Characterizing Ultrafast Equilibrium Dynamics Using Third-Order Nonlinear Mid-Infrared Spectroscopies
Characterizing equilibrium dynamics provides new insight into examining structure-function relationship in chemical reactions and understanding physical properties and processes in various systems. Third-order nonlinear mid-infrared spectroscopic techniques are powerful tools for investigating ultrafast dynamics under thermal equilibrium. In this thesis, vibrational energy relaxation and orientational relaxation dynamics were extracted from polarization-dependent pump probe spectroscopy and local environment dynamics surrounding a vibrational probe were detected using two-dimensional infrared spectroscopy.Effects of ligand structural variation on the ultrafast dynamics of a series of copper complexes were investigated. An azide group was attached to model copper complexes to serve as a vibrational probe. It was found that the peripheral phosphinimine ligands of the copper complex confined the orientational motion of azide and the degree of spatial restriction was quantified using the wobbling-in-a-cone model. The addition of phenyl groups in the ligands acted to create more accessible local environmental configurations for the azide group. The binding configuration and intramolecular motion of the vibrational probe can affect the dynamics detected. The dynamics detected by the azide group bound to the copper complexes were compared to those detected by an isothiocyanate group with a different binding angle. Molecular dynamics simulations showed that intramolecular reorientation of azide sampled a larger space leading to faster orientational relaxation compared to isothiocyanate. The local environment dynamics detected by azide were slower than those detected by isothiocyanate due to the change of local environment dynamics induced by the binding of different vibrational probes and the differences in the local environment dynamics detected along different spatial orientations. The ultrafast dynamics of liquid crystal systems were also investigated. The effects of gold nanoparticles and their capping ligands on the dynamics of 4-cyano-4′-pentylbiphenyl (5CB) during the isotropic-to-nematic phase transition were examined. Our preliminary results showed that the addition of impurities accelerated the dynamics of 5CB, and this acceleration effect was more significant for the ligands than the ligand-capped nanoparticles.