Hole Trapping Sites

Many publications are devoted to the EPR study of the hole trapped centers in Ti02 nanoparticles after low temperature irradiation [36, 37,44-46,49, 63-72], The interpretations of the observed EPR signals and assignments of the photogenerated hole species are less clear than electron traps, and several contradictions can be found in the literature. As it was shown recently [36, 37,46, 54, 55, 57] the main reason for the controversy is that holes are localized in the surface region of the nanoparticles and the structure of the hole centers strongly depends upon surface treatment and modification. 

Charge Separation in the Surface-Modified Ti02 Colloids 

Metal oxide colloids have been effectively coupled with multifunctional ligands containing carboxyl groups that bind to the surface of nanoparticles [46]. One can rationally design optimal photocatalysts by tailoring functional groups for selective adsorption of specific 

These EPR studies have shown that the surface modifier must contain a carboxyl group to bind to the colloid surface and at the same time to bind to the metal ions. The surface modifier has to have a hole trap that enhances photogenerated charge pair separation distance. A mercapto group that is in an a position relative to a carboxyl group enhances adsorption of 

The g-factor for the surface trapped electrons was found to be g = 1.924, the same as in the unmodified Ti02 colloids. These trapping sites are not significantly affected by adsorption of alanine, probably because of the low surface coverage of alanine. However, in the presence of copper, heating of the sample to room temperature resulted in the disappearance of the signal for trapped electrons. Under die same conditions, the reduction of copper ions to a metallic state was confirmed using X-ray absorption spectroscopies (XAS) [27]. 

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For instance, clays are of particular interest with respect to energy storage, because of the frequency and variety of isomorphic, positive charge deficient, cation substitutions. These result in significant numbers of stable hole trapping sites (67-68). The fate of the associated electrons in clays is more speculative (69-70). They... 

Hole trapping sites (in the form of Cz/Cz dimers) able to catch electrons to form blue-emitting excitons... 

The main elements of the energy landscape for positive charge transfer mentioned above are shown in Figure IB, using the fragment of the DNA duplex schematically depicted in Figure 1A as an illustration. In this particular case, the doublet G Gg is an example ofthe deep hole trap, sites G., and G exemplify intermediate states, while sites Tj, T, T, and Tg correspond to "bridging"states. 

In the perfectly paired double strand 22, the yield of product PGgg> which indicates the amount of charge that has reached the hole trap GGG, is 68%. But if the intermediate G C base pair is exchanged by a G T mismatch, the efficiency of the charge transport drops to 23%. With an abasic site (H) opposite to G the hole transport nearly stops at this mismatched site (Fig. 15). We have explained this influence of a mismatch on the efficiency of the charge transport by a proton transfer from the guanine radical cation (G2 +)... 

Site-Selective Hole Trapping by Modified Guanines. 178... 

With the site-selective hole injection and the hole trapping device established, the efficiency of the hole transport between the hole donor and acceptor, especially with respect to the distance and sequence dependence, were examined. Our experiments showed that hole transport between two guanines was extremely inefficient when the intervening sequence consisted of more than 5 A-T base pairs [1]. Hole injection into the DNA n-stack using photoexcited dCNBPU was accompanied by the formation of dCNBPU anion radical. Therefore, hole transport would always compete with the back electron transfer (BET). To minimize the effect of BET, we opted for hole transport between G triplets, that are still lower in oxidation potential than G doublet. With this experimental system, we researched the effect of the bridging sequence between two G triplets on the efficiency of hole transport [2]. 

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