A study on the preparation and photophysical properties of an iridium(III) complexed homopolymer

We have prepared a poly(styrene) [poly(Irppy(2)acac)] that has a bis(2-phenylpyridyl)iridium(III) acetylacetonate complex attached to every 'monomer' via a methylene unit. The polymer is solution processable and can be spin-coated to give good quality neat thin films. We show that despite the close proximity of the phosphorescent chromophores along the polymer backbone there is only a modest reduction in the solution photoluminescence quantum yield (PLQY) relative to a monomer model compound. The PLQY of the 4-ethylbenzyl substituted bis(2-phenylpyridyl)iridium(III) acetylacetonate complex was 52% while the poly(Irppy(2)acac) had a PLQY of 34%. Time-resolved photoluminescence measurements showed that the photoluminescence decay of the poly(Irppy(2)acac) in solution was bi-exponential indicating the presence of more than one emissive species, which was attributed to intra-polymer chromophore interactions. In contrast to the solution measurements the solid-state PLQY was very low (<1%) due to quenching arising from inter-chromophore interactions. When 6 wt% of the poly(Irppy(2)acac) was blended with the small molecule host 4,4'-bis(N-carbazolyi)-2,2'-biphenyl (CBP), which is a ratio similar to that used for small molecule iridium(III) complexes, significant emission from the CBP was observed suggesting that there was not an even distribution of the polymer in the film. In contrast a 19 wt% blend showed almost complete energy transfer [the CBP peak was 100 times smaller than that of the poly(Irppy(2)acac)] and there was an increase in the PLQY relative to the neat films.