D-A pair

Although the quantum efficiency for photoinduced chaige separation is close to unity for a D/A pair, the conversion efficiency in a bilayer heterojunction device is limited ... 

The membrane-separated reductant and oxidant formed upon PET can be used for accomplishment of various catalytic redox reactions which provide conversion of the chemical energy of a (D. ..A ) pair into the chemical energy of a pair of more stable species such, e.g., as H2 and O2 molecules. This stored energy can be released when necessary in the form of high potential heat or electricity via combustion of H2 + 1/2 O2 mixture in a furnace or fuel cell. 

Fig. 3.7.6 DDIF spectra and SPRITE MRI images of Berea obtained in different saturation states. (A) The DDIF spectra during cocurrent imbibition at different water saturation (Sw) levels. Note the similar shape of DDIF spectra at different Sw. (B) The DDIF spectra during counter-current imbibition acquired at different water saturation levels. Note the change in the DDIF spectral shape for the different saturation levels. (C, D) A pair of images show 2D longitudinal slices from 3D...

Mg2+ ion. 49 has been used to deposit MgO by atomic layer epitaxy,222 and is commonly employed as a />-type dopant for semiconductors, particularly GaAs,223 GaN,224,225 and AlGaN.226 In GaN, Mg doping induces a blue 2.8 eV photoluminescence band arising from donor-acceptor (D-A) pair recombination.227 It is likely that isolated Mg... 

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). 

Donor/acceptor association and the electron-transfer paradigm form the unifying theme for the C—C bond cleavage of various benzpinacols and diary-lethane-like donors in the presence of different electron acceptors (such as chloranil (CA), dichlorodicyanobenzoquinone (DDQ), tetracyanobenzene (TCNB), triphenylpyrylium (TPP+), methyl viologen, nitrosonium cation, etc.). Scheme 13 reminds us how this is achieved by either CT photolysis of the D/A pair or via diffusional quenching of the excited electron acceptor A by the electron donor D. 

Diels-Alder cycloaddition reactions of electron-poor dienophiles to electron-rich dienes, which are generally carried out thermally, afford widespread applications for C—C bond formation. On the basis of their electronic properties, numerous dienes can be characterized as electron donors and dienophiles as electron acceptors. Despite the early suggestions by Woodward,206 the donor/ acceptor association and electron-transfer paradigm are usually not considered as the simplest mechanistic formulation for the Diels-Alder reaction. However, the examples of cycloaddition reactions described below will show that photoirradiation of various D/A pairs leads to efficient cycloaddition reactions via electron-transfer activation. 

Donor and acceptors can be covalently linked using a chemical spacer. Assume that we have the same D-A pair Eosin-Phenol Red. In this case we will have a mixture of two linked donor-acceptor species (Eosin-Phenol Red protonated and Eosin-Phenol Red unprotonated) characterized by the same distance distribution and different critical distances (ftoi = 28.3 A and Rm = 52.5 A) for FRET. A distribution of D- to -A distances will be present because the linker is typically flexible. The fractional intensity of the first species at time t = 0 is gi and that of the second species is (1 - 1). The fractional intensity at time t = 0 is equal to fractional concentration of each form, which can be in case of pH indicator (Phenol Red) calculated using Eq. (10.31). The donor fluorescence intensity decay of the mixture is described by the equation... 

AF, FO and FI stand for the antiferro-, ferro- and ferri-magnetic interactions, respectively, expected for the D/A pair. Symbols s, d, t, q and p signify the single, double, triple, quadruple and quintuple degeneracy of the partially occupied orbitals. The superscript number corresponds to the total number of electron spins in those degenerate orbitals. 

EEDQ is a valuable reagent for carboxyl activation in the coupling of amino acids [55]. Ideal d-a pairings of various reactive moieties facilitate the formation of the semicarbonate intermediates. Of course, a major driving force for this transformation is the aromatization of the dihydroquinoline system. 

Decarboxylation of chloroformic esters is greatly catalyzed by dimethylformamide [90]. As indicated in the following equation, an iminium intermediate with contiguous d-a pairs is involved. 

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 energy transfer efficiency is directly proportional to the spectral overlap, and this also directly affects the Forster distance of a particular D-A pair. Figure 10.5 shows the D and A excitation and emission spectra in an ideal energy transfer system, wherein D and A have very distinct excitation spectra (so that A can only be excited by energy transfer and not by direct photon absorption at the wavelengths used to excite D)—the D emission and A excitation spectra overlap strongly—and the D and A emission maxima are well separated, so that the quenching of D fluorescence and the enhancement of A fluorescence can be individually measured.98 99... 

As acceptors, they used 9,10-dicyanoanthracene (DCA) or 2,6,9,10-tetracyanoan-thracene (TCA), the donors were methylated benzenes or naphthalenes. The D/A pair was designed to give radical ion pairs on irradiation. In order to determine sep, they monitored the quantum yield of dimethoxystilbene radical cation formation [70], which intercepts the free radical cation of the donor exclusively. Assuming a constant k, from earlier studies [66b], they indirectly obtained k e, according to Eq. (11) ... 

In late 1996, a new family of DNA-mediated ET experiments began to be reported. In these the ET donor is comprised of one or more DNA nucleotides (either G, GG, GGG, or a covalent thymine dimer) while the other is a covalently attached, photoactivated electron acceptor (either anthraquinone, pyrene, or an tris-polypyridyl M(III) complex, where M = Ru or Rh) [74-80]. These experiments have much in common with the kind of experiments outlined immediately above and with the new hairpin studies recently reported. They will be discussed toward the end of this chapter. They differ from the pre-1997 ET kinetics experiments that will be discussed immediately below in that none of them has reported ET rate measurements. Rather, yields of net photochemistry (either DNA strand cleavage or thymine dimer scission) are presented as evidence that photoinduced ET events occurred. For comparison of experimental results with ET theory it is clear that at a minimum ET rates must be measured and at a maximum several rates should be measured for a given D/A pair at a variety of separation distances. 

The weakest parts of this study are that unknown base sequences intervene between D/A pairs at any given separation distance and an indirect argument must be made to establish the D/A distances. Dye binding studies show that at saturation EB+ and AOH+ have binding ratios of 0.5 dye/base pair, while DAP2+ has a... 

In addition, the spin-inversion symmetry is available for M=0. The operator 6 is defined when it acts on a Slater determinant, transforming all spin-up sites to spin-down sites and vice versa. Thus the space of M=0 is invariant under the group [7,0] which consists of identity and spin-inversion operations. Let us discuss the 20 determinants of M=0 for trimethylene-cyclopropane, the spin-inversion operator transforms A to B, C to D, and E to F, or vice versa, respectively. Under the compounded groups [/,d], a pair of Slater determinants that are transformable via the spin-inversion operation should combine, as represented by A B, C D and E F respectively. Thus blocks A and A2 are further reduced to two 3x3 and two lxl blocks... 

Table 3. Initial Rp (%/min) of Photo-Induced Copolymerizations of D/A pairs ...

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