Charge-transfer donor-acceptor

Specific donor-acceptor charge transfer interactions can lead to a relatively large numerical value of the electronic matrix element, possibly attributable to an increase in V, and, thus, to larger rate constants than those predicted by distance variations alone. 

Radical anions are produced in a number of ways from suitable reducing agents. Common methods of generation of radical anions using LFP involve photoinduced electron transfer (PET) by irradiation of donor-acceptor charge transfer complexes (equation 28) or by photoexcitation of a sensitizer substrate (S) in the presence of a suitable donor/acceptor partner (equations 29 and 30). Both techniques result in the formation of a cation radical/radical anion pair. Often the difficulty of overlapping absorption spectra of the cation radical and radical anion hinders detection of the radical anion by optical methods. Another complication in these methods is the efficient back electron transfer in the geminate cation radical/radical anion pair initially formed on ET, which often results in low yields of the free ions. In addition, direct irradiation of a substrate of interest often results in efficient photochemical processes from the excited state (S ) that compete with PET. 

In this chapter, for space reasons, only a few paradigmatic examples of rotaxanes and catenanes based on donor-acceptor (charge transfer (CT)) and/or hydrogen bonding interactions (systems based on metal-ligand bonding are reviewed in another... 

The most prospective donors are those with ionization potentials of ID < 6.6 eV. Acceptors with electron affinities of EA > 2.6 eV are suitable. When / EA < 4 eV, donor-acceptor interaction leads to strong molecular complexes with a charge-transfer degree >0.5. Donor-acceptor charge transfer often results in the formation of ion radical salts having metallic conductivity. In terms of charge-transfer degree, ion radical salts have values >0.7. 

Conduction in weak donor-acceptor (charge-transfer) complexes 198... 

CONDUCTION IN WEAK DONOR-ACCEPTOR (CHARGE TRANSFER) COMPLEXES... 

Figure 10. Schematic of a general mechanism for extrinsic carrier generation in a donor-acceptor charge transfer system. (Reprinted with permission from Ref [29n].)...

Donor acceptor charge transfer complex based photoreceptors continue to be described in the literature and studied using modern spectroscopic techniques but none has been commercialized. For example, the photoconducting charge transfer complex between poly(V-epoxypropylcarbazole) and TNF has been studied with transient absorption and time-resolved fluorescence. On the basis of Monte Carlo simulations, the results were interpreted in terms of a heterogeneity of charge transfer complexes with different radiative probabilities and a distribution of initial charge pair separation distances [30c]. 

Anesthetics that associate by donor-acceptor (charge transfer) complex formation, either as electron donors (low ionization potential) or as electron acceptors (high electron affinities). There are no confirmed examples of this but fluorocarbon anesthetics containing higher halogens seem to be eligible. 

Levine, B.F. Donor-acceptor charge transfer contributions to the second order hyperpolarizability, Chem. Phys. Lett. 37, 516-520 (1976)... 

Examples are the 1, l -dibenzyl-4, 4 -bipyridinium electron-acceptor dication threaded into the 1, 5-dinaphtho-38-crown-10 (Fig. 2a) [10], and the acyclic polyether containing a dioxybenzene electron-donor unit threaded into the electron-acceptor cyclobis(paraquat-p-phenylene) tetracationic cyclophane (Fig. 2b) [11]. Although in these cases a large contribution to the association driving force comes from the electron-donor/acceptor (charge-transfer, CT) interactions, hydrogen bonding can also play an important role, as clearly shown in the cases of pseudorotaxanes constituted by 4, 4 -bipyridinium [12a] or l,2-bis(pyridinium)ethane [12b] threads and crown ethers. 

Nu, attacks the dication formed by the disproportionation of the cation radical A, + and the half-regeneration mechanism, equations 8 and 9, in which nucleophilic attack takes place directly on the radical cation. A third mechanism, termed the complexation mechanism, is closely related to the disproportionation mechanism but differs from it in that one of the reacting radical cation molecules of equation 6 is complexed to a molecule of the nucleophile in a donor-acceptor charge-transfer tt complex. The role of the nucleophile donor is to facilitate electron transfer to the second radical cation group. The disproportionation step of the complexation mechanism is indicated in equation 10. 

Murata, T., Morita, Y, Yakiyama, Y, Fukui, K., Yamochi, H., Saito, G., and Nakasuji, K. 2007. Hydrogen-bond interaction in organic conductors Redox activation, molecular recognition, structural regulation, and proton transfer in donor-acceptor charge-transfer complexes of TTF-imidazole. /. Am. Chem. Soc. 129 10837-10846. 

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