1. Center for the Study of Excited States of Molecules, Huygens and Gorlaeus Laboratories, University of Leiden, 2300 RA Leiden, The Netherlands, Department of Biomolecular Structure and Dynamics, Faculty of Biology, Free University, 1081 HV Amsterdam, The Netherlands, and Department of Biophysics, Faculty of Exact Sciences, Free University, 1081 HV Amsterdam, The Netherlands