1. Contribution from the Laboratory of Supramolecular Chemistry and Technology and Applied Optics Group (both MESA Research Institute) and Laboratory of Chemical Physics, University of Twente, P.O. Box 217, NL-7500 AE Enschede, The Netherlands, and Laboratory of Chemical and Biological Dynamics, University of Leuven, Celestijnenlaan 200 D, B-3001 Leuven-Heverlee, Belgium