Photosensitized processes

In addition, the results indicated that the efficiency of cis —> trans increased as the initial cis double bond configuration is shifted from the center of the polyenic chain, consistent with the 7j, triplet excited state potential curve that has a very shallow minimum at the 15-cis position compared to the deep minima at the all-trans position. The results strongly suggest that isomerization takes place via the 7j state of the carotenoid even in the case of direct photoexcitation, with their intersystem crossing quantum yield, isc ([Pg.246]

A rule that affects energy transfers in photochemical reactions, particularly photosensitization processes. The total electron spin (/.c., the vectorial overall spin angular momentum of the system) does not change after the electronic energy transfer between an excited molecular entity and another molecular entity. 

Intrinsically carbazole containing polymers are photosensitive in the UV range of spectra. The applications of such polymers in electrophotography and related processes need sensitization to the visual wavelengths. The most acceptable method is charge transfer formation between polymer donor and acceptor molecules. Hoegl pointed out that 0.1-2% of acceptor molecules inserted in the polymer matrix lead to a substantial increase in the photoconductivity especially in the CT bands. Subsequently, a lot of paper were published for CT-carbazole-containing complexes and such materials were used in photosensitive processes. Various types of molecules were used as a photosensitizers. 

Photooxygenation may take place eii.ter as (a) a direct process, where light is absorbed by A designated as the substrate or as (b) an indirect process or photosensitized process where light is absorbed by a molecule other thin that which reacts and is called a sensitizer. The reaction is said to occur from the triplet state of the excited molecule. 

If an appropriate electron acceptor is available, the photoexcited porphyrin may transfer an electron to it. The acceptors may be organic molecules such as JV-alkylnicotinamide, metal complexes such as [Fe(CN)6]3- or another porphyrin molecule.131 The second order reaction rate for the Zn(uroporphyrin)-acceptor system at infinite ionic strength is estimated to be 108M 1s l. The electron travels from Zn(Por) to V-benzylnicotiamide or to [Fe(CN)6]3 over a distance of 15 A or 30 A respectively. Since the reaction creates charged species, polar media accelerate the process. The back reaction or other photosensitization processes such as 02 generation become favourable in nonpolar solvents. 

Thus, for irradiation in accelerators, the effects produced in a medium always have the photoradiation character, which should be taken into account for photosensitive systems. Photosensitive processes are much more efficient in accelerators than in gamma installations. 

Photoredox processes include both photoreduction and photo-oxidation of the excited species. An electron transfer that results from an electronic state produced by the resonant interaction of electromagnetic radiation with matter is called photoinduced electron transfer (PET) [30-32]. This can be done by either a direct or a photosensitized process (see below and Figure 6.6). 

Figure 6.9 Photoinduced electron transfers via (a) direct or (b) photosensitized processes. R, reactant Sens, sensitizer D and sD, donor and sacrificial donor, respectively A and sA, acceptor and sacrificial acceptor, respectively. Reductive PET (photoinduced electron transfer) schemes are drawn in black, whereas oxidative schemes are in red...

The most likely singlet oxygen is formed according to the first mechanism, ie in the photosensitization process [70],... 

Photocatalysis and photosensitization processes are common in natural waters and are of special importance because they can stimulate a transformation of molecules that resist direct photolysis, such as transparent species or chromophores, the reactive states of which are inefficiently populated by sunlight absorption. 

Environmental photosensitization processes may be stimulated not only by natural but also by anthropogenic pollutants, such as phenols and dihydroxyben-zenes, which are efficient photosensitizers. Among others, phenol was found to sensitize the dehalogenation of halogenophenols [99], This behaviour shows that the simultaneous presence of adequate sensitizer and quencher in polluted environment can lead to the photosensitized decomposition of the latter and thereby to abatement of the pollution. 

Jori G, Schindl L, Schindl A, Polo L. Novel approaches towards a detailed control of the mechanism and efficiency of photosensitized processes in vivo. J Photochem Photobiol A Chem 1996 102 101-7. 

QUANTUM YIELD OF A SENSITIZED PROCESS (<(>) the number of molecules which undergo a given photosensitized process per photon absorbed by the sensitizer. 

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). 

Despite the obvious versatility of light-activated key steps and their numerous advantages for the biomimetic modeling of natural systems, up to now, only very few examples are known, where such types of photosensitized processes have been successfully combined to complete reaction cycles with reasonable catalytic turnovers 6). In the last section, we are therefore briefly presenting two case studies which describe some recent work performed in our own group focusing on bioinspired catalytic systems that can be controlled and driven by visible light. 

Moreover, photosensitization processes are observed in which a reaction of a nonabsorbing substrate is induced by energy transfer or electron transfer from an excited light-absorbing photosensitizer (Sens) ... 

A perfect photosensitizer will not be transformed in the photosensitizing process, although in reality, some degradation of the photosensitizer will occur in time. Nonetheless, the rate of the photosensitized reaction will depend directly on the sensitizer concentration at low values where not all of the relevant incident radiation is being absorbed. Most studies of this type of photoreaction use a high concentration of the acceptor A, in which case the transformation of A will follow apparent zero-order kinetics. 

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