Ultrafast spectroscopic studies of molecular interactions and vibrational energy relaxation dynamics in binary solvents and liposomes

Achieving a fundamental understanding of intermolecular interactions and energy transfer processes in fluid systems is key for studies on chemical reaction mechanisms, material properties and the biological dynamics. There is a well established body of work describing transient molecular organization and interactions between dissimilar molecules in neat liquids, the studies in multi-component liquid remain a challenge due to the transient and complex nature of solvent-solvent and solute-solvent interactions.;To gain insight into the solution phase heterogeneity and molecular scale organization, picosecond laser technologies and time resolved spectroscopic approaches have been applied. We selected the ethanol/cyclohexane and n-butanol/cyclohexane binary solvent mixtures as model systems. The polycyclic aromatic hydrocarbon (PAH) perylene has been chosen as the probe molecule to examine local organization in binary solvent systems. The perylene ring breathing mode is nearly degenerate with the ethanol and n-butanol terminal methyl group rocking modes. Steady-state spectroscopic data show that there is a discontinuous dependence of the spectroscopic origin on the binary solvent systems examined. From both orientational and vibrational energy relaxation dynamics measurements of perylene as a function of solution composition, we observed molecular scale heterogeneity in both binary solvent systems. For the ethanol/cyclohexane system, both rotational diffusion and vibrational population relaxation time constants show a clear discontinuity between 5% and 7.5% (v/v) ethanol, suggesting a discontinuous change in the organization of the chromophore local environment. For n-butanol/cyclohexane system, the rotational diffusion results show that perylene reorients as an oblate rotor in neat n-butanol and cyclohexane, but as a prolate rotor in all binary mixtures, and the vibrational population relaxation data show that perylene experiences an n-butanol dominated environment when the n-butanol concentration is 5% (v/v) or above. Taken collectively, both studies demonstrate a non-uniform distribution of alcohol in the binary mixtures, providing experimental evidence on the existence of composition-dependent nano meter scale local organization in these systems.;Another question we addressed in this dissertation using picosecond spectroscopy is the long-term stability of phospholipid vesicles in an aqueous environment, which is an important issue for studies that use phospholipid vesicles. Unilamellar vesicles containing 1,2-dimyristoyl-sn-phosphatidylcholine (DMPC), with and without cholesterol, formed by extrusion in aqueous buffer solution (pH 8) were shown to remain dimensionally stable for periods in excess of hundreds hours by dynamic light scattering (DLS) measurements. The rotational diffusion dynamics of perylene confined in the vesicle acyl chain region revealed structural evolution that was dependent on vesicle composition. We also found that the re-extrusion of the vesicles caused no change in the average diameter or size distribution, but did give rise to diminished organization in the lipid acyl chain region for these vesicles. These findings provide new insight from both macroscopic and microscopic perspectives on the structural stability of phospholipid vesicles.