Investigating vibrational relaxation in cyanide-bridged transition metal mixed-valence complexes using two-dimensional infrared and infrared pump-probe spectroscopies

Using polarization-selective two-dimensional infrared (2D IR) and infrared pump-probe spectroscopies, we study vibrational relaxation of the four cyanide stretching (ν CN ) vibrations found in [(NH 3 ) 5 Ru III NCFe II (CN) 5 ] − (FeRu) dissolved in D 2 O or formamide and [(NC) 5 Fe II CNPt IV (NH 3 ) 4 NCFe II (CN) 5 ] 4− (FePtFe) dissolved in D 2 O. These cyanide-bridged transition metal complexes serve as models for understanding the role high frequency vibrational modes play in metal-to-metal charge transfers over a bridging ligand. However, there is currently little information about vibrational relaxation and dephasing dynamics of the anharmonically coupled ν CN modes in the electronic ground state of these complexes. IR pump-probe experiments reveal that the vibrational lifetimes of the ν CN modes are ∼2 times faster when FeRu is dissolved in D 2 O versus formamide. They also reveal that the vibrational lifetimes of the ν CN modes of FePtFe in D 2 O are almost four times as long as for FeRu in D 2 O. Combined with mode-specific relaxation dynamics measured from the 2D IR experiments, the IR pump-probe experiments also reveal that intramolecular vibrational relaxation is occurring in all three systems on ∼1 ps timescale. Center line slope dynamics, which have been shown to be a measure of the frequency-frequency correlation function, reveal that the radial, axial, and trans ν CN modes exhibit a ∼3 ps timescale for frequency fluctuations. This timescale is attributed to the forming and breaking of hydrogen bonds between each mode and the solvent. The results presented here along with our previous work on FeRu and FePtFe reveal a picture of coupled anharmonic ν CN modes where the spectral diffusion and vibrational relaxation dynamics depend on the spatial localization of the mode on the molecular complex and its specific interaction with the solvent.