Distance Dependence of Electron Transfer

Four Aspects of the Distance Dependence of Electron-Transfer Rates... 

Fig.6 The distance dependence of electron-transfer rates in DNA hairpins [51]. The acceptor is a photoexcited derivatized stilbene (SA) or phenanthrene (PA) the electron donor is guanine (G), deazaguanine (Z), or inosine (I). The decay is much more rapid in the Z-PA couple compared to the G-SA couple because the tunneling energy is further from the bridge states in the case of Z-PA...

The distance dependence of electron transfer has been studied extensively for the homogeneous case. An approximately exponential decay of the electronic coupling has been found with the number of saturated bonds in the spacer unit (see for example [6,7]). The results presented here suggest that an exponential dependence fits also our data for heterogeneous electron transfer in ultra-high vacuum. A different result has been reported for electron transfer from Re complexes to anatase where a local triplet state can play a role [8]. 

Photoinduced Electron Transfer. Monolayer organizates are particularly suited for the investigation of photoinduced electron transfer, since the molecules are fixed and the distance between the planes at which the donor and the acceptor molecules, respectively, are located can be well defined. Therefore, complex monolayers have been arranged in order to study the distance dependence of electron transfer in these systems (2, 20). This strategy has also been used to elucidate the relative contributions of electron injection and energy transfer mechanisms in the spectral sensitization of silver bromide (21). 

A comparison between these two types of mechanisms is shown in Fig. 3.3. Demonstrably, these two different mechanisms will impact the experimental results considerably. The distance dependence of electron transfer is certainly one of the features that are impacting the presented DBA systems. However, further theoretical background is needed to understand the different behaviors and the interplay between the two mechanisms. 

Regarding the correct terminology, there are several references and examples of the term molecular wire . In some cases, it describes a system with a very specific behavior. In others, it simply refers to the structural features of the molecule under consideration. Thus, finding a clear definition is a rather difficult task. In 1998 an attempt was made by Emberly and Kirczenow [1] and a molecular wire has been defined as a molecule between two reservoirs of electrons . Nitzan and Ratner, on the other hand, called it a molecule that conducts electrical current between two electrodes [2], Most appropriate with respect to the topic of this thesis, we should stick to a rather restricted definition by Wasielewski, which classifies a molecular wire as a device that conducts in a regime, wherein the distance dependence (of electron transfer) may be very weak [3]. 

Table 4 Distance Dependence of Electron Transfer in Metal-Organic Dyads ...

Until recently very little quantitative experimental data concerning the distance dependence of electron-transfer rates were available. From experiments on electron transfer between statistically distributed donor and acceptor species in an inert glassy matrix it had been concluded (Miller et al., 1984) that the rate falls of sharply with increasing donor-acceptor separation and that at a given edge-to-edge separation Re (in A) the fastest rate (k in s ) achievable under optimally exothermic conditions would be given by the exponential expression eq. (1) ... 

Issues related to the preferred pathways and the distance dependence of electron transfer in biological systems have been addressed by covalently linking electron-transfer donors or acceptors (e.g. a ruthenium complex) to specific sites (e.g. a histidine) of a protein or an enzyme.The distance dependence of the electron-transfer rate constants is generally fitted to equation (44), with most values of for proteins falling in the range of 1.0 to 1.3 A The protein in... 

Long-distance intervalence electron transfer is now demonstrated with metal-metal distances up to 25 A. Distance dependence of electron transfer has strong implications for the search of, for example, molecular switch devices or biosensors. The bridging ligand dicyanamidobenzene has been extensively studied, and for example, complex (18) exhibits a very strong coupling (0.32 eV) for a metal-metal distance of 19.5 A. ... 

Figure 18.7. Distance Dependence of Electron-Transfer Rate. The rate of electron transfer decreases as the electron donor and the electron acceptor move apart. In a vacuum, the rate decreases by a factor of 10 for every increase of 0.8 A. In proteins, the rate decreases more gradually, hy a factor of 10 for every increase of 1.7 A. This rate is only approximate because variations in the structure of the intervening protein medium can affect the rate.

An adaptation of the temperature-jump method, named indirect laser-induced temperature jump [29], was used in studies of distance dependence of electron transfer at electrodes. A pulsed Nd YAG laser was used to cause a sudden (<5 ns) change in temperature (<5 K) at an electrode/electrolyte interface. The increase in temperature causes a change in the open-circuit potential. The relaxation step is a function of the dissipation of thermal energy and the rate of electron transfer between the electrode and its redox partners. 

Distance dependence of electron transfer across SAMs... 

Recent theoretical developments have been directed at incorporating the distance dependence of electron-transfer rate constants into the kinetic equations for diffusion-controlled reactions on the micelle surface, rather than assuming that these are collisional processes [82]. Both forward and back electron-electron transfers have been considered, with fitting of experimental data by numerical integration of appropriate differential equations. A standard Marcus-type expression was used to describe the electron-transfer rate constants. In the particular case of electron transfer from a donor to a single acceptor located initially at 0, one has instead of Eq. 10 [82c]... 

---