Phthalocyanine conductivity mechanism

Chemical and biological sensors (qv) are important appHcations of LB films. In field-effect devices, the tunneling current is a function of the dielectric constant of the organic film (85—90). For example, NO2, an electron acceptor, has been detected by a phthalocyanine (or a porphyrin) LB film. The mechanism of the reaction is a partial oxidation that introduces charge carriers into the film, thus changing its band gap and as a result, its dc-conductivity. Field-effect devices are very sensitive, but not selective. 

Several metallophthalocyanines have been reported to be active toward the electroreduction of C02 in aqueous electrolyte especially when immobilized on an electrode surface.125-127 CoPc and, to a lesser extent, NiPc appear to be the most active phthalocyanine complexes in this respect. Several techniques have been used for their immobilization.128,129 In a typical experiment, controlled potential electrolysis conducted with such modified electrodes at —1.0 vs. SCE (pH 5) leads to CO as the major reduction product (rj = 60%) besides H2, although another study indicates that HCOO is mainly obtained.129 It has been more recently shown that the reduction selectivity is improved when the CoPc is incorporated in a polyvinyl pyridine membrane (ratio of CO to H2 around 6 at pH 5). This was ascribed to the nature of the membrane which is coordinative and weakly basic. The microenvironment around CoPc provided by partially protonated pyridine species was suggested to be important.130,131 The mechanism of C02 reduction on CoPc is thought to involve the initial formation of a hydride derivative followed by its reduction associated with the insertion of C02.128... 

A detailed examination of the mechanisms of reaction of these vapors with the various types of phthalocyanines is beyond the scope of this work. The reader is directed to the papers by van Ewyk, Chadwick and Wright (14) or Langton and Day (11) for further information in this area. Experiments described here were conducted primarily to survey a series of materials under identical conditions in order to get a relative ranking of candidate materials for practical gas sensors, and to aid decisions about which monolayerforming derivatives to synthesize. 

Compact chemical sensors can be broadly classified as being based on electronic or optical readout mechanisms [28]. The electronic sensor types would include resistive, capacitive, surface acoustic wave (SAW), electrochemical, and mass (e.g., quartz crystal microbalance (QCM) and microelectromechanical systems (MEMSs)). Chemical specificity of most sensors relies critically on the materials designed either as part of the sensor readout itself (e.g., semiconducting metal oxides, nanoparticle films, or polymers in resistive sensors) or on a chemically sensitive coating (e.g., polymers used in MEMS, QCM, and SAW sensors). This review will focus on the mechanism of sensing in conductivity based chemical sensors that contain a semiconducting thin film of a phthalocyanine or metal phthalocyanine sensing layer. 

Space precludes a detailed discussion of the various mechanisms proposed for the conduction of charge through a phthalocyanine lattice. However, some of the more salient points will be mentioned. The conductivity is undoubtedly associated with the mobile w electrons of the phthalocyanine ring (86). Eley proposed (84, 85) that the conductivity arose through thermal or optical excitation of ir electrons from the highest filled to the lowest empty ir orbital. Using the Free Electron Gas approach developed by Kuhn (Section V,B), a value of Eg, the energy difference between the... 

Ambipolar behavior has also been observed in BBL and poly(thiophene-3-propionic acid, anunoninm salt).[297] 51a showed n-channel mobihties of 0.04-0.06 cm V s and 0.02-0.03 cm V s for p-channel operation. 51c showed valnes of 0.5-0.7 cm V s for n channel and 1.2-1.7 cm V s for p-channel with on/off ratios between 2 and 50 for devices operated in air. A similar mechanism involving ion-modulated electrochemical conduction, as described above for water soluble phthalocyanines was proposed by the authors. 

The mechanism of film growth of 56 was discussed in [240,250]. After formation of the first few layers of polymeric phthalocyanines, copper atoms diffuse from the copper film to the growing polymer film surface in order to react with 1,2,4,5-tetracyanobenzene at first to octacyanophthalocyanine and then to oligomeric and polymeric phthalocyanines. By ESCA spectra 0.7% of free Cu in the polymeric films were found. In dependence of the deposited Cu-film thicknesses of 1.5 till 20 nm adhering films of the polymers 56 with thicknesses of 46 till 230 nm were obtained For the ratio of the thickness of the polymer film to the copper film in every case an average value of --25 was determined. The films exhibit good electrical conductivities. 

The room temperature conductivities of polycrystalline samples of [(PcMO)lv] (M = Si, Ge) for various stoichiometries are given in Table 15.1. The nature of the dopant (iodine, bromine, quinones, etc.) has no significant effect on the conductivities [125,128-130], In addition to the main charge transport mechanism via the phthalocyanine system, other mechanisms, e.g, percolation theory and fluctuation-induced carrier tunneling through potential barriers separating metal-like regions, have also been discussed [131]. 

Tollin, G., Keams, D.R., Calvin, M. Electrical properties of (wganic solids. I. Kinetics and mechanism of conductivity of metalo-free phthalocyanine. J. Chem. Phys. 32(4), 1013-1019 (1960)... 

Another interesting composite is that obtained by cospinning of metal phthalocyanines (MPc) or metallo-Pc polymers, e.g, [Si(Pc)0] , with the polyaramid Kevlar. This process produces fibres with conductivities of up to IScm" with good long-term stability and excellent mechanical properties. 

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