Hole-transfers

The spectral dependence of the photoresponse of these bilayer heterojunction devices, illuminated from the 1TO side, is displayed in Figure 15-22. The onset of photocurrent at hv— 1.7 cV follows the absorption of the fullerene, indicating a symmetric hole transfer from the excited fullerene to the MEH-PPV. The minimum in the photocurrent at /iv=2.5 eV corresponds to the photon energy of maximum absorption of MEH-PPV. The MEH-PPV layer, therefore, acts as a filter, which reduces the number of photons reaching the MEH-PPV/C()0 interlace. Thus, the thickness of the MEH-PPV layer determines the anlibatic spectral be-... 

In the first part of the reaction scheme (Eq. 30) the generation (g) of holes (h ) in the valence band and of the surface radical S is described. The holes can also be consumed by recombination with electrons (rate v J or by direct hole transfer to the redox system (v, i). The surface radical can react with an electron from the conduction band (v ,2) or with the redox system (v, 2), processes by which the radical disappears. Accordingly, the original bond is repaired as illustrated in Fig. 7b. 

Fig. 11a and b. Electron and hole transfer at large a) and small b) semiconductor particels to an electron acceptor A and donor D... 

A different strategy has been applied in our work, that emphasizes the importance of DNA stability on hole transfer within double-stranded DNA. This work is based on determination of the overall yield of oxidized nucleosides that arise from the conversion of initially generated purine and pyrimidine radical cations within DNA exposed to two-photon UVC laser pulses. On the one hand, this work benefits from the excellent current knowledge of chemical reactions involving the radical cations of DNA bases, and on the other hand, from major analytical improvements that include recent availability of the powerful technique of high performance liquid chromatography-electrospray ionization-tandem mass spectrometry (CLHP-ESI-MS/MS) [16-18]. 

Relevant kinetic information on two competitive reactions of guanine radical cations within double stranded DNA, namely hydration and hole transfer to another guanine residue, has been examined [13]. Thus, the pseudoorder rate for hydration of guanine radical cations 38 has been estimated to... 

In order to determine the influence of the bridge length on the efficiency of the hole transfer between guanines, we studied double strands where the number of (A T)n base pairs between the guanines were varied (Fig. 9). In these experiments the GGG unit acts as a thermodynamic sink for the charge [17]. 

In extreme cases where all electron transfer steps are reversible and the water trapping reactions are very slow, the charge is distributed over the guanines according to the thermodynamic stabilization. From these experiments one cannot deduce the influence of the sequence on the hole transfer rate. Therefore, using a chemical assay of this type leads to results that have to be discussed with great care. 

These results demonstrate that hole transfer between guanines over short and long (A T)n sequences follow different distance dependencies and there-... 

Fig. 13 Influence of the (A T)n sequence on the hole transfer between a guanine radical cation G + and a GGG triplet... 

Fig. 14 Reaction profile diagram for the hole transfer from a guanine radical cation (G +) to a distant GGG sequence via the activated hopping mechanism, which also involves adenines (A) as charge carriers...

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. 

In conclusion, hole transfer between guanines that are separated by long (A T)n sequences is possible because also the adenines become involved as charge carriers. Such a change of the reaction mechanism can also be explained by calculations [21], and has been described by J. Jortner as well as D. Beratan and M.A. Ratner in their articles of this volume. 

Fig. 15 Influence of mismatches on the efficiency of hole transfer through double strands 23 and 24 where the cytidine (C) is exchanged by thymidine (T) and an abasic site (H), respectively. In 25 guanosine is exchanged by N-methylguanosine (see Scheme 6), and C by an abasic site (H)...

Experiments with methyl guanine (27), in which the acidic proton of the radical cation is exchanged by a methyl group, support this explanation [22]. With this base in a mismatch situation (strand 25) the hole transfer becomes efficient again because a deprotonation cannot occur (Fig. 15). 

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. 

Fig. 16 Influence of the exchange of protons (H20) by deuterons (D20) on the efficiency of the hole transfer, measured by the product ratios Pqgg/Pg...

A multistep hopping mechanism explains not only the long distance hole transfer through DNA, it can also rationalize the electron transfer through reduced DNA as T. Carell and M.D. Sevilla demonstrate in this volume. 

Unlike solid state -stacks, however, double helical DNA is a molecular structure. Here CT processes are considered in terms of electron or hole transfer and transport, rather than in terms of material conductivity. Moreover, the 7r-stack of DNA is constructed of four distinct bases and is therefore heterogeneous and generally non-periodic. This establishes differences in redox energetics and electronic coupling along the w-stack. The intimate association of DNA with the water and counterions of its environment further defines its structure and contributes to inhomogeneity along the mole-... 

Fig. 6 Dynamic molecular motions can gate DNA-mediated charge transport. Two time constants (5 and 75 ps) are observed for hole transfer from photoexcited ethidium, tethered and intercalated near the end of a 14-base pair DNA duplex, to a base analog, 7-deazaguanine, in DNA. The 5 ps time constant, which is independent of distance between 10-17 A, is due to direct hole transfer, while the 75 ps time constant corresponds to reorientation of the ethidium before hole transfer. Adapted from [96]... 

The rate constants for these relatively short range hole transfer reactions generally decrease exponentially with distance. Yet, characterizing these DNA-mediated reactions with the parameter (3 is a simplification and is certainly inappropriate in cases where the Frank-Condon factor varies with distance (such as has been observed for the acridine photooxidant). Keeping these limitations in mind, however, /i-values for DNA-mediated hole transfer of -0.6-0.7 A-1 have been suggested using several different oxidant-DNA assemblies (Ap, St, Ap radical cation). 

---