1. a Organic Materials Research Group , Department of Chemistry , Vanderbilt University , Nashville, TN 37235
2. b Tohoku Pharmaceutical University , 4‐4‐1, Komatsushima, Aoba‐ku, Sendai 981‐8558, Japan
3. c X‐ray Crystallographic Laboratory , Department of Chemistry , University of Minnesota , Twin Cities, MN 55455