Hole traps

Rapid e / h recombination, the reverse of equation 3, necessitates that D andM be pre-adsorbed prior to light excitation of the Ti02 photocatalyst. In the case of a hydrated and hydroxylated Ti02 anatase surface, hole trapping by interfacial electron transfer occurs via equation 6 to give surface-bound OH radicals (43,44). The necessity for pre-adsorbed D andM for efficient charge carrier trapping calls attention to the importance of adsorption—desorption equihbria in... 

At very high dopant concentrations, transport occurs direcdy between the dopant molecules. The polymer acts only as a binder in most cases. Taking TPD-doped PVK as an example, at low TPD concentrations the hole mobihty first decreases from 3 x 10 cm /Vs to 10 cm /Vs with increasing TPD concentration, because TPD molecules act as hole traps (48,49). At higher TPD concentrations, new direct transport channels between the TPD molecules open up and the hole mobihty increases to lO " cm /Vs for ca 60% TPD doping (Table 1, entries 9—11) (48,49). In this case, there is no evidence for unusual interaction between TPD and PVK that affects the hole transport process. 

Figure 9-19. Bund diagram of LPPP with hole traps and gold electrodes with Va<- vacuum level. Ec conduction band, Eva valence band. E, Fermi level. . baudgup energy. and , " trap depths. ,( ) trap distribution, X electron affmity, and All work function of the gold electrodes.

Trilayer structures offer the additional possibility of selecting the emissive material, independent of its transport properties. In the case of small molecules, the emitter is typically added as a dopant in either the HTL or the ETL, near the interface between them, and preferably on the side where recombination occurs (see Fig. 13-1 c). The dopant is selected to have an cxciton energy less than that of its host, and a high luminescent yield. Its concentration is optimized to ensure exciton capture, while minimizing concentration quenching. As before, the details of recombination and emission depend on the energetics of all the materials. The dopant may act as an electron or hole trap, or both, in its host. Titus, for example, an electron trap in the ETL will capture and hold an election until a hole is injected nearby from the HTL. In this case, the dopant is the recombination mmo.-... 

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 +)... 

GG and GGG sequences have been widely used in strand cleavage studies of hole migration in DNA [4-6]. According to conventional wisdom, GG and GGG sequences serve as hole traps . The calculated gas phase ionization potentials reported by Sugiyama and Saito [14] provide relative energies for G vs GG (0.47 eV) and G vs GGG (0.68 eV) that continue to be cited as evi-... 

The difference in stabilities of cation radicals located on G, GG, and GGG sequences was initially investigated by Sugiyama and Saito [14], who employed ab initio methods to calculate the gas phase ionization potentials of nucleobases stacked in B-DNA geometries. Their results indicated large differences in potential for holes on G vs GG (0.47 eV) and GGG (0.68 eV) sequences. A similar G vs GG difference was calculated by Prat et al. [62]. These values suggest that GG and GGG are, in fact, deep hole traps and they have been widely cited as evidence to that effect [54, 63]. 

In order to observe the single-step hole transfer from Py + to inside the DNA, pulse radiolysis of Py-conjugated ODNs with one oxG substituted for G as a hole trap was performed (Scheme 5 Fig. 7). In this case, the oxidation... 

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]. 

The G doublet and triplet effectively functions as a thermodynamic sink in DNA-mediated hole transport. However, the rate determining step of hole trapping at guanine clusters and the rate of hole trapping are not well understood. Furthermore, hole transfer between the donor and acceptor should compete with the hole trapping reaction the relative rate of hole trapping versus the hole transfer rate determines the overall efficiency for the hole transfer. 

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