Pyrene, radicals

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. 

Haidar, M Misra, A., Banerjee, A. K. and Chowdhury, M. (1999) Magnetic field effect on the micellar (C)-pyrene- radical-pair system. J. Photochem. Photobiol. A, 127, 7-12. 

Graceffa, P. and Weitzman, S.A. (1987). Asbestos catalyzes the formation of 6-oxobenzo(a)pyrene radical from 6-hydroxybenzo(a)pyrene. Arch. Biochem. Biophys. 257, 481-484. 

Electron transfer can be accomplished by quenching of a micelle trapped chromophore by ions capable of ion pairing with the micelle surface. For example, excited N-methylphenothiazine in sodium dodecylsulfate (SDS) micelles can exchange electrons with Cu(II). The photogenerated Cu(I) is rapidly displaced by Cu(II) from the aqueous phase so that intramicellar recombination is averted, Fig. 5 (266). Similarly, the quantum yield for formation of the pyrene radical cation via electron transfer to Cu(II) increases with micellar complexation from 0.25 at 0.05 M SDS to 0.60 at 0.8 M SDS (267). The electron transfer quenching of triplet thionine by aniline is also accelerated in reverse micelles by this mechanism (268). 

State they have more than sufficient energy. The excited singlet state of pyrene sensitises the decomposition of [122] and phthaloyl peroxide (Horn and Schuster, 1979). By means of the technique of flash photolysis it was possible to show that these sensitisation reactions produced the pyrene radical cation and heme that the CIEEL mechanisms was operating. 

The absorption band (Amax 488 nm) in the transient spectrum corresponds to the pyrene radical ion (Py ) [155, 156], while the band at Amax 400 nm is assigned to the absorption of the 1-hydro-1-pyrenyl radical (Py ) [157, 158]. Steady-state photolysis of pyrene in the presence of TEA leads to its disappearance, and addition of vinyl monomers decreases the rate of pyrene photoreduction. The photobleaching process follows first-order kinetics. Encinas et al. [154] suggest that the photoinitiation of polymerization by pyrene-TEA is catalyzed by the pyrene radical ion. 

Stable Isotopes other than deuterium have also been used in some novel approaches for studying reactive metabolite formation. Benzo[a]pyrene, another potent polycyclic hydrocarbon carcinogen, was incubated with cofactors and rat liver mlcrosomes in an atmosphere of g Oj to investigate whether or not 6-oxybenzo[a]pyrene radical was formed. Electron spin... 

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

The same reaction was studied by Atik and Thomas [147]. In CTAB micelles a pyrene radical anion lives more than 1 ms and reacts with dimethylviologene, Eu, O2, CO2. Thomas et al. [123] concluded that the dimethylaniline radical cation leaves the micelle within its lifetime. In the case of anionic micelles, the radical cation is trapped and fast recombination takes place. The kinetic scheme is as follows ... 

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