Phenothiazine cation radicals

EGC > EC = C determined using artificial water-soluble phenothiazine radical cations (Salah et al., 1995) and EGCG > EGC > ECG > C determined in a mixture of LDL and VLDL. However, in the oxidation of unilamellar liposomes of phosphatidylcholine initiated with a water-soluble azo compound at 37°C, the antioxidant activities of EGCG and EGC were lower than those of EC and ECG at pH 7.4, and their depletion of EGCG and EGC was faster than that of EC and ECG (Terao et al, 1994). 

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

Bahnemann D, Asmus K-D, Willson RL (1983) Phenothiazine radical-cations electron transfer equilibria with iodide ions and the determination of one-electron redox potentials by pulse radiolysis. J Chem Soc Perkin Trans 2 1669-1673... 

Further interesting examples of C-S bond formation involve the reaction of previously prepared (or in situ anodically generated) thianthrene or phenothiazine radical cations with alkenes or alkynes, to give l,2-bis(hetaryl) alkanes (or the respective alkenes) [82]. With cyclooctene a 1 1 adduct is obtained instead. Another valuable application is the smooth reaction with ketones (Scheme 26). The thian-threnium salts (40) now obtained are readily deprotonated to the corresponding ylides (41) [83]. The latter compounds are directly obtained when yff-dicarbonyls are used. 

A ring carbon can also be involved, however, as in the reaction of the thianthrene and phenothiazine radical cations in neat pyridine or with pyridine in an anhydrous solvent. In this reaction the 1-pyridinium group is inserted on to the benzo ring (43), apparently via nucleophilic attack on di-cations 42, in turn resulting from oxidation of the initially formed radical cation adducts (Scheme 27). In the presence of moisture the sulfoxides are again formed [84]. 

The phenothiazine radical cations are particularly interesting because they have been implicated in charge transfer reactions with several neutral transmitter molecules (68) ... 

After these photophysical studies, Elisei et al. [29] investigated, in the case of PP, FP and TR, the photoionization process in aqueous solutions, which led to the formation of phenothiazine radical cations and solvated electrons. Then, the phototoxicity of these three drugs was tested on various biological substrates through a series of in vitro assays under UVA irradiation. Photohaemolysis of mouse erythrocytes and phototoxicity on cultured murine... 

We can interpret the overall process as follows Ihe phenoAiazine-quinoline (PTZ-Q) repeat unit in the neutral polymer shows a charge transfer-type absorption (PZT-Q ->PZT -Q ) at 425 nm. Following oxidation, the polymer produces phenothiazine radical cation (PZT -Q). Charge transfer is then not possible and this results in a decrease of absorption at 425 nm. At die same time, the absorption of PZT and Q iiKiieties results in the new peak at 540 nm and the increase in absorption at lower wavelengths (Figure 5) (25). 

For the related compounds, phenothiazine and phenoxazine, the reduced form is stable under ambient conditions and oxidation occurs in two one-electron steps. A comparison between the redox behaviour of the two compounds is best made in an antimony trichloride medium where both the radical-cation and the dication levels are stable (Scheme 6.9) [225]. In perchloric acid, phenothiazine shows reversible... 

Preparative scale electrochemical oxidation of phenothiazine in aqueous acetonitrile, with no added acid, leads to the radical formed by proton loss from the radical-cation. The radical dimerizes and further oxidation leads to the green qui-nonoid cation 67 [230]. 

Various phenothiazines (176) have been oxidized in CH3CN-Et4NC104 to a stable radical-cation (R = OCH3). The free radical resulting from deprotonation of the radical-cation gives in those cases where R = H, SCN, or t-Bu a C—N bond dimer, possibly 177272,2-73 [Eq. (107)]. 

Phenoxazines and phenothiazines (446 X = 0, S) may be oxidized to phenoxazonium and phenothiazinium salts (447 X = 0, S). Radical cations are intermediates these lose H+ to form a neutral radical followed by another electron to form the six-rr-electron system (Scheme 53). On careful oxidation, radical cations (446a) can be isolated as deep-colored crystalline salts, stable enough for X-ray analysis (80JHC1053, 81JCS(P2)852, 88CB2059). 

Heterocyclic substrates in SET processes have been widely studied, including the reactions of diliydroiiicotiiiamide,116 pyridine, and quinoline117 and also phenoxazine and phenotiiiazines.118 Phenothiazine has also been shown by ESR analysis to undergo an electron-transfer reaction witii its radical cation with an appreciable 15N/14N isotope effect.119 The reaction of phenazine di-A -oxidc radical cations with hydrocarbons shows evidence of non-radical processes.120... 

Phenothiazine is readily oxidized when irradiated in solution with chlorinated hydrocarbones (8). The reaction has been shown to be an electron transfer generating the phenothiazine (PTH) radical cation (PTH+ ) and the halogen anion as shown in equation 1. hv. ... 

It is now well established that a variety of organic molecules such as polynuclear aromatic hydrocarbons with low ionization energies act as electron donors with the formation of radical cations when adsorbed on oxide surfaces. Conversely, electron-acceptor molecules with high electron affinity interact with donor sites on oxide surfaces and are converted to anion radicals. These surface species can either be detected by their electronic spectra (90-93, 308-310) or by ESR. The ESR results have recently been reviewed by Flockhart (311). Radical cation-producing substances have only scarcely been applied as poisons in catalytic reactions. Conclusions on the nature of catalytically active sites have preferentially been drawn by qualitative comparison of the surface spin concentration and the catalytic activity as a function of, for example, the pretreatment temperature of the catalyst. Only phenothiazine has been used as a specific poison for the butene-1 isomerization on alumina [Ghorbel et al. (312)). Tetra-cyaonoethylene, on the contrary, has found wide application as a poison during catalytic reactions for the detection of active sites with basic or electron-donor character. This is probably due to the lack of other suitable acidic probe or poison molecules. 

In the twentieth century, Kehrmann applied this reagent for the one- and two-electron oxidation of phenothiazine and characterized semiquinoid and holoquinoid species by UV/VIS spectroscopy (Fig. 4) [44,45], In the 1950s colored solutions of aromatic hydrocarbons (perylene, anthracene, etc.) in sulfuric add were found to be paramagnetic [46] and, shortly thereafter, their radical cations were postulated based on optical [47] and ESR spectroscopic data [48]. The detailed reaction mechanisms of these oxidations are still in question molecular oxygen may well be a necessary ingredient... 

Various compounds were shown to sensitize the photochemical decomposition of pyridinium salts. Photolysis of pyridinium salts in the presence of sensitizers such as anthracene, perylene and phenothiazine proceeds by an electron transfer from the excited state sensitizer to the pyridinium salt. Thus, a sensitizer radical cation and pyridinyl radical are formed as shown for the case of anthracene in Scheme 15. The latter rapidly decomposes to give pyridine and an ethoxy radical. Evidence for the proposed mechanism was obtained by observation of the absorption spectra of relevant radical cations upon laser flash photolysis of methylene chloride solutions containing sensitizers and pyridinium salt [64]. Moreover, estimates of the free energy change by the Rehm-Weller equation [65] give highly favorable values for anthracene, perylene, phenothiazine and thioxanthone sensitized systems, whilst benzophenone and acetophenone seemed not to be suitable sensitizers (Table 5). The failure of the polymerization experiments sensitized by benzophenone and acetophenone in the absence of a hydrogen donor is consistent with the proposed electron transfer mechanism. 

IV. Free Radicals, Cations, and Charge-Transfer Complexes within the Phenothiazine Class... 

The intervention of a SET path in (hetero)aromatic nitration has been the subject of an extended debate [65]. The evidence is usually based either on the detection of intermediates or on the regioselectivity of the attack in relation to the spin and charge density on the radical cation. Limiting the attention to heteroaromatic substrates, it should be noted that the radical cations were detected spectroscopically [66] or even isolated as a salt [67] during nitration of, e.g., phenothiazine, phenox-... 

By far the most popular electron donor used in inorganic chromophore-quencher systems has been phenothiazine (PTZ). The pioneering studies of Meyer and coworkers [162, 163] on the chromophore-quencher complex (3) showed rapid (<10 ns) quenching of the MLCT excited state of the chromophore, with formation of a charge-separated state containing a reduced Re(I) complex (i.e., a Re(I)-bpy species) and the PTZ+ radical cation. Charge recombination takes place in several tens of nanoseconds, with rates depending on the X-substituents. More recent work... 

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