Amplified Triplet Emission of Organic Periphery Groups by Exothermic Triplet-Triplet Energy Transfer from the ³MLCT State of an Ir(pmi)₃ Core Complex to the ³LC State of Geometrically Confined Carbazole/Naphthyl Tethers

To investigate the excited-state properties of metal-organic bichromophores, including energy transfer mechanisms, a series of new homoleptic N-heterocyclic carbene (NHC)-based iridium(III) complexes were prepared by incorporating a peripheral naphthalene (Np) (Ir(Nppmi)(3): fac-/mer-Ir(1-Nppmi)(3) and fac-/mer-Ir(2-Nppmi)(3)) or carbazole (Cz) (Ir(Czpmi)(3): fac-/mer-Ir(o-Czpmi)(3), fac-/mer-Ir(m-Czpmi)(3), and fac-/mer-Ir(p-Czpmi)(3)) unit to the phenyl moiety of the phenylimidazole (pmi) ligand. Through a series of photophysical analyses and femtosecond time-resolved absorption (fs-TA) spectroscopy, it was discovered that the phosphorescence of the Ir core, (Ir(pmi)(3)), was considerably quenched, while intense phosphorescence peaks arising from the excited triplet Np (3Np*)/Cz (3Cz*) species were primarily observed at room temperature (r.t.) and low temperature. Such amplified phosphorescence of the tethered organic Np and Cz units originated from triplet-triplet energy transfer (TTET) from the highlying metal-to-ligand charge transfer ((MLCT)-M-3) state of the Ir(pmi)(3) core to the ligand-centered triplet state ((LC)-L-3) of the peripheral Np and Cz units. This result indicates that the exothermic intramolecular energy transfer (IET) in the excited triplet state realizes the efficient phosphorescent emission of geometrically confined organic tethers.