ROTATIONAL DYNAMICS OF EXCITED PROBES: THE ANALYSIS OF EXPERIMENTAL DATA

ABSTRACT

The time course of molecular rotational motion can be observed directly using partially oriented samples produced by photoselection and fast laser techniques. The magnitude of the observed absorption dichroism or fluorescence anisotropy is related to the relative directions of the excitation and observation transition moment vector as well as the degree of orientation. The duration of the observed effect depends on the shape of the probe and the solvent viscosity. It has been the accepted procedure to assume isotropic rotational dynamics of the probe when calculating rotational time constants. This assumption is too severe. It has been shown that the assumption of isotropic rotational dynamics leads to incorrectly determined rotational time constants using simulated data. Systematic errors in these constants render them inappropriate for evaluating rotational dynamics theories such as Debye-Stokes-Einstein theory or extensions thereof.

Keywords:

• Fluorescence anisotropy
• Time-resolved linear dichroism
• Hydrodynamics
• Debye-Stokes-Einstein diffusion theory

ACKNOWLEDGMENTS

Support for this work by the University of Maryland, Baltimore County (DRIF) and NSF (CHE-9985299) are gratefully acknowledged.