Correlating Electronic and Nuclear Motions in Ultrafast Photoinduced Charge Transfer Reactions with Femtosecond Multidimensional Spectroscopies

Abstract

Photochemical reactions in solution are probed using femtosecond multidimensional infrared spectroscopies to elucidate the role high-frequency vibrations play in condensed phase charge transfer processes. In all cases presented in this thesis, electronically excited molecules adjust the position of nuclei in unique ways to accommodate excess electronic energy. In the case of sodium nitroprusside, Na₂[Fe(CN)₅NO], we observe a competition between isomerism (bond rotation) and photodissociation (bond breakage) of the nitrosyl ligand on the sub-picosecond time scale. Photoinduced electron transfer in mixed-valence (MV) systems is explored using four vibrations of the cyano-bridged MV complex [(NC)₅Fe<super>II</super>–CN–Pt<super>IV</super>(NH₃)₄–NC–Fe<super>II</super>(CN)₅]<super>4–</super> (Fe<super>II</super>Pt<super>IV</super>Fe<super>II</super>) in D₂O. Third-order nonlinear infrared (IR) spectroscopies are used to determine the molecular nature of the four high-frequency CN stretching (ν_CN) modes in the ground electronic state (GS), including vibrational energy relaxation time, anharmonic vibrational couplings, relative magnitudes and orientations of the permanent dipole moments, and solvent relaxation time scales. These data, which effectively describe the shape of the Fe<super>II</super>Pt<super>IV</super>Fe<super>II</super> GS potential energy surface (PES), are crucial in subsequent chapters when the metal-to-metal charge transfer (MMCT) transition is initiated with a λ = 400 nm pulse and subsequent non-equilibrium vibrational energy relaxation dynamics are observed. We find that back-electron transfer occurs in 110 ± 10 fs, during which greater than six vibrational quanta of the bridging ν_CN mode are excited. This excess energy quickly transfers into the other ν_CN modes until it eventually is deposited into the solvent. We delved further into the electronic spectroscopy of Fe<super>II</super>Pt<super>IV</super>Fe<super>II</super> by developing a coherent fifth-order visible–infrared spectroscopy in order to obtain a more detailed picture of the complex PES governing the MMCT reaction in solution. The goal is to collect third-order nonlinear IR spectroscopic information from an electronically excited state. Fifth order data of this nature allows for the measurement of time-dependent anharmonic couplings, non-equilibrium frequency correlation functions, incoherent and coherent vibrational relaxation and transfer dynamics, and coherent vibrational and electronic coupling as a function of a photochemical reaction. Results on Fe<super>II</super>Pt<super>IV</super>Fe<super>II</super> provide experimental evidence that the nuclear motions of the molecule are both coherently and incoherently coupled to the electronic charge transfer process. This thesis concludes with the development of a novel coherent IR–Raman spectroscopic technique that combines direct IR excitation with a femtosecond stimulated Raman probe. This technique enables anharmonic couplings between low- and high-frequency modes to be obtained directly. Preliminary results suggest that the high-frequency CN stretching mode of acetonitrile is coupled to the low-frequency CCN bending vibration. It is proposed that this IR–FSRS technique will unravel couplings between localized high-frequency motions and global motions in biological systems such as proteins.