Forster energy-transfer

The distance that separates the two molecules goes from 10 to 60-100 A. Below 10 A, electron transfer may occur between the two molecules, inducing an energy transfer from donor to acceptor, k2 values hold from 0 to 4. For aligned and parallel transition dipoles (maximal energy transfer) k2 is 4, and if the dipoles are oriented perpendicular to each other (very weak energy transfer), k2 is 0. When k2 is not known, its value is considered to be equal to 2/3. This value corresponds to a random relative orientation of the dipoles. 

Orange increases. This type of experiment is one of several ways to show the presence of the energy-transfer mechanism. 

one can put into evidence the energy-transfer mechanism by recording the fluorescence excitation spectrum of the complex (donor-acceptor) (kem is set at a wavelength where only the acceptor emits) and comparing it to the absorption spectrum of the donor alone. In the presence of energy transfer between the two molecules, a peak characteristic of the donor absorption will be displayed in the fluorescence excitation spectrum. 

Membranes fusion can be studied using the energy-transfer mechanism. In fact, membrane vesicles labeled with both NBD and rhodamine probes are fused with unlabeled vesicles. In the labeled vesicles, upon excitation of NBD at 470 nm, emission from rhodamine is observed at 585 nm as a result of energy transfer from NBD to rhodamine. The average distance separating the donor from the acceptor molecules increases with fusion of the vesicules, thereby decreasing the energy-transfer efficiency (Struck et al. 1981). 

To be successfully used as labels in biological assays, rare earth complexes should possess specific properties including stability, high light yield, and ability to be linked to biomolecules. Moreover, insensitivity to fluorescence quenching is of crucial importance when working directly in biological fluids. When complexed with cryptates, however, many of these limitations are eliminated. 

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V.G. Kozlov, V. Bulovic, P.E. Burrows, M. Baldo, V.B. Khalfin, G. Pailhasarathy, S.R. Forrest, Y. You, M. E. Thompson, Study of lasing action based on Forster energy transfer in optically pumped otganic semiconductor thin films, J. Appl. Phys. 1998, 4, 4096. 

Figure 14 Fluorescence intensity ratio of acceptors at 417 nm and donors at 360 nm plotted against the molar fraction of MP-CUA in mixed LB films with (O) PA and ( ) AA The dotted line shows the calculated dependence by Forster energy transfer. 

Pei et al. [412] reported an alternating fluorene copolymer 331 with 2,2 -bipyridyl in a side chain that emitted at 422 nm. Treating this polymer with Eu3+ chelates formed the polymeric complexes 332-334. Their emission was governed by intramolecular Forster energy transfer, whose efficiency depends on the structure of the ligands and the Eu3+ content (Scheme 2.49) [412], The most effective energy transfer manifested itself in a single red emission band at 612 nm for the complex 332 with a maximum intensity achieved at —25 mol% content of Eu3+. 

Another example of efficient Forster energy transfer in Eu3+ complexes of fluorene copolymers (similar to the alternating copolymers described in Scheme 2.49) was demonstrated by Huang and coworkers [414] for random copolymers. They synthesized copolymers 336 with a different ratio between the fluorene and the benzene units in the backbone and converted them into europium complexes 337 (Scheme 2.50) [414]. The complexes 337 were capable of both blue and red emission under UV excitation. In solution, blue emission was the dominant mode. However, the blue emission was significantly reduced or completely suppressed in the solid state and nearly monochromatic (fwhm 4 nm) red emission at 613 nm was observed. 

L. Ding, F.E. Karasz, Z. Lin, M. Zheng, L. Liao, and Y. Pang, Effect of Forster energy transfer and hole transport layer on performance of polymer light-emitting diodes, Macromolecules, 34 9183-9188,2001. 

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 ... 

Triplet decay in the [Mg, Fe " (H20)] and [Zn, Fe (H20)] hybrids monitored at 415 nm, the Fe " / P isosbestic point, or at 475 nm, where contributions from the charge-separated intermediate are minimal, remains exponential, but the decay rate is increased to kp = 55(5) s for M = Mg and kp = 138(7) s for M = Zn. Two quenching processes in addition to the intrinsic decay process (k ) can contribute to deactivation of MP when the iron containing-chain of the hybrid is oxidized to the Fe P state electron transfer quenching as in Eq. (1) (rate constant kj, and Forster energy transfer (rate constant kj. The triplet decay in oxidized hybrids thus is characterized by kp, the net rate of triplet disappearance (kp = k -I- ki -I- kj. The difference in triplet decay rate constants for the oxidized and reduced hybrids gives the quenching rate constant, k = kp — kj, = k, -I- k , which is thus an upper bound to k(. 

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