Photo-reversible and selective Cu2+ complexation of a spiropyran-carrying sulfobetainecopolymer in saline solution

Abstract

A sulfobetaine copolymer (1) carrying a photochromic spiropyran residue was synthesized, which reversibly isomerized between the closed and open forms in pure water and in saline by irradiation with visible light. The thermodynamic stability of the open form of the spiropyran methacrylate (SPMA) units of compound 1 was reduced upon increasing NaCl concentration. In addition, photo-reversible and selective Cu²⁺ complexation in saline solution ([NaCl] = 1.0 wt%) was achieved using compound 1 with metal ions (Cu²⁺, Zn²⁺, Ni²⁺, or Co²⁺). Covalently cross-linked compound 1 showed selective Cu²⁺ adsorption in pure water. In contrast, the other metal ions were increasingly adsorbed as NaCl concentration increased, resulting in lower selectivity of Cu²⁺ ion adsorption with 1, e.g., the ratios of adsorption of Cu²⁺, Zn²⁺, Ni²⁺, and Co²⁺ with 1 in 10 wt% saline were 73, 20, 10, and 3%, respectively, while only Cu²⁺ adsorption was observed in a solution of 1.0 wt% NaCl. Because the stability of the open form of the SPMA units and the metal complexation of 1 were influenced by NaCl concentration, the electrostatically cross-linked networks of the zwitterionic sulfobetaine units of 1 may be loosened by NaCl addition, resulting in easy entry of metal ions into the network. The order of metal complexation among the four metals corresponded to the Irving–Williams series. Hydrogen-bonded networks of water molecules also may contribute to the relatively ineffective selective adsorption of Cu²⁺ ions by 1 compared to the electrically neutral spiropyran-carrying copolymer of N-isopropylacrylamide, 2.