Distance donor-acceptor

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. 

Each of the pharmacophore queries consisted of one donor, one acceptor and one of the two hydrophobic points indicated in Figure 1.16. The directionality of hydrogen bonds was inferred from the X-ray structure and reasonably loose tolerances of 1.5 A were used for donor-acceptor distances and 2-2.5 A to the hydrophobe were chosen to allow for the flexibility seen across kinase structures and to maximise the diversity amongst the identified fragments. Four pharmacophore queries were completed and the results were combined. The chosen fragments were further filtered by molecular weight (between 150 and 250), 51og P (between 2.5 and —2.5) and the presence of... 

Fan C, Plaxco KW, Heeger AJ (2005) Biosensor based on binding-modulated donor-acceptor distances. Trends Biotechnol 23 186-192... 

In contrast to the dipole-dipole interaction, the electron-exchange interaction is short ranged its rate decreases exponentially with the donor-acceptor distance (Dexter, 1953). This is expected since, for the electron exchange between D and A, respective orbital overlap would be needed. If the energy transfer is envisaged via an intermediate collision complex or an exciplex, D + A—(D-------A)- D + A, then Wigner s rule applies there must be a spin com-... 

The bond valence of a hydrogen bond depends on the H "O donor-acceptor distance [144, 145] which in turn also correlates with the proton chemical shift [146]. A combination of Eq. (1) in [144] and Eq. (54) of [146] yields an empirical correlation between a hydrogen bond valence, s0..., H, and proton chemical shifts ... 

Figure 16. The series of dyads used for investigating the switchover from superexchange characteristics to molecular wire behavior. Also shown is a schematic of the photoinduced charge separation rate versus donor-acceptor distance.1331...

All examples discussed above, whether concerning very strong 0-H---0 hydrogen bonds or weak C-H---0 and C-H---F interactions, clearly show that a donor-acceptor distance criterion is an unsafe instrument to check the relevance of hydrogen bonding interactions if the effect of ionic charges is neglected. 

Figure 6.11 The dependence of the efficiency of energy transfer, Er, on the donor-acceptor distance, R, according to the Forster theory...

On the other hand, it is known that PET is essentially a contact process described by an exponential distance dependence, which functions effectively at a donor-acceptor distance considerably shorter than that for FRET processes [72]. Thus, it is also feasible to increase donor-acceptor distance to reduce PET. A poly(fluorene-alt-1,4-phenylene) derivative with spiroanthracenyl groups (12) was synthesized... 

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). 

The sixth power dependence explains why resonance energy transfer is most sensitive to the donor-acceptor distance when this distance is comparable to the Forster critical radius. 

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

This section deals with a single donor-acceptor distance. Let us consider first the case where the donor and acceptor can freely rotate at a rate higher than the energy transfer rate, so that the orientation factor k2 can be taken as 2/3 (isotropic dynamic average). The donor-acceptor distance can then be determined by steady-state measurements via the value of the transfer efficiency (Eq. 9.3) ... 

Time-resolved method 1 decay of the donor fluorescence If the fluorescence decay of the donor following pulse excitation is a single exponential, the measurement of the decay time in the presence (td) and absence (t ) of transfer is a straightforward method of determining the transfer rate constant, the transfer efficiency and the donor-acceptor distance, by using the following relations ... 

This approach is valid if the donor fluorescence decay is not too far from a single exponential, so that an average of the donor-acceptor distance can be estimated. 

The donor-acceptor distance may not be unique, especially when the donor and acceptor are linked by a flexible chain (e.g. end-labeled oligomers of polymethylene or polyethylene oxide, oligopeptides, oligonucleotides, polymer chains). 

The survival probability Gs(t) of the donor molecule (i.e. the probability that when excited at t = 0, it is still excited at time t) is obtained by summation over all possible rate constants kT (given by Eq. 9.1), each corresponding to a given donor-acceptor distance r. For a donor molecule surrounded with n acceptor molecules distributed at random in a spherical volume whose radius is much larger than the Forster critical radius R0, Gs(t) is given by... 

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