Pyrene, 79 Anion radicals

The reactivity of pyrene anion radical toward ArS02R may be demonstrated as follows adding some ArS02R to the pyrene solution leads to an increase of the peak current corresponding to reduction of the pyrene, while this step becomes progressively irreversible. It may also be noted that for small amounts of ArS02R, the specific step for the sulphone more or less vanishes. This corresponds to the following scheme ... 

In one of the first experimental studies where ion radical annihilation in solution was considered as an emissive possibility, Yamamato, Nakato, and Tsubomura61 found that Y,Y,Y, Y -tetramethyl-p-phenyl-enediamine (TMPD) and pyrene when irradiated in the ultraviolet in a glass at low temperature formed Wurster s blue cation radical, pyrene anion radical, and solvated electrons. When the glass was warmed, thermoluminescence was observed. A similar emission was observed when a previously irradiated mixture of TMPD and 2-methylnaph-thalene was warmed. The emission in both instances was ascribed to charge-transfer fluorescence resulting from combination of a cation radical with an anion radical. 

Nearly all the nonfluorescent emission bands reported in these studies have been found at longer wavelengths than the expected fluorescence. They have been variously ascribed to eximers6,15 or to phosphorescence1315 of the electroactive substance. Some apparently do not arise from the electroactive compound itself.16 On oxidation of the pyrene anion radical with Wurster s blue perchlorate, an emission band was obtained which corresponded closely to the known pyrene eximer emission.15 Several spectra obtained from polycyclic aromatic hydrocarbons by electrochemical treatment at constant applied voltage have... 

If the reduction potential of the BX compound is too negative compared to the A compound, the ability of AT to transfer an electron to BX may be enhanced by exciting AT photochemically. Thus pyrene anion radical reacts very slowly with m-chlorotoluene, but shining green light on the red anion-radical makes the reaction proceed very fast (Fig. 5).32,33 Another possiblity is to use the dianion thus perylene anion-radical does not react with 1,4-dichlorobenzene, whereas it is rapidly reduced by perylene dianion.25 (Fig. 6). 

The indirect reduction of tosyl esters can be performed7 in non-aqueous solutions. Thus, for example, the anthracene anion radical formed by cathodic reduction in DMF/TBAB (tetrabutylammonium bromide) electrolyte may reduce tosylates in solution. Similarly, the pyrene anion radical was shown8 (Figure 1) to react also with ethyl tosylate. The redox catalysis general scheme (indirect reduction by a redox P/Q couple) where P is a reducible species and Q its stable reduced form can be written as below ... 

A large number of other sensitizers has been investigated for use in photolytic de-diazoniation. The excited states of these compounds (S ) react either by direct electron transfer (Scheme 10-97), as for pyrene, or by reaction with an electron donor with formation of a sensitizer anion radical which then attacks the diazonium ion (Scheme 10-98). An example of the second mechanism is the sensitization of arenedi-azonium ions by semiquinone, formed photolytically from 1,4-benzoquinone (Jir-kovsky et al., 1981). 

Let us now consider another organic species, such as a sulphone ArS02R known to be irreversibly reduced less easily than pyrene. The basic mechanism for its cathodic reduction has already been presented (reactions 3-6). It is necessary, however, to assume here that the chemical degradation of the anion radical when produced in solution is at least reasonably fast. 

The ratio ARH/ARj (monoalkylation/dialkylation) should depend principally on the electrophilic capability of RX. Thus it has been shown that in the case of t-butyl halides (due to the chemical and electrochemical stability of t-butyl free radical) the yield of mono alkylation is often good. Naturally, aryl sulphones may also be employed in the role of RX-type compounds. Indeed, the t-butylation of pyrene can be performed when reduced cathodically in the presence of CgHjSOjBu-t. Other alkylation reactions are also possible with sulphones possessing an ArS02 moiety bound to a tertiary carbon. In contrast, coupling reactions via redox catalysis do not occur in a good yield with primary and secondary sulphones. This is probably due to the disappearance of the mediator anion radical due to proton transfer from the acidic sulphone. 

Pressure provokes transition of the linear (extended) conformation into the bent (V-like) one. (The V-like form is more compact and occupies a smaller volume.) It is obvious that the V-like form is favorable in respect of intramolecular electron transfer from the donor (the aniline part) to the acceptor (the pyrene part). In the utmost level of the phenomenon, the donor part transforms into the cation-radical moiety, whereas the acceptor part passes into the anion-radical moiety. Such transformation is impossible in the case of the extended conformation because of the large distance between the donor and acceptor moieties. The spectral changes observed reflect this conformational transition at elevated pressures. 

Electron tunneling between organic species was first detected, by direct kinetic experiments, for reactions of the biphenyl anion radical with naphthalene and pyrene [11] and triphenylethylene [12], As is known, upon irradiating vitreous solutions containing biphenyl or pyrene, Py, these acceptors react with electrons to form Ph2 and Py with characteristic optical spectra [13]. Ph2 particles have been found [11] to enter into the electron exchange reactions at 77 K with naphthalene, Nh, and pyrene molecules in mixtures of ethyl alcohol and diethyl ether (2 1). 

During the y-radiolysis of vitreous solutions containing only biphenyl (0.1 M) or only pyrene (0.02 M), the yield of Ph2 and Py- at 77K is high enough for them to be recorded at an irradiation dose of 1019 eV cm-3. At 77 K these particles have been observed to decay spontaneously (Fig. 5), evidently, due to proton transfer from alcohol molecules (the most probable process in the case of Ph2 anion radicals [14]) or to recombination with counterions formed during radiolysis. Naphthalene and pyrene additives to solutions of Ph2 essentially accelerate the decay of the Ph2 anion radical at 77 K which is naturally accounted for by electron transfer from Ph2 to Nh and Py. In agreement with this conclusion the decay of Py in the presence of Ph2 is slower than its spontaneous decay in the absence of Ph2. ... 

The es reacts with PVB to give a polymer anion with a high efficiency [47]. The rate constant was evaluated as 4.7 x 109 mol -1 dm3 s-1 in hexamethylphos-phorictriamide. The absorption specra of the radical anions of PVB [47] and PVP [48] are similar to those of biphenyl anion and pyrene anion, respectively, to mean that the excess electrons trapped by the polymers are essentially localized on the side groups. 

Irradiation with visible light of pyrene and perylene anion radicals produced during a cyclic voltammetric experiment leads to an enhancement of the peak current and photoinduced electron transfer to chlorobenzene as acceptor has been shown to occur directly or via the dianion [179, 180]. 

Okada, Okamoto and Oda [97] have recently described a novel method of decarboxylation through a PET bond cleavage process in IV-acyloxyphthalimides. A photoexcited electron donor such as l,6-bis(dimethylamino)pyrene (BDMAP) transfers an electron to the phthalimide to produce an anion-radical that is protonated to form a radical prior to homolytic N—O bond cleavage. Bond... 

In reaction (2-2a), the electron transfer reaction occurs from Si of pyrene to N,N-dimethylaniline, giving a singlet radical ion pair involving the pyrene anion and N,N-dimethylaniline cation radicals. In reaction (2-2b), the electron transfer reaction occurs from... 

For the most common exciplex-forming systems, like pyrene-dimethylaniline, only a quenching is observed in the micellar phase, but no exciplex emission [25,26]. The interaction of the excited pyrene with dimethylaniline is controlled by diffusion [123] with every collision resulting in quenching, which was also shown by Malaga et al. [25,26] to yield pyrene anion and dimethylaniline radical cation. The quantum yield of radical ions is slightly greater in cationic CTAB micelles than in acetonitrile solutions, in neutral Brij 35 micelles a little less and in anionic SDS micelles considerably less than in acetonitrile. 

It has been reported that Cgo and its derivatives form optically transparent microscopic clusters in mixed solvents [25, 26]. Photoinduced electron-transfer and photoelectrochemical reactions using the C o clusters have been extensively reported because of the interesting properties of C o clusters [25,26]. The M F Es on the decay of the radical pair between a Cgo cluster anion and a pyrene cation have been observed in a micellar system [63]. However, the MFEs on the photoinduced electron-transfer reactions using the Cgo cluster in mixed solvents have not yet been studied. 

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