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

Article abstract of DOI:10.1246/bcsj.57.2144

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) ca. 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 (Expercent 5.9percent) for Cu²⁺, whereas photoisomerized 1 shows the remarkably enhanced affinity (Expercent 93.9percent) 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.