The synthesis and properties of solution processable red-emitting phosphorescent dendrimers

We report methodology for the preparation of symmetric and asymmetric solution processable phosphorescent dendrimers that are comprised of 2-ethylhexyloxy surface groups, biphenyl based dendrons, and iridium(III) complex cores. The symmetric dendrimer has three dendritic 2-benzo[b] thiophene-2'-ylpyridyl (BTP) ligands with the dendritic ligands responsible for red emission. The asymmetric dendrimer has two dendritic 2-phenylpyridyl ligands and one unsubstituted BTP ligand. Iridium( III) complexes comprised of 2-phenylpyridyl ligands are normally associated with green emission whereas those containing BTP ligands emit red light. Red emission is observed from the asymmetric dendrimer demonstrating that emission occurs primarily from the metal-to-ligand charge transfer state associated with the ligand with the lowest HOMO - LUMO energy gap. The photoluminescence quantum yields (PLQYs) of the symmetric and asymmetric dendrimers were strongly dependent on the dendrimer structure. In solution the PLQYs of the asymmetric and symmetric dendrimers were 47 +/- 5% and 29 +/- 3% respectively. The photoluminescence lifetime of the emissive state of both dendrimers in solution was 7.3 +/- 0.1 mus. In the solid state the comparative PLQYs were reversed with the symmetric dendrimer having a PLQY of 10 +/- 1% and the asymmetric dendrimer a PLQY of 7 +/- 1%. The comparatively larger decrease in PLQY for the asymmetric dendrimer in the solid state is attributed to increased core - core interactions. The intermolecular interactions are greater in the asymmetric dendrimer because there is no dendron on the BTP ligand. Electrochemical analysis shows that charge is injected directly into the cores of the dendrimers.