Pyrene electron transfer

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. 

The investigation by Becker et al. (1977 b) also included work on the effect of pyrene added as electron donor. Pyrene has an absorption maximum at 335 nm (e = 55000 M-1cm-1, in petroleum). Much more hydro-de-diazoniation takes place in the presence of pyrene with irradiation at 365 nm, and even more on irradiation with light of wavelength <313 nm. Photoexcited pyrene has a half-life of 300 ns and is able to transfer an electron to the diazonium ion. This electron transfer is diffusion-controlled (k= (2-3) X 1010 m 1s 1, Becker et al., 1977a). The radical pairs formed (ArN2 S +) can be detected by 13C- and 15N-CIDNP experiments (Becker et al., 1983, and papers cited there). 

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). 

Thus, 9,10-diphenylanthracene ( p = — 1.83 V vs. SCE) is reduced at too positive a potential and hence its rate of reaction with the sulphonyl moieties is too low. On the other hand, pyrene (Ep = — 2.04 V) has a too negative reduction potential and exchanges electrons rapidly both with allylic and unactivated benzenesulphonyl moieties. Finally, anthracene Ev = —1.92 V) appears to be a suitable choice, as illustrated in Figure 3 (curves a-d). Using increasing concentrations of the disulphone 17b, the second reduction peak of XRY behaves normally and gives no indication of a fast electron transfer from A. 

In two other studies, it was observed that C60 in LB films can quench the fluorescence of pyrene [293] and of 16-(9-anthroyloxy)palmitic acid [294] by photoinduced electron transfer. In these studies, both C60 and the electron-donating fluorophore were incorporated into a tricosanoic acid LB film in different ratios. 

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. 

Maehashi et al. (2007) used pyrene adsorption to make carbon nanotubes labeled with DNA aptamers and incorporated them into a field effect transistor constructed to produce a label-free biosensor. The biosensor could measure the concentration of IgE in samples down to 250 pM, as the antibody molecules bound to the aptamers on the nanotubes. Felekis and Tagmatarchis (2005) used a positively charged pyrene compound to prepare water-soluble SWNTs and then electrostatically adsorb porphyrin rings to study electron transfer interactions. Pyrene derivatives also have been used successfully to add a chromophore to carbon nanotubes using covalent coupling to an oxidized SWNT (Alvaro et al., 2004). In this case, the pyrene ring structure was not used to adsorb directly to the nanotube surface, but a side-chain functional group was used to link it covalently to modified SWNTs. 

The electron transfer system has not been studied in detail in fish, but the metabolism of compounds such as biphenyl (37), benzo(a)pyrene (21) and 2,5-diphenyloxazole (38) by fish liver microsomes has been shown to require oxygen and NADPH generating system. The metabolism of BP (21), 2,5-diphenyloxazole (Ahokas, unpublished observation) and aldrin (27.) by fish liver microsomal enzyme system is inhibited strongly by carbon monoxide. This information and the fact that cytochrome P-1+50, as well as NADPH cytochrome c reductase system are present in fish, suggest strongly that fish have a cytochrome P-1+50 mediated monooxygenase system which is very similar to that described in mammals. 

Cation-radicals of naphthalene and its homologues, pyrene, or perylene react with NOj" ion in AN, giving electron-transfer products, that is, ArH and NOj. The latter radical is not very active in these reactions and nitration takes place only with extremely reactive compounds such as perylene (Eberson and Radner 1985, 1986). This mechanism is seemingly distinctive of compounds with E° less or equal to 1 V in AN (or in other solvents solvating NOj ions sparingly). 

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. 

A similar supramolecular approach, in which both n-n stacking stacking of pyrene on the SWNT surface and alkyl ammonium-crown ether interactions were used in the self-assembly process of a fullerene derivative with SWNTs, was recently reported (Scheme 9.22).72 The nanohybrid integrity was probed with various spectroscopic techniques, , and electrochemical measurements. Nanosecond transient absorption studies confirmed electron transfer as the quenching mechanism of the singlet excited state of C60 in the nanohybrid resulting in the formation of SWNT"1"/ Pyr-NH3 + /crown- charge-separated state. 

The successful use of - interactions to anchor an electron-donating extTTF to the surface of SWNT was recently demonstrated (Scheme 9.23).73 Interaction between the concave hydrocarbon skeleton of exTTF and the convex surface of SWNT adds further strength and stability to the SWNT/pyrene-exTTF nanohybrid. Because of the close proximity of the exTTF to the electron acceptor SWNT, a very rapid intrahybrid electron transfer affords a photogenerated radical ion pair, whose lifetime is only a few nanoseconds. The present method for the preparation of SWNT/ exTTF nanohybrids nicely complements the covalent approach and bears a strong... 

Similarly, the fluorescence intensity of the 1,4-disubstituted azine with ferrocene and pyrene units (17) can be reversibly modulated by sequential redox reactions of ferrocene moiety. In the neutral state, compound 17 displays weak fluorescence owing to the electron transfer from the ferrocenyl group to the excited pyrene unit or by energy transfer from the excited pyrene unit to the ferrocenyl unit. Oxidation of the ferrocenyl unit, however, leads to remarkable fluorescence enhancement. This is because the ferrocenium cation shows weak electron donating ability and also the corresponding spectral overlap becomes small.27... 

Copolymers of 3-(l -PyrenyDpropyl Methacrylate. The pyrene-containing methacrylate was copolymerized with vinylbenzyltriethylammonium chloride or sodium p-styrenesulfonate. These copolymers were expected to behave very differently from each other as electron transfer sensitizers. Photoreduction of MV2+ sensitized by the copolymers in the presence of EDTA is shown in Figure 5. The photoabsorbing species is exclusively pyrenyl groups. It is noteworthy that polycations which repel MV2+ are more effective sensitizers... 

Instead of MV2+ (in the photo-oxidation of leuco crystal violet (LCV)), a neutral species is sensitized by pyrene containing polymers and the Coulombic effect is not as drastic as in the case of MV2+. As shown in Figure 8, the cationic polymer is more effective than the neutral or anionic polymer. This is attributed to the Coulombic repulsion between LCV- and Py assisted by the cationic environment of the polycation. However, the Coulombic effect occurs only after forward electron transfer. 

With the aim of preparing photoinduced electron transfer devices or simply to be used as fluorescent reporter groups, the synthesis and utility of pyrene-L-alanine have been reported on several occasions. Regarding the development of charge-transfer devices, it was shown that intramolecular electron transfer occurred in 310- and a-helical peptidic systems, such as 182 and 185, between a pyrene and a dimethylaminophenyl chromophore (Scheme 51).[95 101] The rates depend on the relative orientation of the chromophoric side chains and on the framework conformation. Therefore, the preparation of tailor-made charge-transfer devices with tunable properties can be envisaged. 

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. ... 

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). 

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