Photochromism electrical conductivity

For normal chemical systems, the characterization of mixtures of compounds is undesirable and generally unnecessary if means of separation of the components are available. However, photochromic systems inherently display properties of mixtures except when the system is completely converted to either of its forms. This causes measurements of heats of combustion, photoelectric effects, and electrical conductivity to be particularly difficult. A variety of such studies is presented in the following sections to illustrate the utility of these measurements. 

Photochromism is a reversible phototransformation of a chemical species between two forms having different absorption spectra [17-20], Photochromic compounds reversibly change not only the absorption spectra but also their geometric and electronic structures. The geometric and electronic structural changes induce some changes in physical properties, such as fluorescence, refractive index, polarizability and electric conductivity. When the photochromic compounds are used as spin couplers , the magnetic interaction can be controlled by photoirradiation. 

Similarly, Kawai et al. [30] use the concept of changes in electrical conductivity associated with a photochromic response. This polymer (see Fig. 6.8) was found to be soluble in common organic solvents. UV irradiation of a thin film of the polymer turned the film blue. For the measurement of photoinduced conductivity changes, an approximately lOOnm thick polymer film was deposited on an indium tin oxide electrode. A gold electrode about 20 nm thick was then evaporated on top of the polymer film. Before the gold deposition, the polymer film was irradiated with UV... 

A composite comprising paper and a conducting polymer can be prepared by impregnation of paper with a solution of a precursor polymer and heat treating the coated paper. The electrically conducting polymer is located on the surface or between the fibers of the paper [885]. Sexithiophene and other oligo(thiophene)s are ideal photochromic materials for efficient and ultrafast incoherent-to-coherent optical converters [886]. 

Tungsten oxide is an n-type semiconductor that has been used in electrochromic, photochromic, photocatalyst and gas-sensor applications. The inherent electrical conductivity of tungsten oxide stems... 

Examples of such properties are conductivity, refractive index, electrical moment, dielectric constant, chelate formation, ion dissociation, phase transitions, solubility, and viscosity. Certain physical changes that occur when the photochromic entity is chemically attached to the macromolecular backbone of polymers are of special interest (see Chapter 1, Volume 2). 

Films of (or films doped with) photochromic molecules as well as polymers containing photochromic moieties in the main chain have been shown to undergo changes in conductivity upon irradiation. Optical memories and electric circuitries have been built on this base [10]. 

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