1. Contribution from the Laboratory of Supramolecular Chemistry and Technology, University of Twente, P.O. Box 217, NL-7500 AE Enschede, The Netherlands, Laboratory of of Organic Chemistry, NSR Centre, University of Nijmegen, Toernooiveld, NL-6525 ED Nijmegen, The Netherlands, and MicroSpectroscopy Centre, Department of Biomolecular Sciences, Wageningen Agricultural University, P.O. Box 8128, NL-6700 ET Wageningen, The Netherlands