UC San Diego

Vibrational sum frequency generation (VSFG) microscopy is a powerful technique to study molecular vibrational modes at non-centrosymmetric surfaces, interfaces and structures. However, most of the existing VSFG microscopes have 2 limitations: lack of molecular orientation information and lack of ultrafast dynamics information. In this thesis, we report the newly developed VSFG microscopy in our lab, as well as their applications in the study of self-assembly.

First, we developed the self-phase-stabilized heterodyne-detected VSFG microscope that can reveal spectral phase and molecular orientations. In our geometry, the VSFG signal and local oscillator are generated using the same beam path. Therefore, comparing to traditional Michelson interferometric geometry, our phase stability is improved by 9 times. Using this heterodyne VSFG microscope to study a self-assembled material SDS@2β-CD, we successfully identified two molecular domains with different molecular orientations, which is not possible to extract from an ensemble-averaged VSFG spectrum or homodyne-detected VSFG images.

In addition, we extended the spectral coverage and further studied the self-assembled material. We found because of strong hydrogen-bond interactions between water and the self-assembly, water molecules are template to adopt the local mesoscopic ordering of the self-assembly, which allows VSFG to porbe water on nonflat interfaces. We also showed that the origin of the VSFG signal from the self-assembly is the result of combination between individual molecular chirality and highly coordinated ordering, which gives rise of anisotropy. Furthermore, we found heterogeneity among different domains, which can be attributed to variations in the local hydration level. Since the SDS@2β-CD system is a synthetic lattice self-assembly, such heterogeneity could also exist in other natural lattice self-assemblies such as virus and tubulin.

We further developed the first infrared pump, VSFG probe microscope and applied it to study the ultrafast dynamics in the self-assembly. We found that the primary and secondary OH of β-CD exhibit markedly different dynamics, suggesting distinct hydrogen bond environments, despite being separated by only a few angstroms. Another ultrafast dynamics is assigned to weakening and restoration of hydrogen bond between strongly bound water and secondary OH of β-CD, which exhibit spatial uniformity within self-assembled domains but heterogeneity between domain. The ultrafast nature and meso- and microscopic ordering of hydrogen bond dynamics could contribute to the flexibility and crystallinity of the material- two critically important factors for crystalline lattice self-assemblies, shedding light on engineering intermolecular interactions for self-assembled lattice materials.