Colours charge transfer complexes

The nitrosonium cation can serve effectively either as an oxidant or as an electrophile towards different aromatic substrates. Thus the electron-rich polynuclear arenes suffer electron transfer with NO+BF to afford stable arene cation radicals (Bandlish and Shine, 1977 Musker et al., 1978). Other activated aromatic compounds such as phenols, anilines and indoles undergo nuclear substitution with nitrosonium species that are usually generated in situ from the treatment of nitrites with acid. It is less well known, but nonetheless experimentally established (Hunziker et al., 1971 Brownstein et al., 1984), that NO+ forms intensely coloured charge-transfer complexes with a wide variety of common arenes (30). For example, benzene, toluene,... 

Diclofenac sodium, famotidine and ketorolac were analysed utilising their formation of a coloured charge transfer complex with 2,4 dichloro-6-nitrophenol. The complexes were detected by UV/visible spectrophotometry at 450 nm. The method was not affected by the presence of common excipients in the formulations analysed. The precision and accuracy of the method was comparable to that of HPLC methods used to analyse the same samples. ... 

A coloured charge transfer complex formed by absorption of ferrocyanide at the surface of Ti02 particles and electrodes, on photo-excitation injects electrons into the conduction band of Ti02 the conduction band electrons can be used to generate a photocurrent (Gratzel, 1987)... 

Intensely coloured charge-transfer complexes are formed from N0XX.A (X - Al, Ga, Tl X ci F) and benzene or laethyl-substltuted benzenes. The unusual aluminium bls(dlcarbolllde)... 

New stationary phases for specific purposes in chromatographic separation are being continually proposed. Charge transfer adsorption chromatography makes use of a stationary phase which contains immobilised aromatic compounds and permits the separation of aromatic compounds by virtue of the ability to form charge transfer complexes (sometimes coloured) with the stationary phase. The separation is caused by the differences in stability of these complexes (Porath and Dahlgren-Caldwell J Chromatogr 133 180 1977). 

Stoicescu and Dimonie103 studied the polymerization of 2-vinylfuran with iodine in methylene chloride between 20 and 50 °C. The time-conversion curves were not analysed for internal orders but external orders with respect to catalyst and monomer were both unity. Together with an overall activation energy of 2.5 kcal/mole for the polymerization process, these were the only data obtained. Observations about the low DP s of the products, their dark colour, their lack of bound iodine and the presence of furan rings in the oligomers, inferred by infrared spectra (not reported), completed the experimental evidence. The authors proposed a linear, vinylic structure for the polymer, and a true cationic mechanism for its formation and discussed the occurrence of an initial charge-transfer complex on the... 

Because of the dependence of the dissociation on the polarity of the solvent medium, in the less polar acetone solvent the dissolution of [3-2] does not give rise to the green colour of the Kuhn s carbanion [2 ] but simply the pale yellow colour of the hydrocarbon [3-2]. However, when pyrene, which forms a charge-transfer complex with the tropylium ion (Dauben and Wilson, 1968), is added to the acetone solution, it turns green, indicating that the dissociation is induced by pyrene and that the equilibrium is shifted to the ionic side (Okamoto et al., 1985). 

The electronic spectrum of the complex consists of a combination of the spectra of the parent compounds plus one or more higher wavelength transitions, responsible for the colour. Charge transfer is promoted by a low ionization energy of the donor and high electron affinity of the acceptor. A potential barrier to charge transfer of Va = Id — Ea is predicted. The width of the barrier is related to the intermolecular distance. Since the same colour develops in the crystal and in solution a single donor-acceptor pair should be adequate to model the interaction. A simple potential box with the shape... 

The prospective applications ofmolecular assemblies seem so wide that their limits are difficult to set. The sizes of electronic devices in the computer industry are close to their lower limits. One simply cannot fit many more electronic elements into a cell since the walls between the elements in the cell would become too thin to insulate them effectively. Thus further miniaturization of today s devices will soon be virtually impossible. Therefore, another approach from bottom up was proposed. It consists in the creation of electronic devices of the size of a single molecule or of a well-defined molecular aggregate. This is an enormous technological task and only the first steps in this direction have been taken. In the future, organic compounds and supramolecular complexes will serve as conductors, as well as semi- and superconductors, since they can be easily obtained with sufficient, controllable purity and their properties can be fine tuned by minor adjustments of their structures. For instance, the charge-transfer complex of tetrathiafulvalene 21 with tetramethylquinodimethane 22 exhibits room- temperature conductivity [30] close to that of metals. Therefore it could be called an organic metal. Several systems which could serve as molecular devices have been proposed. One example of such a system which can also act as a sensor consists of a basic solution of phenolophthalein dye 10b with P-cyciodextrin 11. The purple solution of the dye not only loses its colour upon the complexation but the colour comes back when the solution is heated [31]. 

Brightly coloured 1 1 charge-transfer complexes have been observed on mixing electron-poor arenediazonium salts, such as 3,5-dinitrobenzenediazonium tetrafluoro-borate, with aromatic hydrocarbons in acetonitrile. These complexes, which involve... 

The ability of Sn(IV) organometallic compounds to act as both electron donors or acceptors has resulted in their use in photochromic systems38. Addition of 2-aminomethylpyridine to a solution of Sn(n-Bu) X4 (X = CH3COO, Cl or F) results in the formation of a deeply coloured solution, possibly the result of complex formation or, more likely, the formation of a charge-transfer complex. Irradiation of the solution efficiently bleaches the colour, which then recovers upon heating. 

Immediately after mixing a colour characteristic of a charge transfer complex was observed. It eventually disappeared at a rate dependent on the solvent, temperature, and vinyl ether. Polar solvents and higher temperatures led to faster rates as did an increase in the size of the alkyl substituent, C(CH3)3 > CH(CH3)2 > C2H5. Tetracyanoethylene also forms charge transfer complexes with these monomers, but the final products are cyclobutane 2 + 2 cyclo addition compounds rather than polymer. 

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