Structural investigation of gas sensing Langmuir-Blodgett films of phthalocyanine ((C6H13)(3)SiOSiPcOGePcOH)

The crystal structure of the phthalocyanine ((C6H13)(3)SiOSiPcOGePcOH) (abbreviated as Si-Ge Pc dimer) Langmuir-Blodgett (LB) film was investigated using X-ray and electron diffraction to provide a structural basis for the observed conductivity change upon gas doping. The structure refinement was done using the linked-atom least-squares method. The structure determined for the Si-Ge Pc dimer is believed to result from the film deposition mechanism and is as follows: crystal system, monoclinic; space group, Pn; a = b = 14.28 Angstrom, c = 23.40 Angstrom, alpha = beta = 90 degrees, V = 4772 Angstrom(3), Z = 2, and the calculated density is 1.015 g cm(-3). The staggering angle between the two Pc rings is 5.5 degrees and the final R value is 9.23%. It is the trihexyl tail group in the molecule that breaks the symmetry of the system and gives rise to the low film density. The phthalocyanine LB films have been used successfully as gas sensors and the gas-sensing mechanism has been modeled as gas adsorption on the film surface and then diffusion through the film. It is the low density and open structure of the film that facilitate the gas diffusion through the film and the charge transfer interactions between the gas molecules and the phthalocyanine rings, which give rise to the high sensitivity and fast response of the gas sensor.