Distance of donor acceptor

Fluorescence resonance energy transfer (FRET) luminescence occurs when donor phosphor decreases its emission intensity and luminescent lifetime, while acceptor phosphor lights up. As the precondition of FRET, the donor emission and the acceptor absorption require adequate spectra overlaps. The spatial distance of donor-acceptor pair is the second factor. Only within a small range, the energy could be transferred from donors to... 

It is important to recognize that the intermolecular long-distance bonding with the participation of halogen derivatives represents a specific example of the broad general area of donor/acceptor interactions. Moreover, the complexes of molecular iodine, bromine and chlorine with aromatic donors represent classic examples of charge-transfer compounds [26-28] that are vital for the development of Mulliken theory of intermolecular association [29-31]. The latter thus provides the convenient framework for the... 

At shorter distances, particularly those characteristic of H-bonded and other charge-transfer complexes, the concepts of partial covalency, resonance, and chemical forces must be extended to intramolecular species. In such cases the distinction between, e.g., the covalent bond and the H-bond may become completely arbitrary. The concept of supramolecular clusters as fundamental chemical units presents challenges both to theory and to standard methods of structural characterization. Fortunately, the quantal theory of donor-acceptor interactions follows parallel lines for intramolecular and intermolecular cases, allowing seamless description of molecular and supramolecular bonding in a unified conceptual framework. In this sense, supramolecular aggregation under ambient thermal conditions should be considered a true chemical phenomenon. 

Chapter 9 is devoted to resonance energy transfer and its applications in the cases of donor-acceptor pairs, assemblies of donor and acceptor, and assemblies of like fluorophores. In particular, the use of resonance energy transfer as a spectroscopic ruler , i.e. for the estimation of distances and distance distributions, is presented. 

Frequency-domain measurements of fluorescence energy transfer are used to determine the end-to-end distance distribution of donor-acceptor D-A) pairs linked by flexible alkyl chains. The length of the linker is varied from 11 to 2B atoms, and two different D-A pairs are used. In each case the D-A distributions are recovered from global analysis of measurements with different values for the FSrster distance, which are obtained by collislonal quenching of the donors. In all cases essentially the same distance distribution Is recovered from the frequency-domain data for each value of tha Ffirster distance. The experimentally recovered distance distributions are compared with those calculated from the RIS model. The experimentally recovered distance distributions for the largest chain molecules are In agreement with the predictions of the RIS model. However, the experimental and RIS distributions are distinct for the shorter D-A pairs. 

The expression for ket in equation (29) is still not a complete expression for the total electron transfer rate constant. Both the electronic coupling term V and A0 are dependent upon the interreactant separation distance r, and, therefore, so is ktt in equation (29). The dependence of /.0 on r is shown in equation (23) in the dielectric continuum limit. The magnitude of V depends upon the extent of donor-acceptor electronic orbital overlap (equation 17) and the electronic wave-functions fall off exponentially from the centers of the reactants. In order to make comparisons between ktt and experimental values of electron transfer rate constants, it is necessary to include the dependence of ktt on r as discussed in a later section. 

Thus, from fluorescence lifetime and transient absorption measurements we gathered the electron-transfer rate constants, i.e. both for charge-separation and for charge-recombination. Next, we plotted these rate constants as a function of donor-acceptor distance. From the resulting linear dependence (Fig. 9.26) it is possible to determine the attenuation factors p for the presented donor-acceptor... 

The analysis of the charge-separation and charge-recombination rate constants enabled us to determine the [j factor for the C6o-oFL -exTTF conjugates. Plotting the rates as a function of donor-acceptor distance RDA results in linear relationships... 

The charge-separation dynamics as deduced from the decays of the Fc radical cation and the C o radical anion characteristics as a function of donor-acceptor distance (Fig. 9.65) yielded a linear relationship. From the slope [1 was determined... 

The donor/acceptor properties and the electronic coupling interactions determine the redistribution of electron density between the aromatic donor and the electron acceptor upon complexation. Significant changes in structure and reactivity of the coordinated arene can be rationalized in terms of spectral and thermodynamic properties within the framework of the CT formalism. This section is devoted to a consideration of the structural effects of arene coordination (in terms of donor/acceptor bond distance and type of bonding, distortion of arene planarity, expansion of the aromatic ring, and re-bond localization). 

Localization of re-bonds (also termed double-bond fixation ) is an important structural feature frequently observed upon arene coordination to a metal center. To rationalize this effect, the formation of covalent (a) bonds between the metal and particular carbon atoms of the arene ring is commonly invoked. However, this cannot explain all the unusual bond distances observed and is not generally applicable to the analogous findings with organic acceptors. Analysis based on the CT concept allows a comparative treatment of all types of donor/acceptor complexes, and predicts a close relationship between the degree of bond localization and the donor/acceptor strengths of the complexed partners. 

R. Wagner, L. Richter, Y. Wu, J. Weissmuller, A. Kleewein, E. Hengge, Appl. Organometal. Chem., 1998, 12(4), 265-276. Silicon-modified carbohydrate surfactants. VII Impact of different silicon substructures on the wetting behavior of carbohydrate surfactants on low-energy surfaces - distance decay of donor-acceptor forces. ... 

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