Photoresponsive crown ethers. Part 3. Photocontrol of ion extraction and ion transport by several photofunctional bis(crown ethers)

Three photoresponsive bis(crown ethers) with an azo-linkage (2)-(4) were synthesised for the purpose of controlling ion extraction and ion transport by an on-off light switch. The compound trans-(2), in which azobenzene is linked with two monoaza-15-crown-5 compounds via an amide linkage, extracted K ⁺ and Rb⁺ in 6.7% and 0.5%, respectively, whereas the extractive abilities (ex%) of photoisomerised cis-(2) were higher (16.7% and 8.0%, respectively). Similarly, the ex% of cis- (3) (methylene linkage between azobenzene and two monoaza-15-crown-5-5 compounds) was greater than that of trans-(3). The results suggest that in the photoisomerised cis-forms alkalimetal cations are extracted as intramolecular 1:2 cation-crown complexes. On the other hand, the ex% of compound (4), which has an unsymmetrical structure was scarcely affected by photoirradiation. The rate of ion transport across a liquid (o-dichlorobenzene) membrane with compounds (2) and (3) as ion carriers was accelerated by u.v. irradiation, but retarded by u.v. and visible irradiation. This implies that ion transport is accelerated by the photoconversion of the trans- to the cis-forms in the membrane phase. The difference in the ex% and the ion- transport ability is discussed in relation to the crown structure.