No abstract

We report a new family of homoleptic iridium(III) complexes that emit blue phosphorescence at room temperature. The iridium(III) complexes are comprised of phenyltriazole ligands and were easily prepared via short synthetic routes. The parent fac-tris(1-methyl-5-phenyl-3-propyl-[1,2,4]triazolyl)iridium(III) complex exhibits blue photoluminescence (PL) with emission peaks at 449 and 479 nm and has a solution PL quantum yield of 66%. The emission was sequentially blue-shifted by the attachment of one and two fluorine atoms to the ligand phenyl ring with the fac-tris{1-methyl-5-(4,6-difluorophenyl)-3-propyl-[1,2,4]triazolyl}iridium(III) complex having the 1931 Commission Internationale de l'Eclairage coordinates of (0.16, 0.12) at room temperature. In contrast, when the phenyl ring of the ligands was substituted by trifluoromethyl, the PL spectrum was red-shifted when compared to the parent compound whereas if the trifluoromethyl group was attached to the triazole ring, the emission was blue-shifted. The radiative rates of these new blue iridium(III) complexes were found to be in the range of 2−6 × 105 s-1, indicating that the emission had varying amounts of metal-to-ligand charge-transfer character. Molecular orbital calculations showed that for the fluorinated complexes the contribution of the ligand triplet character to the emissive energy state increased with the hypsochromic shift in emission. This was confirmed by time-resolved PL measurements, which showed that the complex with the deepest blue emission had the slowest radiative decay rate.

Dendrimers are now an important class of light‐emitting material for use in organic light‐emitting diodes (OLEDs). Dendrimers are branched macromolecules that consist of a core, one or more dendrons, and surface groups. The different parts of the macromolecule can be selected to give the desired optoelectronic and processing properties. The first light‐emitting dendrimers were fluorescent but more recently highly efficient phosphorescent dendrimers have been developed. OLEDs containing light‐emitting dendrimers have been reported to have external quantum efficiencies of up to 16 %. The solubility of the dendrimers opens the way for simple processing and a new class of flat‐panel displays. In this Review we show how the structure of the light‐emitting dendrimers controls key features such as intermolecular interactions and charge transport, which are important for all OLED materials. The advantages of the dendrimer architecture for phosphorescent emitters and the way the structure can be varied to enhance materials performance and device design are illustrated.

A simple convergent procedure has been developed for the preparation of solution processable phosphorescent dendrimers with biphenyl-based dendrons and fac-tris(2-phenylpyridyl)iridium(III) cores. We found that the attachment point and branching of the dendrons are important for controlling the color of the light emission. Photoluminescence excitation measurements showed that energy could be transferred efficiently from the dendrons to the core. Solution photoluminescence quantum yield (PLQY) measurements of the dendrimers were of order 70%, showing that the attachment of the dendron did not decrease the luminescence efficiency of the core iridium complex. The PLQYs of the neat dendrimer films increased with generation with the second-generation dendrimer having a neat film PLQY of 31%, 1 1 /2 times higher than the first-generation dendrimers and almost 3 times that of the nondendritic iridium complex, demonstrating the power of the dendrimer architecture to control intermolecular interactions. Electrochemical experiments showed that charge was injected directly into the core of the dendrimers.