Real-time observation of tight Au-Au bond formation and relevant coherent motion upon photoexcitation of [Au(CN)₂^-] oligomers

Structural dynamics involving tight Au-Au bond formation of excited-state oligomers [Au(CN)₂^-]_n was studied using picosecond/femtosecond time-resolved emission and absorption spectroscopy. With selective excitation of the trimer ([Au(CN)₂^-] ₃) in aqueous solutions, transient absorption due to the excited-state trimer was observed around 600 nm. This transient exhibited a significant intensity increase (τ = 2.1 ps) with a blue shift in the early picosecond time region. Density functional theory (DFT) and time-dependent DFT calculations revealed that the observed spectral changes can be ascribed to a structural change from a bent to a linear staggered structure in the triplet excited-state trimer. The transient absorption also exhibited a clear modulation of the peak position, reflecting coherent nuclear wave packet motion induced by photoexcitation. The frequencies of the coherent motions are 66 and 87 cm ^-1, in very good accord with the frequencies of two Au-Au stretch vibrations in the excited state of the trimer calculated by DFT. Time-resolved emission spectra in the subnanosecond time region showed that association of the excited-state trimer with the ground-state monomer proceeds with τ = 2.0 ns, yielding the excited-state tetramer.