UV band splitting of chromogenic azo-coupled calix[4]crown upon cation complexation

Calixcrown-6 compounds carrying a pair of phenylazo moieties on the upper rim and two OH groups, one OH group, and two OR groups on the lower rim have been prepared in both the cone and partial cone conformations. UV/vis spectral measurements showed a red shift upon the addition of Ca²⁺ to the calixcrown carrying two OH groups and a blue shift for the calixcrown carrying two OR groups. For the compounds with two OR groups on the lower rim and a fixed partial cone conformation, a blue shift caused by electrostatic interaction between the oxygen atoms of OR and the metal ion as well as a red shift caused by the π-metal complexation between the rotated calix benzene and the metal ion were observed.