Photoresponsive tweezers for alkali metal ions. Photochromic diarylethenes having two crown ether moieties

Alkali metal ion extraction was carried out using photochromic dithienylethene having two benzo-12-crown-4 ethers (1), benzo-15-crown-5 ethers (2), or benzo-18-crown-6 ethers moieties (3) and bis(benzothienyl)ethene having two benzo-18-crown-6 ether moieties (4). Two crown ether moieties of the open-ring form captured large metal ions cooperatively like tweezers, while the photogenerated closed-ring form released the ions. The reduced affinities for metal picrates were fully recovered by irradiation with visible light (>480 nm). The H-1 NMR spectrum measurement of the solutions (CDCl3/CD3CN: 5/1) containing both alkali metal picrates and 2 or 3 confirmed that the large alkali metal ions were captured by the two intramolecular crown ethers,