Tracking Photoinduced Au-Au Bond Formation through Transient Terahertz Vibrations Observed by Femtosecond Time-Domain Raman Spectroscopy.

Real-time observation of chemical bond formation and subsequent nuclear rearrangements is an ultimate goal of chemical science. Yet, such attempts have been hampered by the technical difficulty of triggering bond formation at well-defined, desired timing. The trimer of dicyanoaurate complex ([Au(CN)2-]3) is an ideal system for achieving this aim because the tight covalent Au-Au bonds are formed upon photoexcitation. Despite the apparent simplicity of the system, however, recent time-resolved studies failed to construct a consistent picture of its ultrafast dynamics. Here, we report femtosecond time-domain Raman tracking of ultrafast structural dynamics of the [Au(CN)2-] trimer upon photoinduced Au-Au bond formation. The obtained Raman data reveal that the Au-Au breathing vibration at ∼90 cm-1 exhibits a gradual frequency upshift in a few picoseconds, demonstrating a continuous bent-to-linear structural change on the triplet-state potential energy surface upon the Au-Au bond formation. The comprehensive ultrafast spectroscopic study settles the controversy on this prototypical molecular assembly.