Forster transfer

Knox R S and Gulen D 1993 Theory of polarized fluorescence from molecular pairs—Forster transfer at large electronic coupling Photochem. Photobiol. 57 40-3... 

If the conditions for Forster transfer are not applicable, then the theory must be extended. There is recently experimental evidence that coherent energy transfer participates in photosynthesis [74, 75], In this case, the participating molecules are very close together. The excited state of the donor does not completely relax to the Boltzmann distribution before the energy can be shared with the acceptor, and the transfer can no longer be described by a Forster mechanism. We will not discuss this case. There has been active discussion of coherent transfer and very strong interactions in the literature for a longer time [69], and references can be found in some more recent papers [70-72, 76, 77],... 

The acronym FRET and extensions beyond Forster transfer... 

Fig. 19.12 Characteristics of the nonradiative Forster transfer, (a) FRET spectra show that e 100%, 91%, and 63%, respectively, when the pump intensity is below, near, and above the R6G lasing threshold, (b) FRET spectra show a decreased s (61%, 41%, and 34%) when R6G concentration increases while LDS722 concentration and the pump power remain the same, (c) LDS722 emission as a function of its concentration. Reprinted from Ref. 20 with permission. 2008 International Society for Optical Engineering...

Another major energy transfer process, the so-called Forster transfer mechanism is based on a dipole-dipole interaction between the host excited state and the guest ground state (Figure 4.2) [24], It does not include the transfer of electrons and may occur over significantly larger distances. The rate constant of the Forster energy transfer is inversely proportional to the sixth power of the distance R between the molecules ... 

T Forster, Transfer mechanisms of electronic excitation, Discussions Faraday Soc., 27 7-17, 1959. 

The emission of the Alq3 DCM system appears orange and not red as the spectrum of the pure DCM dye because of a nonideal overlap of the energy states for Forster transfer. A purer red emission can be obtained in a three-component system with both rubrene (62) and DCM2 (62b) as dopants. The energy transfer occurs in two steps via rubrene to DCM2 [150]. Here, the emission of DCM2 is at... 

Another class of red dopants, tetraphenylporphyrins (63), offer a direct energy transfer from blue to red [151], The absorption bands comprise the sharp porphyrin Soret band at 418 nm and the weaker Q bands at 512 and 550 nm. The photoluminescence shows two sharp transitions at 653 and 714 nm and can be induced from a blue emitting host by Forster transfer to the Soret band and internal conversion to the Q bands. 

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. 

In other white light devices, blue, green and red emitters are combined. Kido et al. [169, 170] designed multilayer systems using 6 (TPD) for blue, metal-chelate complexes for green and red emission, respectively. Similar devices have been developed by other groups, using Forster transfer or exciton confinement for the creation of the three primary colors [171, 172]. Exciplex emission was... 

Since one of the most investigated systems for the incorporation of laser resonators is the Forster-transfer couple Alq3 DCM [188, 189], we will discuss this as a representative example for low molecular glass lasers. 

Some tryptophans do not exhibit phosphorescence because of quenching by specific sites from within the protein. The absence of phosphorescence could be due to quenching of either the singlet state or the triplet state. For example, in horse heart cytochrome c the tryptophan is adjacent to the heme, and its fluorescence is quenched by Forster transfer to the heme. Since the singlet state is populating the triplet state, the lack of observable phosphorescence is likely to be due to an unpopulated triplet state. Another example where the redox center of the protein interacts with the tryptophan excited states is found in azurin. The copper(II) quenches both the singlet and triplet states.(28)... 

More convincing proof for a particle-enhanced energy transfer mechanism comes from a study of the concentration dependence of the transfer. Bulk Forster transfer leads to a linear dependence on acceptor concentration with constant donor-to-acceptor ratio. The resonance mechanism would be expected to saturate at (relatively) high concentrations and fall off linearly at very low concentrations. 

The preceding analysis is valid in the region of acceptor concentration below that where bulk Forster transfer occurs. 

Forster Transfer. Singlet energy transfer has actually been known for many years, as evidenced by numerous studies of the sensitized fluorescence of dyes.161,162 In these cases strong overlap of donor emission with acceptor absorption results in Forster type transfer but the acceptor undergoes no photochemistry. 

The only recent example of Forster transfer of photochemical importance is the demonstration by Saltiel163 that the ability of azulene to increase the photostationary transjcis ratio in direct photoisomerization of the stilbenes is due entirely to radiationless transfer of excitation from traw.y-stilbene singlets to azulene. As expected for Forster transfer, this azulene effect did not depend upon solvent viscosity. The experimental value of R0, the critical radius of transfer in Forster s formula,181 was 18 A, in good agreement with the value calculated from the overlap of stilbene emission and azulene absorption. 

Rice [147] has noted the similarity of form between the time-dependent rate coefficients from the Smoluchowski, eqn. (19), Debye— Smoluchowski, eqn. (53), diffusion and Forster transfer equations,... 

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