PICOSECOND STUDIES OF SOLVENT-SOLUTE INTERACTIONS

Two types of solvent-solute interactions were studied with emphasis on measuring their dynamics. Using the solute ANF (2-amino, 7-nitroflourene), the relaxation process caused by the interaction of the solute dipole and the solvent dielectric was investigated. Studies of the solute acridine were performed to demonstrate how hydrogen bond formation between solvent and solute can cause inversion of solute energy levels. Both types of processes were monitored by time resolving solute fluorescence in a variety of solvents and solvent mixtures. A new method for picosecond time resolution of flourescence was developed, called frequency conversion gating, which surpasses the commonly used Kerr cell in sensitivity and versatility. To study ANF, time resolved measurements were made of the temperature dependent Stokes shift of the fluorescence spectrum. Fluorescence time profiles were measured at both the high and low frequency edges of the spectrum at several temperatures and found to vary by more than an order of magnitude. A model was proposed that relates the measured decay times at high frequencies and the corresponding risetimes at low frequencies to the rate of solvent dielectric relaxation. Using a binary solvent, the measured decay times reflected not only the relaxation of the polar solvent molecules in the vicinity of the ANF, but also the diffusion of the polar solvent component to the solute. Flourescence decay times and radiative lifetimes were measured for acridine in a variety of hydrogen bonding and non-hydrogen bonding solvents. The temperature dependence of the quantum yields and decay times were also measured. A new method was described for measuring radiative lifetimes based on absolute flourescence intensity. For the hydrogen bonding solvents, the radiative lifetimes were constant while the florescence decay times varied with solvent hydrogen bonding strength and temperature. The radiative lifetimes of the non-hydrogen bonding solvents were seen to vary with solvent polarizability and polarity, while the decay times were temperature independent and varied little with solvent. This data is discussed in terms of a model where hydrogen bond formation between solvent and solute causes the singlet energy levels to invert