Photoexcited electron donors

Figure 2 shows the structures of the bifunctional model systems l(n), (n=4,6,8,10,12), investigatedThe synthesis of these systems and a discussion of their electrochemical properties as well as of their electronic absorption and emission spectra have been given elsewhere (Oevering et al., 1987). These molecules contain a 1, 4-dimethoxynaphthalene unit as the photoexcitable electron donor (E./2 =+1.1 V vs. see in acetonitrile) and a 1,1-dicyano-vinyl moiety as a moderately powerful electron acceptor (Ev=-. l V) separated by an array of at least n C-C sigma-bonds. The centre-to-centre separation (Rc) and the edge-to-... 

Okada, Okamoto and Oda [97] have recently described a novel method of decarboxylation through a PET bond cleavage process in IV-acyloxyphthalimides. A photoexcited electron donor such as l,6-bis(dimethylamino)pyrene (BDMAP) transfers an electron to the phthalimide to produce an anion-radical that is protonated to form a radical prior to homolytic N—O bond cleavage. Bond... 

Semiconducting, conjugated polymers as photoexcited electron donors... 

Sassoon and Rabani [79, 83] constructed an intriguing photoinduced ET system in which the back ET was greatly retarded by the electrostatic repulsion between two different polycations. They prepared poly(3,3-ionene) covalently linked with Ru(bpy)f + (26) and with an iY,Af,/V, Ar -tetraalkyl-/>-phenylenediamine derivative (27). The latter is an electron donor quencher toward the photoexcited Ru(II) complex. 

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... 

Sulfonamides are very difficult to hydrolyze. However, a photoactivated reductive method for desulfonylation has been developed.240 Sodium borohydride is used in conjunction with 1,2- or 1,4-dimethoxybenzene or 1,5-dimethoxynaphthalene. The photoexcited aromatic serves as an electron donor toward the sulfonyl group, which then fragments to give the deprotected amine. The NaBH4 reduces the radical cation and the sulfonyl radical. 

Square-planar zinc compounds predominate with these ligand types as would be predicted. This is in contrast to the prevalence of tetrahedral or distorted tetrahedral geometries for four-coordinate species that have been discussed thus far. Zinc porphyrin complexes are frequently used as building blocks in the formation of supramolecular structures. Zinc porphyrins can also act as electron donors and antenna in the formation of photoexcited states. Although the coordination of zinc to the porphyrin shows little variation, the properties of the zinc-coordinated compounds are extremely important and form the most extensively structurally characterized multidentate ligand class in the CSD. The examples presented here reflect only a fraction of these compounds but have been selected as recent and representative examples. Expanded ring porphyrins have also... 

A number of fluorescent dyes with internal charge transfer mechanism allow the molecule to twist (rotate) between the electron donor and electron acceptor moieties of the fluorescent dipole. In most cases, the twisted conformation is energetically preferred in the excited Si state, whereas the molecule prefers a planar or near-planar conformation in the ground state. For this reason, photoexcitation induces a twisting motion, whereas relaxation to the ground state returns the molecule to the planar conformation. Moreover, the Si — So energy gap is generally smaller in the twisted conformation, and relaxation from the twisted state causes either a... 

Many fluorophores are sensitive to changes in the hydropho-bicity of the immediate environment. Therefore, bringing these fluorophores into a different environment may also produce a change in FRET, when a second fluorophore is affected by the emission change of the first. Fluorophores like Nile Red with changes of up to 100 nm when transferred from water to an aprotic organic solvent are principally suitable for such an approach [71], Molecular rotors have the characteristic of having a quantum yield that depends on the viscosity. Such dyes are formed by an electron donor unit and an electron acceptor unit that can rotate relative to each other upon photoexcitation with a behavior that depends on the viscosity of the environment. These dyes have been included in FRET probes for viscosity studies [53],... 

Since amines generally have low oxidation potentials, they are good electron donors in their ground state, and the donor ability is further enhanced by photoexcitation. The chemical consequence of this single electron transfer (SET) is the generation of the amine radical cations (aminium radicals) and an earlier review on the aminium radicals is available1. 

Zero field splitting (zfs) values in photoexcited triplets of primary donor bacteriochlorophyll a in photosynthetic bacteria are much lower than those found for vitro BChla triplets. There is a pronounced difference in kinetics of population and depopulation of the triplet sublevels as well. The differences have been attributed to the effect of BChla dimerization and it is now generally accepted that the primary electron donor in photosynthetic bacteria consists of a BChla dimer (special pair)(l- ). 

Polysilanes are cr-conjugated polymers composed of Si-Si skeletons and organic pendant groups. They are insulators with filled intramolecular valence bands and empty intramolecular conduction bands. However, because of strong cr conjugation, they have rather narrow band gaps of less than 4 eV [24,25] and are converted to conductors by photoexcitation or by doping electron donors or acceptors. Recently they have attracted much attention because of their potential utility as one-dimensional conductors, nonlinear optical materials, and electroluminescent materials [26-28]. 

Fig.6 The distance dependence of electron-transfer rates in DNA hairpins [51]. The acceptor is a photoexcited derivatized stilbene (SA) or phenanthrene (PA) the electron donor is guanine (G), deazaguanine (Z), or inosine (I). The decay is much more rapid in the Z-PA couple compared to the G-SA couple because the tunneling energy is further from the bridge states in the case of Z-PA...

Type IT. When Ed > ER and AG < 0, the reaction should be possible thermodynamically but may not occur because of a high energy of activation AG. The role of photoexcitation in this case is to provide an easier route for the reaction. These are called photocatalyzed reactions. The photoreduction of methylene blue by EDTA or other electron donors like stannous chloride fall in this category. The reaction is pH and concentration dependent and the reductunt is consumed during the reaction. Dyes in the reduced state can act as very powerful reducing agents. 

Electron donor and acceptor capabilities are enhanced significantly by photoexcitation in comparison with those of the ground states. This is because one-electron redox potentials of excited states are shifted drastically... 

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