Pyrene cation

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. 

When an ion-pair recombines, it may form an excited state which can luminesce. The intensity of luminescence is a direct monitor of the competing ratio of recombination and luminescence. With steady-state conditions, the luminescence intensity is proportional to the rate of recombination. For instance, Morrow et al. [380] have radio lysed solutions of pyrene in cyclohexane. Solvated electrons and pyrene cations are produced. On recombination, an excited singlet state is produced which can fluoresce. If two pyrene molecules are in (or near) contact when one or other molecule is in the excited singlet state, then excimer fluorescence may be observed. The intensity of fluorescence can be decreased by application of an electric field, since fewer ion-pairs recombine to form the excited state. Jarnagin [381] and Holroyd and Russell [382] have photoionised iVjA iV. iV -tetramethyl-p-phenylenedia-mine (TMPD) with light (of photon energy 5.5—6 eV) in hydrocarbon solvents and measured the photocurrent at various electric field strengths. 

Fig. 3 shows the spectra of short lived species produced in the laser flash photolysis of PN+ in water, NaLS, and in silica particles. Previous work enable us to identify the excited triplet and singlet states of the pyrene moiety, together with the pyrene cation and hydrated electron, e g q... 

A spectroscopic investigation of the formation of THF-Cu"Cl2 complexes has been described, and irradiation of [Cu(Dto)2] (Dto = dithiooxalate) has been found to induce an intramolecular Dto Cu two-electron transfer with cleavage of the C-C bond in the Dto ligand and the formation of SCO. The kinetics of photo-oxidation of pyrene by Cu" in SDS micelles have been measured, but oxidants such as Eu" and Hg do not produce pyrene cations. A non-exponential decay of fluorescence is observed, and this is interpreted in terms of a model due to Tachiya which restricts the numbers of quenchers in a micelle. Transient Cu"-alkyl species are formed on flash photolysis of Cu -bis(amino-acid) complexes such as those of serine and valine, and pseudo first order rate constants for the decay of the transients have been obtained. 

One of the important features of the coupling reaction that was brought out by Eberson and Radner (19, 20) is that it does not work for all ArH +. In particular, the perylene and pyrene cation radicals did not couple with N02. Reaction of the pyrene-dimer cation radical, (pyrene)2 +PF6 , with N02, for example, gave mainly tarry materials and less than 1% of 1-nitro-pyrene (19, 20). Eberson and Radner (19, 20) concluded that only cation... 

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. 

Imidazole derivatives 380 Imperatorin 65 Impregnation of TLC layers 86 -with caffeine 86 -with silver nitrate 86 -with tungstate 86 Indeno(l,2,3-cd)pyrene 39,85 Indicators, pH- 303 -, reagents 45 Indium cations 144 Indoleacetic acid 45... 

Purified ligninase H8 produced by P. chrysosporium in stationary cultures oxidized pyrene to pyrene-1,6- and pyrene-l,8-quinones in high yield, and experiments with showed that both quinone oxygen atoms originated in water (Figure 8.25). It was suggested that initial one-electron abstraction produced cation radicals at the 1 and 6 or 8-positions (Hammel et al. 1986), whereas in... 

The pyrene molecule is transferred by irradiation to its cation radical [29]. This reacts at the oil/water interface by nucleophilic attack from the cyanide ion. Typically, the cyanated product remains in the organic phase. 

Keywords Pulse radiolysis Pyrene Phenothiazine Radical cation Hole transfer rate... 

For unsubstituted PAH, such as benzo[a]pyrene (BP), pyridinium or acetoxy derivatives are formed by direct attack of pyridine or acetate ion, respectively, on the radical cation at C-6, the position of maximum charge density (Scheme 1). This is followed by a second one-electron oxidation of the resulting radical and loss of a proton to yield the 6-substituted derivative. For methyl-substituted PAH in which the maximum charge density of the radical cation adjacent to the methyl group is appreciable, as in 6-methylbenzo[a]-pyrene (6-methylBP) (Scheme 2), loss of a methyl proton yields a benzylic radical. This reactive species is rapidly oxidized by iodine or MnJ to a benzylic carbonium ion with subsequent trapping by pyridine or acetate ion, respectively. 

This list includes BP, 7,12-dimethylbenz[a]anthracene, 3-methylchol-anthrene, dibenzo[a,i]pyrene and dibenzo[a,h]pyrene. These PAH can be activated both by one-electron oxidation and/or monooxygenation. There are a few PAH with low IP which are inactive (Table I), such as perylene, or weakly active, such as anthanthrene. This indicates that low IP is a necessary, but not sufficient factor for determining carcinogenic activity by one-electron oxidation. These inactive or weakly active PAH have the highest density of positive charge delocalized over several aromatic carbon atoms in their radical cations, whereas the active PAH with low IP have charge mainly localized on one or a few carbon atoms in their radical cations. 

Chemically inert triplet quenchers e.g. trans-stilbene, anthracene, or pyrene, suppress the characteristic chemiluminescence of radical-ion recombination. When these quenchers are capable of fluorescence, as are anthracene and pyrene, the energy of the radical-ion recombination reaction is used for the excitation of the quencher fluorescence 15°). Trans-stilbene is a chemically inert 162> triplet quencher which is especially efficient where the energy of the first excited triplet state of a primary product is about 0.2 eV above that of trans-stilbene 163>. This condition is realized, for example, in the energy-deficient chemiluminescent system 10-methyl-phenothiazian radical cation and fluoranthene radical anion 164>. 

A first generation poly(amido amine) dendrimer has been functionalized with three calyx[4]arenes, each carrying a pyrene fluorophore (4) [30]. In acetonitrile solution the emission spectrum shows both the monomer and the excimer emission band, typical of the pyrene chromophore. Upon addition of Al3+ as perchlorate salt, a decrease in the excimer emission and a consequent revival of the monomer emission is observed. This can be interpreted as a change in the dendrimer structure and flexibility upon metal ion complexation that inhibits close proximity of pyrenyl units, thus decreasing the excimer formation probability. 1H NMR studies of dendrimer 4 revealed marked differences upon Al3+ addition only in the chemical shifts of the CH2 protons linked to the central amine group, demonstrating that the metal ion is coordinated by the dendrimer core. MALDI-TOF experiments gave evidence of a 1 1 complex. Similar results have been obtained for In3+, while other cations such as Ag+, Cd2+, and Zn2+ do not affect the luminescence properties of... 

PET-19 (Figure 10.16) consists of a diazacrown ether with two pendant pyrene groups. As expected, cation binding results in a large change in the monomer/... 

E-3 (Figure 10.26) is the first example of an ionophoric calixarene with appended fluorophores, demonstrating the interest in this new class of fluorescent sensors. The lower rim contains two pyrene units that can form excimers in the absence of cation. Addition of alkali metal ions affects the monomer versus excimer emission. According to the same principle, E-4 was designed for the recognition of Na+ the Na+/K+ selectivity, as measured by the ratio of stability constants of the complexes, was indeed found to be 154, while the affinity for Li+ was too low to be determined. 

Anodic conversion of aromatics proceeds in most cases by le-transfer to the anode to form a radical cation (34) (Scheme 9). Oxidation is facilitated by extension of the 7T-system ( 1/2 vs. Ag/Ag+ benzene 2.08 V, pyrene 0.86 V) and by electron donating substituents ( 1/2 vs. Ag/Ag+p-phenylenediamine —0.15 V). Oxidation potentials of polycyclic aromatics and substituted benzenes are collected in Ref [140-142]. 

Lee and Meisel incorporated Py, at levels of 10 M or more, into 1200 EW acid form samples that were swollen with water and with ferf-butyl alcohol. It was concluded based on the /3//1 value for water swollen samples that the Py molecules were located in the water clusters and were most likely near fluorocarbon—water interfaces. It was also concluded, based on both absorption and emission spectra, that the probes had strong interactions with the SO3 groups that were exchanged with Ag+ and Pb + cations in the case of water containing samples. Likewise, the pyrene molecules were rationalized as being surrounded by terf-butanol molecules in that case. However, excimer formation (due to the presence of adjacent pyrene molecules) in the ferf-butyl alcohol system suggested the loss of cluster morphology-... 

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). 

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. 

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