Photoresponsive Crown Ethers. 5. Light-Driven Ion Transport by Crown Ethers with a Photoresponsive Anionic Cap

Three photoresponsive ionophores (Cr-o, Cr-m, and Cr-p) in which benzo-18-crown-6 and a phenol group are linked with a photoisomerizable azo linkage were synthesized. On the irradiation of UV light, the azo linkage isomerized to the cis forms (46–73%) in which the crown ring wears an anionic phenoxide cap. The cis forms isomerized back to the trans forms slowly in the dark and rapidly by the irradiation of visible light. In solvent extraction, the extractability of alkali and alkaline earth metal cations with Cr-o, Cr-m, and Cr-p, which have the cap of 2-hydroxy-5-nitrophenyl, 3-hydroxyphenyl, and 4-hydroxyphenyl, respectively, was in most cases enhanced by UV light irradiation. In particular, cis-formed Cr-o exhibited markedly improved extractabilities for Na⁺ and Ca²⁺. The result suggests that the enhanced extractability is due to the formation of an intramolecular sandwich-type complex like PhO⁻⋯M⁺⁽⁺⁾⋯crown. In ion transport across a liquid membrane (90 vol % o-dichlorobenzene + 10 vol % n-butyl alcohol) in a U-tube, the rates of Na⁺ and Ca²⁺ transport were enhanced by 4.0-and 276-fold, respectively, by adding 0.01 N HCl into the second OUT aqueous phase. The rates were further enhanced by irradiating the liquid membrane phase by UV light (1.7- and 2.8-fold, respectively), and the rate of Ca²⁺ transport was greater than that of Na⁺ transport in spite of the lower concentration in the first IN aqueous phase ([Na⁺] = 0.10 M, [Ca²⁺] = 0.0225 M). The finding implies that transport of the Ca²⁺ ion is efficiently mediated with an anionic, photoresponsive ionophore Cr-o under a countercurrent of proton flux and UV light irradiation. This is the first example of the light-driven Ca²⁺ transport across a liquid membrane.