Metal-Metal Bond Formations in [Au(CN)2-] n ( n = 3-5) Oligomers in Water Identified by Coherent Nuclear Wavepacket Motions.

Large oligomers of [Au(CN)2-] n including the pentamer were favorably formed in an aqueous solution containing tetra-ethylammonium chloride (1.0 mol/dm3), and intense transient absorption in the visible region was recorded by selective photoexcitation of the oligomers. Distinct oscillations at ∼40-100 cm-1 were clearly observed in the temporal profile of the excited-state absorption signal, and the frequency-wavelength two-dimensional analysis of the oscillation clearly distinguishes the coherent nuclear motion of different oligomers. The observed nuclear motions were assigned to Au-Au stretch vibrations in the trimer, tetramer, and pentamer induced by the bond formation in the excited states. The transient absorption exhibits significant changes with a time constant of 3-20 ps, reflecting intersystem crossing and structural change.