Donor-acceptor pair spectra

Equilibrium constants for complex formation (A") have been measured for many donor-acceptor pairs. Donor-acceptor interaction can lead to formation of highly colored charge-transfer complexes and the appearance of new absorption bands in the UV-visible spectrum may be observed. More often spectroscopic evidence for complex formation takes the font) of small chemical shift differences in NMR spectra or shifts in the positions of the UV absorption maxima. In analyzing these systems it is important to take into account that some solvents might also interact with donor or acceptor monomers. 

Forster (1968) points out that R0 is independent of donor radiative lifetime it only depends on the quantum efficiency of its emission. Thus, transfer from the donor triplet state is not forbidden. The slow rate of transfer is partially offset by its long lifetime. The importance of Eq. (4.4) is that it allows calculation in terms of experimentally measured quantities. For a large class of donor-acceptor pairs in inert solvents, Forster reports Rg values in the range 50-100 A. On the other hand, for scintillators such as PPO (diphenyl-2,5-oxazole), pT (p-terphenyl), and DPH (diphenyl hexatriene) in the solvents benzene, toluene, and p-xylene, Voltz et al. (1966) have reported Rg values in the range 15-20 A. Whatever the value of R0 is, it is clear that a moderate red shift of the acceptor spectrum with respect to that of the donor is favorable for resonant energy transfer. 

The excitation intensity was also varied in order to determine the effect this had on the PL spectrum. PL1 and PL3 bands had a blueshift per decade of 3.7 meV and 5.5 meV, respectively, with an increase in excitation intensity. The blueshifts were attributed to donor-acceptor pair (DAP) recombination.68,69 PL2 did not show any excitation power dependency, whereas the analysis of the PL4 band was not attempted because of the uncertainty in its precise location. The effect of increasing excitation intensity can be clearly observed in Fig. 6.29. 

The electronic spectrum of the complex consists of a combination of the spectra of the parent compounds plus one or more higher wavelength transitions, responsible for the colour. Charge transfer is promoted by a low ionization energy of the donor and high electron affinity of the acceptor. A potential barrier to charge transfer of Va = Id — Ea is predicted. The width of the barrier is related to the intermolecular distance. Since the same colour develops in the crystal and in solution a single donor-acceptor pair should be adequate to model the interaction. A simple potential box with the shape... 

Excitation and emission spectra of molecules for donor-acceptor pairs can be found at one of the following Web sites Becton-Dickinson Fluorescence Spectrum Viewer (http //www. bdbiosciences.com/spectra), Invitrogen-Molecular Probes Fluorescence Spectra Viewer (http //www.probes.invitrogen. com/servlets/spectraviewer). 

Weller24 has estimated enthalpies of exciplex formation from the energy separation vg, — i>5 ax of the molecular 0"-0 and exciplex fluorescence maximum using the appropriate form of Eq. (27) with ER assumed to have the value found for pyrene despite the doubtful validity of this approximation the values listed for AHa in Table VI are sufficiently low to permit exciplex dissociation during its radiative lifetime and the total emission spectrum of these systems may be expected to vary with temperature in the manner described above for one-component systems. This has recently been confirmed by Knibbe, Rehm, and Weller30 who obtain the enthalpies and entropies of photoassociation of the donor-acceptor pairs listed in Table XI. From a detailed analysis of the fluorescence decay curves for the perylene-diethyl-aniline system in benzene, Ware and Richter34 find that... 

It has been noted by Potasek [105] that electron tunneling in the donor-acceptor pair D-A may lead to the appearance of a charge transfer band in the absorption spectrum of this pair. The author obtained the following formula describing the dependence of the extinction coefficient, , of this band on the energy, E, of the absorbed light quantum... 

Figure 2 FRET characteristics, (a) The FRET efficiency as a function of R/Rq is shown f = 1/(1 + (R/Rq) ). Proper selection of FRET pairs so that distances of interest lie near Rg where the FRET efficiency-distance slope is greatest will give the most sensitivity, (b) The spectral overlap requirement for FRET between the donor emission and the acceptor fluorescence spectrum is shown for the organic cyanine Cy3-Cy5 donor-acceptor pair.

The EPR spectrum of the intrinsic STH was also recorded by ODMR in studies of nominally pure AgCl [171,172] and was later identified [173-175]. The self-trapped hole and shallowly trapped electron undergo donor-acceptor pair recombination which contributes to a blue-green (500 nm) luminescence from AgCl [69]. The species observed in the ODMR spectrum has g-factors, hyperfine, and superhyperfine matrices that are identical, within experimental error, to those observed earlier by EPR methods [68]. 

The broad PL band at 0.988 eV is registered in PL spectra of the annealed samples (spectrum 2). The high-energy tail of this band contains many narrow lines (shown in the inset more detail). A similar emission band and narrow lines were detected previously in silicon crystals doped with P and In by thermal diffusion. As shown in [2], the recombination involving P and In centres separated by distances from 0.77 to 2 nm is responsible for the observed sharp line structure. In our experiment part of In and As atoms occupy regular sites in the Si lattice after annealing. In our opinion these spectra (narrow lines and broad band at 0.985 eV) are due to donor-acceptor pair recombination between In (acceptor) and As (donor) separated by the distance from 0.6-2.5 nm. 

Figure 3.12 10 K PL spectrum for a forming gas-anneaied ZnO substrate in the region where donor-acceptor-pair transition and LO-phonon replicas are expected to appear. (After Ref [50].)...

In electron donor-acceptor (EDA) complexes, there is always a donor molecule and an acceptor. The donor may donate an unshared pair (an n donor) or a pair of electrons in a ti orbital of a double bond or aromatic system (a it donor). One test for the presence of an EDA complex is the electronic spectrum. These complexes generally exhibit a spectrum (called a charge-transfer spectrum) that is not the same as the sum of the spectra of the two individual molecules. Because the first excited state of the complex is relatively close in energy to the ground state, there is usually a... 

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