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(RC∧N∧N)­(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 RC∧N∧N 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
RC∧N∧N 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 RC∧N∧N 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.