Examining the Vibrational Couplings and Dynamics in Cyanide-Bridged Transition Metal Mixed Valence Complexes Using Ultrafast Nonlinear Infrared Spectroscopy
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Charge transfer processes are essential to any number of functions that are necessary for everyday life. Photosynthesis, respiration, and obtaining energy from solar cells are just a small sample of functions that rely on moving electrons around. Cyanide-bridged mixed valence complexes serve as useful models to study the coupling between electronic and nuclear motions during charge transfer reactions by using the cyanide stretching (νCN) vibrations as probes. However, little is known about the νCN modes for these classes of molecules on the electronic ground state. Polarization-selective two-dimensional infrared (2D IR) spectroscopy was used to measure anharmonic couplings and angles between the transition dipole moments of the four νCN vibrations found in [(NH3)5RuIIINCFeII(CN)5]− (FeRu) and [(NC)5FeIICNPtIV(NH3)4NCFeII(CN)5]4− (FePtFe) by fitting the experimental spectra. For this study, FeRu was dissolved in both formamide and D2O, and FePtFe was dissolved in D2O. IR pump-probe and 2D IR spectroscopies were also used to study vibrational relaxation of the νCN modes in both complexes. The experimental 2D IR spectra of FeRu and FePtFe and their fits reveal a set of weakly coupled anharmonic νCN modes with the vibrational mode anharmonicities ranging from 14 to 28 cm−1 and the mixed-mode anharmonicities ranging from 2 to 14 cm−1. Measurement of the relative transition dipole moments of the four νCN modes reveal that FeRu appears almost linear in solution when dissolved in formamide, but bent when dissolved in D2O. Fits of FePtFe dissolved in D2O reveal that the molecule also appears bent. IR pump-probe experiments reveal that the vibrational lifetime of the radial, axial, and trans νCN modes are ~2 times faster when FeRu is dissolved in D2O versus formamide, and the lifetimes of the νCN modes of FePtFe are ~4 times faster than FeRu when both are dissolved in D2O. Finally, the synthesis and basic characterization of new cyanide containing RuII polypyridine complexes are discussed. These complexes contain a metal-to-ligand charge transfer that relaxes into a longer lived triplet state. This longer lifetime will allow for a more in-depth analysis of vibrational modes on an electronic excited state as well as simplifying the analysis by reducing the number of νCN modes from the four seen in FeRu and FePtFe to one or two in the Ru polypyridine complexes.
- Chemistry