state species, such as an excimer.

We demonstrate a novel organic light-emitting diode (LED) heterolayer structure that contains a conjugated dendrimer as the light-emitting molecule. The LED was prepared by spin-coating two dendrimer layers from the same solvent. The device consists of a graded bilayer structure formed from a neat dendrimer film covered with a film consisting of the same dendrimer but doped with the electron-transporting material 2-(4-biphenylyl)-5-phenyl-1,3,4- oxadiazole (PBD). In this device, the heterojunction interface present in conventional bilayer organic light-emitting diodes is eliminated and is replaced by a graded interlayer. By optimizing the concentration of PBD in the dendrimer, a peak electroluminscence (EL) external quantum efficiency of 0.16% at 600 cd m(-2) was obtained. The EL quantum efficiency is significantly enhanced in comparison with devices based on a single layer, a conventional bilayer, and a single-layer doped with PBD. The EL quantum efficiency is a factor of eight larger than that of a conventional bilayer LED made with the conjugated dendrimer as the emissive layer and poly(methylmethacrylate) (PMMA doped wit PBD as the electron-transporting layer. The best blended device exhibited only one third of the efficiency of the graded device. The improvement in the operating characteristics of the graded device is attributed to the efficient device structure, in which exciton formation is improved by a graded doping profile of electron- and hole-transporting components.

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.

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).

Conjugated dendrimers are of interest as novel materials for light-emitting diodes. They consist of a luminescent chromophore at the core with highly branched conjugated dendron sidegroups. In these materials, light emission occurs from the core and is independent of generation. The dendron branching controls the separation between the chromophores. We present here a family of conjugated dendrimers and investigate the effect of dendron branching on light emission and charge transport. We apply a number of transport measurement techniques to thin films of a conjugated dendrimer in a light-emitting diode configuration to determine the effect of chromophore spacing on charge transport. We find that the mobility is reduced by two orders of magnitude as the size of the molecule doubles with increased branching or dendrimer generation. The degree of branching allows a unique control of mobility by molecular structure. An increase in chromophore separation also results in a reduction of intermolecular interactions, which reduces the red emission tail in film photoluminescence. We find that the steady-state charge transport is well described by a simple device model incorporating the effect of generation, and use the materials to shed light on the interpretation of transient electroluminescence data. We demonstrate the significance of the ability to tune the mobility in bilayer devices, where a more balanced charge transport can be achieved.

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.

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.

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.

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.

Summary form only given. The degree of order and interaction between molecules is a crucial factor in determining the photophysical and charge-transporting properties of organic materials. Strong intermolecular interactions may be favourable for charge transport but reduce the efficiency of luminescence. We explore this trade-off using a family of conjugated dendrimers in which the generation number (G) provides an elegant way of controlling the degree of interaction between chromophores. For low generation numbers a red component of the electroluminescence (EL) spectrum due to intermolecular excited states is observed. As the generation number is increased, reducing interaction between the emissive chromophores, this feature disappears, and the spectrum resembles the solution PL spectrum. The effect of the increase in localisation of excitations is clearly seen in the transport properties of LEDs based on these materials: the EL quantum efficiency increases exponentially with generation and the majority carrier current-flow is inhibited. Time-of-flight measurements show highly dispersive transport and a strong increase of transit times and photocurrent decay transients with generation. The increase in hopping distance gives rise to a decrease in mobility by almost 2 orders of magnitude as the generation is increased. This gives rise to a scaling of the mobility with dendrimer radius comparable to the concentration dependence of mobility previously observed in doped polymer host systems. The dendrimer generation hence provides a unique method for tuning the intermolecular electronic overlap integral in undiluted systems.

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.

We have successfully synthesised exo and endo diphenylacetylene linked bis-porphyrin dimers. We found that the exo-bis-porphyrin dimer could be easily prepared in good yield whilst the endo-dimer was harder to form due to the steric constraints of the meso-substituents. Electrochemical studies indicated that the porphyrin centred reductions of the bis-porphyrin dimers were dominated by the bis-porphyrin moieties. We found that the first four one-electron reductions of the exo- and endo-bis-porphyrin dimers occurred at similar potentials. Finally, at more negative potentials the bis-porphyrin dimers were reduced further which significantly changed the subsequent oxidation processes. This is thought to be due to a conformational change in the molecule which causes a change in orbital density in the "excited state".

meso-Functionalised porphyrin-α-diones have been prepared as the basic building blocks for bis-porphyrin arrays. meso-Halophenyl porphyrins 4 and 7 were prepared by condensation of 3,5-di-tert-butylbenzaldehyde with dipyrromethanes 1, 2, and 3. Conversion of 4 and 7 to the corresponding meso-halophenylporphyrin-α-diones was achieved in five steps in overall yields of up to 58%. The reaction sequence consisted of chelation of the porphyrin with copper(II), nitration, hydroxylation with the benzaldoximate anion, demetallation, followed by oxidation with the Dess–Martin periodinane. The key hydroxylation step was found to proceed chemoselectively, displacing the β-pyrrolic nitro group whilst leaving the meso-halophenyl group intact.

Porphyrin-alpha-diones have been used for the preparation of extended rigidly oriented porphyrin systems. Copper 2, free-base 3, and zinc 4 2-hydroxyporphyrins have been prepared by an improved method and treated with a range of oxidants to form the corresponding porphyrin-alpha-diones, 5, 6, and 7 in yields of up to 94%, We have found that the free-base 2-hydroxyporphyrin 3 was easily oxidised and usually gave the best yields of porphyrin-alpha-dione. In this study the most effective general oxidant was determined to be the Dess-Martin periodinane which oxidised all three 2-hydroxyporphyrins to their respective porphyrin-alpha-diones in good yields.