Oxymethylcrowned chromene: photoswitchable stoichiometry of metal ion complex and ion-responsive photochromism

Abstract

Chromene derivatives bearing oxymethyl-12-crown-4 ( 1 ), -15-crown-5 ( 2 ), -18-crown-6 ( 3 ) ether moieties, and non-cyclic analogue ( 4 ) were synthesized, and their metal ion binding properties and photochromism were examined. NMR titration with alkali metal ions revealed that 1 formed a 1:2 complex (metal ion: ligand) with Na +, while Li + afforded a 1:1 complex of 1 . In cases of K + and Rb +, the complexes were a mixture of 1:1 and 1:2 complexes, but the formation of 1:1 complex was observed again with Cs +. Under UV irradiation, however, the complex stoichiometry of 1 with all alkali metal ions was 1:1. As a comparison of NMR spectra between the Li + and Na + complexes of 1 indicated considerable upfield shift for the chromene moiety of the Na + complex, π–π stacking of the chromene moiety seems to induce formation of the 1:2 complex. These results indicate that the chromene moiety is not only to show photochromism but also to induce aggregation to form the 1:2 complex resulted in switching of the complex stoichiometry by UV irradiation. The formation of 1:2 complex appeared only with 1 because flexibility of the crown moieties for 2 and 3 interfered the formation of 1:2 complex. Studies on photochromism in the presence of a metal ion demonstrated that the chromene derivatives bearing crown ether moieties show ion-responsive photochromism depending on the metal ion binding ability of their crown ether moieties.