Photoexcited electron acceptor reaction

The radical cation can also be obtained by interaction of bicyclobutane with a photoexcited electron acceptor to which it transfers an electron Examples of electron acceptors used are 1-cyanonapthalene and 9,10-dicyanoanthracene. The radical cations obtained in this way undergo various dimerization reactions (the course of which depends on the bridgehead substituents ) and in the presence of nucleophiles such as MeOH or CN " undergo nucleophilic addition reaction (equation 89). 

If an appropriate electron acceptor is available, the photoexcited porphyrin may transfer an electron to it. The acceptors may be organic molecules such as JV-alkylnicotinamide, metal complexes such as [Fe(CN)6]3- or another porphyrin molecule.131 The second order reaction rate for the Zn(uroporphyrin)-acceptor system at infinite ionic strength is estimated to be 108M 1s l. The electron travels from Zn(Por) to V-benzylnicotiamide or to [Fe(CN)6]3 over a distance of 15 A or 30 A respectively. Since the reaction creates charged species, polar media accelerate the process. The back reaction or other photosensitization processes such as 02 generation become favourable in nonpolar solvents. 

Photoexcitation of n-type semiconductors renders the surface highly activated toward electron transfer reactions. Capture of the photogenerated oxidizing equivalent (hole) by an adsorbed oxidizable organic molecule initiates a redox sequence which ultimately produces unique oxidation products. Furthermore, specific one electron routes can be observed on such irradiated surfaces. The irradiated semiconductor employed as a single crystalline electrode, as an amorphous powder, or as an optically transparent colloid, thus acts as both a reaction template and as a directed electron acceptor. Recent examples from our laboratory will be presented to illustrate the control of oxidative cleavage reactions which can be achieved with these heterogeneous photocatalysts. 

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). 

Unfortunately, the experimental data concerning the distances at which electron exchange reactions in the membranes take place are very scarce. Tsuchida et al. have shown [147], that even when the photoexcited Zn porphyrin embedded in the membrane cannot approach the membrane // water interface closer than 12 A, the electron transfer is still possible to MV2+ located in the water phase outside the membrane. However, when the distance of the closest approach of these reactants is increased up to 17 A, the electron transfer is totally stopped. Examples of electron transfer proceeding presumably via electron tunneling across molecular layers about 20 A thick, which separate electron donor and acceptor molecules, can be found in papers by Mobius [230, 231] and Kuhn [232, 233]. Note, that in... 

The photosynthetic process involves photochemical reactions followed by sequential dark chemical transformations (Fig. 3). The photochemical processes occur in two photoactive sites, photosystem I and photosystem II (PS-I and PS-II, respectively), where chlorophyll a and chlorophyll b act as light-active compounds [6, 8]. Photoinduced excitation of photosystem I results in an electron transfer (ET) process to ferredoxin, acting as primary electron acceptor. This ET process converts light energy to chemical potential stored in the reduced ferredoxin and oxidized chlorophyll. Photoexcitation of PS-II results in a similar ET process where plastoquinone acts as electron acceptor. The reduced photoproduct generated in PS-II transfers the electron across a chain of acceptors to the oxidized chlorophyll of PS-I and, consequently, the light harnessing component of PS-I is recycled. Reduced ferredoxin formed in PS-I induces a series of ET processes,... 

To study the regularities of photoexcited electron relaxation in the reaction of the electron transfer by the method of flash photolysis in microsecond timescale, we had to change the electron acceptor concentration in a liquid phase. The ability of the acceptor molecules to adsorb at the surface of the semiconductor colloidal particle was found to determine the character of changes in the photobleaching relaxation kinetic curves. 

As mentioned above, the slowest reaction step determines the rate of the total reaction. For instance, if no electron acceptor is present in the solution, then the photoexcited electrons may be trapped in surface sites via... 

Photoinduced electron transfer reaction between an electron donor molecule (D) and an electron acceptor molecule (A) can be initiated by the photoexcitation of either a D or A molecule. Various reactive species are generated in the course of this process. Important reactive species involved in photoinduced electron transfer reactions are shown in Scheme 1. 

Use of photoexcited fullerenes (i.e., the singlet or triplet excited state) widens the scope of electron-transfer reactions. This assumption is because excitation of fullerenes enhances both the electron-acceptor and -donor behavior of the photoexcited fullerenes. For example, the triplet excited state of C o, which is formed by efficient intersystem crossing (i.e. with a quantum yield close to unity) [18, 19] has a reduction potential of E°red = 1.14 V relative to the SCE [18, 19]. This potential is clearly more positive than the reduction potential of the ground state (—0.43 V) [20]. Thus, the triplet excited state of Ceo can be reduced with a variety of organic compounds yielding the Cgo radical anion and the oxidized donor [18]. 

In green plants, algae, and cyanobacteria, the primary photochemical events of photosynthesis occur in the protein-pigment complex called photosystem II (PSII). PSII consists of more than ten polypeptide chains and a number of co-factors important for electron transport.(i, 6) The co-factors are believed bound to two homologous polypeptides approximately 32 kD in size (D1 and D2). Photoexcitation of the PSD reaction center drives single electron transfer from the primary electron donor, P, (probably a dimer of chlorophyll a) to the primary electron acceptor, one of two pheophytin a molecules. The reduced pheophytin transfers the electron on to a primary plastoquinone... 

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