Development of nonlinear infrared spectroscopic methods to probe molecular dynamics in functional materials [electronic resource]
Keywords: Chemistry.

A variety of unique functional materials have been invented over past decades following the development of new chemical strategies to design and synthesize the materials. These functional materials typically come in the form of films or powders, leading to the impression that the materials are static. Despite their macroscopic appearance, in many cases molecules constituting a film or powder constantly change their microscopic configurations on ultrafast time scales. The molecular dynamics are often intimately related to the functionality of the material. Probing the ultrafast dynamics of molecules in these materials is of fundamental interest. Nonlinear infrared spectroscopy has been successful in elucidating complex dynamics occurring in liquid phases, such as bulk water, room-temperature ionic liquids, and liquid crystals. Nonlinear infrared spectroscopy exploits interactions between infrared fields and molecular vibrations to infer dynamics occurring in a system of interest. In particular, polarization-selective pump-probe spectroscopy is capable of monitoring orientational motions of molecules, while two-dimensional infrared spectroscopy characterizes the interaction between a molecule and its surroundings by observing the fluctuation of the molecular vibrational frequency. These two methods are particularly informative and have been fully developed to study liquid dynamics. However, the application of these methods to functional materials such as films or powders has been severely limited due to both experimental and conceptual problems. To overcome the challenges, we developed novel nonlinear infrared spectroscopic methods and associated theoretical frameworks which can characterize molecular dynamics in films or powders. Using the newly developed methods, we obtained new insights into unique molecular dynamics occurring in a broad range of materials. First, infrared polarization-selective angle-resolved pump-probe (PSAR-PP) spectroscopy was theoretically formulated and experimentally implemented to study orientational dynamics of a catalytic head group tethered to a flat surface. Conventional polarization-selective pump-probe spectroscopy is applicable only to three-dimensionally isotropic samples such as liquids and cannot sufficiently characterize anisotropic dynamics occurring on a surface. The new method can independently address the in-plane and out-of-plane dynamics of the head group, revealing the highly anisotropic nature of the dynamics happening at the surface. A major challenge of applying two-dimensional infrared spectroscopy to a monolayer or thin film is the detection of a small signal arising from an inherently limited number of molecules. Near-Brewster's angle reflection pump-probe geometry was found to dramatically enhance the detection of signals from thin films. The enhancement was experimentally demonstrated for a molecular monolayer and subsequently extended to study two types of functional thin films, namely a lead halide perovskite film and an ionic liquid thin film. Finally, the application of two-dimensional infrared spectroscopy to powdered samples was hampered by its vulnerability to scattered pump pulses. The scattering problem was overcome by combining an acousto-optic modulator (AOM) pulse shaping system, a polarization filter, and a new phase cycling sequence involving chopping of the probe pulse. The scatter-free two-dimensional infrared spectroscopy was applied to study molecular dynamics in metal-organic frameworks (MOFs). Both the structural fluctuations of the elastic frameworks and the dynamical nature of framework-guest interactions were revealed by the new method.