UC San Diego

In this thesis, two multidimensional spectroscopic methods — two-dimensional infrared (2D IR) spectroscopy and heterodyne 2D sum frequency generation (HD 2D SFG) spectroscopy — are applied to investigate ultrafast chemical exchange of organometallics in bulk solution and structural orientation of surface catalysts at interface, respectively.

2D IR spectroscopy is used to study the ultrafast chemical exchange of two organometallics compound ( Ru(S2C2(CF3)2)(CO)(PPh3)2 and Co(CNArMes2)4 ) occurring on the picosecond time scale. Combined with DFT simulation results, 2D IR provides direct evidence of the existence of various isomers. For the five-coordinate Ru complex, one isomers that serves as the intermediate of the axial-equatorial exchange was observed for the first time. Furthermore, our 2D IR works find the kinetic barriers of dynamic exchange between multiple isomers surprisingly low, consider both of the transition metal complexes under study have relatively large ligands. The low kinetic barriers are attributed to the small core-angle movement involved in the chemical exchange.2D IR is also applied to W(CO)6/dual cavity system to observe ultrafast intercavity nonlinear polariton interactions. Combined with a newly developed theory model, we show that the nonlinear interaction is realized by shared molecular anharmonicities among cavity modes, e.g. through mode delocalization, some molecules (with anharmonicity) are coupled by cavity modes adjacent to each other.

HD 2D SFG spectroscopy is used to study a model CO2 reduction catalyst, Re(diCNbpy)(CO)3Cl, as a monolayer on a gold surface. We show that short-range interactions with the surface can cause substantial line-shape differences between vibrational bands from the same molecules. This interaction can be explained as the result of couplings between CO vibrational modes of the catalyst molecules and the image dipoles on gold surface, which are sensitive to the relative distance between the molecule and the surface. Thus, by analysis of HD 2D SFG lineshape differences and polarization dependences of IR spectra, the ensemble-averaged orientation of the molecules on the surface can be determined unambiguously. The high sensitivity of HD 2D SFG spectra to short-range interactions can be applied to many other adsorbate–substrate interactions and therefore serve as a unique tool to determine adsorbate orientations on surfaces.

Meanwhile, surface molecules of the monolayers can adopt conformations with many different orientations. Thus, it is necessary to describe the orientations of surface molecular monolayers using both mean tilt angle and orientational distribution, which together we refer to as orientation heterogeneity. Orientation heterogeneity is difficult to measure. In most cases, in order to calculate the mean tilt angle, it is assumed that the orientational distribution is narrow. This assumption causes ambiguities in determining the mean tilt angle and loss of

orientational distribution information, which is known as the “magic angle” challenge. Using HD 2D SFG spectroscopy, we report a novel method to solve the “magic angle” challenge, by simultaneously measuring mean tilt angle and orientational distribution of molecular monolayers. Although applied to a specific system, this method is a general way to determine the orientation heterogeneity of an ensemble-averaged molecular interface.