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.

A simple two-step convergent iterative procedure has been developed for the preparation of stilbene dendrons containing an aldehyde at the center which can be coupled in a single step to give dendrimers that contain luminescent chromophores. The stilbene units in the dendrons are linked in a meta arrangement at the branching phenyl units allowing them to be treated, to a first approximation, as isolated chromophores. All the dendrimers are luminescent with emission observed from the core. The cores prepared were distyrylbenzene, distyrylanthracene, and meso-tetraphenylporphyrin which photoluminesce blue, yellow-green, and red, respectively. We found that all the dendrimers, independent of generation or core, could be spin-coated from solution to form good quality thin films, which shows that the processing conditions have been disengaged from the electronic properties in this series of materials.

The synthesis and characterisation of precursor 2,5-dialkoxy-poly-p-phenylenevinylenes (PPVs) 1-5 (1: R=n-hexyl, R'=methyl; 2: R=R′=n-hexyl; 3: R=R′=methyl; 4:R=n-octyl, R′=methyl; 5:R=R′=2-ethylhexyl) is reported. The monomers were prepared by alkylation of hydroquinone followed by chloro- or bromomethylation. They were then polymerized by base-catalyzed elimination of HX (X=C1, Br) to give the precursor polymers for the corresponding 2, 5-dialkoxy-poly-p-phenylenevinylenes(PPVs).

Conjugated dendrimers are an interesting class of materials for light emitting diodes, and can be used either as light-emitting or charge-transporting layers. We report a study of the use of a solution-processible dendrimer with distyrylbenzene core and stilbene dendrons. We find that its use in bilayer LEDs with polypyridine gives efficiencies ten times greater than for single layer polypyridine LEDs. The light emission can be either from the polypyridine or the dendrimer layer, depending on the layer thicknesses. This offers a way of tuning the colour of bilayer LEDs.

We have compared the optical and electronic properties of a series of porphyrin centered dendrimers containing stilbene dendrons. The first and second generation dendrimers could be spin-coated from solution to form good quality thin films. Incorporation into single layer light-emitting diodes gave red-light emission with maximum external quantum efficiencies of 0.02% and 0.04% for the first and second generation dendrimers respectively. We have determined by photoluminescence studies that energy can be transferred efficiently from the stilbene dendrons to the porphyrin core and that PL emission is from the core. Cyclic voltammetry studies on the dendrimers show that the reductions are porphyrin centered with the dendrons only affecting the rate of heterogeneous electron transfer between the electrode and the dendrimers. This suggests that charge mobility within a dendrimer film in an LED will be affected by the porphyrin edge to porphyrin edge distance. We have studied the hydrodynamic radii of the dendrimers by gel permeation chromatography and found as expected that the average porphyrin edge to dendron edge distance increases with generation. This is consistent with the slowing of heterogeneous electron transfer observed in the cyclic voltammetry on increasing the generation number and suggests that the dendrons are interleaved in the solid state to facilitate charge transport.

Conjugated dendrimers are a promising new class of material for organic LEDs. The authors report here an investigation into a new family of conjugated light-emitting dendrimers based on the phenylenevinylene structure, and the effect of generation number (level of branching) on both the electroluminescence and photophysical properties of the materials. High fluorescence was found for all three of the first, second, and third generation dendrimers, while concentration quenching effects are substantially avoided by the dendritic molecular architecture.

We have co-dissolved poly(p-pyridine) (PPY) and polymethylmethacrylate in formic acid and cast films from these blends. We observe on increasing dilution that PPY absorption and emission changes from that characteristic of its solid state to that predominantly of its solution state. This is ascribed to increasing inter-ring torsion angle as PPY-PPY inter-chain interactions are diluted.

Dendrimers offer the opportunity to vary the electronic properties of an electroactive material without changing the processing procedure. Poly(1,4-phenylenevinylene) and its derivatives have been widely studied for use in light-emitting diodes. We have investigated two different iterative routes for the formation of phenylenevinylene based dendrimers to compare their properties with the linear counterparts. Our approach involves palladium-catalysed coupling of styryl derivatives to halo-benzenes which has enabled the rapid construction of a family of dendrimers.

The use of phenylenevinylene based dendrimers for organic LEDs was investigated. We have found when a family of distyrylbenzene centred dendrimers were used as the light-emitting layers in LEDs that the efficiency was dependent on the dendrimer generation number. A maximum efficiency of 0.09% was observed in an ITO/ second generation dendrimer/Ca device.

The use of conjugated dendrimers as a novel molecular architecture for organic LEDs is investigated. Control of colour is demonstrated using materials based on a porphyrin, anthracene and distyrylbenzene core with stilbene dendrons. The resulting dendrimers are solution processible and give photoluminescence and electroluminescence in the red, green and blue regions of the spectrum.

We have recently shown that poly(2,5-pyridine diyl) (PPY) can be synthesized to yield a polymer with high photoluminescence quantum yield (PLQY) in the solid state, and that it is an excellent electron transport material. To explore the photophysical properties of PPY further, we have used a range of acidic “dopants” to protonate the nitrogen sites on each ring and made observations on how this affects the optical properties of the resultant protonated PPY films. In general, we find that sulphonic acids have the greatest effect, causing perturbations to both the ground-state and excited-state properties of the PPY. These changes occur with only moderate reduction of the PLQY, whereas nonsulphonic acids cause a larger reduction in PLQY without significantly affecting the ground- or excited-state energy levels. These aspects of the photophysics of PPY can be described using a simple ring torsion argument. This model can also account for the observed shifts between solution state and solid-state emission wavelengths.

The efficient operation of polymer light-emitting diodes (LEDs) requires balanced injection and transport of electrons and holes. This has stimulated much research into suitable electron-injecting and transporting materials. We report the use of polypyridine as an efficient electron-transporting polymer. We have achieved much-improved LED performance by incorporating polypyridine as an electron-transporting layer in a poly(p-phenylene vinylene) (PPV) LED and optimizing layer thicknesses to balance transport of electrons and holes. The external quantum efficiency of these LEDs is 0.25%, 60 times greater than similar devices without the electron-transporting layer.

Langmuir-Blodgett (LB) layers of poly(2-methoxy,5-(2′-ethylhexyloxy)-p-phenylene vinylene) (MEH-PPV) have been built up on quartz and indium-tin oxide (ITO) substrates. The optical properties of the films were characterized using time-correlated single-photon counting, and absorption and steady-state fluorescence spectroscopy. Electroluminescence (orange-yellow light) from ITO/MEH-PPV LB/A1 devices under forward bias was observed. The dependence of the electrical and electroluminescent characteristics on LB film thickness is reported. The quantum efficiency was 7 × 10−3% for a device containing 64 LB layers.

In this paper we report the absolute photoluminescence quantum yield of poly(p-pyridine) (PPy), which is a promising electroluminescent polymer. The influence of vacuum and air on the quantum yield of the PPy is investigated. We find a significant decay of the efficiency under vacuum whereas it is stable in air. We compare the results with those obtained on poly(p-phenylenevinylene)(PPV).

To probe the role nitrogen lone pair electrons play in the photophysics of poly(p-pyridine) (PPY) we have used a range of acidic 'dopants' to protonate the nitrogen sites on each ring and made observations on how this affects the optical properties of the resultant protonated PPY films. In general we find that sulphonic acids have the greatest effect, causing perturbations to both the ground and excited state properties of the PPY. These changes occur with only moderate reduction of the photoluminescence quantum yield (PLQY), whereas non-sulphonic acids cause a larger reduction in PLQY without significantly effecting the ground or excited state energy levels. These aspects of the photophysics of PPY can be described using a simple inter-ring torsion argument.