Controlled ionochromism with polythiophenes bearing crown ether side chains

Regioregular poly(3-alkoxy-4-methylthiophene)s bearing crown ethers of different sizes (12-crown-4), PT12C4; and (15-crown-5) PT15C5) have been synthesized and their thermochromic and ionochromic behaviors investigated. These polythiophene derivatives exhibit a highly conjugated form in the solid state at room temperature (absorption maximum around 550 nm) and a less conjugated form upon heating (absorption maximum around 425 nm). In acetone or ethyl acetate ionochromic responses have been observed upon addition of some alkali metal cations. Initially, the absorption maximum is located around 425 nm (yellow) but moves to 550 nm (violet). These results indicate the formation of planar (highly conjugated) polymeric assemblies upon the addition of alkali metal cations. PT12C4 is more sensitive to sodium salts while PT15C5 gives more intense ionochromic effects with potassium salts. These effects can be explained by a more stable 2:1 complexation between a given crown ether and an appropriate alkali metal cation.