Photoresponsive crown ethers. Part 14. Photoregulated crown–metal complexation by competitive intramolecular tail(ammonium)-biting

Abstract

New photoresponsive crown ethers (1H⁺) having a crown ether ring and an ammoniumalkyl [H₃N⁺-(CH₂)_n, n= 4,6,10] group attached to the two sides of an azobenzene have been synthesized. These photoresponsive ‘tail-biting’ crown ethers have been designed so that intramolecular ‘biting’ of the ammonium group to the crown can only occur upon photoisomerisation to the cis-forms. In the thermal cis–trans isomerisation, the first-order rate constants for cis-(1H⁺) were smaller by 1.6–2.2-fold than those for (1)(the analogous free amines). Moreover, this rate increased with increasing added K⁺ concentration. This suggests that the ammonium tail is intramolecularly bound to the crown ether ring in cis-(1H⁺). This intramolecular ‘biting’ is further reflected in the relative affinities of (1H⁺) for alkali-metal cations. The affinities were markedly reduced by u.v,-light irradiation and in particular, cis-(1H⁺)(n= 6) and cis-(1H⁺)(n= 10) showed almost no metal-binding ability. This marked difference in the metal-binding ability was used to generate the light-controlled systems for the passive or active ion-transport of ions across a liquid membrane.