Electron donor-acceptor systems

Senesi and Testini [147,156] and Senesi et al. [150,153] showed by ESR the interaction of HA from different sources with a number of substituted urea herbicides by electron donor-acceptor processes involves organic free radicals which lead to the formation of charge-transfer complexes. The chemical structures and properties of the substituted urea herbicides influence the extent of formation of electron donor-acceptor systems with HA. Substituted ureas are, in fact, expected to act as electron donors from the nitrogen (or oxygen) atoms to electron acceptor sites on quinone or similar units in HA molecules. 

Horak, J., Maier, N.M., and Lindner, W., Investigations on the chromatographic behavior of hybrid reversed-phase materials containing electron donor-acceptor systems ii. Contribution of pi-pi aromatic interactions, J. Chromatogr. A, 1045, 43, 2004. 

In an electron donor-acceptor system (D-A) in which an electron donor (D) and an electron acceptor (A) are separated by a carbon chain, the exciplex state may be considered as a resonance hybrid of the electron transfer configuration (D A" ) mixed with the locally excited configuration (D A) or (DA ) [211]. 

Masuhara H, Mataga N (1981) Ionic photo-dissociation of electron-donor-acceptor systems in solution. Acc Chem Res 14 312... 

Masuhara H, Hino T, Malaga N (1975) Ionic photodissociation of excited electron donor-acceptor systems. 1. Empirical equation on relationship between yield and solvent dielectric-constant. J Phys Chem 79 994... 

II. GENERATION OF REACTIVE SPECIES IN THE ELECTRON-DONOR-ACCEPTOR SYSTEM... 

The reactive species generated by the photoexcitation of organic molecules in the electron-donor-acceptor systems are well established in last three decades as shown in Scheme 1. The reactivity of an exciplex and radical ion species is discussed in the following sections. The structure-reactivity relationship for the exciplexes, which possess infinite lifetimes and often emit their own fluorescence, has been shown in some selected regioselective and stereoselective photocycloadditions. However, the exciplex emission is often absent or too weak to be identified although the exciplexes are postulated in many photocycloadditions [11,12], The different reactivities among the contact radical ion pairs (polar exciplexes), solvent-separated radical ion pairs, and free-radical ions as ionic species... 

Photoinduced electron-transfer reaction of aromatic compounds with amines is one of the most fundamental reactions in the electron-donor-acceptor systems, which was recently reviewed by Lewis [35], Because of the low oxidation potentials of the amines, the photoinduced one-electron transfer from the amines to the excited singlet states of aromatic hydrocarbons ( Aril ) readily occurs to give the radical cations of amines and the radical anions of aromatic compounds even in the less polar solvents. 

Scheme 6. Primary electron transfer of electron donor — acceptor systems in polar solvents (HF = hyperfine interaction) [K. Schulten, A. Weller, Biophys. J. 24 (1978), 296].

In all covalent electron donor-acceptor systems produced earlier, triplet states observed by EPR were formed via a spin-orbit intersystem crossing (SO-ISC) mechanism. Another possible mechanism of triplet formation is RP-ISC, mentioned above, which results from radical ion pair recombination, and which had been observed previously by time-resolved electron paramagnetic resonance spectroscopy (TREPR) only in bacterial reaction centers and in the green plant Photosystem I and II reaction centers. These two mechanisms can be differentiated by the polarization pattern of the six EPR transitions at the canonical orientations. In SO-ISC,... 

Electronic absorption spectra (Table 2) were also used for analysis of the electron-donor-acceptor systems represented by compounds 9-14 and displayed a broad low-energy ICT band in the visible region (500-700 nm) the energy ( i ict 1-7-2.5eV) and intensity ( ss5000-50000M cm ) depended on the nature of both 1,3-dithiole electron donor and dicyanomethylene or 9-fluorenyl acceptor moieties and on the structure of the linker unit <2001EJ02671>. 

M. Mizoguchi and N. Ohta. Fluorescence of an electron donor-acceptor system in a polymer film under the simultaneous application of electric field and magnetic field. Chem. Phys. Lett., 372(l-2) 66-72, 2003. 

The most difficult chemical step in precursor Z synthesis is the cleavage of the C2 -C3 bond with subsequent insertion of the C8 atom. While the mechanistic details of this process are unknown, it appears that strict radical transfer from dAdo to 5 -GTP must occur with significant rearrangement of active site molecules in a manner that prevents unwanted side reactions. The C-terminal [4Fe S] cluster may play an important, heretofore unidentified role during catalysis, either functioning as an electron donor/acceptor system or coordinating reaction intermediates. 

The photoinduced intramolecular electron transfer reactions of some poly-(ethylene glycol)-linked 9-aminoacridine-benzoate electron donor-acceptor systems have been described. Photosensitized oxidation of 5-methyl-2 -deoxy-cytidine using menadione (2-methylnaphthalene-l,4-dione) gives 5,6-dihydroxy-5,6-dihydro-5-methyl-2 -deoxycytidine in what is thought to be an electron... 

Films prepared by co-deposition of a polycyclic aromatic hydrocarbon (PAH) and an electron-poor PAH (EPPAH) (Fig. 27) exhibit an oblique arrangement reflecting the formation of epitaxial composite layers of the electron donors and acceptors [81]. Interestingly, on top of a layer of donor molecules, within the same layer between two physisorbed donor molecules, acceptor molecules are co-adsorbed in a well-defined arrangement. When the acceptor co-crystallizes, the second PAH layer is stabilized (Fig. 27). These studies were extended to covalently linked electron donor-acceptor systems [82,83]. 

Within three series of pyridyl polyene chromophores 4, the /Iq values obtained by the HRS technique or Stark spectroscopy (about 100-600 x 10 ° esu) maximize when n = 2. This is in contrast with the behavior of known organic chromophores based on an electron-donor-acceptor system linked by a polyene bridge, in which /lo(HRS) increases steadily with the increase of the n delocalization of the polyene bridge [48, 49]. TD-DFT calculations show that the HOMO level of these... 

The second term on the right side of equation 19 ultimately determines the usefulness of the molecule when incorporated into a polymer for second-order applications (96). A DC field aligns the dipole moment fx so that the first hyperpolarizability, /3, can contribute to the bulk response. A similar expression is applied to poled polymers (24). The assumption that y is negligible compared to p is usually valid for nonlinear chromophores, but not for extended -electron donor-acceptor systems because y increases with conjugation length faster than p (97). 

Energetic and kinetic aspects of these electron transfer processes have been considered in detail and correlated with each other, by which microscopic elucidation of these processes has been made possible. Environmental conditions in polymers such as micropolarity and microviscosity, molecular motions, and interchromophoric interactions are different from those in typical small molecule electron donor-acceptor systems. As a consequence, the structure and dynamics of polymer exciplexes and ion radicals are more complicated compared to those of aromatic hydrocarbons. In the following sections these topics are described and discussed. 

Lee, K. and Falvey, D. E., Photochemically removable protecting groups based on covalently linked electron donor-acceptor systems, /. Am. Chem. Soc., 122, 9361—9366, 2000. 

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