Photo-Induced Charge Transfer Reactions

When the semiconductor-electrolyte junction is illuminated with light, photons having energies greater than the semiconductor band gap are absorbed and create electron-hole pairs in the semiconductor. Photons absorbed in the depletion layer produce electron-hole pairs that separate under the influence of the electric field present in the space charge region. Electron-hole pairs produced by absorption of photons beyond the depletion layer will separate if the minority carriers can diffuse to the depletion layer before recombination with the majority carriers occurs. 

The photoproduction and subsequent separation of electron-hole pairs in the depletion layer cause the Fermi level in the semiconductor to return toward its original position before the semiconductor-electrolyte junction was established, i.e., under illumination the semiconductor potential is driven toward its flat-band potential. Under open circuit conditions between an illuminated semiconductor electrode and a metal counter electrode, the photovoltage produced between the electrodes is equal to the difference between the Fermi level in the semiconductor and the redox potential of the electrolyte. Under close circuit conditions, the Fermi level in the system is equalized and no photovoltage exists between the two electrodes. However, a net charge flow does exist. Photogenerated minority carriers in the semiconductor are swept to the surface where they are subsequently injected into the electrolyte to drive a redox reaction. For n-type semiconductors, minority holes are injected to produce an anodic oxidation reaction, while for p-type semiconductors, minority electrons are injected to produce a cathodic reduction reaction. The photo-generated majority carriers in both cases are swept toward the semiconductor bulk, where they subsequently leave the semiconductor via an ohmic contact, traverse an external circuit to the counter electrode, and are then injected at the counter electrode to drive a redox reaction inverse to that occurring at the semiconductor electrode. 

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Reaction Mechanism of Vinyl Polymerization with Amine in Redox and Photo-Induced Charge-Transfer Initiation Systems... 

A substantial number of photo-induced charge transfer polymerizations have been known to proceed through N-vinylcarbazole (VCZ) as an electron-donor monomer, but much less attention was paid to the polymerization of acrylic monomer as an electron receptor in the presence of amine as donor. The photo-induced charge-transfer polymerization of electron-attracting monomers, such as methyl acrylate(MA) and acrylonitrile (AN), have been recently studied [4]. In this paper, some results of our research on the reaction mechanism of vinyl polymerization with amine in redox and photo-induced charge transfer initiation systems are reviewed. 

The initiation mechanism of photo-induced charge-transfer copolymerization of donor-acceptor monomer pairs is clarified by integrating the results from organic chemistry and polymer chemistry. A reasonable suggestion for the initiating species in certain cases may be a tetramethylene 1,4-diradical. The excited complex of the donor/acceptor monomers undergoes multiple follow-up reactions to produce the... 

For electron transfer reaction involving neutral species such as BP and LCV alone, ionic field effects are confined to reactions after primary electron transfer. When reacting species bears charge before redox reaction, situation is more complicated particularly in the case of photo-induced electron transfer reactions. Both formation and back electron transfer processes will be influenced by ionic field in different manners. For example, the well-known positive effect of polyelectrolyte bearing cation on reactions between anions(or vice versa) will not be applicable if the overall reaction including back electron transfer process is under consideration. 

The donor-acceptor complexes [Ir(/r-dmpz)(CO)(PPh2 0(CH2)2R )]2 exhibit photo-induced electron-transfer rate constants of 1012s—1 and charge recombination rates slower than 2 x 10los-1 when R = pyridine and 4-phenylpyridine.534 Further studies on these complexes revealed that recombination reactions were temperature dependent and slower for the deuterated acceptors.535... 

A theory used to study and interpret the photo-induced electron transfer in solution was described by Marcus.19-25 In this theory, the electron transfer reaction can be treated by transition state theory where the reactant state is the excited donor and acceptor and the product state is the charge-separated state of the donor and acceptor (D+-A ), shown in Figure 15. 

For photo-induced electron transfer (ET) reactions [53], there exist three cases depending on their mechanism (1) non-adiabatic, diabatic, or weak coupling case, (2) adiabatic, or strong coupling case, and (3) charge transfer complex case. This section shall focuses on case (1) to which perturbation theory can be applied. 

Photo-induced electron transfer between [Ru(bpy)3]2+-like centres covalently bound to positively-charged polymers (N-ethylated copolymers of vinylpyridine and [Ru(bpy)2(MVbpy)]2+) and viologens or Fe (III) has been studied using laser flash photolysis techniques. It is found that the backbone affects the rates of excited state quenching, the cage escape yield, and the back electron transfer rate because of both electrostatic and hydrophobic interactions. The effect of ionic strength on the reactions has been studied. Data on the electron transfer reactions of [Ru(bpy)3]2+ bound electrostatically or covalently to polystyrenesulphonate are also presented. 

Three main changes need to be made to ary of the homogeneous equations to make them appropriate for heterogeneous, i.e. electrochemical, electron transfer or photo-induced electron transfer (Chidsey, 1991 Marcus, 1996 Royea et al., 1997). First, because the reaction involves movement of an electron across a charged interface, the potential drop across which changes with the electrode potential U, the free energy of reaction is a function of U. For the reduction (cathodic) reaction Ox + e Rd we may write... 

Figure 19.28 Energy transfer from accessory pigments to reaction centers. Light energy absorbed by accessory chlorophyll molecules or other pigments can be transferred to reaction centers, where it drives photo induced charge separation. The green squares represent accessory chlorophyll molecules and the red squares represent carotenoid molecules the white squares designate protein.

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