Organic donor-acceptor complexes

It is interesting that X values in modified proteins are comparable to those values (0.9-1.25 eV) extracted from studies of organic donor-acceptor complexes [3, 30], The electronic couplings estimated for close contact, however, are dramatically smaller than typical values ( 300 to 1900 cm obtained from experiments on donor(spacer)acceptor molecules [3]. The intrinsic (close contact) electronic coupling also varies significantly from protein to protein. 

In Section P-68.1 of Ref. 4, a slightly different nomenclature is presented for organic donor-acceptor complexes. 

Based on Hydrogen Bonding or on Organic Donor-Acceptor Complexes... 

Elegant evidence that free electrons can be transferred from an organic donor to a diazonium ion was found by Becker et al. (1975, 1977a see also Becker, 1978). These authors observed that diazonium salts quench the fluorescence of pyrene (and other arenes) at a rate k = 2.5 x 1010 m-1 s-1. The pyrene radical cation and the aryldiazenyl radical would appear to be the likely products of electron transfer. However, pyrene is a weak nucleophile the concentration of its covalent product with the diazonium ion is estimated to lie below 0.019o at equilibrium. If electron transfer were to proceed via this proposed intermediate present in such a low concentration, then the measured rate constant could not be so large. Nevertheless, dynamic fluorescence quenching in the excited state of the electron donor-acceptor complex preferred at equilibrium would fit the facts. Evidence supporting a diffusion-controlled electron transfer (k = 1.8 x 1010 to 2.5 X 1010 s-1) was provided by pulse radiolysis. 

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 ... 

The BFT, PFg" and SbCl salts of cation radicals are readily prepared by oxidation of organic donors with the corresponding NO+ salts in a relatively nonpolar solvent such as dichloromethane. For example, a solution of the hydroquinone ether MA in anhydrous (deaerated) dichloromethane turns purple upon the addition of crystalline NO+BFT at low temperature ( 50°C).173 The coloration is due to formation of the donor/acceptor complex [MA, NO+] (equation 34). 

Importantly, the purple color is completely restored upon recooling the solution. Thus, the thermal electron-transfer equilibrium depicted in equation (35) is completely reversible over multiple cooling/warming cycles. On the other hand, the isolation of the pure cation-radical salt in quantitative yield is readily achieved by in vacuo removal of the gaseous nitric oxide and precipitation of the MA+ BF4 salt with diethyl ether. This methodology has been employed for the isolation of a variety of organic cation radicals from aromatic, olefinic and heteroatom-centered donors.174 However, competitive donor/acceptor complexation complicates the isolation process in some cases.175... 

Electron donor-acceptor complexes, electron transfer in the thermal and photochemical activation of, in organic and organometallic reactions, 29, 185 Electron spin resonance, identification of organic free radicals, 1, 284 Electron spin resonance, studies of short-lived organic radicals, 5, 23 Electron storage and transfer in organic redox systems with multiple electrophores, 28, 1... 

Electron transfer, in thermal and photochemical activation of electron donor-acceptor complexes in organic and organometallic reactions, 29,185 Electron-transfer, single, and nucleophilic substitution, 26,1 Electron-transfer, spin trapping and, 31,91 Electron-transfer paradigm for organic reactivity, 35, 193... 

Electron Transfer in the Thermal and Photochemical Activation of Electron Donor-Acceptor Complexes in Organic and Organometallic Reactions... 

A new wave of interest and productive research was aroused by synthesis of tricyclic condensed derivative benzobis(l,3,2-dithiazole) (BBDTA) which was shown experimentally to afford at least three distinct oxidation states dication, radical cation, and diradical <86SM233, 86SM239). This has been helpful for developing an approach to ferromagnetic organic metals in which homomolecular stacks are formed from donor-acceptor complexes in which the donor is a triplet and the acceptor a radical ion derived from the donor <86SM233>. Initial experiments showed conductivities lO " -10 S for complexes of BBDTA radicals with TCNQ <86SM239>. 

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