Exploring the versatile role of doubly resonant sum frequency spectroscopy in molecular characterization of self-assembled and solid interfaces
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Vibrational sum frequency generation (VSFG) spectroscopy is a nonlinear linear optical technique used, in general, to study the resonant infrared frequencies of chemical systems which lack centrosymmetry at the molecular level. We specialize in a specific variant of this spectroscopy whereby electronic and vibrational resonances are dually probed in a process called doubly resonant sum frequency generation spectroscopy, or DR-SFG. Using this approach, we probed C60 thin films and demonstrate the occurrence of electron–phonon couplings at the C60/substrate interface and shed light on the impact of these couplings on the optical response of electronically excited fullerene. This coupling may influence charge and energy transport in organic electronic devices mediated by vibrational motions. Similarly, we use DR-SFG to study poly(3-hexylthiophene) (P3HT) films and learn that hydrating the polymer impacts the surface vibronic response which may affect the development of proposed biosensing probes that are coated with this type of material. Additionally, VSFG is employed to probe the Jacobsen Catalyst; a MnIII(salen) complex used in oxygen transfer reactions. This study focuses on the Mn-oxo bond which occurs in the reactive MnV(O)(salen) intermediate and opens the door to future VSFG studies on surface-bound high-valent transition-metal complexes. We also present work on the fullerene/water interface probed in the same range as the aforementioned study and we look at low and high temperature effects on the C60 film’s spectral features. Finally, as supported catalysis becomes an increasingly attractive choice in large-scale, high-throughput industrial synthesis, a need to understand the nature of the supports themselves exists. We present preliminary VSFG work on sulfonated phenyalkyl-silane monolayers relevant to silica-bound coordination complex-mediated catalysis.