Electron Transfer Products Research Articles

Cycloelimination reactions of cyclic aryl pinacol sulfites and carbonates via photosensitized reversible electron transfer

Evidence is presented that the singlet and triplet photosensitized cycloelimination reactions of cyclic aryl pinacol sulfites and carbonates proceed by reversible electron transfer and yield products which are different from those formed on direct photolysis.

Photochemical reactions of amorphous and crystalline titanium dioxide powder suspensions in water

Photochemical and photophysical properties of crystalline and amorphous TiO2 colloids in water are compared. Crystallinic TiO2 colloids show a broad emission with a maximum at 375 nm, which corresponds to the band gap of the semiconductor, and the material absorbs anionic, cationic and neutral molecules at pH 5.6. Instantaneous formation of primary electron-transfer products is observed on laser excitation; these subsequently decay away in short time periods (µs). Doping with small amounts of metal-oxide impurities increases the steady-state photolysis yields of products. Aqueous solutions of amorphous TiO2 colloids do not luminesce and do not adsorb cations and neutral molecules. However, anions such as EDTA4–, I– or SCN– are adsorbed at the surface of these particles. Photoexcitation of methyl viologen, MV2+, with amorphous TiO2 samples leads to a growth of MV+ formation which lasts many microseconds. This behaviour is contrary to that observed with crystalline TiO2.

CIDNP Effects of Sensitized Photochemical Dediazoniation of Arene Diazonium Salts Manipulating CIDNP Intensities by the Experimental Conditions

13C and 15N photo-CIDNP effects were determined for the reversible electron transfer from pyrene to arene diazonium salts on excitation of the charge transfer band at 360 nm. The diazonium salts being the products of back electron transfer (“cage products”) show enhanced absorption for 13 C(1) and the 15N-enriched diazonium group, whereas the escape products, ArH or 15N2, respectively, yield emission signals. It was shown that the intensities of the CIDNP effects depend on the rates of intersystem crossing kisc within the geminate radical pair, i.e. on the magnetic nucleus used as a probe of the CIDNP effect. Using 1H, 13C or 15N the time domain of observation can be manipulated in the ranges of 90-100 ns, 15-20 ns and 3-5 ns, respectively. Furthermore, the CIDNP intensities depend on the proper balance of the rate of electron back transfer, k-e, and the rate kp of formation of the escape product. Since k-e increases with increasing energy of the geminate radical pair, this balance and therefore the CIDNP intensities vary according to the substituent present and the electron donor used

Quenching of the triplet states of carbonyl compounds by nitroxyl radicals

1. Quenching of triplet states of ketones by nitroxyl radicals occurs by charge transfer and by energy transfer from the triplet molecule to the radical. The relative contributions of these mechanisms depends on the triplet energy of the ketone and the electron donor ability of the radical. 2. Quenching of the triplet state of anthanthrone by nitroxyl radicals occurs by charge transfer from the radical to the triplet molecule. Products of electron or hydrogen atom transfer in this case are not formed as a rule. Their formation is possible under conditions where the radical contains a fragment which is a stronger quencher than the radical center.

Direct electrochemical oxidation of Clostridium pasteurianum ferredoxin : Identification of facile electron-transfer processes relevant to cluster degradation

Rapid oxidation processes relevant to the degradation of [4Fe4S] clusters in Clostridium pasteurianum ferredoxin were studied via direct (unmediated) heterogeneous electron transfer at a pyrolytic graphite electrode. Differential-pulse voltammograms of native [4Fe4S] ferredoxin showed two well-defined oxidation peaks corresponding to apparent E-values of +793 and +1120 mV at 5°C. Direct involvement of the cluster was established through parallel experiments with the 2[4Fe4Se] derivative for which peak positions were shifted. Square-wave voltammetry showed that the product of the first electron transfer, which may correspond to the ‘super-oxidised’ [4Fe4S] 3+ oxidation level, undergoes rapid degradation ( t 1 2 < 1.6 ms at 5°C). The second oxidation process, as characterised by a significant (⪢100 mV) negative shift upon selenium substitution, very likely represents oxidation of S(Se) still associated with the protein and possibly contained within the remaining FES(Se) substructure.

Picosecond spectroscopic study of chlorophyll-based models for the primary photochemistry of photosynthesis

A series of covalently linked dimers and trimers of chlorophyllide derivatives was investigated by time-resolved absorption and fluorescence spectroscopy (3 to 10/sup 4/ ps). For these compounds, the free energy difference between the singlet excited state of the electron donor and the anticipated cation-anion photoproduct (..delta..G/sub ET/) is estimated to range from +200 to -400 MeV. For the dimers studied, the singlet-excited-state lifetimes range from 1 to 7 ns and depend inversely on the solvent's static dielectric constant. Since no decrease in lifetime or fluorescence quantum yield was found as ..delta..G/sub ET/ became more negative, this effect is unlikely to be due to slow electron transfer. It may be a result of fluctuating intramolecular association of the nonpolar macrocycles in solvents with a high dielectric constant. We also studied two trimers, each having the same chlorophyllide a dimer as the electron donor, but with pyropheophorbide a or pheophorbide a as the electron acceptor (the latter is 90 MeV easier to reduce than the former). For the trimer with pheophorbide a as the acceptor, there is evidence for a new path of radiationless decay which may involve an electron-transfer product. However, the rate of formation of this product is slow (lessmore » than or equal to 10/sup 10/ s/sup -1/), and its yield is low (less than or equal to 50%). Taken together, these results suggest that chlorophyll-based, donor-acceptor pairs connected by flexible chains longer than five atoms are not likely to duplicate the highly efficient excited-singlet-state electron-transfer reactions characteristic of the primary photochemistry of photosynthetic organisms.« less

ChemInform Abstract: PHOTOINDUCED REDOX REACTIONS IN AQUEOUS MICELLES. QUENCHING, BACK‐REACTION, AND COMPETING PROCESSES FOR TETRAANIONIC PORPHYRINS WITH ALKYLVIOLOGENS

Photoinduced redox reactions employing the anionic porphyrins meso-tetrakis(p-sulfonatophenyl)porphyrin (1) and its palladium(II) complex (2) as excited substrates and various alkylviologens as quenchers have been studied in homogeneous aqueous solution and in the presence of the charged detergents sodium dodecyl sulfate (SDS) and cetyltrimethylammonium chloride (CTAC) above the critical micelle concentration (cmc). In homogeneous solution strong ground-state association between the porphyrins and the viologens occurs; even dynamic quenching processes result in no separation of electron transfer product ions. Addition of either SDS or CTAC reduces the quenching rates but allows the detection of product ions surviving up to ca. 100 ms in some cases. The back-reactions are sensitive to a variety of factors including added electrolyte, hydrophobicity of the viologen and its one-electron reduction product, and the charge on the surfactant. The long-lived products can be selectively intercepted by a variety of reagents. In the case of quenching of the palladium porphyrin (2) by methylviologen, the oxidized porphyrin can be reduced by iodide while the reduced viologen can be oxidized via either a colloidal catalyst or reaction with molecular oxygen. These reactions illustrate the possibility for obtaining net chemical conversion in a process whereby both the light-absorbing substrate and the quenchermore » are recycled.« less

The reaction of tris(4-methoxyphenyl)methyl chloride with sodium methoxide—non-operation of single electron transfer pathways

Tris(4-methoxyphenyl)methyl chloride (2) and sodium methoxide furnish, in methanol as well as in aprotic solvents, the normal substitution product methyl tris(4-methoxyphenyl)methyl ether (3) with partial replacement of the p-methoxy-groups of the substrate by the reagent. No reduction to tris(4-methoxyphenyl)methane takes place unlike the reaction in the absence of sodium methoxide (during which reaction exchange of methoxy-groups between substrate and methanol occurs to a much greater extent) and no single electron transfer products are formed unlike the reaction of triphenylmethyl halides (1a, b) and sodium methoxide. These differences are rationalized by invoking the irreversibility of formation of compound (3) by attack of methoxide at the central carbon atom of cation (12) as contrasted to the reversibility of formation of the conjugate acid of (3), (14), resulting from attack of methanol at the same site, and also the different stabilities and tendencies of formation of the tris(4-methoxyphenyl)carbenium (12) and triphenylcarbenium ions, respectively.

A Novel Effect of a Polyelectrolyte on a Photochemically Induced Electron Transfer Process Involving a Zwitterion

AbstractA photoelectron transfer system containing the photosensitizer tris (2,2′‐bipyridyne) ruthenium II chloride, and the quencher N, Nî, di(4‐sulfonato toluyl) 4,4′‐bipyridine is described. Following a laser pulse the exited state of the photosensitizer is quenched by BSV giving a net yield of electron transfer of about 0.1. Addition of the polyelectrolyte polyvinyl sulfate (PVS) markedly enhances the rate of quenching. The decay of the electron transfer products Ru(bpy)3+3 and BSV+ is considerably retarded in the same system upon addition of PVS.

Excited singlet and triplet state electron-transfer reactions of aluminium(III) sulphonated phthalocyanine

Conventional and nanosecond flash photolysis has been used to study the excited-state reactions of aluminium(III) sulphonated phthalocyanine (AlSP) with benzohydroquinone (BQH2), methyl viologen (MV), benzyl viologen (BV) and anthraquinone-2,6-disodium sulphonate (AQDS). The fluorescent state of AlSP was quenched by all of these reagents at rates close to the diffusion-controlled limit; however, no separated electron-transfer products were detected from the reaction with BV, MV or AQDS despite the electrostatic repulsion between the phthalocyanine and semiquinone radical anions. In contrast, quenching by BQH2 produced a transient (τ≃ 100 ns), which is attributed to a long-lived geminate ion pair, from which separated ions escaped.The triplet state of AlSP did not react with AQDS, BV or MV, but it was quenched by oxygen and BQH2. Separated ions were produced three times as efficiently from the triplet reaction of AlSP with BQH2 than from the excited-singlet-state reaction.

Photoinduced redox reactions in aqueous micelles. Quenching, back-reaction, and competing processes for tetraanionic porphyrins with alkylviologens

Photoinduced redox reactions employing the anionic porphyrins meso-tetrakis(p-sulfonatophenyl)porphyrin (1) and its palladium(II) complex (2) as excited substrates and various alkylviologens as quenchers have been studied in homogeneous aqueous solution and in the presence of the charged detergents sodium dodecyl sulfate (SDS) and cetyltrimethylammonium chloride (CTAC) above the critical micelle concentration (cmc). In homogeneous solution strong ground-state association between the porphyrins and the viologens occurs; even dynamic quenching processes result in no separation of electron transfer product ions. Addition of either SDS or CTAC reduces the quenching rates but allows the detection of product ions surviving up to ca. 100 ms in some cases. The back-reactions are sensitive to a variety of factors including added electrolyte, hydrophobicity of the viologen and its one-electron reduction product, and the charge on the surfactant. The long-lived products can be selectively intercepted by a variety of reagents. In the case of quenching of the palladium porphyrin (2) by methylviologen, the oxidized porphyrin can be reduced by iodide while the reduced viologen can be oxidized via either a colloidal catalyst or reaction with molecular oxygen. These reactions illustrate the possibility for obtaining net chemical conversion in a process whereby both the light-absorbing substrate and the quenchermore » are recycled.« less

Electron-transfer quenching of excited Ru(bpy)32+ ions adsorbed on ion-exchange surfaces

Both the decay of the /sup 3/CT Ru(bpy)/sub 3//sup 2 +/ emission at 600 nm and the bleaching of Ru(bpy)/sub 3//sup 2 +/ ground-state optical absorption have been investigated. The ionic species were adsorbed on a negative ion exchange resin (Sephadex SP-C50) which is a tridimensional dextran gel with sulfopropyl groups attached. The /sup 3/CT Ru(bpy)/sub 3//sup 2 +/ ions are quenched by Cu/sup 2 +/, Fe/sup 3 +/, ferric nitrilotriacetate (FeNTA), O/sub 2/, benzoquinone, and Cr(acac)/sub 3/ in a dynamic process. Except for O/sub 2/ and benzoquinone, electron-transfer products are produced with quantum yields ranging from 0.1 for FeNTA and Cr(acac)/sub 3/ to 0.68 for Cu/sup 2 +/ and 1.0 for Fe/sup 3 +/. The rates of quenching for the positive ions, based on the total volume of the swollen resin, are higher by about one order of magnitude as compared to the appropriate rates in water. Rates of quenching by the uncharged species seem to be comparable to the rates in polyelectrolyte solutions. The reversed electron-transfer reactions were also measured for Fe/sup 3 +/, Cu/sup 2 +/, and FeNTA. In all three cases, the back reaction is very fast, obeying fairly closely a second-order rate law. Reaction ratemore » constants are 3.4 x 10/sup 8/, 4.1 x 10/sup 9/, and 4 x 10/sup 9/ M-/sup 1/s-/sup 1/ for Fe/sup 3 +/, Cu/sup 2 +/, and iron(III) nitrilotriacetate, respectively. At sufficiently high coverage by Ru(bpy)/sub 3//sup 2 +/ and sufficiently high laser pulse intensities, triplet-triplet annihilation was observed for /sup 3/CT Ru(bpy)/sub 3//sup 2 +/, similarly to the effect measured previously in micelle solutions and in a polyelectrolyte solution. Unlike the complicated nature of the enhanced decay of /sup 3/CT Ru(bpy)/sub 3//sup 2 +/ in the polyelectrolyte, the enhanced decay in the ion-exchange resin obeys a simple second-order rate law. The results are discussed in the light of previous work in water, polyelectrolytes, and an ion exchanger.« less

A comparison of ultrafast electron transfers in porphyrin—quinone and magnesium—free-base diporphyrin molecules: mimicking photosynthetic charge separations

Picosecond absorption spectroscopy was used to investigate the excited state relaxations in meso-triphenyl( p-benzoquinone)porphine(P 3QP) and in solutions of meso-tetraphenylporphine with added amounts of p-benzoquinone (BQ). P 3QP has an observable S 1 state with a lifetime of less than 6 ps which decays exclusively to its lowest energy triplet T 1 and ground S 0 electronic states. The T 1 state decays in about 30 ps to yield both long-lived (τ > 5 ns) electron transfer (ET) products and S 0 states. The lack of formation of a detectable ET product 1[ −·P 3QP· +] from the quenching of the S 1 state of P 3QP contrasts strikingly with an earlier report of 1[ +·MgH 2· −] formation from the S 1 state of MgH 2 in methylene dichloride (CH 2Cl 2). (MgH 2 is a doubly linked cofacial diporphyrin comprising magnesium porphyrin and free-base porphyrin subunits.) Here BQ quenching of the excited states of MgH 2 in tetrahydrofuran (THF) and CH 2Cl 2 provides spectral and kinetic evidence which confirms the prior assignment of ET in CH 2Cl 2 and 1(π,π *) formation in THF. Also, analysis of the excited state couplings in P 3QP and MgH 2 illustrates the important role of nuclear distortions in controlling the lifetimes of singlet biradical ET products and suggests a reason for the apparent absence of quinones as primary electron acceptors in natural photosystems.

Redox potential–structure relationships in metal complexes. Part 3. The electrochemical activation of co-ordinated hydrazido(2–)-ligands

The complexes [Mo(N2RR′)(S2CNMe2)3]+, R,R′= alkyl or aryl, undergo two successive reversible one-electron reductions in non-aqueous electrolytes at platinum or vitreous carbon electrodes. The initial electron-transfer product [Mo(N2RR′)(S2CNMe2)3]0 disproportionates to [Mo(N2RR′)(S2CNMe2)3]+ and to the product of the second electron transfer, [Mo(N2RR′)(S2CNMe2)3]–. Two-electron reduction of the cation leads to electroactivation of the N2RR′ ligand towards attack by electrophiles and reduction/protonation is shown to liberate the free hydrazine, NH2NMe(Ph), in ca. 57% yield. A plausible mechanism for this electroactivation is discussed, and, from studies of related complexes, it is suggested that the redox orbitals of the [Mo(N2R2)(S2CNMe2)3]+ complex are associated with the MoN–N function.

Electron-transfer reactions of quinones, hydroquinones and methyl viologen, photosensitized by tris(2,2′-bipyridine)-ruthenium(II)

The photosensitized reactions of tris(2,2′-bipyridine)-ruthenium(II)[Ru(bpy)32+] with quinones and hydroquinones have been studied by conventional and nanosecond flash photolysis. Quenching of the excited state [*Ru(bpy)32+] by quinone occurs rapidly (kq > 2 × 109 dm3 mol–1 s–1) but the yield of electron-transfer products observed by nanosecond absorption spectroscopy is small ( 2 × 109 dm3 mol–1 s–1) with QH2 and Q, respectively.Ru(bpy)33+ is also formed when Ru(bpy)32+ is quenched by methyl viologen (MV2+) and QH2 can act as electron donor for the Ru(bpy)32+/MV2+ photoredox couple. No permanent storage of energy is achieved, however, since MV[graphic omitted] reacts rapidly (k > 2 × 109 dm3 mol–1 s–1) with quinones which are produced by disproportionation of Q[graphic omitted] or are present in the original hydroquinone samples.

CHLOROPHYLL PHOTOCHEMISTRY IN CONDENSED MEDIA—II. TRIPLET STATE QUENCHING AND ELECTRON TRANSFER TO QUINONE IN LIPOSOMES*

Abstract—The fate of excitation energy and electron transfer to quinones within Chl‐a‐containing phosphatidyl choline liposomes has been investigated. The bilayer membrane of the liposome stabilizes the Chl triplet state, as evidenced by a three‐fold increase in the lifetime over that observed in ethanol solution. The relative triplet yield follows the relative fluorescence yield, indicative of quenching at the singlet level. Triplet state lifetimes are markedly shortened as the Chl concentration is increased, demonstrating that quenching occurs at the triplet level as well. This process is shown to be due to a collisional de‐excitation. In the presence of quinones, the Chl triplet reduces the quinone resulting in production of long‐lived electron transfer products. The percent conversion of Chl triplet to cation radical when benzoquinone is employed as acceptor is approximately 60 ± 10%, which is slightly less than in ethanol solution (70 ± 10%). The lifetime of the radical, however, can be as much as 1900 times longer. With respect to potentially useful photochemical energy conversion, the magnitude of this increased lifetime is far more significant than is the decreased radical yield.

Studies of Ruthenium‐Oxy Species in Basic Solutions by Cyclic Voltammetry

Ruthenium (VI) solutions in alkaline media have been investigated by cyclic voltammetry using platinum and ruthenium dioxide electrodes. Four pairs of peaks were observed. The oxidation states of the electrolysis products were evaluated by coulometry and absorption spectra. The first and second pairs of peaks correspond, respectively, to the oxidation reduction of Ru(VIII)/ Ru(VII) and Ru(VII)/Ru(VI) with a catalytic reaction. The third pair shows reduction of Ru(VI) to Ru(IV) with a following reaction giving ruthenium dioxide. The electron transfer product is postulated as . The last or most cathodic pair revealed a reduction peak composed of two waves. The first is from the reduction of the electrode surface oxide layer or adsorbed oxygen and the second, the reduction of Ru(IV) to Ru(III). The corresponding pairs of peaks are also observed from ruthenium dioxide electrodes in sodium hydroxide solutions.

Mechanism of the quenching of the emission of substituted polypyridineruthenium(II) complexes by europium(II)

The emission from the charge-transfer excited states of poly(pyridine)ruthenium(II) complexes (RuL/sub 3//sup 2 +/, L = a 2,2'-bipyridine or 1,10-phenanthroline derivative) is quenched by aqueous europium(II) at rate constants ranging from 1.5 x 10/sup 6/ to 1.0 x 10/sup 8/ M/sup -1/s/sup -1/(25/sup 0/C, 0.5 M ionic strength) depending on the identity of L. The quenching gives rise to the electron-transfer products RuL/sub 3//sup +/ and Eu(III) which undergo back-reaction to form RuL/sub 3//sup 2 +/ and Eu(II) at rate constants ranging from 2 to 6 x 10/sup 7/ M/sup -1/s/sup -1/. Energy-transfer and electron-transfer mechanisms are considered for the Eu(II) quenching process and the free energy dependences observed in europium outer-sphere electron-transfer reactions are reviewed. 4 figures, 3 tables.

ChemInform Abstract: QUENCHING OF THE LUMINESCENCE OF THE TRIS(2,2′‐BIPYRIDINE) COMPLEXES OF RUTHENIUM(II) AND OSMIUM(II). KINETIC CONSIDERATIONS AND PHOTOGALVANIC EFFECTS

The quenching of the charge-transfer excited states of Ru(bpy)/sub 3//sup 2 +/ and Os(bpy)/sub 3//sup 2 +/ by O/sub 2/, Fe/sup 3 +/, Co(phen)/sub 3//sup 3 +/, Ry(NH/sub 3/)/sub 6//sup 3 +/, Os(bpy)/sub 3//sup 3 +/, and Fe(CN)/sub 6//sup 3 +/ has been studied by a spectrofluorometric method. The lifetime of the emitting state of the osmium (II) complex and the rate of its reaction with Ru(NH/sub 3/)/sub 6//sup 3 +/ were measured by a single-photon counting technique. A value of -0.96 V was estimated for the reduction potential of the Os(bpy)/sub 3//sup 3 +/ parallel *Os(bpy)/sub 3//sup 2 +/ couple. Emission intensity measurements were used to determine the steady-state concentrations of the Fe/sup 2 +/ and Ru(bpy)/sub 3//sup 3 +/ formed in the reaction of Fe/sup 3 +/ with the emitting state of Ru(bpy)/sub 3//sup 2 +/. The steady-state concentrations of the electron-transfer products increase in the order HCl < H/sub 2/SO/sub 4/ < HClO/sub 4/ approx. CF/sub 3/SO/sub 3/H and in 0.5 M HClO/sub 4/ amount to about 25 percent of the initial Ru(bpy)/sub 3//sup 2 +/ concentration under the conditions used. The Ru(bpy)/sub 3//sup 2 +/-Fe/sup 3 +/ system exhibits a large photogalvanic effect. The mechanisms ofmore » the reactions are discussed and general equations describing the steady-state characteristics of these systems are derived. (auth)« less

Electron-donor and electron-acceptor properties of carotenoids electrochemical study of carotenes

The methods of polarography, voltammetry at rotating solid electrodes and cyclic voltammetry with triangle and trapezoidal pulses have been employed to study the electrochemical behavior of carotenes in various solvents (in mixtures of dimethyl-formamide and acetonitrile with benzene, in dichloroethane) at different electrodes (Hg, Pt, graphite). The capacity of carotenes to accept or donate electrons reversibly was revealed. Contrary to the general scheme, electrooxidation of carotenes was shown to proceed as a one-stage two-electron reversible process in an available range of time measured (at scan rates up to 1000 V s −1 ) Formation of a number of short-living intermediate particles, products of electron transfer (anion-radicals, anions, cations) was recorded. The kinetics of the reactions with the participation of these particles was studied. Normal potentials of reversible redox transitions including those in which unstable intermediates had formed were calculated. Ionization potentials and electron affinity of β-carotene and donor—acceptor activity of carotenes was revealed: carotenes possess stronger electrondonor properties than electron-acceptor. With chlorophylls the similar asymmetry has not been observed.