Photoelectrons transfer systems

This section described the fabrication of ITO electrodes modified with porphyrin-terminated M(tpy)2 complex wires by the stepwise coordination method, and it is demonstrated that the electronic nature of the molecular wire is critical to the photoelectron transfer from the porphyrin to ITO. These results suggest that the new facile fabrication method of molecular assemblies is effective in the construction of photoelectron transfer systems. The system could be upgraded by extending the wire length, embedding the redox potential step in the wire, increasing the photoreceptors in the wire, and/or incorporating donors and acceptors. 

Another approach is to perform ex situ reactions and insert the sample into a high vacuum system without exposure to ambient conditions. Incorporating N2 glove boxes or reactor systems with X-ray photoelectron spectroscopy (XPS) sample handling can also provide information that is closer to operational conditions. In a similar manner ex situ reactions and sample handling are starting to be apphed to electron microscopy studies. Commercially available sample transfer systems will accelerate the application of this methodology. 

DNA mediated photoelectron transfer reactions have been demonstrated60 . Binding to DNA assists the electron transfer between the metal-centered donor-acceptor pairs. The increase in rate in the presence of DNA illustrates that reactions at a macromolecular surface may be faster than those in bulk homogeneous phase. These systems can provide models for the diffusion of molecules bound on biological macromolecular surfaces, for protein diffusion along DNA helices, and in considering the effect of medium, orientation and diffusion on electron transfer on macromolecular surfaces. 

Figure 1 Schematic of the experimental UHV/electrochemical transfer system used for studies on modified platinum single-crystal surfaces. (From Ref. 26.) The UHV system has facilities for X-ray photoelectron spectroscopy (XPS), low-energy ion scattering spectroscopy (LEISS), low-energy electron diffraction (FEED), and temperature-programmed desorption (TPS). The electrochemical chamber allows the electrochemical cell, 0 with integral counter, reference, and secondary working electrode, to be brought to the surface allowing contact of the electrolyte with the transferred surface.

Addition Reactions.- The photoelectron transfer process of the iminium salt (38) with the 3-butenoate anion results in the formation of the allylated product (39). The reaction involves decarboxylation of the 3-butenoate followed by a radical coupling reaction. The photoaddition of halogenated alkenes to the tetraraza phenanthrene (40) yields products (41) of (2+2)-addition. The Eu(III)/Eu(II) photoredox system has been studied with regards to its reactivity toweu ds a-methylstyrene. Irradiation of the system at > 280 nm in methanol yielded the products (42) and(43). ... 

A laser flash photolysis study of the behaviour of the lowest excited triplet state and semiquinone radical anion of hypocrellin A (HA") suggests that, in the presence of substrates such as ascorbic acid and cysteine, formation and decay of (HA") occurs by electron transfer. The production of superoxide radical anion (O2") was also confirmed, and the conclusion is drawn from the experimental results that an electron transfer (Type I) mechanism may be important in the photodynamic interaction between HA and some biological substrates. Photoelectron transfer and hydrogen abstraction in the phenothiazine/p-benzoquinone system proceeds competitively, and a series of porphyrin quinones (5 = H,... 

Photoelectron transfer from the excited singlet state of zinc(II) tetraphenylpor-phyrin (ZnTPP) to MV " in a compartmentalised system in which the ZnTPP moieties are covalently attached to amphiphilic sodium polysulfonates carrying lauryl 2-(naphthyl)methyl, or cyclododecyl groups, is reported to be much slower... 

Remarkably, not only are both systems effective at carrying out this process, but the photoelectron transfer rate constants of 8.1 x 10 and 4.3 x 10 for 14 and 15, respectively, suggest that the carboxylic acid double hydrogen bond is more effective at transmitting the excited electron than the a-framework in 15. despite the fact that some dissociation might occur in solution. 

Ruthenium hexacyanides have been used in these photoelectron transfer reactions because of the availability of the (Ru(II)/Ru(III)) redox system, which differs by one electron. For the cyanide complex, (RuCCNji ) " =0.86 V. An example... 

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