Donor-acceptor distance, electron-transfer

The effects of deviations from the Born-Oppenheimer approximation (BOA) due to the interaction of the electron in the sub-barrier region with the local vibrations of the donor or the acceptor were considered for electron transfer processes in Ref. 68. It was shown that these effects are of importance for long-distance electron transfer since in this case the time when the electron is in the sub-barrier region may be long as compared to the period of the local vibration.68 A similar approach has been used in Ref. 65 to treat non-adiabatic effects in the sub-barrier region in atom transfer processes. However, nonadiabatic effects in the classically attainable region may also be of importance in atom transfer processes. In the harmonic approximation, when these effects are taken into account exactly, they manifest themselves in the noncoincidence of the... 

In contrast to the dipole-dipole interaction, the electron-exchange interaction is short ranged its rate decreases exponentially with the donor-acceptor distance (Dexter, 1953). This is expected since, for the electron exchange between D and A, respective orbital overlap would be needed. If the energy transfer is envisaged via an intermediate collision complex or an exciplex, D + A—(D-------A)- D + A, then Wigner s rule applies there must be a spin com-... 

Distance The affects of electron donor-acceptor distance on reaction rate arises because electron transfer, like any reaction, requires the wavefunctions of the reactants to mix (i.e. orbital overlap must occur). Unlike atom transfer, the relatively weak overlap which can occur at long distances (> 10 A) may still be sufficient to allow reaction at significant rates. On the basis of work with both proteins and models, it is now generally accepted that donor-acceptor electronic coupling, and thus electron transfer rates, decrease exponentially with distance kji Ve, exp . FCF where v i is the frequency of the mode which promotes reaction (previously estimated between 10 -10 s )FCF is a Franck Condon Factor explained below, and p is empirically estimated to range from 0.8-1.2 with a value of p 0.9 A most common for proteins. 

Davis WB, Ratner MA, Wasielewski MR (2001) Conformational gating of long distance electron transfer through wire-like bridges in donor-bridge-acceptor molecules. J Am Chem Soc 123 7877-7886... 

For example, in 1963 the photochemistry of magnesium phthalocyanine with coordinated uranium cations was studied in pyridine and ethanol and indicated the occurrence of PET to the uranium complex . A rapid photoinduced electron transfer (2-20 ps) followed by an ultrafast charge recombination was shown for various zinc and magnesium porphyrins linked to a platinum terpyridine acetylide complex . The results indicated the electronic interactions between the porphyrin subunit and the platinum complex, and underscored the potential of the linking para-phenylene bisacetylene bridge to mediate a rapid electron transfer over a long donor-acceptor distance. 

After the ESIPT the molecule exhibits a pronounced ringing in this mode. The 295 cm-1 mode is a symmetric in-plane stretching vibration (see Fig. 3). The corresponding contraction of the molecule reduces the donor acceptor distances in both chelate rings simultaneously and initiates the electronic configuration change of the concerted ESIPT. The concerted double proton transfer leads therefore to a ringing of the molecule in this second mode. 

As follows from the data from Sect. 2, the primary photochemical stage in the majority of the membrane systems studied is the redox quenching of the excited photosensitizer by an electron acceptor or donor leading to electron transfer across the membrane // water interface. For electron transfer to occur from the membrane-embedded photosensitizer to the water soluble acceptor, it is necessary for the former to be located sufficiently close to the membrane surface, though the direct contact of the photosensitizer with the aqueous phase is not obligatory. For example, Tsuchida et al. [147] have shown that electron transfer to MV2 + from photoexcited Zn-porphyrin inserted into the lecithin membrane, is observed only until the distance from the porphyrin ring to the membrane surface does not exceed about 12 A. 

Thus, from fluorescence lifetime and transient absorption measurements we gathered the electron-transfer rate constants, i.e. both for charge-separation and for charge-recombination. Next, we plotted these rate constants as a function of donor-acceptor distance. From the resulting linear dependence (Fig. 9.26) it is possible to determine the attenuation factors p for the presented donor-acceptor... 

FRET occurs when the electronic excitation energy of a donor chromophore is transferred to an acceptor molecule nearby via a through-space dipole-dipole interaction between the donor-acceptor pair.80 The strong dependence of the FRET efficiency on donor-acceptor distance has been widely exploited in studying the structure and dynamics of proteins and nucleic acids, in the detection and visualization of inter-molecular association, and in the development of intermolecular binding assays.81,82... 

Studies on electron transfer rates through proteins, DNA and in supramolecular systems have shown that generally such rates decrease exponentially with increasing donor-acceptor distance according to the following equations [10] ... 

Electrochemical studies on SAMs have proven invaluable in elucidating the impact of various molecular parameters such as bridge structure, molecular orientation or the distance between the electroactive species and electrode surface. As described above in Section 5.2.1, the kinetics of heterogeneous electron transfer have been studied as a function of bond length for many systems. Similarly, the impact of bridge structure and inter-site distances have been studied for various supramolecu-lar donor-acceptor systems undergoing photoinduced electron transfer in solution. In both types of study, electron transfer is observed to increase as the distance between the donor and acceptor decreases. As discussed earlier in Chapter 2, the functional relationship between the donor-acceptor distance and the electron transfer rate depends on the mechanism of electron transfer, which in turn depends on the electronic nature of the bridge. 

Fig. 25 The series of dyads, 29(n), possessing the oligo-p-phenylenevinylene bridge that were used to investigate the switchover from superexchange characteristics to molecular wire behaviour in the photoinduced electron transfer reaction, from the locally excited state of tetracene (TET) donor to the pyromellitimide (PI) acceptor.148 Also, shown are a schematic of the photoinduced charge separation rate versus, donor-acceptor distance (lower left-hand side) and the LUMO energies of TET and the various bridges (lower right-hand side).

Electron transfer theory predicts that the rate of an electron transfer reaction will vary predictably with temperature (T), AG, and donor-acceptor distance (r) according to the relationships given in Eq. 4 and 5 (Marcus and Sutin, 1985), where h is Planckis constant, R is... 

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