Electron transfer photoinitiated

Figure C3.2.13. Orientation in a photoinitiated electron transfer from dimetliylaniline (DMA) solvent to a coumarin solute (C337). Change in anisotropy, r, reveals change in angle between tire pumped and probed electronic transition moments. From [46],...

The addition followed a radical chain mechanism initiated by photoinitiated electron transfer from the tertiary amine to the excited aromatic ketone and occurred with complete facial selectivity on the furanone ring (99TL3169). The yields increased and best results were obtained with sensitizers (4-methoxyacetophenone,... 

Cytochromes, catalases, and peroxidases all contain iron-heme centers. Nitrite and sulfite reductases, involved in N-O and S-O reductive cleavage reactions to NH3 and HS-, contain iron-heme centers coupled to [Fe ] iron-sulfur clusters. Photosynthetic reaction center complexes contain porphyrins that are implicated in the photoinitiated electron transfers carried out by the complexes. 

A remarkable reorganization has been observed for 2,2-diarylmethylene-spiropentanes 113 after photoinitiated electron transfer from 9,10-dicyanoan-thracene (DCA) leading to the reversible formation of 2,2-diarylbicyclopropyl-idenes 114, the proportion of which decreases with an increase of the electron-donating ability of the aryl substituents (Scheme 22) [109]. 

K. M. Kersey, D. E. Cragg, D. W. Minsek, Longdistance photoinitiated electron transfer through polyene molecular wires Ed. J. R. Norris, Elsevier, New York, NY Chem. Div., Argonne Natl. Lab., Argonne, IL, 60439, USA, 1989, pp 135-47. 

This review highlights recent studies of synthetic, covalently linked multicomponent molecular devices which mimic aspects of photosynthetic electron transfer. After an introduction to the topic, some of the salient features of natural bacterial photosynthetic reaction centers are described. Elementary electron transfer theory is briefly discussed in order to provide a framework for the discussion which follows. Early work with covalently linked photosynthetic models is then mentioned, with references to recent reviews. The bulk of the discussion concerns current progress with various triad (three-part) molecules. Finally, some even more complex multicomponent molecules are examined. The discussion will endeavor to point out aspects of photoinitiated electron transfer which are unique to the multicomponent species, and some of the considerations important to the design, synthesis and photochemical study of such molecules. 

When the donors and acceptors lack spherical symmetry, there will also be an orientation dependence. In cases such as those to be discussed below, where the donor and acceptor moieties are linked by covalent bonds, there is considerable evidence that in certain situations the electron transfer occurs through the linkage bonds [22]. Although such linkages are not present in photosynthetic reaction centers, it has been proposed that the accessory Bchl or other intervening material may still take part in electron transfer through a superexchange mechanism [8, 26]. The distance dependence of photoinitiated electron transfer has recently been reviewed [13]. 

The first reaction in each sequence is the photoinitiated electron transfer, and can in principle occur from either an excited singlet state, or a triplet state. The second reaction is the charge recombination reaction or back reaction , as it is sometimes... 

In general, it has been found that under the correct combination of electronic coupling, thermodynamic driving force, solvent and temperature, P-Q systems readily undergo photoinitiated electron transfer as shown in Fig. 2. Singlet excitation localizes on the porphyrin, which has the energetically lowest-lying... 

As discussed above, the photosynthetic reaction center solves the problem of rapid charge recombination by spatially separating the electron and hole across the lipid bilayer. In order to achieve photoinitiated electron transfer across this large distance, the reaction center uses a multistep sequence of electron transfers through an ensemble of donor and acceptor moieties. The same strategy may be successfully employed in photosynthesis models, and has been since 1983 [42-45]. The basic idea may be illustrated by reference to a triad Dj-D2-A, where D2 represents a pigment whose excited state will act as an electron donor, Di is a secondary donor, and A is an electron acceptor. Excitation of D2 will lead to the following potential electron transfer events. 

The general strategy of multistep electron transfers in triad-type molecules has proven to be a useful one for maximizing the quantum yield of charge separated states formed by photoinitiated electron transfer, the lifetimes of these states, and the amount of energy stored therein. As a result, the strategy has been exploited in a variety of molecular devices, many of which are reviewed below. 

Fig. 4. Photoinitiated electron transfer rate constant (natural logarithm) vs. edge-to-edge porphyrin-quinone separation for C-P-Q triads 4-8 and related P-Q molecules. The separations shown are derived from H-NMR measurements...

Recently, a new type of C-P-Q triad was reported in which the carotenoid and quinone moieties were linked to the tetraarylporphyrin via basket handle linkages between opposite aryl groups [65]. This linkage positioned the carotenoid and quinone species above and below the plane of the porphyrin, rather than to the side, as confirmed by H-NMR studies. Evidence for photoinitiated electron transfer in this molecule was provided by incorporating it into a phospholipid bilayer and detecting a light-induced photocurrent. Similar experiments had previously been reported with the other C-P-Q triads discussed above [55],... 

In this connection, the authors also report an analog of 14 in which the two porphyrins and the acceptor assume a more extended conformation wherein the moieties are side-by-side, rather than stacked. The electronic coupling is not as good in this case, as demonstrated by the absorption spectrum. In this analog, both the photoinitiated electron transfer and the recombination of the P+-P-AT state are substantially retarded [66]. 

Figure 16,6 Molecular structures of typical calcium PET (photoinitiated electron transfer) sensors (a) FLUO-3, (b) Calcium Green 1, and (c) Calcium Green 2 are commonly used in medical analytics...

Figure 16.7 Molecular structures of the most common calcium PET (photoinitiated electron transfer) sensors...

Photoinitiated electron transfer reactions are among the earliest photochemical reactions documented in the chemical literature and (ground state) electron donor-acceptor interactions have been known for over one hundred years. Some aspects of plant photosynthesis were already known to Priestly in the eighteenth century. The photooxidation of oxalic acid by metal ions in aqueous solution was discovered by Seekamp (UVI) in 1803 and by Dobereiner (Fe,n) in 1830. The electron donor-acceptor interactions between aromatic hydrocarbons and picric acid were noticed by Fritzsche in the 1850s the quinhydrones are even older,... 

In this article, we discuss several early examples of photoinitiated electron transfer reactions. We also follow the development of alternative methods to achieve one-electron oxidation and reduction. This general reaction type, of which the photo-induced reaction is a special case, pervades organic, bio-, and inorganic chemistry. 

Each of the terms in Eq. (1) must be known in order to calculate the free energy change for a photoinitiated electron transfer process (Eq. (2)). 

Indeed, reactions of this type can be observed if the porphyrin and quinone species are judiciously chosen. However, such simple systems have several serious drawbacks as models for photosynthesis. In the first place, the photoinitiated electron transfer step from the porphyrin to the quinone cannot occur faster than diifusion of the donor and acceptor. Because the... 

In spite of this success, dyads like 1 suffer from a major limitation as mimics of the natural electron transfer process. The very structural and electronic features which ensure rapid photoinitiated electron transfer, and consequently a high quantum yield, in these molecules also favor rapid charge recombination (step 3 in Figure 2). Thus, the P -Q " state lives at most a few hundred picoseconds in solution. The P-Q systems, and indeed other dyad-type artificial photosynthetic molecules, are unable to reproduce the long-lived charge separation characteristic of the reaction center. The stored energy is quickly lost as heat. 

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