Photoresponsive Crown Ethers. 2. Photocontrol of Ion Extraction and Ion Transport by a Bis(crown ether) with a Butterfly-like Motion

A photoresponsive bis(crown ether) with an azo linkage (1) was synthesized, trans-1 was isomerized by UV light to cis-1 and cis-1 was isomerized thermally to trans-1, the interconversion being reversible such as the motion of a butterfly. We found that (i) the concentration of cis-1 under the photostationary state is markedly enhanced by added Rb ⁺ and Cs ⁺ , (ii) the rate of the thermal isomerization (cis ↔ trans) is suppressed by added alkali metal cations, the order of the inhibitory effect being Rb ⁺ > Cs ⁺ > K ⁺ > Na ⁺ , and (iii) Na ⁺ is extracted efficiently from an aqueous phase to an organic (odichlorobenzene) phase by trans-1, whereas K ⁺ , Rb ⁺ , and Cs ⁺ are extracted efficiently by cis-1, and the selectivity between Na ⁺ and K ⁺ is 238-fold. These results consistently indicate that cis-1 forms a stable sandwich-type 1:1 cation/1 complex with large alkali metal cations. The K ⁺ transport across a liquid (odichlorobenzene) membrane in a U-tube established that the transport rate is suppressed by light when a hydrophobic counteranion is used, whereas it is accelerated by light when a relatively hydrophilic counteranion is used. Most interestingly, even the transport velocity of potassium picrate (most hydrophobic anion used) was accelerated when the liquid membrane in contact with the IN aqueous phase was partially photoirradiated. This implies that ion transport can be facilitated by the interconversion between cis-l and trans-1 occurring in the membrane phase. We thus consider that 1 acts as a useful “phototweezers” in solvent extraction and ion-transport systems.