Electronic excited states of [Au2(dmpm)3](ClO4)2 (dmpm= bis(dimethylphosphine)methane)

We present studies of the resonance Raman and electronic luminescence spectra of the [Au-2(dmprp)(3)](ClO4)(2) (dmpm = bis(dimethylphosphine) methane) complex, including excitation into an intense band at 256 nm and into a weaker absorption system centered about similar to300 nm. The resonance Raman spectra confirm the assignment of the 256 nm absorption band to a (1)(dsigma(star) --> psigma) transition, a metal-metal-localized transition, in that nu(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 nu(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 A from the ground-state value of 3.05 Angstrom. The similar to300 nm absorption displays a different enhancement pattern, in that resonance-enhanced Raman bands are observed at 103 and 183 cm(-1) in addition to nu(Au-Au) at 79 cm(-1) The compound exhibits intense, long-lived luminescence (in room-temperature CH3CN, for example, tau = 0.70 mus, phi(emission) = 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(dsigma(star) --> psigma) but instead derives from (3)(d(x2-y2.xy) --> psigma) excitations. We compare the Au(I)-Au(I) interaction shown in the various transitions of the [Au-2(dmpm)(3)](ClO4)(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.