Acceptor-donor molecules

Yang X, Jiang X, Zhao C, Chen R, Qin P, Sun L (2006) Donor-acceptor molecules containing thiophene chromophore synthesis, spectroscopic study and electrogenerated chemiluminescence. Tetrahedron Lett 47 4961 -964... 

Much fundamental work yet remains in the study of intramolecular donor-acceptor molecules to find out what structural parameters of the donor, acceptor and particularly the linkage enhance the efficiency of forward electron transfer while at the same time inhibiting the rate of reverse electron transfer. Progress so far is very promising. 

Energy transfer [3,12] between molecules has also been used in the design of optical sensors. Here, an excited molecule (donor) can transfer its electronic energy to another species (acceptor). This process occurs without the appearance of a photon and results from dipole-dipole interaction between the donor-acceptor molecules. The rate depends on the fluorescence quantum yield of the donor, the overlap of the emission spectra of the donor with the absorption of the spectrum of the acceptor, and their relative orientation and distance. It is the overlap of... 

Intramolecular charge transfer in conjugated donor-acceptor molecules may be accompanied by internal rotation leading to TICT (twisted intramolecular charge transfer) states. A dual fluorescence may be observed as in PCT-5 (Letard et al., 1994) (which resembles the well-known DMABN (see section 3.4.4) containing a dimethylamino group instead of the monoaza-15-crown-5) the short-wavelength... 

Z. R. Grabowski, K. Rotkiewicz, and A. Siemiarczuk, Dual fluorescence of donor-acceptor molecules and the twisted intramolecular charge transfer (TICT) states, J. Lumin. 18, 420 (1979). 

We recently prepared a series of restricted distance porphyrin-quinone donor-acceptor molecules designed to study the dependence of the rate of electron transfer proceeding from the lowest excited singlet state of the porphyrin on the free energy of reaction.(3-4) We now report measurement... 

This decrease is more pronounced than that observed in small aromatic donor-acceptor molecules.(18) In these molecules the energy of the lowest excited singlet state of the electron donor is about 3.5 eV and the energy of... 

Coumarins are one of several classes of fluorescent structures containing the carbonyl group as an essential molecular substructure and showing the features typical of donor-acceptor molecules. Others include naphthalimides, perylene esters, perylenediimides, benzanthrones, anthraquinones, benzoxanthones and benzothioxanthones (see below). 

The squarylium (4.6) and croconium (4.7) dyes are closely related structurally to cyanines but are in fact donor-acceptor molecules and consequently the design principles of near-IR absorbers based on these chromophores are different. The synthesis of these chromophores is achieved easily by reacting either squaric acid, or preferably an alkylated derivative, e.g. di-n-butyl squarate, or croconic acid with electron-donor molecnles. The croconium dyes absorb at significantly longer wavelengths than the sqnarylinms as shown in Figure 4.2. 

In chemical terms the photoinduced electron transfer results in transfer of an electron across the photosynthetic membrane in a complex sequence that involves several donor-acceptor molecules. Finally, a quinone acceptor is reduced to a semiquinone and subsequently to a hydroquinone. This process is accompanied by the uptake of two protons from the cytoplasma. The hydroquinone then migrates to a cytochrome be complex, a proton pump, where the hydroquinone is reoxidized and a proton gradient is established via transmembrane proton translocation. Finally, an ATP synthase utilizes the proton gradient to generate chemical energy. Due to the function of tetrapyrrole-based pigments as electron donors and quinones as electron acceptors, most biomimetic systems utilize some... 

Figure 1. Contributions to /3XXX (solid) in a 2-orbital donor-acceptor molecule arise from a product of the terms Xg - Xex (dash) and x2gex (dash-dot), and l/(Eg - Eex)2 (dot).

Incorporation of an ionic component into a donor/acceptor molecule is a very effective way of suppressing electron back-transfer. One interesting example consists of the photo-oxidation of leuko crystal violet (LCV) to crystal violet (CV, the dye) by benzophe-none bearing a quaternary ammonium ion (Tazuke Kitamura 1984). In this case, the cation radical of LCV formed is repulsed by the ammonium positive charge. At the same time, the benzophenone anion radical remains stabilized by the attached cationic atmosphere (Scheme 5-16). As shown in the scheme, two favorable results are achieved the stabilization of an ion radical pair by counterion exchange and the charge separation by coulombic repulsion between the two positive charges. This leads to 100% efficiency of the photo-oxidation. With unsubstituted benzophenone itself, the efficiency does not exceed 20%. 

The protons are dissociated away in contact with the water in the internal channels. Center. A covalent bonding of proton donor-acceptor molecules and a sufficiently dense stacking leads to a solvent free proton transport. Bottom. In the soggy sand electrolytes anions are absorbed at the surfaces of the insulating matrix (e.g., SiOJ. The respective cations (e.g., Li+) are free while far away from the matrix essentially associated in form of ions pairs if the solvent is a weak dielectric. 

The following ten types of triad donor-acceptor molecules may be differentiated ... 

Keeping in mind that all three x-rayed flavodoxins (7, 8, 9, 10, 11) exhibit C(7,8) as the only possible point of direct outer contact while all the rest of the flavin molecule is buried in the protein, we must finally admit that chemically C(8) is the reasonable site of orbital overlap between flavin and secondary heteroaromatic le -donor-acceptor molecules such as flavin itself and, above all, cytochrome. Flavodoxins represent the case of flavoproteins scheduled for le -only transfer by a hydrogen bond directed from the apoprotein towards N(5), stabilizing the radical Hp. The bond is strong enough to maintain the active protein reduced in the lower le -shuttle between Flred and Fl. 

NON-ADIABATIC PHOTOPROCESSES OF FUNDAMENTAL IMPORTANCE TO CHEMISTRY FROM ELECTRONIC RELAXATION OF DNA BASES TO INTRAMOLECULAR CHARGE TRANSFER IN ELECTRON DONOR-ACCEPTOR MOLECULES... 

In the classical Forster treatment of energy transfer between two well-separated donor/acceptor molecules the rate of energy transfer depends on 1/r6. Though there is a strong distance (r) dependence it still... 

Fig. 9. Charge-transfer excited states in a schematic donor-acceptor molecule and in a stilbazole.

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