Light absorbing species

The most valuable bromine compound probably is silver bromide, which is the light-absorbing species on which most film photography is based (see Box 20.2, Chemistry and Technology, on page 895). Bromine compounds are also used extensively as fire retardants, particularly in carpets, rugs, and clothing... 

Elemental carbon is believed to be the major significant light-absorbing species in particles. In remote regions, absorption may only account for 5-10% of the total extinction, whereas in urban areas, its contribution is greater, up to 50%. 

Colorimetry. Colorimetric methods, that is, wet chemical methods in which reagents are added to generate a light-absorbing species whose absorbance can... 

The intensity of any light absorbed by a light-absorbing species (chro-mophores) follows Lambert-Beer s law ... 

The molar extinction coefficient varies with the nature of the absorbing compound, the solvent, and the wavelength, and also with pH if the light-absorbing species is in equilibrium with an ionization state that has different absorbance properties. 

Absorbance is so important because it is directly proportional to the concentration, c. of the light-absorbing species in the sample (Color Plate 14). 

The presence of sunlight on the surface of the Earth has played an absolutely essential role in the development of life. It should be remembered in this respect that the quality5 of this sunlight may have varied greatly in the course of time, since it is determined largely by the presence of light-absorbing species in the atmosphere. 

Because the cast films are relatively thin, the optical density of the light absorbing species can he low and can vary with time of exposure. Additionally, the depth penetration of the absorbed light can be inhomogeneous in some systems. However, thin films can be mounted directly in UV/visible or infrared spectrometers, and so the course of the photopolymerization (and the rate) can be monitored directly in some systems. The most common observation made is the disappearance of monomer (e.g., loss of double bond absorption in the IR) as a function of irradiation time. It must be emphasized that in most thin film compositions important industrially, the monomers used are multifunctional. The polymer which results is then highly cross-linked and simple kinetic arguments are usually not valid. 

Table 7. Photochemical transformations catalyzed by meatllotetrapyrroles as light absorbing species... 

The reasons for light sensitivity in these enz5unes are not always well imderstood, and several other examples will probably be discovered when the nature of the light-absorbing species is clarified. It therefore seems appropriate to provide a brief overview on the basic t5q)es of inorganic chromophores occurring in biological metal sites. 

Type IT. Either the acceptor radical anion or the donor radical cation is retransformed into its original state during subsequent reaction steps. If this molecule corresponds to the light-absorbing species, it serves as sensitizer for PET processes and can be used in catalytic amounts. 

It is useful to collect here a few more definitions normally used in the discussion of optical properties. One is the molar extinction coefficient Gm = g/c, where c is the concentration in moles/liter of the absorbing material which implies the assumption that the absorption of light is due to specific light absorbing species. Absorbance (A) and optical density (D) are the other two quantities. They are related as follows ... 

C represents an electronically excited solid species, for example, a semiconductor, and C. .Q, a solute Q that is adsorbed at the surface of C. If C is reconverted to its original oxidation state, either by oxidation or by reduction, C can be considered as a photocatalyst according to the following definition A photocatalyst, PC, is a light-absorbing species that enables the redox reaction but that remains unchanged after the overall process ... 

In photoredox reactions occurring at the surface of hydrous metal oxides, a surface compound can have different functions it can act merely as electron donor or acceptor without being involved as the chromophore, or it can act as electron donor or acceptor and as the light absorbing species, relevant for the heterogeneous photoredox reaction. 

According to Beer s law, the measured absorbance of a solution of a single light-absorbing species is directly proportional to its concentration. For a solution containing a mixture of absorbing species, the measured absorbance is then simply the linear combination of the absorbances of all species in that solution, each measured at the same wavelength. When the different species in the mixture have different spectra, we can do a multi-component analysis and extract the concentrations of the individual species. 

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