Two-Dimensional Vibrational-Electronic Spectroscopy: The Design and Development of a Novel Multidimensional Spectroscopic Technique to Directly Measure Coherent Coupling Between Vibrational and Electronic Degrees of Freedom

Abstract

Two-dimensional vibrational-electronic (2D VE) spectroscopy fulfills the need for a direct measure of how vibrational motions effect electron transfer. This third-order Fourier transform technique probes electronic transitions that occur after excitation of an infrared (IR) pump. The information contained within a 2D VE spectrum is outlined alongside the details of how this spectroscopy is implemented. The mixed-valent molecule [(CN)6FeII(CN)RuIII(NH3)5] ̅ (FeRu), which features a cyanide bridged metal-to-metal charge transfer (MMCT) from iron to ruthenium, serves as a model to demonstrate how 2D VE can be used to study how high frequency C≡N vibrations couple to the MMCT. The vibrational modes found to couple most strongly with the MMCT are the ones that modulate the distance between the two metal centers, specifically the bridging mode, νbridge. Spectra taken with varying delay times between excitation and detection demonstrate changes in lineshape indicative of fluctuating correlation as a function of the time between the vibrational excitation and the electronic transition. Improvements to the experiment including incorporating continuous, fast scanning and polarization selectivity allow more spectra to be taken in a shorter period of time while looking at the orientational dependence of 2D VE signal. FeRu exhibits strong orientational dependence because of the large angles found around the metal centers. The polarization dependent 2D VE data is fit to a model which includes the orientational response function, allowing the separation of signal amplitude resulting from strong vibronic coupling and orientational alignment. The νradial mode are shown to align nearly perpendicularly to the MMCT, while the νbridge and νtrans modes are found to be 3° and 9° relative to the MMCT respectively. This also improves the understanding of the coupling strength, independent of orientational factors. The νbridge mode is shown to be the dominating vibration when it comes to coupling with the MMCT with a coupling strength almost 3 times that of the similarly oriented νtrans and more than 5 times stronger than the νradial mode. By separating the orientational and coupling strength components to 2D VE signal amplitude, quantitative models can be developed to fully describe 2D VE phenomena and better the understanding of vibrational-electronic coupling.