1. Department of Physics, Blackett Laboratory, Imperial College London, Prince Consort Road, London SW7 2BW, United Kingdom, Department of Chemistry, Imperial College London, Exhibition Road, London SW7 2AZ, United Kingdom, Merck Chemicals, Chilworth Science Park, Southampton, United Kingdom, and Organic Nanoelectronics Laboratory, Department of Chemical Engineering, Kyungpook National University, Daegu 702-701, South Korea