Liquid-liquid extraction of alkali metal ions with photochromic crowned spirobenzopyrans

On the liquid-liquid extraction using 1,2-dichloroethane as an organic solvent, the crowned spirobenzopyrans exhibited textractability in the following order: Li+ much greater than Na+ > K+ greater than or equal to tetramethylammonium ion (TMA(+)), Li+ > Na+ much greater than K+ > TMA, and Na+ > K+ much greater than Li+ > TMA(+) for spirobenzopyran derivatives bearing monoaza-12-crown-4, 1; monoaza-15-crown-5, 2; and monoaza-18-crown-6, 3; respectively, under dark conditions. The ion selectivity of 1 depends on the metal-ion complexing ability of monoaza-12-crown-4. Even 2, which carries a 15-crown-5 moiety, showed Li+ selectivity because of the much stronger interaction of Li+ with the phenolate ion of the merocyanine form of 2 than that of Na+. The Na+ selectivity of 3 is also attributed to the ionic interaction with the phenolate ion of the merocyanine form, since the ionic interaction prefers Na+ to K+ regardless of the higher affinity of the 18-crown-6 ring itself to the latter ion. The Li+ extraction into the organic phase with 1 was enhanced by UV irradiation (300-400 nm), while some depression in the extraction was found by visible irradiation (>500 nm). The effect of visible irradiation on the Li+ complexing ability of I was also examined with electrospray ionization mass spectrometry.