A study of triplet-triplet exciton annihilation and nonradiative decay in films of iridium(III)-centered phosphorescent dendrimers is reported. The average separation of the chromophore was tuned by the molecular structure and also by blending with a host material. It was found that triplet exciton hopping is controlled by electron exchange interactions and can be over 600 times faster than phosphorescence quenching. Nonradiative decay occurs by weak dipole-dipole interactions and is independent of exciton diffusion, except in very thin films (<20 nm) where surface quenching dominates the decay. DOI: 10.1103/PhysRevLett.100.017402 PACS numbers: 78.66.Qn, 73.50.Gr, 78.40.Me, 78.55.Kz Excitation energy transfer is an important process in organic semiconductors and has to be taken into account when designing new optoelectronic materials and devices. In photovoltaic devices, the neutral excited state is generated by light absorption and must diffuse to a heterojunction with another material to be separated into charge carriers and so provide photocurrent. In organic light emitting diodes (OLEDs), exciton diffusion can lead to a decrease of the electroluminescence efficiency due to quenching of the emitting state by intermolecular interactions and defects [1,2], exciton interactions with charge carriers [2,3], and exciton-exciton annihilation [4,5]. These quenching effects are more pronounced in phosphorescent OLEDs than in fluorescent devices because of the longer excited state lifetime. Nevertheless, phosphorescent OLEDs show much higher internal quantum efficiencies due to their ability to convert both singlet and triplet excitons into light [6 -8]. A photoluminescence (PL) study of iridium(III) complexes dispersed into a wide energy gap host suggested that intermolecular quenching of phosphorescence in films is controlled by the dipole-dipole interactions between emitters [9]. However, the impact of exciton migration on phosphorescence quenching has not been considered.In this Letter, we compare the dynamics of triplet exciton diffusion and quenching in several fac-tris(2-phenylpyridyl)iridium(III) [Ir ppy 3 ]-cored phosphorescent dendrimers. Dendrimers provide a convenient way of changing the spacing of the core chromophores in the solid state and hence of studying the effect of spacing on the physics of exciton diffusion and light emission. The triplet exciton diffusion rates are extracted from the measurements of triplet-triplet annihilation and have an exponential dependence on chromophore spacing. This shows that diffusion is controlled by nearest-neighbor electron exchange interactions [10]. Nonradiative decay in 180 nm thick films is governed by much weaker dipole-dipole interactions and does not depend on the triplet diffusion rate. In much thinner films (<20 nm), phosphorescence quenching is found to depend on exciton diffusion and can be modeled using the diffusion equation together with quenching at the film surface.Five green Ir ppy 3 -cored dendrimers were used in this study and their chemical st...

Intermolecular interactions play a crucial role in the performance of organic light‐emitting diodes (OLEDs). Here we report the photophysical and electroluminescence properties of a fac‐tris(2‐phenylpyridyl)iridium(III) cored dendrimer in which highly branched biphenyl dendrons are used to control the intermolecular interactions. The presence of fluorene surface groups improves the solubility and enhances the efficiency of photoluminescence, especially in the solid state. The emission peak of the dendrimer is around 530 nm with a PL quantum yield of 76 % in solution and 25 % in a film. The photophysical properties of this dendrimer are compared with a similar dendrimer with the same structure but without the fluorene surface groups. Dendrimer LEDs (DLEDs) are prepared using each dendrimer as a phosphorescent emitter blended in a 4,4′‐bis(N‐carbazolyl)biphenyl host. Device performance is improved significantly by the incorporation of an electron‐transporting layer of 1,3,5‐tris(2‐N‐phenylbenzimidazolyl)benzene. A peak external quantum efficiency of 10 % (38 Cd A–1) for the dendrimer without surface groups and 13 % (49.8 Cd A–1) for the dendrimer with fluorene surface groups is achieved in the bilayer LEDs.

A simple convergent procedure has been developed for the formation of sterically encumbered phosphorescent dendrimers. The procedure is demonstrated with the preparation of a first-generation dendrimer composed of a fac-tris(2-phenylpyridyl)iridium(III) core and three dendrons. Each dendron is comprised of a branching phenyl unit with a further four phenyl groups attached. The lack of surface groups on the dendrons was found to reduce solubility and also reduced the level of control over the intermolecular interactions of the emissive and electroactive core in films. The 6-fold decrease in photoluminescence quantum yield in going from solution (69%) to the solid state (11%) showed that there were strong intermolecular interactions of the emissive cores in the solid state. Single-layer devices with the dendrimer blended with 4,4′-bis(N-carbazolyl)biphenyl showed an external quantum efficiency of 1.7% (5.4 cd/A) at 100 cd/m 2 and 11.4 V, giving a power efficiency of 1.5 lm/W.

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.