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

Karla M Slenkamp,Michael S Lynch,Munira Khalil,Stephanie L Daifuku,Caitlin C Bannan,Jennifer F Brookes

doi:10.1063/1.4943766

Karla M Slenkamp, Michael S Lynch + Show 4 more

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https://doi.org/10.1063/1.4943766

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Abstract

Using polarization-selective two-dimensional infrared (2D IR) and infrared pump-probe spectroscopies, we study vibrational relaxation of the four cyanide stretching (νCN) vibrations found in [(NH3)5RuIIINCFeII(CN)5]− (FeRu) dissolved in D2O or formamide and [(NC)5FeIICNPtIV(NH3)4NCFeII(CN)5]4− (FePtFe) dissolved in D2O. 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 νCN modes in the electronic ground state of these complexes. IR pump-probe experiments reveal that the vibrational lifetimes of the νCN modes are ∼2 times faster when FeRu is dissolved in D2O versus formamide. They also reveal that the vibrational lifetimes of the νCN modes of FePtFe in D2O are almost four times as long as for FeRu in D2O. 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 νCN 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 νCN 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.

Highlights

Moieties.5–11 These studies highlight the importance of understanding the time-dependent intraand intermolecular vibrational energy transfer and relaxation pathways to develop a microscopic understanding of light-induced phenomena in small molecules and materials for efficient photochemical energy conversion
The results presented here along with our previous work on FeRu and formamide and [(NC)5FeIICNPtIV(NH3)4NCFeII(CN)5]4À (FePtFe) reveal a picture of coupled anharmonic cyanide stretching (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
Electron withdrawing effect carries along the metalto-metal charge transfer (MMCT) axis, the strength of the CN bond could increase for the terminal cyanide ligands explaining why the t mode (blue arrows in Figs. 1(a) and 1(b)) in FePtFe is at a much higher frequency than it is in FeRu and why the frequency ordering of the four CN modes change between the two complexes

Summary

Introduction

Moieties. These studies highlight the importance of understanding the time-dependent intraand intermolecular vibrational energy transfer and relaxation pathways to develop a microscopic understanding of light-induced phenomena in small molecules and materials for efficient photochemical energy conversion. Previous ultrafast studies of cyanide-bridged mixed-valence complexes have explored the role played by high-frequency CN stretching (CN) vibrations in the photo-induced metalto-metal charge transfer (MMCT) and the subsequent back electron transfer process.. Previous ultrafast studies of cyanide-bridged mixed-valence complexes have explored the role played by high-frequency CN stretching (CN) vibrations in the photo-induced metalto-metal charge transfer (MMCT) and the subsequent back electron transfer process.7–9,12–21 These studies have shown that there is a high degree of vibrational excitation and measured how the vibrational transfer and relaxation pathways evolve as the molecule equilibrates in the ground electronic state following back-electron transfer. The intra- and intermolecular vibrational relaxation dynamics of the CN modes in the ground electronic state of cyanide bridged transition metal mixed valence complexes have not been explored. Electron withdrawing effect carries along the MMCT axis, the strength of the CN bond could increase for the terminal cyanide ligands explaining why the t mode (blue arrows in Figs. 1(a) and 1(b)) in FePtFe is at a much higher frequency than it is in FeRu and why the frequency ordering of the four CN modes change between the two complexes

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