Donor-acceptor absorption

Donor-acceptor absorption can also be observed in semiconductors, but this process is weak because of the small overlap of the wavefunctions (like an n -> n transition). Donor-acceptor absorption is best monitored through the emission, that is by excitation spectra. In the normal situation, the donor-acceptor absorption can be observed but the valence band-to-donor and the acceptor-to-conduction band transitions can also be seen, as they also contribute to the luminescence. All three of these transitions are weak but of similar strengths [6]. In undoped AgCl and AgBr, only a very weak excitation spectrum is seen, which consists of a relatively sharp line near the band edge. In Cd2 + doped AgBr both the sharper line, whose onset is about... 

In addition to the donor-acceptor absorption, a new ultraviolet band (230 to 400 nm, marked CT in Figure 6.11) appears upon adduct formation. This absorption is associated with the transition a— a between the two orbitals formed by the interaction between the frontier orbitals. Because the donor orbital (in this case, from the solvent or I ) contributes the most to lower a adduct orbital, and the I2 LUMO contributes the most to the a adduct orbital, the CT transition transfers an electron from an orbital that is primarily of donor composition to one that is primarily of acceptor composition hence, the name charge transfer (CT) for this transition. The energy of this transition is less predictable, because it depends on the energy of the donor orbital. These transitions result in electron density being shifted... 

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. 

In these dye-functionalized dendrimers, light absorbed by the numerous peripheral coumarin-2 units is funneled to the coumarin-343 core with remarkably high efficiency (toluene solution 98% for the first three generations 93% for compound 8). Given the large transition moments and the good overlap between donor emission and acceptor absorption, energy transfer takes place by Forster mechanism [34]. 

Proteases are enzymes that break peptide bonds in proteins. As such they lend themselves to a variety of homogeneous assay techniques. Most employ labeling both ends of the substrate with a different tag, and looking for the appearance (disappearance) of the signal generated in the intact substrate (product). As an example, for a fluorescence quench assay, the N-terminal of a peptide is labeled with DNP and the C-terminal with MCA. As such, the peptide is fluorescently silent since the fluorescence from DNP is quenched by absorption by the MCA. Another very popular donor/acceptor pair is EDANS 5-[(2-aminoethyl)amino] naphthalene-1-sulfonic acid and DABCYL 4-(4-dimethylaminophenylazo)benzoic acid) (a sulfonyl derivative (DABSYL) [27], Upon peptide cleavage, the two products diffuse, and due to a lack of proximity, the fluorescence increases. 

In many other situations of donor-acceptor solutes in aprotic solvents, such as quaternary alkylammonium salts (R4NX), a UV absorption shift to higher wavelength has proved the occurrence of simple cation-anion ion... 

Fig. 4) related to the 1 1 complex [Br /TCP] [23]. Additions of chloride or iodide salts to the same acceptor also result in the immediate appearance of new absorptions characteristic of the donor/acceptor complexes. Importantly, the band maxima are red-shifted in [r/TCP] and blue-shifted in [cr/TCP] relative to that in [Br /TCP]. 

In a similar way, the formation of halide complexes with other jt-acceptors in Fig. 3 are revealed by the appearance of new absorption bands in the electronic spectra to reflect the yellow to red colorations of the mixtures. The spectral data thus indicate that halide salts form well-defined electron donor/acceptor complexes with organic jt-acceptors, as typified by Eq. 2 ... 

Have we pushed the use of time resolved transient absorption with kinetic modeling to its limit We think not. Studies of the temperature dependence of hole transport in hairpins such as 3GAZ should provide additional details about the mechanism of this processes. The use of donor-acceptor triplexes... 

Energy transfer measurements were used, together with fluorescence and absorption spectral data of the donor and acceptor moieties, to calculate the donor-acceptor separation via the Forster equation. The average values of R obtained assuming random donor-acceptor orientations were 21.3 1.6 for (1) and 16.7 + 1.4 for (2). The average separation obtained from molecular models is 21.8 + 2.0 for (1) and 21.5 2.0 for (2). The somewhat low calculated separation between the groups of (2) may be due to nonrandom donor-acceptor orientations. 

Inasmuch as K and L are constants not readily available from experimental data, only the form of Eq. (6.22) is of interest. For example, the rate constant should decrease exponentially with increasing separation R between the donor and acceptor. Also, because donor and acceptor multiplicities can change during the transfer, the overlap integral is calculated with both the donor emission and acceptor absorption normalized to unity. 

Absorption studies of 2-cyclohexenone-ethoxylethylene solutions failed to reveal evidence of donor-acceptor complex formation. It should be noted, however, that photocycloaddition from ground state 7r-complexes (such as would be observed from absorption studies) does not correctly predict the observed orientational effects. 

It is very difficult to excite the donor without also exciting some of the acceptor population. This is because the absorption spectra of dyes extend significantly into the blue side of their absorption maxima, so the absorption spectra of the donor and acceptor usually overlap. The donor fluorescence can typically be observed without acceptor fluorescence interference therefore, when measuring FRET efficiency by observing the donor fluorescence, this overlap is not important. However, when observing the acceptor fluorescence the overlap of the donor and acceptor absorption must be taken into account. The total steady-state fluorescence of the acceptor, assuming that [A] = [D (i.e., a equal donor and acceptor concentrations, and 100% labeling) is... 

What is the donor/acceptor ratio in a given cell Again, this ratio cannot be directly derived because it concerns two quantities that stem from fluorophores with different properties (absorption coefficient, quantum yield, spectra) and that emit into two channels differing in gain, filters, and excitation intensity. Thus, the (overlap corrected) intensity of acceptors in channel A will be a factor k times that of donors in D, at equimolar concentrations,3 or ... 

In Eq. (4.5) the donor emission spectrum/ and the acceptor absorption spectrum eA are separately normalized to unity, so that the transfer rate is independent of the oscillator strength of either transition. Unfortunately, the constants W and L are not easily determined by experiment. Nevertheless, an exponential dependence on the distance is expected. It should be noted that this type of transfer involves extensive orbital overlap and is guided by Wigner s (1927) spin rule. 

Various enol silyl ethers and quinones lead to the vividly colored [D, A] complexes described above and the electron-transfer activation within such a donor/acceptor pair can be achieved either via photoexcitation of charge-transfer absorption band (as described in the nitration of ESE with TNM) or via selective photoirradiation of either the separate donor or acceptor.41 (The difference arising in the ion-pair dynamics from varied modes of photoactivation of donor/acceptor pairs will be discussed in detail in a later section.) Thus, actinic irradiation with /.exc > 380 nm of a solution of chloranil and the prototypical cyclohexanone ESE leads to a mixture of cyclohexenone and/or an adduct depending on the reaction conditions summarized in Scheme 5. 

The scope of the Patemo-Buchi cycloaddition has been widely expanded for the oxetane synthesis from enone and quinone acceptors with a variety of olefins, stilbenes, acetylenes, etc. For example, an intense dark-red solution is obtained from an equimolar solution of tetrachlorobenzoquinone (CA) and stilbene owing to the spontaneous formation of 1 1 electron donor/acceptor complexes.55 A selective photoirradiation of either the charge-transfer absorption band of the [D, A] complex or the specific irradiation of the carbonyl acceptor (i.e., CA) leads to the formation of the same oxetane regioisomers in identical molar ratios56 (equation 27). 

Thermal or photochemical activation of the [D, A] pair leads to the contact-ion pair D+, A-, the fate of which is critical to the overall efficiency of donor/acceptor reactivity as described by the electron-transfer paradigm in Scheme 1 (equation 8). In photochemical reactions, the contact ion pair D+, A- is generated either via direct excitation of the ground-state [D, A] complex (i.e., CT path via irradiation of the charge-transfer (CT) absorption band in Scheme 13) or by diffusional collision of either the locally excited acceptor with the donor (A path) or the locally excited donor with the acceptor (D path). 

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