Crowned and Liquid-Crystalline Phthalocyanines as Gas-Sensor Materials

Semiconducting films of crown-ether substituted metal-free phthalocyanines are shown to have unique properties of fast response and reversal to NO2 at room temperature. Increasing the size of the crown ring or complexing the phthalocyanine with Cu(II) gives slower responses. Complexing the crown-ether rings with simple metal salts leads to very rapid reversible conductivity decreases in 1-5 ppm NO2 at room temperature. Discotic liquid-crystal phases of octaoctyl phthalocyanine also respond rapidly and reversibly to NO2. The origins of these effects are discussed with reference to reduced local polarizability, increased molecular separation and improved molecular assembly, and experiments exploring the influence of molecular design and crystal engineering on these sensing properties are described.