Light-emission from conjugated dendrimers and polymers

Abstract

This thesis reports the photophysical and electroluminescence studies undertaken on two types of material: polymeric and dendritic. The dendritic architecture is a recent concept adopted to develop new materials for light-emitting diodes. The dendritic structure offers a combination of properties of both polymers and small organic molecules whilst having their own interesting characteristic of optimising processibility, charge transport, and optical properties independently. The dendritic structure consists of functional surface groups, conjugated dendrons and a conjugated core. Initial optical (absorption and photoluminescence) studies revealed that the dendrimer emission originates from the core and is independent of excitation wavelength. This was investigated further in distyrylbenzene based dendrimers where the effect of dendrimer generation number on photoluminescence and electroluminescence properties was studied. All dendrimers emit blue electroluminescence with, in some cases, reasonable electroluminescence quantum efficiency in the range of 0.09 % and brightness up to 150 Cd m(^-2). Having established that the furmel effect, where excitation is successfully transferred to the dendrimer core in both PL and EL, different chromophores were incorporated in the dendrimer structure. Colour control was thus demonstrated in EL devices of the different dendrimers, showing the possibility of using a large number of chromophores in a processible form for EL applications. Conjugated polymers were also studied to investigate the nature of the emitting species (poly(p-pyridine)) and the effect of side- chains (poly(p-phenylenevinylene)). In poly(p-pyridine) the emission was found to be strongly dependent on pyridyl ring rotation affecting the emission and its quantum yield while the side-chains in the poly(p-phenylenevinylene) derivatives were found to affect polymer properties such as degree of conversion of non-conjugated to conjugated polymer. The PL quantum yield system was set-up and proved useful in assessing synthesis of new materials.