1. Contribution from the Molecular Photonics Laboratory, School of Natural Sciences, Bedson Building, University of Newcastle, Newcastle upon Tyne NE1 7RU, United Kingdom, and Laboratoire de Chimie Moléculaire, Ecole de Chimie, Polymères et Matériaux, CNRS, 25 rue Becquerel, 67087 Strasbourg Cedex 02, France