A Class of Luminescent Cyclometalated Alkynylgold(III) Complexes:  Synthesis, Characterization, and Electrochemical, Photophysical, and Computational Studies of [Au(CNC)(C⋮CR)] (CNC = κ3C,N,C Bis-cyclometalated 2,6-Diphenylpyridyl)

A new class of luminescent cyclometalated alkynylgold(III) complexes, [Au(RCN(R‘)CR)(C⋮CR‘ ‘)], i.e., [Au(CNC)(C⋮CR‘ ‘)] (HCNCH = 2,6-diphenylpyridine) R‘ ‘ = C6H5 1, C6H4Cl-p 2, C6H4NO2-p 3, C6H4OCH3-p 4, C6H4NH2-p 5, C6H4C6H13-p 6, C6H13 7, [Au(tBuCNCtBu)(C⋮CC6H5)] 8 (HtBuCNCtBuH = 2,6-bis(4-tert-butylphenyl)pyridine), and [Au(CNTolC)(C⋮CC6H4−C6H13-p)] 9 (HCNTolCH = 2,6-diphenyl-4-p-tolylpyridine), have been synthesized and characterized. The X-ray crystal structures of most of the complexes have also been determined. Electrochemical studies show that, in general, the first oxidation wave is an alkynyl ligand-centered oxidation, while the first reduction couple is ascribed to a ligand-centered reduction of the cyclometalated ligand with the exception of 3 in which the first reduction couple is assigned as an alkynyl ligand-centered reduction. Their electronic absorption and luminescence behaviors have also been investigated. In dichloromethane solution at room temperature, the low-energy absorption bands are assigned as the π−π* intraligand (IL) transition of the cyclometalated RCN(R‘)CR ligand with some mixing of a [π(C⋮CR‘ ‘) → π*(RCN(R‘)CR)] ligand-to-ligand charge transfer (LLCT) character. The low-energy emission bands of all the complexes, with the exception of 5, are ascribed to origins mainly derived from the π−π* IL transition of the cyclometalated RCN(R‘)CR ligand. In the case of 5 that contains an electron-rich amino substituent on the alkynyl ligand, the low-energy emission band was found to show an obvious shift to the red. A change in the origin of emission is evident, and the emission of 5 is tentatively ascribed to a [π(C⋮CC6H4NH2) → π*(CNC)] LLCT excited-state origin. DFT and TDDFT computational studies have been performed to verify and elucidate the results of the electrochemical and photophysical studies.