Forster transfer, efficiency

Figure 6.8. Dependence of the energy transfer efficiency (E = etin the figure) on distance. The slope of 5.9 is in excellent agreement with the r e dependence for Forster-type transfer. From Stryer and Haugh-land.(45) Reprinted by permission of Proc. Nat. Acad. Sci. U.S.

Posokhov, Y. O., Merzlyakov, M., Hristova, K. and Ladokhin, A. S. (2008). A simple proximity correction for Forster resonance energy transfer efficiency determination in membranes using lifetime measurements. Anal. Biochem. 380, 134—6. 

Note that the transfer efficiency is 50% when the donor-acceptor distance is equal to the Forster critical radius. Equation (4.83) shows that the distance between a donor and an acceptor can be determined by measuring the efficiency of transfer, provided that r is not too different from Ro (which is evaluated by means of Eq. 4.80). 

Fig. 9.1. Variations in the transfer efficiency as a function of the ratio donor—acceptor distance/Forster critical radius.

Baldo et al. [ 164] used the platinum complex of 2,3,7,8,12,13,17,18-octaethyl-21 //,23//-porphine (PtOEP, 66) as efficient phosphorescent material. This complex absorbs at 530 nm and exhibits weak fluorescence at 580 nm but strong phosphorescence from the triplet state at 650 nm. Triplet transfer from a host like Alq3 was assumed to follow the Dexter mechanism. Dexter-type excitation transfer is a short-range process involving the exchange of electrons. In contrast to Forster transfer, triplet exciton transfer is allowed. 

Bifluorophores consisting of two different fluorescent dyes linked by a flexible spacer containing heteroatoms (oxygen, nitrogen or sulfur atoms) can bind cations. This results in a decrease of the distance between the two fluorophores and, consequently, to an increase in efficiency of photoinduced energy transfer between the two moieties (Figure 2.13) provided that the emission spectrum of the donor (D) overlaps the absorption spectrum of the acceptor (A).(36) The transfer efficiency depends on the distance according to Forster s theory ... 

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 validity of the Forster theory was tested and confirmed in a number of model studies with compounds that contained a donor and an acceptor separated by well-defined rigid spacers. This work has been reviewed 5] In a classical study, a naphthyl group (donor) was attached to the C-terminal and a dansyl group (acceptor) to the N-terminal of poly-L-proline oligomers (1-12 proline residues) 61 These proline oligomers assume a trans helical conformation in ethanol and thus represent spacers of well-defined length (12-46 A). A continuous decrease in the transfer efficiency from 100% at a donor-acceptor separation of... 

An important condition for energy transfer in this manner is an overlapping of the fluorescence emission spectrum of the donor with the absorption spectrum of the acceptor. A small distance (less than 10 nm) between donor and acceptor is likewise important because according to Forster the efficiency of energy transfer decreases with the inverse sixth power of the distance between donor and acceptor ... 

Excited-state proton transfer relates to a class of molecules with one or more ionizable proton, whose proton-transfer efficiency is different in the ground and excited states. The works of Forster [2-4] and Weller [5-7] laid the foundation for this area on which much of the subsequent work was based. Forster s work led to the understanding of the thermodynamics of ESPT. He constructed a thermodynamic cycle (Forster cycle) which, under certain acceptable approximations, provides the excited-state proton-transfer equilibrium constant (pK f,) from the corresponding ground-state value (pKa) and electronic transition energies of the acid (protonated) and base (deprotonated) forms of the ESPT molecule ... 

The energy transfer efficiency is directly proportional to the spectral overlap, and this also directly affects the Forster distance of a particular D-A pair. Figure 10.5 shows the D and A excitation and emission spectra in an ideal energy transfer system, wherein D and A have very distinct excitation spectra (so that A can only be excited by energy transfer and not by direct photon absorption at the wavelengths used to excite D)—the D emission and A excitation spectra overlap strongly—and the D and A emission maxima are well separated, so that the quenching of D fluorescence and the enhancement of A fluorescence can be individually measured.98 99... 

The Forster distance R0 (A) at which the energy-transfer efficiency is 50% is calculated using Equation (14.5) ... 

---