“…In this respect, macrocyclic azo compounds, discriminating ions according to their size, serve as a group of molecules of interesting sensing properties and applications [ 21 , 22 ]. Due to presence of an azo moiety, macrocycle-metal cation interaction can be followed by the change of absorption spectra in the visible range of electromagnetic radiation [ 23 , 24 , 25 , 26 , 27 , 28 ]. Until now, a series of chromogenic crown ethers with two azo groups and phenol [ 23 ], pyrrole [ 24 , 25 , 26 , 27 ] or imidazole [ 28 ] residue as a part of macrocycle were synthesized and studied in our group as metal cation complexation agents in solution and as ionophores in membrane ion selective electrodes.…”