Abstract
We study the influence of the film thickness on the time-resolved phosphorescence and the luminescence quantum yield of fac-tris(2-phenylpyridyl)iridium(iii) [Ir(ppy)3]-cored dendrimers deposited on dielectric substrates. A correlation is observed between the surface quenching velocity and the quenching rate by intermolecular interactions in the bulk film, which suggests that both processes are controlled by dipole-dipole interactions between Ir(ppy)3 complexes at the core of the dendrimers. It is also found that the surface quenching velocity decreases as the refractive index of the substrate is increased. This can be explained by partial screening of dipole-dipole interactions by the dielectric environment.
Highlights
Triplet exciton diffusion in iridium(III) complex materials is mediated by electron exchange interactions,[12] the concentration quenching of the phosphorescence in the bulk films and near the dielectric surfaces is controlled by dipole–dipole interactions between iridium(III) complex dendrimer cores
The Forster-type energy transfer rate is known to be proportional to 1/n4 where n is the refractive index of the medium[31,32] and the dipole–dipole interactions can be sensitive to the boundary conditions of an interface.[33,34,35]
The results show that this effect strongly depends on the exciton diffusion and is larger in materials with high non-radiative decay rate and low concentration quenching
Summary
We report a study on the thickness dependence of the decay of the dendrimer thin film phosphorescence and describe the results using a model based on surface quenching of triplets at the top and bottom interfaces of the film. The roles of exciton diffusion, the bulk photophysical properties of the dendrimer films and the refractive index of the substrates on the surface quenching effects are discussed, providing important insights into the triplet quenching mechanisms in phosphorescent organic thin films. Our results broaden our understanding of triplet-state photophysics, but are of practical interest for the development of light-emitting devices, in particular electrophosphorescent organic field-effect transistors.[20]
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