Luminescent cyclometalated alkynylgold(III) complexes with 6-phenyl-2,2′-bipyridine derivatives: Synthesis, characterization, electrochemistry, photophysics, and computational studies

A novel class of luminescent gold(III) complexes containing various tridentate cyclometalating ligands derived from 6-phenyl-2,2′-bipyridine and alkynyl ligands, [Au(RĈN̂N)(C≡C-R')] PF6, has been successfully synthesized and characterized. One of the complexes has also been determined by X-ray crystallography. Electrochemical studies show a ligand-centered reduction originated from the cyclometalating RĈN̂N ligands as well as an alkynyl-centered oxidation. The electronic absorption and photoluminescence properties of the complexes have also been investigated. In acetonitrile at room temperature, the complexes show intense absorption at higher energy region with wavelength shorter than 320 nm, and a moderately intense broad absorption band at 374-406 nm, assigned as the metal-perturbed intraligand π-π* transition of the cyclometalating RĈN̂N ligand, with some charge transfer character from the aryl ring to the bipyridine moiety. Most of the complexes have been observed to show vibronic-structured emission bands at 469-550 nm in butyronitile glass at 77 K, assigned to an intraligand excited state of the RĈN̂N ligand, with some charge transfer character from the aryl to the bipyridyine moiety. Insights into the origin of the absorption and emission have also been provided by density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations.