Luminescent Cyclometalated Dialkynylgold(III) Complexes of 2-Phenylpyridine-Type Derivatives with Readily Tunable Emission Properties

A novel class of luminescent dialkynylgold(III) complexes containing various phenylpyridine and phenylisoquinoline-type bidentate ligands has been successfully synthesized and characterized. The structures of some of them have also been determined by X-ray crystallography. Electrochemical studies demonstrate the presence of a ligand-centered reduction originating from the cyclometalating (CN)-N-boolean AND ligand, whereas the first oxidation wave is associated with an alkynyl ligand-centered oxidation. The electronic absorption and photoluminescence properties of the complexes have also been investigated. In dichloromethane solution at room temperature, the low-energy absorption bands are assigned as the metal-perturbed pi-pi* intraligand (IL) transition of the cyclometalating (CN)-N-boolean AND ligand, with mixing of charge-transfer character from the aryl ring to the pyridine or isoquinoline moieties of the cyclometalating (CN)-N-boolean AND ligand. The low-energy emission bands of the complexes in fluid solution at room temperature are ascribed to originate from the metal-perturbed pi-pi* IL transition of the cyclometalatng (CN)-N-boolean AND ligand. For complex 4 that contains an electron-rich amino substituent on the alkynyl ligand, a structureless emission band, instead of one with vibronic structures as in the other complexes, was observed, which was assigned as being derived from an excited state of a [pi(C CC6H4NH2)->pi*((CN)-N-boolean AND)] ligand-to-ligand charge-transfer (LLCT) transition.