Molecular designing for Intensely emissive transition metal complexes

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Molecular designing of transient-metal compounds with blue phosphorescence is one of the most important problems in recent development of phosphorescent OLED devices. While pyrazole ligands have been often used to increase emission energy of phosphorescent iridium(III) complexes, introduction of these ligands is known to increase nonradiative transition rates remarkably. In this work, kinetic parameters for the nonradiative process have been determined from emission lifetimes of iridium complexes with two pyrazole-iridium bonds were measured over a wide temperature range. The obtained activation energy of 1800 cm-1 indicates that the emissive phosphorescent state is quenched via closely lying dd state.To get insights into the quenching process, the potential energy surface of the lowest triplet state of this complex was calculated using unrestricted DFT calculation and the geometrical changes associated with the quenching via the dd state were studied. It was revealed that at the geometry where the iridium-pyrazole bonds are elongated by 0.6nm compare to that for the phosphorescent state the energies of both the triplet excited state and the ground state become degenerating and therefore very fast deactivation occurs. Both the calculated emission energy and activation energy were in good agreement with those observed, which greatly validates the DFT calculations.