Pyrene radical-cation salts

Table II. Crystallographic Data of Pyrene Radical-Cation Salts...

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. 

Conducting radical-cation salts have been obtained not only for benzenoid hydrocarbons, but also for their non-benzenoid analogues such as the pyrene... 

Three studies on radical cations discuss the characterization of polynuclear aromatic radical cation salts as organic metals (8), the reactions of cation radicals with neutral radicals (9), and the magnetic-electrical properties of perfluoroaromatic radical-cation salts (10). Chapters on polynuclear aromatic compounds in nonvolatile petroleum products (II) and in coal-based materials (12) present reviews of the subject and new findings. The remaining chapters in this book discuss the thermal conversion of polynuclear aromatic compounds to carbon (13), the nitration of pyrene by mixtures of N02 and N204 (14), the spectra, structures, and chromatographic retention times of large polycyclic aromatic hydrocarbons (15), the desulfurization of polynuclear thiophenes correlated with tt electron densities (16) and simple theoretical methods to predict and correlate polynuclear benzenoid aromatic hydrocarbon reactivities (IT). 

The possibility of preparing radical-cation salts of these nonplanar aromatic compounds is an interesting prospect. Many of these compounds contain pyrene and perylene substructures, both of which form well-defined salts when the procedures outlined and reviewed in the chapter by Enkel-mann (8) are used. However, the morphologies of crystals of radical-cation salts derived from the nonplanar compounds are difficult to envisage. Consequently, one cannot be certain that the resulting salts will have useful conducting properties. 

For instance, NOBF4 oxidation of benzo[a]pyrene (BP, the additional benzene ring is fused at positions 7 and 8 of pyrene) generates the BP+ BF4 salt. When this cation-radical salt is attacked with nucleophiles of various strengths, the pattern of nucleophilic substitution reflects the distribution of a positive charge in the cation-radical part of the salt. This positive charge is localized mainly at the meio-anthracenic position, that is, at the C-6 atom. Nucleophiles (Nu ) such as OH , AcO , and F enter this position (Scheme 3.68). 

Measurement of the influence of different micellar environments on proton transfer from excited states of 3-hydroxyflavone allows estimates to be made of micelle concentrations from measurement of the tautomer emission yield. Proton transfer reactions of benzimidazole excited singlet states have also been studied in ionic micelles. Magnetic fields are found to affect the behaviour of radicals generated by the photodissociation of benzil in micellar media. The starburst dendrites which are formed by anionic macromolecules in interaction with both anionic and cationic surfactants have been examined by pyrene fluorescence. Benzo[k]fluoranthrene fluorescence has served as a probe of the effects of metal salts on bile salt aggregation. The incorporation and distribution of benzoquinone into liposomes containing amphilic Zn(II) porphyrin has been followed by its effect on the quenching of the excited state °. A comparison of the photochromism of spirobenzpyran derivatives in unilamellar surfactant vesicles and solvent cast surfactant films has also been reported. ... 

The spectrophotometric technique determines K whenever the fraction of free ions is very low. The concentration of the free ions may be reduced to an insignificant level by the addition of some readily dissociated salt sharing a common cation with the investigated radical anion. On the other hand, the potentiometric technique yields K, and its value can be used to calculate K if the necessary dissociation constants are known. These constants may be derived from conductometric data (5). For an anthracene and pyrene pair incorporating Na+ as the counterion and tetrahydrofuran (THF) as the solvent, the ratio of equation 8 is only 1.6, which is equivalent to 10 millivolts (mV). However, for an anthracene and naphthalene pair, the ratio is 30.3, which is equivalent to 90 mV. 

The room temperature conductivity of polycrystalline samples of the naphthalene cation radical salts lies between 0.1 and 1 (Q cm) . Single crystals measured along the stack direction show a conductivity of several 100 (Q cm) depending on the quality of the crystals and to some extent on the nature of the counterions, solvent inclusion etc. The naphthalin cation radical salts can be stored for sometime at room temperature, if moisture is excluded. The corresponding radical ion complexes of perylene, pyrene, fluoranthene, and other arenes of higher number of fused rings exhibit a much greater stability and can be handled under normal laboratory conditions. 

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