1. Division of Frontier Materials Science, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531 Japan and Department of Chemistry, Division of Molecular and Nanomaterials, Laboratory of Photochemistry and Spectroscopy, and Institute of Nanoscale Physics and Chemistry, Katholieke Universiteit Leuven (KULeuven), Celestijnenlaan 200 F, B-3001 Leuven, Belgium