Sensitizer-acceptor system electron transfer

Photochemical addition of ammonia and primary amines to aryl olefins (equation 42) can be effected by irradiation in the presence of an electron acceptor such as dicyanoben-zene (DCNB)103-106. The proposed mechanism for the sensitised addition to the stilbene system is shown in Scheme 7. Electron transfer quenching of DCNB by t-S (or vice versa) yields the t-S cation radical (t-S)+ Nucleophilic addition of ammonia or the primary amine to (t-S)+ followed by proton and electron transfer steps yields the adduct and regenerates the electron transfer sensitizer. The reaction is a variation of the electron-transfer sensitized addition of nucleophiles to terminal arylolefins107,108. 

Photoinduced Electron Transfer. Monolayer organizates are particularly suited for the investigation of photoinduced electron transfer, since the molecules are fixed and the distance between the planes at which the donor and the acceptor molecules, respectively, are located can be well defined. Therefore, complex monolayers have been arranged in order to study the distance dependence of electron transfer in these systems (2, 20). This strategy has also been used to elucidate the relative contributions of electron injection and energy transfer mechanisms in the spectral sensitization of silver bromide (21). 

The described problem was encountered in investigations of cis-trans isomeriza-tions and cycloadditions of donor olefins D and acceptor olefins A in acetonitrile. All polarizations, both of the cycloadducts and of the starting and isomerized olefins, could be traced to radical ion pairs D" + A" formed by photoinduced electron transfer. As, however, exciplexes are frequently discussed as percursors to the products in such systems,and CIDNP does not respond to exciplexes because no diffusive separation is possible, the question as to the relative contributions of the radical ion and exciplex pathways arose. To answer it, we employed photoinduced electron transfer sensitization (PET-sensitization). 

The majority of the research on the photochemistry of porphyrins linked to other moieties has been in the area of photoinduced electron transfer, and the systems studied are all in some sense mimics of the photosynthetic process described above. The simplest way to prepare a system in which porphyrin excited states can act as electron donors or acceptors is to mix a porphyrin with an electron acceptor or donor in a suitable solvent. Experiments of this type have been done for years, and a good deal about porphyrin photophysics and photochemistry has been learned from them. Although these systems are easy to construct, they have serious problems for the study of photoinduced electron transfer. In solution, donor-acceptor separation and relative orientation cannot be controlled. As indicated above, electron transfer is a sensitive function of these variables. In addition, because electron transfer requires electronic orbital overlap, the donor and acceptor must collide in order for transfer to occur. As this happens via diffusion, electron transfer rates and yields are often affected or controlled by diffusion. As mentioned above, porphyrin excited singlet states typically have lifetimes of a few nanoseconds. Therefore, efficient photoinduced electron transfer must occur on a time scale shorter that this. This is difficult or impossible to achieve via diffusion. Thus, photoinduced electron transfer between freely diffusing partners is confined mainly to electron transfer from excited triplet states, which have the required long lifetimes (on the micro to the millisecond time scale). 

Recently, we (82) and others (82-84) have shown that similar hetero-structures can be prepared by using two-dimensional inorganic sheets (made by exfoliation of various lamellar solids) in place of the organic polyanion. This technique offers a potentially powerful alternative to the construction of multi-component electron transfer systems, because it can, in principle, be used to stack up an arbitrary number of redox-active polymers without interpenetration (85). This chapter describes the preparation and photochemistry of simple multilayer composites on high-surface-area silica. Specifically, the synthesis and electron transfer kinetics of systems containing a polycationic sensitizer, poly-[Ru(bpy)2(vbpy)(Cl)2] (1), (abbreviated [Ru(bpy)3 ]n bpy = 2,2 -bipytidine and vbpy = 4-vinyl-4 -methyl-2,2 -bipyridine), and an electron-acceptor polycation poly[(styrene-co-]V-vinylbenzyl-N -methyl-4,4 -bipyridine)(Cl)2] (2), (PS-MV ) are presented. Using a solution-phase electron donor, 3, as the third electroactive component, it was possible to prepare and study the photoinduced electron transfer reactions of several different diad and triad combinations. 

When a molecule is photoexcited, the excited state is better electron donor as well as better electron acceptor than the ground state. In the presence of donor and acceptor which are not capable of conducting dark electron transfer reaction with each other or with sensitizer, the sensitizer may be recycled via radical cation or radical anion(Fig. 2). Detailed kinetic study on electron transfer sensitization of photooxidation of leuco crystal violet(LCV) in the presence of perylene(Pe), and 9-cyanoanthracene(CNA) or 1,4-dicyanobenzene(DCB) as sensitizer and acceptor, respectively, (3) provides the following information, i) Reaction between Pe and DCB is ca. 2.5 times faster than reaction between Pe and LCV. ii) The turnover number of Pe could be infinite in vacuo, iii) Oxygen could substitute the role of DCB. iv) The quantum efficiency of Pe-LCV-DCB ternary system is better than that by direct excitation of LCV-DCB system. v) When other solvents are used, solvents having high... 

Some bichromophoric systems, whose structure is based on the donor-bridge-acceptor principle, can undergo complete charge transfer, i.e. electron transfer. The resulting huge dipole moment in the excited state explains the very high sensitivity to solvent polarity of such molecules. An example is FP (l-phenyl-4-[(4-cyano-l- ... 

The electronic coupling between an initial (reactant) and a final (product) state plays a key role in many interesting chemical and biochemical photoinduced energy and electron transfer reactions. In excitation (or resonance) energy transfers (EET or RET) [1,2], the excitation energy from a donor system in an electronic excited state (D ) is transferred to a sensitizer (or acceptor) system (A). Alternatively, in photoinduced electron transfers (ET) [3,4], a donor (D) transfers an electron to an acceptor (A) after photoexcitation of one of the components (see Figure 3.50). 

A schematic cycle describing the principle of light capture and energy storage via a photosensitized electron transfer process In an artificial device Is presented In Figure 2. In this system a synthetic sensitizer, S, substitutes for the natural chlorophyll as the light capturing entity. Excitation of the sensitizer, followed by an electron transfer to electron acceptor. A, results In the oxidized sensitizer and a reduced species, A. Oxidation of an electron donor, D, recycles the sensitizer and produces an... 

For this purpose an electron transfer across the bilayer boundary must be accomplished (14). The schematic of our system is presented in Figure 3. In this system an amphiphilic Ru-complex is incorporated Into the membrane wall. An electron donor, EDTA, is entrapped in the inner compartment of the vesicle, and heptylviolo-gen (Hv2+) as electron acceptor is Introduced into the outer phase. Upon illumination an electron transfer process across the vesicle walls is initiated and the reduced acceptor (HVf) is produced. The different steps involved in this overall reaction are presented in Figure 3. The excited sensitizer transfers an electron to HV2+ in the primary event. The oxidized sensitizer thus produced oxidizes a Ru located at the inner surface of the vesicle and thereby the separation of the intermediate photoproducts is assisted (14). The further oxidation of EDTA regenerates the sensitizer and consequently the separation of the reduced species, HVi, from the oxidized product is achieved. In this system the basic principle of a vectorial electron transfer across a membrane is demonstrated. However, the quantum yield for the reaction is rather low (0 4 X 10 ). 

The photosensitized electron transfer reaction forms the reduced lipophilic electron acceptor BNA which is ejected into the continuous organic phase and thus separated from the oxidized product. In order to monitor the entire phase transfer of the reduced acceptor, BNA, a secondary electron acceptor, p-dlmethyl-amlnoazobenzene (dye),was solubilized in the continuous oil phase. The photochemically induced electron transfer reaction in this system results in the reduction of the dye (0 = 1.3 x 10 3). Exclusion of the sensitizer or EDTA or the primary electron acceptor, BNA, from the system resulted in no detectable reaction. Substitution of the primary acceptor with a water soluble derivative, N-propylsulfonate nicotinamide, similarly results in no reduction of the dye. These results indicate that to accomplish the cycle formulated in Figure 6A the amphiphilic nature of the primary electron acceptor and its phase transfer ability in the reduced form are necessary requirements. 

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