Positive hole transfer

In spite of all this research, systematic criteria for trapping of organic radical cations produced by positive hole transfer in solid argon are not yet fully understood. It would be useful to establish such criteria to evaluate the prospects of characterization of chemically important aliphatic radical cations under the conditions of classic matrix isolation experiments. 

In a heterogeneous photocatalytic reaction, in which multiple photons are absorbed due to the relatively large size of particles compared with the size of molecules, multiple-electron (positive hole) transfer may occiu. For example, photocatalytic silver metal deposition accompanied by molecular oxygen (O2) liberation proceeds with the following stoichiometry (28) ... 

In the inhomogeneous coulombic field generated by the trapped cations and anions formed by irradiation, positive holes will migrate in the direction of trapped anions. When, as a result of this process, a matrix (RjH) radical cation becomes adjacent to a solute (RiiH) molecule, positive-hole transfer to the solute may occur. 

Also, proton transfer from matrix radical cations to solute molecules takes place in competition with positive-hole transfer (reaction (5.19) in the same direction. 

The site of proton donation in the proton transfer from solute radical cations to matrix molecules has been investigated using y-irradiated pentane-dj2 containing 0.5 mol% octane as well as trapped CO2 [31]. Octane radical cations are formed by positive-hole transfer from matrix cations in this system and part of them react by proton transfer to matrix molecules. 

Concerning cellular DNA, a two-component hypothesis has been developed. According to this hypothesis, the electron loss centers (radical cations) end up with the purines, particularly with the guanine moiety, whereas the final site of deposition of the ejected electron is with the pyrimidines, particularly with thymine [127]. The two-components hypothesis implies that in DNA there are mechanism of electron and positive hole transfer by which the initially generated and randomly distributed electron gain and loss centers are tunneled into the T and G traps respectively. 

L + I-> l3 (counterion) -i- 1 h" (positive hole transfered locally to the polymer chain in polaronic form, as established during chemical doping [94])... 

In Ref 13, the electron conduction mechanisms of Section 21-4 are modified to allow for positive hole transfer as well as electron transfer. The modified antibonding molecular orbital mechanism is displayed in Figure 21-2. 

According to the electron-transfer mechanism of spectral sensitization (92,93), the transfer of an electron from the excited sensitizer molecule to the silver haHde and the injection of photoelectrons into the conduction band ate the primary processes. Thus, the lowest vacant level of the sensitizer dye is situated higher than the bottom of the conduction band. The regeneration of the sensitizer is possible by reactions of the positive hole to form radical dications (94). If the highest filled level of the dye is situated below the top of the valence band, desensitization occurs because of hole production. 

This almost distance independent hole transfer over (A T)n sequences where adenines are charge carriers is very surprising. Maybe the transfer of a positive charge between adenines of an (A T)n sequence is extremely fast, as recent calculations of M.D. Sevilla predicted [20], One could also speculate that the positive charge is delocalized over more than one A T base pair so that polaron hopping, which is discussed in this volume by G.B. Schuster as well as E.N. Conwell, might make the hole transport in oxidized (A T)n sequences very efficient. 

These experiments demonstrate the importance of proton transfer processes during hole transfer through DNA. S. Steenken has already remarked that a proton shift between the G C bases stabilizes the positive charge [23]. If such a proton shift is coupled with the hole shift, a deuterium isotope effect should arise. Actually, H/D isotope effects are described by V. Shafiro-vich, M.D. Sevilla as well as H.H. Thorp in their articles of this volume. Experiments with our assay [22] also demonstrate (Fig. 16) that hole transfer in protonated DNA (H20 as solvent) is three times more efficient than in deuterated DNA (D20 as solvent). If this reflects a primary isotope effect, it shows that the charge transfer is coupled with a proton transfer. 

Oxides play many roles in modem electronic technology from insulators which can be used as capacitors, such as the perovskite BaTiOs, to the superconductors, of which the prototype was also a perovskite, Lao.sSro CutT A, where the value of x is a function of the temperature cycle and oxygen pressure which were used in the preparation of the material. Clearly the chemical difference between these two materials is that the capacitor production does not require oxygen partial pressure control as is the case in the superconductor. Intermediate between these extremes of electrical conduction are many semiconducting materials which are used as magnetic ferrites or fuel cell electrodes. The electrical properties of the semiconductors depend on the presence of transition metal ions which can be in two valence states, and the conduction mechanism involves the transfer of electrons or positive holes from one ion to another of the same species. The production problem associated with this behaviour arises from the fact that the relative concentration of each valence state depends on both the temperature and the oxygen partial pressure of the atmosphere. 

The resolution of this apparent contradiction to the thermodynamic expectations for this transfer is that the ionic membrane will always contain a small electron/positive hole component in the otherwise predominantly ionic conductivity. Thus in an experiment of very long duration, depending on the ionic transport number of the membrane, the eventual transfer would be of both oxygen and sulphur to the manganese side of the membrane. The transfer can be shown schematically as... 

Hole transfer (HT) is the movement of a positive chaige, say in cation radicals. In reality, it is still an ET process but it is often more convenient to focus on the migration of the positive hole (vacated by an electron) rather than the electron (which moves in the opposite direction to the hole). 

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