Donor-acceptor transfer

The FRET efficiency Ed as determined above is the fraction of energy quanta absorbed by all donor molecules that is transferred to acceptors. For a given pixel, Ed effectively reflects both the efficiency with which paired donor-acceptors transfer energy (E) and the fraction of molecules in that pixel that pair up (/)>). This means, for example, that a pixel with ED = 0.2 may result from 100% of donors having =0.2, or from 20% of donors having E = 1, or anything in between. The FRET efficiency E of a donor/acceptor pair (termed characteristic FRET efficiency, Ec in some literature [2, 3]) is most often unknown. 

Let the electron-acceptor interaction be described by the short-range potential UsA(f—fA) where rA is the center of acceptor coordinate. It is supposed that the wave function of the electron bound state on the acceptor with energy E, TG(F — ta, E), is an exact one, i.e. it is considered not only UA(f—rA) but the donor-electron interaction also. Then, the exact value of the matrix element of the electron donor-acceptor transfer is equal [1] ... 

As expected, the inferred value for rF turned out to be larger than the value based upon the spectral overlap integral because that analysis ignores donor-donor transfer prior to donor-acceptor transfer. The current theory takes proper account of this fact. 

If measurements of 4>p and/or Tp are to be used to evaluate intramolecular relaxation rates, the effect of intermolecular processes on these quantities must be assessed. In addition, the eflBciency of excitation energy transfer is of intrinsic interest, especially in connection with solid state photochemical and photophysical processes. If excitation energy is transferred to an identical center, i,e, the same species in precisely the same environment, then all of the relaxation rates k2-ks are unaffected, and no change in measureable quantities (except polarization) is expected. However, if transfer occurs between non-identical centers, then observable changes will occur. Two situations can be distinguished (36) (a) single step donor-acceptor transfer and (b) migration transfer. 

Measurement of the donor lifetime, which typically is 2-25 nsec, requires adequate time resolution. Two techniques, time-correlated singlephoton counting and frequency-domain fluorimetry modulation, can be used (see A. R. Holzwarth, this volume [14]). Excellent books have been written which include discussion of each technique, and Lakowicz and co-workers have discussed advances infrequency-domain instrumentation and applications to FRET. Donor lifetime measurements, unlike steady-state measurements, are capable of detecting multiple donor-acceptor transfer efficiencies in the sample. These lead to multiexponential decays. Donor lifetime measurements are also not affected by an inner-filter effect... 

The energy transfer mechanism in dilute systems has been summarized by Watts (1975). At high donor concentrations and at elevated temperatures the donor-donor transfer may be appreciable. The fluorescence decay curves of the donors behave differently in the two cases mentioned above. If we write the donor-acceptor transfer rate as a R and the donon-donor rate as b R where R is the separation between the interacting ions and s equals 6, 8, and 10, for dipole-dipole, dipole-quadrupole, and quadrupole-quadrupole interactions, respectively, then two limiting cases can be considered (i) h/a = 0, where donor-donor interaction is absent, and (ii) b a>l, where donor-donor interaction is predominant. In the former case the decay curve of the donor fluorescence is nonexponential, being the sum of the decay of an isolated donor ion and the energy transfer to various accepted ions characterized by the factor exp(-Af ). In the opposite limit, which corresponds to rapid donor-donor transfer, the decay is exponential at all times, with a rate equal to the total donor-acceptor transfer... 

Wynne K, Galli C and Hochstrasser R M 1994 Ultrafast charge transfer in an electron donor-acceptor complex J. Cham. Phys. 100 4796-810... 

Figure C 1.2.9. Schematic representation of photo induced electron transfer events in fullerene based donor-acceptor arrays (i) from a TTF donor moiety to a singlet excited fullerene and (ii) from a mthenium excited MLCT state to the ground state fullerene.

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. 

Figure C3.2.7. A series of electron transfer model compounds with the donor and acceptor moieties linked by (from top to bottom) (a) a hydrogen bond bridge (b) all sigma-bond bridge (c) partially unsaturated bridge. Studies with these compounds showed that hydrogen bonds can provide efficient donor-acceptor interactions. From Piotrowiak P 1999 Photoinduced electron transfer in molecular systems recent developments Chem. Soc. Rev. 28 143-50.

Current research aims at high efficiency PHB materials with both the high speed recording and high recording density that are required for future memory appHcations. To achieve this aim, donor—acceptor electron transfer (DA-ET) as the hole formation reaction is adopted (177). Novel PHB materials have been developed in which spectral holes can be burnt on sub- or nanosecond time scales in some D-A combinations (178). The type of hole formation can be controlled and changed between the one-photon type and the photon-gated two-photon type (179). 

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