1. Contribution from the Dipartimento di Chimica “Giacomo Ciamician”, Università di Bologna, via Selmi 2, I-40126, and Istituto ISOF-CNR, via Gobetti 101, I-40129, Bologna, Italy, and the California NanoSystems Institute and Department of Chemistry and Biochemistry, University of CaliforniaLos Angeles, 405 Hilgard Avenue, Los Angeles, California 90095-1569