Photoinduced processes sensitization

Given the availability of a suitable chromophore, the use of light offers the mildest possible thermal environment in which to effect a synthetic transformation insofar as photoinduced processes may be activated at temperatures well below 0 °C. Not unexpectedly then, one finds the photosynthetic procedure to be, thus far, the only one which has been successfully applied to the construction of the relatively sensitive parent (unrestricted) 77- excessive heteroannulenes as shown in Scheme 9a. Similarly, exposure of the tetracyclic azides depicted in Scheme 9b to low-pressure mercury irradiation leads to deazotation and the formation of the pyridine-like 14- and 18-membered parents (49) and (75). 

In terms of photophysics, electron transfer reactions create an additional non-radiative pathway, so reducing the observed emission lifetimes and quantum yields in A-L-B dyads in comparison with a model compound. However, there are other processes, such as molecular rearrangements, proton transfer and heavy-atom effects, which may decrease the radiative ability of a compound. One of the most important experimental methods for studying photoinduced processes is emission spectroscopy. Emission is relatively easy to detect and emission intensities and lifetimes are sensitive to competing processes. Studying parameters such as emission quantum yields and lifetimes for a given supramolecular species and associated... 

Studies in these fields [16] have shown that transition metal complexes may play the role of sensitizers both in the photoinduced processes schematized by Eq. 15 and in the light generating processes schematized by Eq. (16). In the first case, the complex can be called a light absorption sensitizer (LAS), while in the second case it can be called a light emission sensitizer (LES). High luminescence efficiency is a useful property for a LAS, and a necessary requirement for a LES. 

We present our recent research of excitation transfer and charge separation in conjugated polymers. We continue by summarizing our studies of ultrafast photoinduced processes in dye-sensitized nanocrystalline large band-gap semiconductor films - a key part of the GrStzel solar cell. Finally, our recent studies of energy transfer in transition metal supramolecular complexes, a kind of artificial antenna, are presented. 

Photoinduced electron transfer from eosin and ethyl eosin to Fe(CN)g in AOT/heptane-RMs was studied and the Hfe time of the redox products in reverse micellar system was found to increase by about 300-fold compared to conventional photosystem [335]. The authors have presented a kinetic model for overall photochemical process. Kang et al. [336] reported photoinduced electron transfer from (alkoxyphenyl) triphenylporphyrines to water pool in RMs. Sarkar et al. [337] demonstrated the intramolecular excited state proton transfer and dual luminescence behavior of 3-hydroxyflavone in RMs. In combination with chemiluminescence, RMs were employed to determine gold in aqueous solutions of industrial samples containing silver alloy [338, 339]. Xie et al. [340] studied the a-naphthyl acetic acid sensitized room temperature phosphorescence of biacetyl in AOT-RMs. The intensity of phosphorescence was observed to be about 13 times higher than that seen in aqueous SDS micelles. 

Radical anions are produced in a number of ways from suitable reducing agents. Common methods of generation of radical anions using LFP involve photoinduced electron transfer (PET) by irradiation of donor-acceptor charge transfer complexes (equation 28) or by photoexcitation of a sensitizer substrate (S) in the presence of a suitable donor/acceptor partner (equations 29 and 30). Both techniques result in the formation of a cation radical/radical anion pair. Often the difficulty of overlapping absorption spectra of the cation radical and radical anion hinders detection of the radical anion by optical methods. Another complication in these methods is the efficient back electron transfer in the geminate cation radical/radical anion pair initially formed on ET, which often results in low yields of the free ions. In addition, direct irradiation of a substrate of interest often results in efficient photochemical processes from the excited state (S ) that compete with PET. 

Figure 6 Photoinduced stepwise electron-transfer process in Ti02-sensitizer-donor and Ti02 acceptor-sensitizer heterotriads.

Acid-catalyzed photohydration of styrenes19 and additions to cyclohexenes20 leading exclusively to the Markovnikov products are also possible. Sensitized photoaddition, in contrast, results in products from anti-Markovnikov addition. The process is a photoinduced electron transfer21 taking place usually in polar solvents.22,23 Enantiodifferentiating addition in nonpolar solvents has been reported.24 The addition of MeOH could be carried out in a stereoselective manner to achieve solvent-dependent product distribution 25... 

Panchromatic sensitization of vinyl addition polymerization requires the presence of suitable dyes as primary absorbers of radiation. In 1949 Bamford and Dewar (5) reported that styrene could be polymerized by irradiation of a variety of dyes. It was later shown by Koizumi, Watanabe, and Kuroda (6) that these reactions, and others uncovered by the Japanese group, were the result of photoinduced decomposition of the dyes rather than a true sensitization process. 

Photoinduced electron transfer in the presence of a sensitizer (9,10-diphenylan-thracene) also generates the same anion radical. However, its disintegration proceeds within the solvent (acetonitrile) cage. Inside the cage, 4-nitrobenzyl radical and thiocyanate ion unite anew, but in this case by their soft-to-soft ends. This nucleophilic reaction takes place faster than the electron back-transfer occurs. The final, stable product of the whole process is 4- n i tro benzyl- iso- th iocy an ate (Wakamatsu et al. 2000) ... 

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