Structural Ion Selectivity of Thia Crown Ether Compounds with a Bulky Block Subunit and Their Application as an Ion-Sensing Component for an Ion-Selective Electrode

Several 14-membered thia crown ether ionophores having a bulky block subunit were synthesized, and their chemical structures and ion selectivities were examined in detail when these compounds were used as an ion-sensing component of an ion-selective electrode. The ionophores of both cyclic and noncyclic thia ethers exhibited a high selectivity for silver ion (Ag⁺), in which the sulfur atom in the ionophore molecule plays a role as the effective coordination donor site for the silver ion. The best Ag⁺-selective electrode was prepared with the 14-membered thia crown ether having one sulfur atom, three oxygen atoms, and a bulky pinan subunit. The ion selectivity of this electrode for Ag⁺ was over 10⁴ times that for other metal cations. In the case where the sulfide in the thia ether ionophore was changed to sulfoxide by oxidation, ion selectivity for mercury ion became higher; therefore, the sulfoxide was found to be an effective coordination site for the mercury ion. The ion selectivity features of noncyclic sulfide, sulfoxide, and sulfone were also examined and compared with the results of the cyclic and noncyclic thia ethers.