Synthesis of an optically switchable salicylaldimine substituted naphthopyran for selective and reversible Cu²⁺ recognition in aqueous solution

A light controlled reversible switch for copper ion was synthesized by substituting photochromic naphthopyran with a salicylaldimine moiety. The naphthopyran based photoreversible receptor was characterized using IR, NMR, HRMS and single crystal X-ray crystallographic techniques. The photochromic properties of the receptor under light irradiation were investigated by UV-visible spectroscopy. The affinity towards transition metal ions in both closed and open forms were determined. The open form of the receptor displayed an increased affinity towards copper ions. A 3.25 × 10⁴ fold difference in binding affinity for copper ions between the closed and open forms in aqueous methanol solution suggested that the receptor can act as a selective photoswitch for copper ions. Theoretical investigations at the molecular level supported the experimental observations.