Aromatic charge transfer complexes

Hooper, H. O. 1964. Lack of Charge Transfer in Aromatic Charge-Transfer Complexes. J. Chem. Phys. 41, 599. 

Fluorescence quenching of fluorene, dibenzofuran, and dibenzothiophens by aromatic nitriles and aliphatic amines is the result of electron transfer with exciplex formation,177 and ion-pair formation in pyromellitic dianhydride-ethylbenzene is followed by dissociation into separated ion pairs in their highly excited states.178 Photochemical iodination of aromatic hydrocarbons may proceed by way of an electronically excited Ia-aromatic charge-transfer complex.179 Modulation excitation spectrophotometry has been used to analyse the nature of some complexes between polycyclic aromatic hydrocarbons and chloranil.180... 

D. Background Information on Aromatic Charge Transfer Complexes. 824... 

Its charge transfer complexes with aromatic hydrocarbons have characteristic melting points and may be used for the identification and purification of the hydrocarbons. 

As well as the cr-complexes discussed above, aromatic molecules combine with such compounds as quinones, polynitro-aromatics and tetra-cyanoethylene to give more loosely bound structures called charge-transfer complexes. Closely related to these, but usually known as Tt-complexes, are the associations formed by aromatic compounds and halogens, hydrogen halides, silver ions and other electrophiles. 

Charge-Transfer Compounds. Similat to iodine and chlorine, bromine can form charge-transfer complexes with organic molecules that can serve as Lewis bases. The frequency of the iatense uv charge-transfer adsorption band is dependent on the ionization potential of the donor solvent molecule. Electronic charge can be transferred from a TT-electron system as ia the case of aromatic compounds or from lone-pairs of electrons as ia ethers and amines. 

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). 

In many cases, the effects of the nucleophile observed with carbo-aromatics can be expected to carry over to heteroaromatics, e.g., changing the relative reactivity of leaving groups (Section II, D, 2, b) deceleration by charge-transfer complexing with the substrate... 

As the preceding section correctly suggests, aromatic rings a-hound in compounds that show biologic activity. The reasons for this are many the role of the pi electrons in some form of charge transfer complex ranks among the more Important. There ire few monocyclic alicyclic compounds known that are used as medicinal agents. 

Charge-transfer complexes with pyrimidine and purine bases as well as with solvents like hexa-methylphosphoramide and dimethyl sulfoxide are reported in Ref 66. The action of aromatic amines (primary, secondary, or tertiary) resulted in fume-offs or unidentifiable tars, in all cases purple or red colors developed prior to more violent reactions (Ref 66)... 

Nitration versus alkylation. Upon the CT irradiation of an orange solution of the charge-transfer complex, the color bleaches rapidly, and either an aromatic nitration product (i.e. 3-nitro-4-methoxytoluene) or an aromatic alkylation product (i.e. 3-trinitromethyl-4-methoxytoluene) is obtained in high yield depending on the reaction conditions summarized in Scheme 22.4lc... 

Donor-acceptor association. It is experimentally well established that nitrosonium cation forms vividly colored charge-transfer complexes with a wide variety of aromatic donors243 (equation 85). 

A. Weller and K. Zachariasse 157-160) thoroughly investigated this radical-ion reaction, starting from the observation that the fluorescence of aromatic hydrocarbons is quenched very efficiently by electron donors such as N,N diethylaniline which results in a new, red-shifted emission in nonpolar solvents This emission was ascribed to an excited charge-transfer complex 1(ArDD(H )), designated heteroexcimer, with a dipole moment of 10D. In polar solvents, however, quenching of aromatic hydrocarbon fluorescence by diethylaniline is not accompanied by hetero-excimer emission in this case the free radical anions Ar<7> and cations D were formed. 

In 1977, Koo and Schuster studied the CL emission produced when diphe-noyl peroxide was decomposed at 24°C in dichloromethane in the dark producing benzocoumarin and polymeric peroxide [111, 112]. No CL emission was observed directly as benzocoumarin is nonfluorescent however, in the presence of aromatic hydrocarbons light was produced because of the fluorescence of these hydrocarbons. The explanation of this phenomenon was based on the above-mentioned CIEEL the aromatic hydrocarbons, which have a low oxidation potential, transfer one electron to diphenoyl peroxide to form a charge-transfer complex, from which benzocoumarin and the corresponding hydrocarbon in the excited state are produced (Fig. 13). 

Peroxyoxalate-based CL reactions are related to the hydrogen peroxide oxidation of an aryl oxalate ester, producing a high-energy intermediate. This intermediate (l,2-dioxetane-3,4-dione) forms, in the presence of a fluorophore, a charge transfer complex that dissociates to yield an excited-state fluorophore, which then emits. This type of CL reaction can be used to determine hydrogen peroxide or fluorophores including polycyclic aromatic hydrocarbons, dansyl- or fluores-camine-labeled analytes, or, indirectly, nonfluorescers that are easily oxidized (e.g., sulfite, nitrite) and quench the emission. The most widely used oxalate... 

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