Photoresponsive crown ethers. Part 18. Photochemically ‘switched-on’ crown ethers containing an intra-annular azo substituent and their application to membrane transport

Abstract

Photoresponsive crown ethers (3) having an intra-annular azo substituent have been synthesized. These crown ethers have been designed so that the crown-metal interaction can only occur upon photoisomerisation of the intra-annular azo substituent from trans to cis. In thermal cis-to-trans isomerisation, the first-order rate constants (k) were suppressed to 14–66% by the addition of Na⁺ or K⁺, indicating that an additional free energy of activation (1.0–5.0 kJ mol^–1) is required to disrupt the favourable crown-metal interactions. From the plots of k vs. [M⁺] the association constants (K_c) for the cis-forms were estimated to be 10^4.07– 10^4.81 I mol^–1, which are comparable with the K_c for regular crown ethers. The spectral study established that the high metal affinity of the cis-forms is due to the co-ordination of one of the azo nitrogens to the metal ion. In two-phase extraction of alkali-metal cations the Ex% values for cis-(3) were always higher than those for trans-(3). In particular, the dramatic photoirradiation effect was observed for Na⁺: cis-(3) extracts Na⁺(8.1–14.1%), whereas trans-(3) extracts no Na⁺. This system was applied to ion transport across a liquid membrane. As expected, Na⁺ transport occurred only when the membrane phase was irradiated by u.v. light. The marked difference can be used to control the ion permeability by an on-off light switch.