1. Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Cientificas, Cantoblanco Madrid 28040, Spain, Research Institute for Solid State Physics and Optics, HAS, P.O. Box 49, H-1525 Budapest, Hungary, and Department of Organic Chemistry, Eötvös Loránd University, P.O. Box 32, H-1518 Budapest, Hungary