Photoresponsive Crown Ethers. 8. Azobenzenophane-Type “Switched-On” Crown Ethers Which Exhibit an All-or-Nothing Change in Ion-Binding Ability

Three new azobenzenophane-type crown ethers, cr(07), cr(O8), and cr(O9), in which 4,4’ positions of azobenzene are linked by a polyoxyethylene chain, were synthesized. The trans isomers were isomerized by UV light to the cis isomers, and the cis isomers were isomerized thermally or by visible light to the trans isomers, the interconversion being completely reversible. The solvent extraction showed that the trans isomers totally lack affinity toward metal ions, whereas the cis isomers are able to bind considerable amounts of alkali metal cations. The result is in accord with the prediction by the CPK model building that the polyoxyethylene chains of the trans isomers are linearly extended, whereas those of the cis isomers form the crown-like loops. The cis isomers showed spheric recognition patterns in the binding of alkali metal cations, typical of crown ethers in solution; the metal cations which provided the maximum extractability are Na+ for cis-cr(O7), K+ for cis-cr(O8), and Rb+ for cis-cr(09). The rate of the thermal cis-to-trans isomerization was significantly suppressed by added alkali metal cations, indicating that the additional energy is required to disrupt the crown-metal interaction. The order of inhibitory effect is well correlated with that of extractability. From plots of the rate constant against metal concentration, we estimated association constants (K) and rate constants for the crown-metal complexes (K). Conceivably, the increment in free energy of activation in the presence of metal cations reflects the magnitude of the “template effect” of metal ions. This is the first example for photoresponsive crown ethers which exhibit an “all-or-nothing” change in ion-binding ability.