Nathan P. Wells

Excited State Hydrogen Bond Dynamics: Coumarin 102 in Acetonitrile−Water Binary Mixtures

The time dependent change in the intermolecular response of solvent molecules following photoexcitation of Coumarin 102 (C102) has been measured in acetonitrile-water binary mixtures. Experiments were performed on mixtures of composition x(CH3CN) = 0.25, 0.50, 0.75, and 1.00. At low water concentrations (x(H2O) < or = 0.25) the solvent response is consistent with previous measurements probing dipolar solvation. With increasing water concentration (x(H2O) > or = 0.50) an additional response is found subsequent to dipolar solvation, exhibited as a rapid gain in the solvents polarizability on a approximately 250 fs time scale. Monte Carlo simulations of the C102:binary mixture system were performed to quantify the number of hydrogen-bonding interactions between C102 and water. These simulations indicate that the probability of the C102 solute being hydrogen bound with two water molecules, both as donors at the carbonyl site, increases in a correlated fashion with the amplitude of the additional response in the measurements. We conclude that excitation of C102 simultaneously weakens and strengthens hydrogen bonding in complexes with two inequivalently bound waters.

Exciton relaxation and energy transfer dynamics in size selected polythiophenes

Using fluorescence upconversion and two-color photon echo spectroscopy we have investigated the initial relaxation and subsequent energy transfer dynamics in a series of size-selected polythiophenes with and without fullerene termination.

3D Tracking of Antibody-Receptor Dynamics on RBL Cells.

We have previously demonstrated the ability to track single quantum dots freely diffusing in three dimensions using a custom confocal microscope. The microscope is capable of both following the trajectory of a single (fluorescent) molecule and recording the time resolved (fluorescent) photon stream. With both positional and lifetime information, the microscope can provide fluorescent lifetime trajectories (FLT) analogous to fluorescent lifetime imaging (FLIM). Using this microscope we have recently begun to study the signal cascade process in the IgE-FceRI antibody-receptor system in live RBL cells. We have demonstrated the ability to track fluorescently labeled IgE docked to the FceRI receptors and observe 3D motion on the membrane. We are currently working to observe and track the endocytosis of the antibody receptor complex on a single molecule basis.