A class of luminescent cyclometalated alkynylgold(III) complexes: synthesis, characterization, and electrochemical, photophysical, and computational studies of [Au(C^N^C)(C≡C-R)] (C^N^C = κ(3)C,N,C bis-cyclometalated 2,6-diphenylpyridyl)

Abstract

A new class of luminescent cyclometalated alkynylgold(III) complexes, [Au(RC^N(R')^CR)(C≡CR'')], i.e., [Au(C^N^C)(C≡CR'')] (HC^N^CH = 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(tBuC^N^CtBu)(C≡CC6H5)] 8 (HtBuC^N^CtBuH = 2,6-bis(4-tert-butylphenyl)pyridine), and [Au(C^NTol^C)(C≡CC6H4-C6H13-p)] 9 (HC^NTol^CH = 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 RC^N(R')^CR ligand with some mixing of a [π(C≡CR'') -> π*(RC^N(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 RC^N(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*(C^N^C)] LLCT excited-state origin. DFT and TDDFT computational studies have been performed to verify and elucidate the results of the electrochemical and photophysical studies.