Metallophthalocyanines - Gas sensors, resistors and field effect transistors

The conventional metallophthalocyanines (PcM, M=Cu, Zn, Ni ...) are insulators when undoped. However, they possess accessible pi and pi* orbitals which make at least partial oxidization or reduction of the macrocycle possible with gases such as NO2 or O-3. This yields more or less conductive molecular materials. Their resistance is, therefore, a measurement of the concentration of the oxidizing or reducing species present in the surrounding atmosphere (resistors). In the case of rare earth bisphthalocyanines (in particular the lutetium derivative), the intrinsic density of charge carriers is no longer negligible and the corresponding thin films or crystals are semiconductors. Gases such as NO2, at moderate concentrations and temperatures, can afford completely oxidized materials: the doping process which increases the conductivity is therefore followed by an increase in the resistance. The same type of studies have been carried out using metallophthalocyanine as the electroactive (semiconductive) part of a field effect transistor (FET). After a schematic description of the chemical phenomena arising in molecular FET, a detailed chronology of their discovery is given. The influence of gases on FET characteristics is then reported. (C) 1998 Elsevier Science S.A. All rights reserved.