Donor acceptor pair emission

FIGURE 9.1 Schematic of emissive transitions of electrons in semiconductors (a) band-band emission (b) a free electron recombines with a trapped hole (c) a trapped electron recombines with a free hole (d) donor-acceptor pair emission. 

Another type of recombination in semiconductors is donor-acceptor pair emission. In this type of emission an electron trapped at a donor and a hole trapped at an acceptor recombine. Again GaP is a very nice example (Fig. 3.24). The donor-acceptor pair emission in this figure is due to a Sp-Zncg pair transition. The lines are due to the fact that the di.stance between the donor Sp and the acceptor Znca varies due to their statistical distribution over the lattice, so that the binding energy of electron and hole varies with the distance between the centers where they are trapped. 

Also in other semiconductors such donor-acceptor pair emissions have been found. A well-known example is ZnS Cu,Al where Alzn is the donor and Cuzo the acceptor. This material is used as the green-emitting phosphor in color television tubes. The blue emission of ZnS Al is due to recombination in an associate consisting of a zinc vacancy (acceptor) and Alzn (donor). Since these centres occur as coupled defects, their distance is restricted to one value only (this is sometimes called a molecular center). Due to strong electron-lattice coupling the emissions from ZnS consist of broad bands. 

Fig. 3.25. Emission tran.sitions in a semiconductoi (schemalical representation). The band gap Eg separates the valence band (VB) and the conduction band (CB). Excitation over the band gap (/) creates electrons in CB and holes in VB. Optical recombination is shown in processes 2-6 (2) a free hole recombines with an electron trapped in a shallow trap level (near-edge emission) (3) the same with a deep electron-trapping level (4) a free electron recombines with a trapped hole (5) donor-acceptor pair emission (6) electron-hole recombination in an associate of a donor and an acceptor...

Finally nonradiative transitions which are specific for semiconductors are mentioned. In order to do so, we consider a specific radiative transition, viz. the donor-acceptor-pair emission (Sect 3.3.9). In the excited. state the donor and acceptor are occupied (Fig. 4.14). This excited luminescent center may show radiative and nonradiative transitions within the center itself (similar to the discussion in Sect. 4.2). In addition, however, there are other processes which are related to the valence and/or conduction band. 

Fig. 4.14. Nonradiativc transitions in a semiconductor. The donor-acceptor pair emission (DA) can be quenched by thermal ionization of one of the centers (a) or by an Auger process (6). In the latter case a conduction electron is promoted high into the conduction band...

Fig. 7.6. Rneigy level scheme for the donor-acceptor pair emission of ZnS Ag. The valence and conduction bands of ZnS are indicated by VB and CB, respectively. D is the shallow donor (aluminium or chlorine), A is the acceptor (silver)...

The other well-known emission bands in GaN, in lower energy regions such as donor-acceptor pair emission together with its phonon replicas, as well as the blue-green, yellow, and red emission bands, have also been observed in GaN films with nonpolar orientations and their origin was, in principle, related to the same defects as the respective emissions in polar materials [95, 99]. 

Let us consider tire case of a donor-acceptor pair where tire acceptor, after capturing excitation from tire donor, can emit a photon of fluorescence. If tire excitation light is linearly polarized, tire acceptor emission generally has a different polarization. Common quantitative expressions of tliis effect are tire anisotropy of fluorescence, r, or tire degree of polarization,... 

CFP-YFP donor-acceptor pair, YFP is several times brighter than CFP [62]. Lastly, for studying dynamic protein associations in plants, the presence of chlorophyll pigments in leaf and stem cells is an additional limitation. These pigments directly absorb the fluorescence, which decreases blue fluorescence intensity for BFP and CFP donors that can be erroneously interpreted as reduced donor fluorescence quantum yield caused by FRET [18]. If sensitized emission or FSPIM is the only available method for quantifying FRET, then it is very important to restrict measurements to chlorophyll free areas within the cells. 

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. 

For the design of complexing bifluorophores, much attention should be paid to the Forster critical radius of the donor-acceptor pair as compared to the interchromophoric distance (Figure 2.13). This critical radius depends on the donor quantum yield and on the spectral overlap between donor emission and acceptor emission. Complex-... 

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... 

This latter expression is very useful, as Ro is characteristic of each donor-acceptor pair, so it can be calibrated and then be used to predict distances from EET measurements. In Fig. 6 we show a schematic representation of the spectral overlap between donor emission and acceptor absorption given by Eq. 2 as well as a plot of the EET efficiency as a function of the donor-acceptor separation. 

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... 

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.

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