Synthesis and properties of a twistophane ion sensor: A new conjugated macrocyclic ligand for the spectroscopic detection of metal ions

The synthesis of a structurally new type of conjugated macrocyclic ligand (1) is reported that comprises a dehydroannulene framework incorporating two 2,2'-bipyridine units. Modeling studies showed the ligand to possess an unusual chirally twisted and relatively rigid architecture capable of binding metal ions in an enforced tetrahedral coordination geometry. The macrocycle was prepared in seven steps from (2-bromophenylethynyl)-trimethylsilane (2) and characterized by spectroscopic techniques. The pyridine H3 protons in the H-1 NMR spectrum of 1 showed a marked temperature dependencey that may be related to conformational opening and closing motions of the macrocyclic ring. Ligand 1 was found to spectroscopically detect the presence of Co2+, Ni2+, Cu2+, and Zn2+ and, in particular, to function as a multiple readout sensor, giving different sequences of signal output depending upon the type of metal ion analyte with which the system was addressed. Macrocycle 1 also gave a highly characteristic and specific visual output response in the presence of Zn2+ consisting of a bright turquoise fluorescence and in this respect may find applications in the sensing of this biologically important metal ion.