Dumbbell-shaped dinuclear iridium complexes and their application to light-emitting electrochemical cells

A novel family of dumbbell-shaped dinuclear complexes in which an oligophenyleneethynylene spacer is linked to two heteroleptic iridium(III) complexes is presented. The synthesis, as well as the electrochemical and photophysical characterization of the new complexes, is reported. The experimental results are interpreted with the help of density functional theory calculations. From these studies we conclude that the lowest triplet excited state corresponds to a (3)pi-pi* state located on the conjugated spacer. The presence of this state below the (3)MLCT/(3)LLCT emitting states of the end-capping Ir(III) complexes explains the low quantum yields observed for the dinuclear complexes (one order-of-magnitude less) with respect to the mononuclear complexes. The potential application of the novel dinuclear complexes in optoelectronic devices has been tested by using them as the primary active component in double-layer light-emitting electrochemical cells (LECs). Although the luminance levels are low, the external quantum efficiency suggests that a near-quantitative internal electron-to-photon conversion occurs in the device. This indicates that the emission inside the device is highly optimized and that the self-quenching associated with the high concentration of the complex in the active layer is minimized.