Catalytic interfaces involving surface-bound molecular catalysts often exhibit a large structural heterogeneity from uncontrolled variation in surface morphology. Conventional spectroscopic techniques typically average over these different structural motifs within the sample, making it difficult to link the underlying surface morphology to the properties of the immobilized catalyst. Here we present the first direct comparison of the vibrational dynamics of a CO2 reduction catalyst bound to two different single-crystalline TiO2 surfaces, rutile (001) and (110), probed with transient surface-specific sum-frequency generation spectroscopy. We find that the change in surface structure between crystallographic faces alters both the vibrational frequency and relaxation time of the symmetric carbonyl stretching mode of the catalyst, with (001) displaying a lower frequency and longer relaxation time. This results from a change in the catalyst electronic structure and indicates that the molecular properties of the catalyst, likely including the catalytic properties, depend on the specific TiO2 surface to which it is bound. The comparison of the molecular properties on these two single crystal surfaces is an essential step toward understanding how semiconductor surface structure influences catalyst behavior and identifying optimal surface structures for improved catalytic performance.

Until now, ultrafast IR spectroscopy has been limited by the bandwidth of optical parametric amplifiers, typically 100-400  cm(-1). Here we present the first example of transient IR spectroscopy using a continuum laser source to probe the entire mid-IR region with ultrafast time resolution. The continuum source is based on focusing the fundamental, second harmonic, and third harmonic of 1 mJ, 25 fs, 800 nm pulses in air, generating ∼150  fs continuum mid-IR pulses that span the frequency range of <400 to >5000  cm(-1) or, conversely, <2 to >25  μm. We characterize the spectral and temporal properties of dicarbonylacetonato rhodium(I) in hexane. We further demonstrate the versatility of the method by measuring the very fast and broad (>1500  cm(-1)) spectral changes following IR excitation associated with the 7-azaindole-acetic acid heterodimer in carbon tetrachloride.

Cyclic hydrogen-bonded structures are common motifs in biological systems, providing structural stability and mediating proton transfer for redox reactions. The mechanism of proton transfer across hydrogen-bonded interfaces depends on the strength of the intermolecular coupling between bridging OH/NH vibrational modes. Here we present a novel ultrafast continuum mid-IR spectroscopy experiment to study the vibrational dynamics of the 7-azaindole-acetic acid (7AI-Ac) heterodimer as a model system for asymmetric cyclic hydrogen-bonded structures. In addition to spreading of the excitation across the whole OH band within the time resolution of the experiment, excitation of a 300 cm(-1) region of the ∼1000 cm(-1) broad OH stretching mode of the acetic acid monomer leads to a frequency shift in the NH stretching mode of the 7AI monomer. This indicates that the NH and OH stretching modes located on the two monomers are strongly coupled despite being separated by 750 cm(-1). The strong coupling further causes the OH and NH bands to decay with a common decay time of ∼2.5 ps. This intermolecular coupling is mediated through the hydrogen-bonded structure of the 7AI-Ac heterodimer and is likely a general property of cyclic hydrogen-bonded structures. Characterizing the vibrational dynamics of and the coupling between the high-frequency OH/NH modes will be important for understanding proton transfer across such molecular interfaces.