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 inves tigated using X-ray and electron diffraction to provide a structural b asis for the observed conductivity change upon gas doping. The structu re refinement was done using the linked-atom least-squares method. The structure determined for the Si-Ge Pc dimer is believed to result fro m the film deposition mechanism and is as follows: crystal system, mon oclinic; space group, Pn; a = b = 14.28 Angstrom, c = 23.40 Angstrom, alpha = beta = 90 degrees, V = 4772 Angstrom(3), Z = 2, and the calcul ated density is 1.015 g cm(-3). The staggering angle between the two P c rings is 5.5 degrees and the final R value is 9.23%. It is the trihe xyl tail group in the molecule that breaks the symmetry of the system and gives rise to the low film density. The phthalocyanine LB films ha ve been used successfully as gas sensors and the gas-sensing mechanism has been modeled as gas adsorption on the film surface and then diffu sion through the film. It is the low density and open structure of the film that facilitate the gas diffusion through the film and the charg e transfer interactions between the gas molecules and the phthalocyani ne rings, which give rise to the high sensitivity and fast response of the gas sensor.