1. Contribution from the Laboratoire de Chimie Moléculaire, Centre National de la Recherche Scientifique (CNRS), École de Chimie, Polymères, Matériaux (ECPM), 25 rue Becquerel, 67087 Strasbourg Cedex 02, France, and Molecular Photonics Laboratory, School of Natural Sciences, Bedson Building, University of Newcastle, Newcastle upon Tyne, NE1 7RU, United Kingdom