Analysis of the vibronic structure in the emission and absorption spectra of (mu-1,1-dicyanoethylene-2,2-dithiolato-S,S')bis(triphenglphosphine)digold(I) and assignment of the emissive state

Both the 77 K single crystal absorption and 20 K emission spectrum of (mu-1,1-dicyanoethylene-2,2-dithiolato-S,S')bis(triphenylphosphine)digold(I), (AuPPh(3))(2)[i-MNT], show resolved vibronic structure. Progressions in the 1410 cm(-1) C=C stretching mode of the dithiolate ligand, and in the 480 cm(-1) mode, which involves gold-dithiolate stretching, are observed in the emission spectrum. The resonance Raman spectra of the title compound and related compounds were used to identify the modes that give rise to the vibronic structure observed in the emission spectrum. The emission spectrum is fit using the time dependent theory of electronic spectroscopy. The theoretical fit to the spectrum requires distortions in vibrations involving both the metal-sulfur and dithiolate centered modes. These distortions show that the transition is a charge transfer involving the gold and the dithiolate ligand. The emission spectrum of an analogous complex, (AuAsPh(3))(2)[i-MNT], is red shifted relative to the title complex. This red shift allows the direction of the charge transfer emission in the title complex to be assigned as a dithiolate to gold, ligand to metal charge transfer.