That is, the red shift indicates that the energy gap of ICT band decreases, upon binding Cu2þ ion to the electron withdrawing moieties. Likewise, we as- sume that the red shift of 1eCu2þ complex was induced by binding of Cu2þ ion to the electron withdrawing groups such as the C]N group. Therefore, the change of ICT band might be responsible for the dramatic color change from yellow to purple. These results indicate that receptor 1 could serve as a selective chromogenic sensor for Cu2þ ion.