Color change on deprotonation of 5-(8,8-dicyanoheptafulven-3-yl)-2-hydroxy-1,3-xylyl-18-crown-5

Abstract

Compound 5-(8,8-dicyanoheptafulven-3-yl)-2-hydroxy-1,3-xylyl-18-crown-5 is a novel non-benzenoid crown ether dye derived from 8,8-dicyanoheptafulvene together with a crown ether having a phenolic hydroxide group in its cavity. The color change on deprotonation has been UV/Vis-spectroscopically investigated with special attention to the metal-selective coloration, using a variety of alkali metal bases. Deprotonation brought about a significant color change from red to blue ( λ max≈430→640 nm), but quite irrespective of the cation species of base added. In addition, the coloration is found to be very similar to that of the uncrowned 8,8-dicyanoheptafulvene-substituted phenol. Interestingly, geometry optimization by MO calculations revealed that the crown ether ring is heavily deformed due to lone pair repulsion of the oxygen atoms in the cavity, making the hydroxide group directly exposed to the surrounding environment. Thus, the electronic state of the title compound is almost equivalent to that of the uncrowned compound. This explains why no cation-dependent coloration takes place and why the bathochromic displacement is mainly due to π-electron delocalization of the phenoxide (O −) into the heptafulvene chromophore.