Efficient Intramolecular Vibrational Excitonic Energy Transfer in Ru₃(CO)₁₂ Cluster Revealed by Two-Dimensional...

Trinuclear transition-metal carbonyl complex dodecacarbonyl triruthenium (Ru₃(CO)₁₂) is considered as one of the paradigms in cluster chemistry, which plays an important role in photocatalysis, photoenergy conversion, and synthetic chemistry. Due to structural symmetry (D_3h point group), 12 carbonyl (C≡O) groups in the Ru₃(CO)₁₂ complex contribute to mainly three excitonic carbonyl stretching modes: E' (radial), A₂″ (axial), and E' (axial). In this work, efficient intramolecular vibrational energy redistribution (IVR) processes among the three modes in this Ru-CO complex were observed to occur on the time scale of tens of picoseconds. The IVR processes were characterized in detail using a kinetic model and fitting to the waiting-time-dependent diagonal and off-diagonal signals of ultrafast two-dimensional infrared spectroscopy. In addition, the diagonal anharmonicities of the three C≡O stretching modes were determined to be quite close to one another, and the coupling-induced cross peaks were invariant because this Ru₃(CO)₁₂ cluster does not show picosecond fluxionality and hence their contributions were neglected in modeling the IVR processes. Our results provide a benchmark for understanding the excitonic nature of the vibrational excited states of the carbonyl vibrators and the associated efficient vibrational energy-flow pathways, in such multicentered transition-metal complexes, which are of key importance to their functions.