Electronic excited states of [Au2(dmpm)3](CIO4)2 (dmpm = bis(dimethylphosphine)methane)

Abstract

We present studies of the resonance Raman and electronic luminescence spectra of the [Au 2(dmpm) 3](CIO 4) 2 (dmpm=bis(dimethylphosphine) methane) complex, including excitation into an intense band at 256 nm and into a weaker absorption system centered about ∼300 nm. The resonance Raman spectra confirm the assignment of the 256 nm absorption band to a 1(dσ* → pσ) transition, a metal-metal-localized transition, in that v(Au-Au) and overtones of it are strongly enhanced. A resonance Raman intensity analysis of the spectra associated with the 256 nm absorption band gives the ground-state and excited-state v(Au-Au) stretching frequencies to be 79 and 165 cm -1, respectively, and the excited-state Au-Au distance is calculated to decrease by about 0.1 Å from the ground-state value of 3.05 Å. The ∼300 nm absorption displays a different enhancement pattern, in that resonance-enhanced Raman bands are observed at 103 and 183 cm -1 in addition to v(Au-Au) at 79 cm -1 The compound exhibits intense, long-lived luminescence (in room-temperature CH 3CN, for example, τ= 0.70, μs, Φemission = 0.037) with a maximum at 550-600 nm that is not very medium-sensitive. We conclude, in agreement with an earlier proposal of Mason (Inorg. Chem. 1989, 28, 4366-4369), that the lowest-energy, luminescent excited state is not 3(dσ* → pσ) but instead derives from 3(d x 2- y 2,xy → pσ) pGr) excitations. We compare the Au(I)-Au(I) interaction shown in the various transitions of the [Au 2(dmpm) 3](CIO 4) 2 tribridged compound with previous results for solvent or counterion exciplexes of [Au 2(dcpm) 2] 2+ salts (J. Am. Chem. Soc. 1999, 121, 4799-4803; Angew. Chem. 1999, 38, 2783-2785; Chem. Eur. J. 2001, 7, 4656-4664) and for planar, mononuclear Au(I) triphosphine complexes. It is proposed that the luminescent state in all of these cases is very similar in electronic nature.