Interaction of electron-accepting vapours with phthalocyanine and related compounds: a novel calorimetric study

Heats of interaction of nitrogen dioxide with thin films of copper and lead phthalocyanine (CuPc and PbPc) and tetraazadibenzo[14]annulene (TADA) have been studied using a novel interferometric fibre-optic microcalorimeter. For CuPc the heat is consistent with weak chemisorption on a film composed of microcrystals of dimensions smaller than the film thickness of 700 Å. Comparison of simultaneous studies of the rates of heat evolution and electrical conductivity change for CuPc films at ambient temperature following exposure to NO 2 shows that the conductivity change is substantially slower than the initial adsorption process. PbPc and TADA films react with NO 2 (3 mbar) undergoing oxidative ring cleavage with partial nitration, with release of heats in excess of 800 kJ mol −1 in some cases, although these reactions do not always occur immediately following exposure to the gas. These results are discussed in the context of the use of such materials as semiconducting gas-sensing films.