1. Contribution from the Department of Chemistry, Physical Chemistry I, University of Dortmund, Otto-Hahn-Strasse 6, D-44227 Dortmund, Germany, Max-Planck-Institute of Molecular Physiology, Otto-Hahn-Strasse 11, D-44227 Dortmund, and Laboratory for Fluorescence Dynamics, University of Illinois, Urbana, Illinois 61801