Photoresponsive Crown Ethers. 12. Photocontrol of Metal Ion Complexation with Thiacrown Ethers

Abstract

Thiacrown ethers with a photofunctional azolinkage were synthesized to control their binding ability by an on-off light switch: They are the N2S4 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 Hg2+ is N,N′-dibenzoylated N2S4 crown (4)≈photoirradiated 1>trans-1>photoirradiated 2>trans-2, (ii) neither trans-2 nor 4 shows significant affinity toward heavy metal ions such as Cu2+, Co2+, Ni2+, and Pb2+, (iii) trans-1 has a slight affinity (Ex% 5.9%) for Cu2+,whereas photoisomerized 1 shows the remarkably enhanced affinity (Ex% 93.9%) with Cu2+, and (iv) such light-induced improvement is not seen for 2. The relatively poor binding ability of 2 and 4 is rationalized in terms of the unfavorable inside-out conformation of the N2S4 crown ring. The remarkable affinity change in cis-1 is probably due to the cooperative contribution of the thiacrown and the azopyridine cap constructing a new ligand site. 1 was used as a carrier for Cu2+ transport through a liquid membrane. It was found that the rate is efficiently accelerated by UV-light irradiation.