Photoresponsive crown ethers. Part 6. Ion transport mediated by photoinduced cis—trans interconversion of azobis(benzocrown ethers)

Abstract

In the light-driven membrane (o-dichlorobenzene) transport with azobis(benzo-15-crown-5)(Cr(5,5)), we have found that by alternate irradiation with u.v. light and visible light the rate of K⁺ is considerably accelerated while the rates of Na⁺ and Rb⁺ transport are accelerated only a little. This result indicates that efficient light-driven ion-transport is achieved with moderately stable complexes (e.g. K[cis-Cr(5,5)]) rather than with unstable (Na[cis-Cr(5,5)]) or very stable (Rb[cis-Cr(5,5)]) complexes. The rate ratio of K⁺versus Na⁺ independent transport in the dark ([M⁺] in the aqueous phase/[Cr(5,5)] in the membrane phase = 80) was only 2.7-fold, whereas that in competitive ion transport ([M⁺] in the aqueous phase/[Cr(5,5)] in the membrane phase = 1 000) was enhanced up to 27-fold. In competitive ion transport, the rates of Na⁺ and K⁺ transport were both suppressed by u.v. light irradiation and significantly enhanced under alternate irradiation with u.v, and visible light. Since the photoirradiation effect is similar to that of K⁺ transport, it is proposed that the rate of competitive ion transport is primarily governed by the affinity of K⁺ for the carrier molecule.