Charge-transfer complexes and radical ion salts

Dithiafulvene derivatives behave as -electron donors and form stable charge-transfer complexes and radical ion salts with a wide variety of organic and inorganic acceptor species. 

CONDUCTION IN STRONG (IONIC) DONOR-ACCEPTOR (CHARGE-TRANSFER) COMPLEXES AND RADICAL IONS SALTS... 

Charge transfer complexes and radical ion salts can be prepared either as single crystals and polycrystalline powder or as thin films. Single crystals can be obtained by the following methods ... 

Electroconductive charge-transfer complexes and ion-radical salts from 4,4 -bipyranilidenes and bi(chalcogenopyranilidenes) 93MI17. 

Semi-amphiphilic molecules are used to develop conductive LB films. In this case, unsubstituted active molecules are attached to amphiphiles through Coulomb interactions (Figure 14.5). This method is employed for the construction of the LB films of charge-transfer complexes and ion radical salts. In the former case the amphiphile should be electrically active and in the latter inert molecules can be used. By using amphiphilic donor and acceptor, a superlattice of alternate layers with non-centrosymmetric structure is obtained (Figure 14.6). 

Bulk crystalline radical ion salts and electron donor-electron acceptor charge transfer complexes have been shown to have room temperature d.c. conductivities up to 500 Scm-1 [457, 720, 721]. Tetrathiafiilvalene (TTF), tetraselenoful-valene (TST), and bis-ethyldithiotetrathiafulvalene (BEDT-TTF) have been the most commonly used electron donors, while tetracyano p-quinodimethane (TCNQ) and nickel 4,5-dimercapto-l,3-dithiol-2-thione Ni(dmit)2 have been the most commonly utilized electron acceptors (see Table 8). Metallic behavior in charge transfer complexes is believed to originate in the facile electron movements in the partially filled bands and in the interaction of the electrons with the vibrations of the atomic lattice (phonons). Lowering the temperature causes fewer lattice vibrations and increases the intermolecular orbital overlap and, hence, the conductivity. The good correlation obtained between the position of the maximum of the charge transfer absorption band (proportional to... 

In the context of mechanistic studies, the electrochemical behavior and reactions with nucleophiles of 4-chloro-2,6-diphenylpyrylium and 4-chloro(bromo)flavylium have been studied <1999CHE653>. The proposed mechanism for nucleophilic substitution in halogen-substituted pyrylium and flavylium salts passes through formation of a charge-transfer complex that is converted into an ion-radical pair by simple electron transfer. Heterocyclic cleavage of the C-halogen bond occurs at the stage of the radical or the adduct from the reaction of the pyrylium salt and the nucleophile. In this study, an amine nucleophile was used however, the data are likely relevant for other types of nucleophiles as well (Scheme 5). 

As in the case of carbenium ions, one must distinguish between stable cation-radical salts which can be prepared and characterised without excessive precautions, and which owe their stability to considerable delocalisation of the positive charge and the unpaired electron, and transient, reactive cation-radicals (often without counterion) which undergo rapid secondary reactions immediately after their formation. While recently a few instances have been reported of initiation by the first type of species (see Sect. V-E), the presence of the less stable entities is characteristic in such initiation processes as the activation of charge-transfer complexes, the anodic oxidation of suitable anions... 

Reduction of the dication 138 by excess lithium iodide gives a complex which seems to exist as an equilibrium mixture of a neutral charge-transfer complex 139 and ion-radical salt 140. The ratio of iodine, = 2.8, is estimated by elemental analysis. ... 

Knowledge of how aluminum chloride oxidizes aromatics to cation radicals is practically non-existent. At one time it seemed that a nitro compound was a necessary co-acceptor (Buck et al., 1960) and that, whereas with mononuclear alkylaromatics, the Lewis acid-nitro compound pair formed only charge transfer complexes (Brown and Grayson, 1953), complete electron transfer occurred with more easily oxidized aromatics. But, cation-radical formation from perylene, anthracene, and chrysene was found to occur in carbon disulfide, chloroform, and benzene solutions, too (Rooney and Pink, 1961) and even occurs on warming anthracene and naphthacene with solid aluminum chloride (Sato and Aoyama, 1973). There is no doubt that a nitro compound enhances electron transfer, however (Sullivan and Norman, 1972). Cation radical formation in AlCl3-nitromethane has been estimated as approximately 100% as compared with 1% in sulfuric acid oxidation of dialkoxybenzenes (Forbes and Sullivan, 1966). Unfortunately, aluminum halide salts have not been isolated and, therefore, even the beginnings of analytical data have yet to be collected. There is no definite knowledge of either the nature of the counter ion or the fate of the electrons in these cation-radical formations. 

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