Localization of the Au–Au bond strength in the triplet excited state of the [Au(CN)2−] oligomers revealed by ultrafast time-domain Raman spectroscopy

Abstract

Photo-induced Au–Au bond formation and subsequent ultrafast dynamics of the [Au(CN)₂⁻] oligomers have been attracting much interest. To fully understand this unique photochemical process, it is important to elucidate the oligomer size dependence of the structure and dynamics in the excited state. In this study, we use time-resolved impulsive stimulated Raman spectroscopy to obtain Raman spectra of large [Au(CN)₂⁻] oligomers, the tetramer and pentamer, in the lowest excited triplet (T₁) state. We observe a frequency shift of the Au–Au stretch vibration, reflecting an ultrafast bent (zigzag)-to-linear structural change for the T₁ tetramer, as in the case of the T₁ trimer, indicating that this type of structural change commonly occurs with the photo-induced Au–Au bond formation in the [Au(CN)₂⁻] oligomers. With the addition of the Raman spectrum of the T₁ dimer newly measured, we compare the Raman spectra to discuss the structure of T₁ [Au(CN)₂⁻]_n (n = 2–5) with the help of complementary DFT calculations. The results of time-domain Raman spectroscopy and theoretical calculations indicate that the Au–Au bond strength(s) in the T₁ tetramer and T₁ pentamer are significantly stronger in the central dimer-like and trimer-like “core” parts, respectively, compared to the Au–Au bonds connecting two monomer units at both ends. This localization of bond strength highlights the distinct nature of the p_z–p_z σ chemical bond formed in the T₁ [Au(CN)₂⁻]_n oligomers, as a new type of chemical bond. This study expands our understanding of the photo-induced Au–Au bond formation process and the resulting excited-state structure of metal complex assemblies.