Synthesis and Photophysical Study of [(C^N^C)Au(L)]n+ Pincer Complexes, L = Alkynyl (n = 0), Isocyanide (n = +1): Comparative Analysis of Their Aggregation Behavior

Two series of Au(III) pincer complexes [(C^N^C)AuL]n+ (C^N^C = 2,6-diphenylpyridine, L = alkynyl, n = 0; L = isocyanide, n = 1) have been synthesized and characterized in solution by NMR spectroscopy and mass spectrometry. The solid-state structures of alkynyl complexes were also analyzed with XRD crystallography. In a dilute CH2Cl2 solution, both alkynyl and isocyanide complexes display a similar weak blue phosphorescence originating from a metal-perturbed 3ILCT state localized on the pincer ligand, but their solid-state emission characteristics differ significantly. The isocyanide complexes exhibit strong bathochromic shifts of emission band maxima compared to those observed in solution, which can be assigned to the formation of aggregates that afford excimers with short Au-Au contacts in the T1 emissive state. Among the neutral alkynyl analogues, two compounds retain essentially solution-like emissions, since their rigid crystalline environment precludes the excited-state transformation required for metal–metal bonding. Only the electron-rich dimethylamino derivative, which crystallizes as loosely packed dimers, exhibits a red-shifted solid-state band, and even this shift is substantially smaller than that of its isocyanide congener. The electronic structure of the complexes has been analyzed using DFT calculations, which corroborate the proposed rationalization of the observed trends in the emission characteristics and the difference in the aggregation behavior between the alkynyl and isocyanide chromophores.