Thiacrown ethers with a photofunctional azolinkage were synthesized to control their binding ability by an on-off light switch: They are the N 2 S 4 crowns capped with 2, 2′- azopyridine (1) or azobenzene (2). The cis-trans isomerism occurred reversibly by photoirradiation. The binding ability, evaluated by solvent extraction, showed that (i) the order of the affinity for Hg 2+ is N, N′-dibenzoylated N 2 S 4 crown (4)≈ photoirradiated ...