Sensitizer, in photochemistry

McEvoy AJ, Gratzel M (1994) Sensitization in photochemistry and photovoltaics. Sol Energy Mater Sol Cells 32 221-227... 

McEvoy A. J. and Gratzel M. (1994), Sensitization in photochemistry and photo-voltaics . Solar Energy Mat. Solar Cells 32, 221-227. 

RuLs ) therefore became the prototype sensitizer in photochemistry. The subsequent development of sensitizers through to the panchromatic black dye that constitutes the current state of the art is dealt with in detail in a separate chapter. 

Ito has also extended this type of photochemistry to the electron-beam-induced catalytic acidolysis of acid-labile main chain acetal linkages in polyphthaldehyde. These polymers, like the poly(2-methylpentene-l-suIfone) (PMPS) sensitizer in NPR resist described earlier have ceiling temperatures on the order of -40 °C. As normally used, the polyaldehydes are end-capped by acylation or alkylation and are thus quite stable. The main chain bonds are very sensitive to acid-catalyzed cleavage which in turn allows the whole chain to revert to monomer in an unzipping sequence similar to that occuring in irradiated PMPS. Irradiation of polyphthaldehyde containing 10% of a suitable sensitizer such as triphenylsulfonium hexafluoroarsenate with either deep UV... 

Armesto, D., Ortiz, M.J., and Agarrabeitia, A.R. (2004) Novel di-Jt-methane rearrangements promoted by photoelectron transfer and triplet sensitization, in CRC Handbook of Organic Photochemistry and Photobiology, 2nd edn (eds W. M. Horspool and... 

Of significant interest are the Ru(bipy)3"+ and Cr(bipy)3 + complexes. The former, along with derivatives, can be used as sensitizers in photolytic systems such as the photolysis of water, and the Ru /Ru couple can be tuned by varying the nature of the bipy ligand. The Cr(bipy)3 + cation is one of the standard substances for probing excited state photochemistry see Photochemistry of Transition Metal Complexes)... 

As already mentioned above, spectral sensitization may also become indispensable when the light absorption properties of a potentially photoreactive compound do not permit direct excitation in the desired wavelength region. By application of sensitizers with adjusted excited state properties, it is, for example, possible to induce photochemical reactions of otherwise colorless compounds with visible light. Another important application in photochemistry is the sensitized population of excited state levels, which are not easily reached by direct absorption of light due to the limitations of quantum chemical selection rules. This phenomenon has been extensively exploited in mechanistic and synthetic organic photochemistry, where enhanced yields of triplet state population could be achieved in various dye-photosensitized processes (98). 

Sensitization by mercury is useful for performing the photoreaction on all those 1,4-dienes that lack chromophoric groups, e.g. alkyl and/or cycloalkyl substituted systems. These reactions appear to proceed by triplet energy transfer from the mercury to give vibrationally exited diene triplets. Thus, at variance with solution phase photoreactions, the exited states possess greater vibrational energy and substantially different products are possible. While such comparison is impossible for the parent system because no solution phase photochemistry has been reported, the di-ir-methane product (4) was not observed when irradiation of the 3,3-dimethyl-substituted diene (3) (equation 3) was carried out under acetone sensitization in solution. Hydrocarbon (4) was the major component under mercury-sensitized conditions in the vapor phase. ... 

Figure 3.64. Sketch of two possible reactions of a sensitizer in combination with a coinitiator relying on redox photochemistry with the possibility to oxidize the sensitizer (route A) or to reduce the sensitizer (route B).

As can be seen from the few examples cited above SET processes are now fairly common in organic photochemistry. One of the areas where considerable study has taken place is the process referred to as a photo-NOCAS. Within this framework Albini and coworkers have shown that the products formed from the reaction of 2,3-dimethylbut-2-ene with 1,4-dicyanobenzene are compounds (22)- 25). The reaction was brought about using phenanthrene as the initial light absorber. This technique leads to cleaner reactions than those where the 1,4-dicyanobenzene is irradiated directly. The solvent system used is methanol/ acetonitrile and products (24) and (25) are the result of solvent incorporation. A further example of photo-NOCAS chemistry has been reported by Arnold and coworkers.Typical of the examples studied is the reaction illustrated in Scheme 2. The cyclization of the dienes (26) was also examined. This specific example deals with the generation of radical cations from (/ )-(+)-a-terpineol and (/ )-(+)-limonene with 1,4-dicyanobenzene as the electron accepting sensitizer. In another detailed study on reactions of this type the factors that control the regiochemistry in photo-NOCAS processes have been assessed. ... 

Zimmerman and Hofacker have studied the photochemically induced SET reactivity of the 1,4-dienes (74). The sensitizers used were dicyanoanthracene and dicyanonaphthalene. The radical cations of the 1,4-dienes undergo regioselective cyclization to the cyclic radical cations (75) which ultimately afford the final products (76). The SET-induced photochemistry of other non-conjugated dienes such as geraniol (77) has been studied. The results demonstrate that with DCA as the sensitizer in methylene chloride a contact radical-ion pair is involved and this yields the cyclopentane derivatives (78) and (79) in the yields shown. The cyclization is the result of a five-centre cyclization. With the more powerful oxidant dicyanobenzene as the sensitizer and in acetonitrile as solvent, separated radical-ion pairs are involved and this leads to the formation of the bicyclic ethers (80) and (81). DCA-sensitized reactions of the dienes (82) and E,E-(S3) and the bicyclohexane (84) have been studied. At low conversion the irradiation of (84) under these conditions affords a mixture of the dienes (82) and , -(83) in ratios that are independent of temperature. 

However, in photochemistry both excited states, singlet and triplet, of the precursor can be reached, e g. by direct irradiation and by spontaneous intersystem crossing (ISC) or by sensitization. The multiplicity of the state that undergoes the cleavage determines that of the cation. In the case of electron-donating substituted aryl halides fragmentation occurs in most cases from the triplet and fonns the triplet cation (Scheme 11). "... 

Luminescence appears in mechanistic chemistry in two distinct contexts. The details of the emission process itself are important in photochemistry, which is considered in Section 8. In other applications, fluorescence is simply an assay for qualitative and quantitative analyses. Good selectivity and sensitivity have earned it a role in commercial apparatus. However, the sensitivity is compromised by the small windows of a high-pressure cell, which limit the solid angle available. Laser sources help, but they restrict luminescence methods to specialized facilities. It is important to understand the polarization artefacts which may be introduced by thick windows and to appreciate that they may change with pressure. Many optical elements, including monochromators, have transmission properties that depend on the polarization. 

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