Photoresponsive Crown Ethers. 1. Cis-Trans Isomerism of Azobenzene as a Tool to Enforce Conformational Changes of Crown Ethers and Polymers

An azobenzene-bridged crown ether (2) and a polymer containing both crown ether unit and azobenzene unit (4) were synthesized. 2 bridged with trans-azobenzene (trans-1) bound an ammonium cation (5) and methyl orange salts of Li+ and Na+ preferably, whereas 2 bridged with photo- (UV) isomerized cA-azobenzene (cis-2) bound methyl orange salts of K+ and Rb+ preferably. The result was rationalized in terms of photoinduced expansion of the crown ether size. The binding ability of polymer 4 was also subject to photoirradiation. In particular, the crown ether units adjoining the cA-azobenzene unit lost the affinity toward Na+ ion. The conformational changes of the crown ether and the polymer were conveniently monitored by following the spontaneous regeneration of trans-azobenzene in the dark. It was established on the basis of the rate measurements that (i) the conformational fluctuation of the crown ether is suppressed by K+ and ammonium cations, (ii) the extension of the polymer chain in solution accompanies a large entropy increase, and (iii) ∆H* and ∆S* for the rates of regeneration of rrarw-azobenzene are subject to an isokinetic relationship with β = 328 K. These results indicate that photoinduced cis-trans isomerism of azobenzene, which has been found in former days, is useful as a new tool to enforce the conformational changes of crown ethers and polymers. The findings provide a possibility of controlling chemical functions of crown ethers and polymers by light.