Sensitization electron transfer mechanism

Iron(II) ediylenediaminetetraacetic acid [15651 -72-6] Fe(EDTA) or A/,Ar-l,2-ethaiiediylbis[A[-(carboxymethyl)glyciQato]ferrate(2—), is a colorless, air-sensitive anion. It is a good reducing agent, having E° = —0.1171, and has been used as a probe of outer sphere electron-transfer mechanisms. It can be prepared by addition of an equivalent amount of the disodium salt, Na2H2EDTA, to a solution of iron(II) in hydrochloric acid. Diammonium [56174-59-5] and disodium [14729-89-6] salts of Fe(EDTA) 2— are known. 

According to the electron-transfer mechanism of spectral sensitization (92,93), the transfer of an electron from the excited sensitizer molecule to the silver haHde and the injection of photoelectrons into the conduction band ate the primary processes. Thus, the lowest vacant level of the sensitizer dye is situated higher than the bottom of the conduction band. The regeneration of the sensitizer is possible by reactions of the positive hole to form radical dications (94). If the highest filled level of the dye is situated below the top of the valence band, desensitization occurs because of hole production. 

The advantage of employing periodic perturbation of light intensity, e.g., using a chopper, and phase-sensitive detection are beyond a simple enhancement of the signal-to-noise ratio. For photoinduced electron-transfer mechanisms, as schematized in Fig. 11, the... 

Many carboxylic acids lose carbon dioxide on either direct or sensitized irradiation, and in some cases (4.10 the evidence points to the operation of an initial electron-transfer mechanism rather than primary a-deavage. Cleavage occurs readily with acyl halides, and this can [ead to overall decarbonylation (4.11). Aldehydes also cleave readily, since the (0=)C—H bond is more prone to homolysis than the (0= C-C bond. This offers a convenient method for replacing the aldehydic hydrogen by deuterium in aromatic aldehydes (4.12. and a similar initial reaction step accounts for the production of chain-Iengtheped amides when formamide is irradiated in the presence of a terminal alkene (4.13). 

From the data of refs. 64 and 65 it follows that, depending on the nature of the reagents, the metalloporphyrin-sensitized electron transfer from donor to acceptor particles by the tunneling mechanism in vitreous matrices can proceed via the formation of both cation radicals and anion radicals of me tal loporphyrins. 

Fig. 9. Proposed electron-transfer mechanism for the sensitization of YbIn luminescence by the excited state of tryptophan (Trp). Figures are energies of the states in eV. Redrawn from (deW. Florrocks et al., 1997).

Figure 2.21 Schematics illustrating photosensitized heterogeneous electron transfer mechanisms (a) a directly bound photosensitizer, which photoinjects directly to the electrode (b) a remotely bound bridge sensitizer, which undergoes a prior photoinduced electron transfer, followed by ground-state heterogeneous electron transfer to the electrode...

Dye sensitization of inorganic photoconductors was recently discussed by Tani126 who also accepted the electron transfer mechanism of sensitization. 

On this ground, they suggested for the two sensitizers the classical Foote proposal for the DCA system [84] and a chain electron-transfer mechanism, like the... 

The very high limiting quantum yield calculated for the DCA-sensitized photooxygenation of 54 ( = 23.8) [115], together with the cyclic voltammetric spectra (Fig. 2a, b) recorded under inert and/or oxygen atmosphere [146], strongly support the following chain electron-transfer mechanism (Scheme 9). 

The results of these studies emphasize that compounds which can react with cyanoaromatic sensitizers, both by electron-transfer mechanism and by singlet oxygen reactions, are likely to show a very complex mechanistic outcome not easy to predict in advance. Anyway, if no reasonable doubts exist about singlet oxygen involvement in the cyanoaromatic sensitized oxygenations, there are some problems regarding the mechanism of its production. 

Various compounds were shown to sensitize the photochemical decomposition of pyridinium salts. Photolysis of pyridinium salts in the presence of sensitizers such as anthracene, perylene and phenothiazine proceeds by an electron transfer from the excited state sensitizer to the pyridinium salt. Thus, a sensitizer radical cation and pyridinyl radical are formed as shown for the case of anthracene in Scheme 15. The latter rapidly decomposes to give pyridine and an ethoxy radical. Evidence for the proposed mechanism was obtained by observation of the absorption spectra of relevant radical cations upon laser flash photolysis of methylene chloride solutions containing sensitizers and pyridinium salt [64]. Moreover, estimates of the free energy change by the Rehm-Weller equation [65] give highly favorable values for anthracene, perylene, phenothiazine and thioxanthone sensitized systems, whilst benzophenone and acetophenone seemed not to be suitable sensitizers (Table 5). The failure of the polymerization experiments sensitized by benzophenone and acetophenone in the absence of a hydrogen donor is consistent with the proposed electron transfer mechanism. 

Pyridinyl radicals react with haloorganic compotinds at ratra that are very sensitive to the strength of the halogen-carbon bond. Both atom-transfer and electron-transfer mechanisms for the reaction have been detected, with the latter exhibiting a large response to solvent polarity change and to variations in the electron affinity of the organic halide. 

An electron transfer mechanism has been proposed to account for the formation of carbon dioxide on irradiation of alkyl pyruvates the yield of carbon dioxide is enhanced by the presence of electron acceptors such as methyl viologen. The dye-sensitized photo-oxidation of a-oxo-carboxylic acids and esters also leads to the production of carbon dioxide. An initial dye-substrate interaction rather than singlet oxygen appears to be responsible for this fragmentation. 

This substrate, along with the corresponding substrate 6, which has a cyclo-butanation rate of 1.2 x 10 , have been proposed as sensitive cation radical probes, for detecting the presence of cation radical intermediates in various reactions [74]. An interesting example is the reaction of tetracyanoethylene (TCNE) with electron rich alkenes, a reaction for which an electron transfer mechanism had been considered (Scheme 39) [75],... 

Sensitization and interfacial electron transfer mechanisms have been described by Gerischer [4-6]. A basic assumption is that electron transfer, like light absorption, occurs under the restriction of the Franck-Condon principle. The time-scale for interfacial electron transfer is much shorter than that for nuclear motion. This means that the energy terms for electron transfer are different from the thermodynamic formal reduction potentials described above. Gerischer considered the appropriate energy levels and derived a distribution of energy levels when the sensi-... 

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