Donor-acceptor distances, energy-transfer

Energy Transfers Donor-Acceptor Distances and Control of the Photophysical Properties of the Acceptor by the Donor... 

Much of chemistry occurs in the condensed phase solution phase ET reactions have been a major focus for theory and experiment for the last 50 years. Experiments, and quantitative theories, have probed how reaction-free energy, solvent polarity, donor-acceptor distance, bridging stmctures, solvent relaxation, and vibronic coupling influence ET kinetics. Important connections have also been drawn between optical charge transfer transitions and thennal ET. 

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-... 

Figure 6.11 The dependence of the efficiency of energy transfer, Er, on the donor-acceptor distance, R, according to the Forster theory...

The sixth power dependence explains why resonance energy transfer is most sensitive to the donor-acceptor distance when this distance is comparable to the Forster critical radius. 

This section deals with a single donor-acceptor distance. Let us consider first the case where the donor and acceptor can freely rotate at a rate higher than the energy transfer rate, so that the orientation factor k2 can be taken as 2/3 (isotropic dynamic average). The donor-acceptor distance can then be determined by steady-state measurements via the value of the transfer efficiency (Eq. 9.3) ... 

Energy transfer experiments permit the measurement of distances because the rate of transfer is proportional to the inverse sixth power of the distance between donor and acceptor. According to the theory of Forster the donor-acceptor distance r is related to the transfer efficiency E by eq 16... 

The design of dendritic multiporphyrin systems [18] permits energy transfer over longer distances. The outer shell of the dendrimer shown in Fig. 5.14 is made up of eight porphyrin-zinc complexes as energy donor units. Excitation of the units of the outer shell leads to fluorescence emission of the metal-free porphyrin core as a result of energy transfer from the periphery to the energy acceptor [19]. 

The feasibility of intramolecular electron- and energy-transfer depends on distance and is usually studied in covalently linked systems. However, donor-acceptor dyads can be also arranged by self-assembly what resembles the situation of electron transfer in biological systems. Artificial dyads tethered by a small number of hydrogen bonds immediately dissociate in methanol or water. To improve the binding while keeping the reversibility, a photoinducible electron donor-acceptor dyad linked by a kinetically labile bond was designed. [19]... 

Fig. 6 Schematic representations of (a) the spectral overlap between donor emission and acceptor absorption spectra promoting energy transfer (b) energy transfer efficiency as a function of the donor acceptor distance.

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... 

Forster energy transfer or energy transfer at a distance occurs between two molecules, a donor (the excited fluorophore), and an acceptor (a chromophore or fluorophore). Energy is transferred by resonance, i.e., the electron of the excited molecule induces an oscillating electric field that excites the acceptor electrons. As a result of this energy transfer, the fluorescence intensity and quantum yield of the emitter will decrease. Energy transfer is described in Chapter 14. 

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