Photoexcitation electron transfer

Similarly, the cathodic photoexcited electron transfer of the h3 rogen reaction shown in Eqn. 10-19 can occur at p-type semiconductor electrodes at which the cathodic hydrogen reaction is thermodynamically impossible in the dark. 

Alkyl Nicotinamides. Benzyl nicotinamide can be used as a photochemical model for biological reductions. Upon photoexcitation, electron transfer causes reduction of a variety of acceptors. The radical anionic intermediates thus formed can fragment or participate in further chemical reduction. Several examples of observable chemistry initiated in this way include eqs. 69 (208), 70 (209), and 71 (210) ... 

Scheme 14. Electron transfer in the ground state of a biselectrophoric monoradical A A bridged by spacer L and photoexcited electron transfer in donar-acceptor bridged molecules A-L-D...

Some of the photoreceptor proteins experience a photoexcited electron transfer reaction at the chromophore site, while others change their molecular conformation as a consequence of the photoexcitation (Figure 6.2). These reactions can be described using the well-defined language of physics and chemistry. 

Functionalization of nanoparticles with porphyrins and analogous macrocycles opened the way to important applications, such as, to name only a few gelators [8], phototherapy of cancer [9], degradation of organic pollutants by singlet oxygen photosensitization [10], and photoexcited electron transfer [11]. The interest in porphyrin-based nanoreactors lies in the possibility of using a variety of metals that can enter in catalytic processes [12]. 

Dye-Sensitized Eiectrode, Photoanode, Fig.1 Photoexcited electron transfer scheme for dye sensitization of n-type semiconductor electrode... 

Ashkenazi, G., Kosloff, R., Ratner, M.A. Photoexcited electron transfer Short-time dynamics and turnover control by dephasing, relaxation, and mixing. J. Am. Chem. Soc. 121, 3386-3395 (1999)... 

Finally, imines can be used in the synthesis of nitrogen-containing heterocycles. One particularly interesting method involves a sequential photoexcitation-electron-transfer desUylation method for generating a diradical species capable of forming a spirocyclic product (eq 4). Notably, the ionic cyclization method involving fluoride ion leads, in this case, to mixtures of products including only small amounts (ca. 7%) of the desired spirocycle. ... 

Hwang K C and Mauzerall D C 1992 Vectorial electron transfer from an interfacial photoexcited porphyrin to ground-state Cgg and C g and from ascorbate to triplet Cgg and C g in a lipid bilayer J. Am. Chem. Soc. 114 9705-6... 

Another common loss process results from electron—hole recombination. In this process, the photoexcited electron in the LUMO falls back into the HOMO rather than transferring into the conduction band. This inefficiency can be mitigated by using supersensitizing molecules which donate an electron to the HOMO of the excited sensitizing dye, thereby precluding electron—hole recombination. In optimally sensitized commercial products, dyes... 

Dicarbocyanine and trie arbo cyanine laser dyes such as stmcture (1) (n = 2 and n = 3, X = oxygen) and stmcture (34) (n = 3) are photoexcited in ethanol solution to produce relatively long-Hved photoisomers (lO " -10 s), and the absorption spectra are shifted to longer wavelength by several tens of nanometers (41,42). In polar media like ethanol, the excited state relaxation times for trie arbo cyanine (34) (n = 3) are independent of the anion, but in less polar solvent (dichloroethane) significant dependence on the anion occurs (43). The carbocyanine from stmcture (34) (n = 1) exists as a tight ion pair with borate anions, represented RB(CgH5 )g, in benzene solution photoexcitation of this dye—anion pair yields a new, transient species, presumably due to intra-ion pair electron transfer from the borate to yield the neutral dye radical (ie, the reduced state of the dye) (44). 

It has generally been concluded that the photoinitiation of polymerization by the transition metal carbonyls/ halide system may occur by three routes (1) electron transfer to an organic halide with rupture of C—Cl bond, (2) electron transfer to a strong-attracting monomer such as C2F4, probably with scission of-bond, and (3) halogen atom transfer from monomer molecule or solvent to a photoexcited metal carbonyl species. Of these, (1) is the most frequently encountered. 

Meanwhile, it was found by Asai and colleagues [48] that tetraphenylphosphonium salts having such anions as Cl, Br , and Bp4 work as photoinitiators for radical polymerization. Based on the initiation effects of changing counteranions, they proposed that a one-electron transfer mechanism is reasonable in these initiation reactions. However, in the case of tetraphenylphosphonium tetrafluoroborate, it cannot be ruled out that direct homolysis of the p-phenyl bond gives the phenyl radical as the initiating species since BF4 is not an easily pho-tooxidizable anion [49]. Therefore, it was assumed that a similar photoexcitable moiety exists in both tetraphenyl phosphonium salts and triphenylphosphonium ylide, which can be written as the following resonance hybrid [17] (Scheme 21) ... 

Sub-picosecond photoinduced absorption studies were employed to demonstrate the speed of the photoinduced electron transfer. Upon addition of C(M to P30T, the P1A spectrum, decay kinetics, and intensity dependence all change dramatically 36J. Already at 1 ps after photoexcitation by a 100 fs pump pulse at... 

MEH-PPV and P3MBET, were used. As a measure of the efficiency of the photo-induced charge transfer, the degree of luminescence quenching and the ratio of the charged photoexcitation bands to the neutral photoexcitation bands were taken. These two numbers are plotted in Figure 15-15 versus the electrochemical reduction potential. A maximum in the photoinduced electron transfer was determined for Cbo. 

It lias also been suggested that photoexcited benzoyl peroxide is somewhat more susceptible to induced decomposition processes involving electron transfer than the ground state molecule. Rosenthal et c//.15 reported on redox reactions with certain salts (including benzoate ion) and neutral molecules (e.g. alcohols). 

Visible light systems comprising a photoreducible dye molecule e.g. 87)293 or an a-diketone e.g. 85)2% and an amine have also been described. The mechanism of radical production is probably similar to that described for the ketone amine systems described above (i.e. electron transfer from the amine to the photoexcited dye molecule and subsequent proton transfer). Ideally, the dye molecule is reduced to a colorless byproduct. 

The investigation by Becker et al. (1977 b) also included work on the effect of pyrene added as electron donor. Pyrene has an absorption maximum at 335 nm (e = 55000 M-1cm-1, in petroleum). Much more hydro-de-diazoniation takes place in the presence of pyrene with irradiation at 365 nm, and even more on irradiation with light of wavelength <313 nm. Photoexcited pyrene has a half-life of 300 ns and is able to transfer an electron to the diazonium ion. This electron transfer is diffusion-controlled (k= (2-3) X 1010 m 1s 1, Becker et al., 1977a). The radical pairs formed (ArN2 S +) can be detected by 13C- and 15N-CIDNP experiments (Becker et al., 1983, and papers cited there). 

Research on the molecular basis of photoexcitation and electron transfer, including interactions of electron donor and acceptor molecules, could lead to new photochemicals. Development of model photosensitive compounds and methods of incorporating them into membranes containing donor, acceptor, or intermediate excitation transfer molecules, and... 

However, some data have been more difficult to incorporate into the mechanism shown in Figs. 8 and 9. As reported 21) in Section II,B the Fe protein can be reduced by two electrons to the [Fe4S4]° redox state. In this state the protein is apparently capable of passing two electrons to the MoFe protein during turnover, although it is not clear whether dissociation was required between electron transfers. More critically, it has been shown that the natural reductant flavodoxin hydroquinone 107) and the artificial reductant photoexcited eosin with NADH 108) are both capable of passing electrons to the complex between the oxidized Fe protein and the reduced MoFe protein, that is, with these reductants there appears to be no necessity for the complex to dissociate. Since complex dissociation is the rate-limiting step in the Lowe-Thorneley scheme, these observations could indicate a major flaw in the scheme. 

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