1. Laboratory for Photonics and Interfaces, Institute of Chemical Sciences and Engineering, School of Basic Sciences, Swiss Federal Institute of Technology, CH-1015 Lausanne, Switzerland, and Centers of Molecular Devices, Organic Chemistry, Royal Institute of Technology (KTH) Teknikringen 30, 100 44 Stockholm, Sweden, and Department of Physical and Analytical Chemistry, Uppsala University, Box 259, 75105 Uppsala, Sweden