Hole photo-induced electron transfer

Scheme 6 Photo-induced electron transfer and hole transfer in DNA...

An alternative inexpensive organic polymer-based photovoltaic solar cell has been invented. In this device, p-type and n-type semiconductors are sequentially stacked on top of each other. In such devices, absorption of a photon by a ji-conjugated polymer results in the formation of an excited state, where coulom-bicaUy bound electron-hole pair (exciton) is created. This exciton diffuses to a region of interface of n-type semiconductor where exciton dissociation takes place and transport of charge to the respective electrodes occurs. For example, the photo-induced electron transfer from a donor layer (p-type) to acceptor layer (n-type) takes place in a polymer/fullerene-based organic bilayer solar cell, MDMO-PPV PCBM, with power conversion efiiciency of 2.5 % (Fig. 11.8) [13]. 

Figure 6-3. Schematic view of photo-induced electron or hole transport through the DNA-based molecular chain. Reversible random walk of the charge along the chain is interrupted by irreversible chemical degradation ( quenching ) at given sites. Charge i.e. electron or hole) injection at the terminals corresponds to interfacial charge transfer in the in situ STM or nanogap electrode configurations to be discussed in Section 4.

It is believed that surface localized electron-hole pairs produced under light in SC nanoparticles participate in photo-induced processes of charge transfer between nanoparticles. These processes most probably of quantum tunnel type determine photoconductivity of composite films containing SC nanoparticles in a dielectric matrix. The photocurrent response time in this case should correspond to the lifetime ip of such pairs, which is of the order nanosecond and even more [6]. This rather long ip makes photo-induced tunnel current in composite film possible. 

Micic, O. I. Zhang, Y. Cromack, K. R. Trifunac, A. D. Thumauer, M. C. Photo-induced hole transfer from Ti02 to methanol molecules in aqueous solution studied by electron paramagnetic resonance, J. Phys. Chem. 1993, 97, 13284. 

In order to account for such a mechanism, photochemical excitation of a semiconductor surface might induce the promotion of an electron from the valence band to the conduction band. Since relaxation of the high-energy electron is inhibited by the absence of intra-states, if the lifetime of this photo generated electron-hole pair is sufficiently long to allow the interfacial electron transfer from an accumulation site to an electron acceptor, as well as the interfacial electron transfer from an adsorbed organic donor to the valence-band hole, the irradiated semiconductor can simultaneously catalyze both oxidation and reduction reactions in a fashion similar to multifunctional enzymes reactions [232]. 

Although photoelectrochemistry has been known as a field for over thirty years, its full impact on organic synthesis has yet to be revealed. This article has dealt with a variety of examples that show how chemical conversions can be induced by photo-electrochemical activation of light-sensitive semiconductor surfaces. Photoexcitation causes the promotion of an electron from the valence band to the conduction band, thus producing a surface-confined electron-hole pair. The charges represented by this pair are then trapped by interfacial electron transfer. The oxidized and reduced... 

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