1. Contribution from the Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138, Dipartimento di Chimica, Centro di Fotochimica CNR, Universita' di Ferrara, 44100 Ferrara, Italy, Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Avenue SE, Minneapolis, Minnesota 55455, and Department of Chemistry, Northwestern University, Evanston, Illinois 60208