1. Department of Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium, Department of Chemistry, University of Antwerp, B-3610 Wilrijk, Belgium, and Agfa-Gevaert N.V., RDM/TI 3342, Septestraat 27, B-2640 Mortsel, Belgium