1. Department of Chemistry, Faculty of Science, Okayama University, Tsushima‐naka 3‐1‐1, Okayama 700‐8530, Japan, Fax: +81‐86‐251‐7842
2. Department of Chemistry, Aichi University of Education, Kariya, Aichi 448‐8542, Japan
3. Department of Chemistry, Faculty of Science, Kumamoto University, Kurokami 2‐39‐1, Kumamoto 860‐8555, Japan