Jonathan Pillow

Conjugated dendrimers for LEDs: Control of colour

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.

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

The effect of dendrimer generation on LED efficiency

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 effect of intermolecular interactions on the electro-optical properties of porphyrin dendrimers with conjugated dendrons

We have synthesised a new family of dendrimers with stilbene dendrons attached to a porphyrin core via a stilbene unit and compared their properties with a family of dendrimers with the same core and dendrons but with the dendrons attached via a phenyl unit. The oxidation and reduction half potentials of the two dendrimer families were found to be the same and independent of generation indicating that the dendrons were not creating a micro-environment for the core. However, the rate of heterogeneous electron transfer was found to be strongly dependent on link type and generation. The photoluminescence quantum yield (PLQY) of the dendrimers was also found to be strongly dependent on the method of attachment of the core. In solution the dendrimers with the stilbene link between core and dendrons had PLQYs 1.5 times higher than their phenyl counterparts but in the solid state the trend was reversed with the phenyl linked dendrimers generally having a higher PLQY. The difference in properties has been assigned to the comparative openness of the dendrimer architectures and the effect of the dendrons on the shape of the porphyrin core.

Synthetic routes to phenylene vinylene dendrimers

Abstract 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

Effect of generation on the electronic properties of light-emitting dendrimers

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.