Photoprocesses, sensitization

It is clear from these results that the relative sensitivity of the copolymers is unchanged in the very thin photoresist films. Quite high quantum yields can be obtained for photoprocesses which do not require large amounts of free volume. 

Regeneration of consumed (i.e., given off an electron to the electrode or, on the contrary, acquired an electron) photoactive substance (sensitizer) in the solution is a very important matter from the practical point of view. As soon as all the near-the-electrode (adsorbed) layer of this substance is oxidized (or reduced) the photoprocess ceases. To obtain a continuous photocurrent, the amount of the initial reactant, sensitizer, near the electrode surface should be renewed. 

In the presence of the dye the photocurrent at both semiconductors is observed in the visible region of the spectrum, i.e., in that region where crystal violet absorbs light. Thus, sensitization is observed. Photoprocesses that take place at zinc oxide and gallium phosphide can be represented by schemes of Figs. 23c and 23b, respectively. 

Quantum yield of the sensitized photocurrent is usually small, of the order of 0.01. Such a low efficiency can be explained by the fact that the layer of the adsorbed dye, taking part in the photoprocess, is very thin—of the order of the molecule size—so it absorbs only an insignificant fraction of the incident light. But this layer cannot be made thicker, because the organic substance (the dye) is an insulator. Thus, photosensitization by adsorbed dyes appears to have a relatively low efficiency for sensitizing wide-gap semiconductors to visible (solar) light. 

Photoprocessing operations involve the developing of images in a light-sensitive emulsion that has been coated onto paper, plastic film, or a glass base. This emulsion is generally composed of silver halide salts. 

A triplet route using benzophenone as sensitizer is also possible, but with reduced efficiency.42 The kinetic scheme for the photoprocess is shown in Figure 6.3 (see also Refs. 3-7). 

Silva, E., Riickert, V., Lissi, E., and Abuin, E. (1991) Effects of pH and ionic micelles on the riboflavin-sensitized photoprocess of tryptophan in aqueous solution, J. Photochem. Photobiol. B Biol., 11, 57-68. 

Figure 18.2.10 Dye sensitization of a photoprocess at a semiconductor electrode. For example, n-ZnO/rose bengal/I /Pt (75).

Topics which have formed the subjects of reviews this year include excited state chemistry within zeolites, photoredox reactions in organic synthesis, selectivity control in one-electron reduction, the photochemistry of fullerenes, photochemical P-450 oxygenation of cyclohexene with water sensitized by dihydroxy-coordinated (tetraphenylporphyrinato)antimony(V) hexafluorophosphate, bio-mimetic radical polycyclisations of isoprenoid polyalkenes initiated by photo-induced electron transfer, photoinduced electron transfer involving C o/CjoJ comparisons between the photoinduced electron transfer reactions of 50 and aromatic carbonyl compounds, recent advances in the chemistry of pyrrolidino-fullerenes, ° photoinduced electron transfer in donor-linked fullerenes," supra-molecular model systems,and within dendrimer architecture,photoinduced electron transfer reactions of homoquinones, amines, and azo compounds, photoinduced reactions of five-membered monoheterocyclic compounds of the indigo group, photochemical and polymerisation reactions in solid Qo, photo- and redox-active [2]rotaxanes and [2]catenanes, ° reactions of sulfides and sulfenic acid derivatives with 02( Ag), photoprocesses of sulfoxides and related compounds, semiconductor photocatalysts,chemical fixation and photoreduction of carbon dioxide by metal phthalocyanines, and multiporphyrins as photosynthetic models. 

The ready accessibility of only one coordination (via loss of CO) site about copper makes Cu[HB(pz)3]CO (Figure 2) potentially attractive as a specific sensitizer for photorearrangements of olefins. For example, endo-dicyclopentadiene in the presence of Cu(03SCF3) undergoes virtually exclusive photodimerization (Reaction 7, path i), presumably via prior formation of a 2 1 olefin Cu(I) complex (17). Since the analogous bis-olefin complex involving the Cu[HB(pz)3] moiety is sterically improbable, the most likely photoprocess is internal cyclization (Reaction 7, path ii). Preliminary studies in our laboratory indicate that Cu[HB-(pz)3]CO sensitizes the NBD-to-Q conversion (Reaction 3) with respectable quantum efficiency (21). 

Wavelength sensitivity refers to the efiScacy of various regions or wavelengths of the electromagnetic spectrum in causing a given photoreaction or a photoprocess. Data can be expressed either in relative or absolute terms. 

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