Ruthenium-manganese complexes for artificial photosynthesis: factors controlling intramolecular electron transfer and excited-state quenching reactions

Continuing our work toward a system mimicking the electron-transfer steps from manganese to P(680)(+) in photosystem II (PS II), we report a series of ruthenium(II)-manganese(II) complexes that display intramolecular electron transfer from manganese(II) to photooxidized ruthenium(III). The electron-transfer rate constant (k(ET)) values span a large range, 1 x 10(5)-2 x 10(7) s(-1), and we have investigated different factors that are responsible for the variation. The reorganization energies determined experimentally (lambda = 1.5-2.0 eV) are larger than expected for solvent reorganization in complexes of similar size in polar solvents (typically lambda approximately 1.0 eV). This result indicates that the inner reorganization energy is relatively large and, consequently, that at moderate driving force values manganese complexes are not fast donors. Both the type of manganese ligand and the link between the two metals are shown to be of great importance to the electron-transfer rate. In contrast, we show that the quenching of the excited state of the ruthenium(II) moiety by manganese(II) in this series of complexes mainly depends on the distance between the metals. However, by synthetically modifying the sensitizer so that the lowest metal-to-ligand charge transfer state was localized on the nonbridging ruthenium(II) ligands, we could reduce the quenching rate constant in one complex by a factor of 700 without changing the bridging ligand. Still, the manganese(II)-ruthenium(III) electron-transfer rate constant was not reduced. Consequently, the modification resulted in a complex with very favorable properties.     

Comparison of photoinduced electron transfer reactions of C60 and aromatic carbonyl compounds

The one-electron reduction potential of the triplet excited state of C₆₀ is similar to those of some aromatic carbonyl compounds. Thus, photoinduced electron transfer is expected to occur from the common electron donors to both C₆₀ and aromatic carbonyl compounds. In this paper comparison is made between photoinduced electron transfer from organosilanes and organostannanes used as the electron donors to the triplet excited states of C₆₀ and aromatic carbonyl compounds, providing valuable insight into their common mechanistic features for the C-C bond formation via photoinduced electron transfer as well as the new functionalization method of C₆₀.     

Comparison of photoinduced electron transfer reactions of aromatic carbonyl vs. cyano compounds with electron donors in condensed phase:the importance

Photoinduced electron transfer reactions in acetonitrile with bensopheneone, anthraquinone, 9-cyanoanthracene and 9,10-dicyanoanthracene as electron acceptors, and with 1,4-diasabicyclo[2,2,2]octane and N,N-dimethylaniline as electron donors have been studied with ns-laser flash photolysis and fluorescence quenching measurements. For these systems the resulting free ion yield depends on the spin state of the geminate ion pair: its separation is very efficient if formed in a triplet state (carbonyl compounds/donors), while it is very inefficient if formed in a singlet state (cyanoanthracenes/donors). In the triplet systems, geminate back electron transfer is limited by the rate of spin flip.     

Some charge transfer reactions of excited aromatic compounds with sulphides

Photo-reductive dehalogenation of aromatic halides to their parent hydrocarbons is stimulated in the presence of electron donors such as di-ethyl sulphide. Evidence is presented for the involvement of radicals ions in these reactions.     

Photoinduced electron transfer reactions of rose bengal and selected electron donors

The photoinduced electron transfer reactions of the triplet state of rose bengal (RB) and several electron donors were investigated by the complementary techniques of steady state and time-resolved electron paramagnetic resonance (EPR) and laser flash photolysis (LFP). The yield of radicals varied with the light fluence rate, RB concentration and, in particular, the electron donor used. Thus for L-dopa (dopa, dihydroxyphenylalanine) only 10% of RB anion radical (RB√−) was produced, with double the yield observed with NADH (NAD, nicotinamide adenine dinucleotide) as quencher and more than three times the yield observed with ascorbate as quencher. Quenching of the RB triplet was both reactive and physical with total quenching rate constants of 4 × 10⁸ mol⁻¹ dm³ s⁻¹ and 8.5 × 10⁸ mol⁻¹ dm³ s⁻¹ for ascorbate and NADH respectively. The rate constant for the photoinduced electron transfer from ascorbate to RB triplet was 1.4 × 10⁸ mol⁻¹ dm³ s⁻¹ as determined by Fourier transform EPR (FT EPR). FT EPR spectra were spin polarized in emission at early times indicating a radical pair mechanism for the chemically induced dynamic electron polarization. Subsequent to the initial electron transfer production of radicals, a complex series of reactions was observed, which were dominated by processes such as recombination, disproportionation and secondary (bleaching) reactions.

It was observed that back electron transfer reactions could be prevented by mild oxidants such as ferric compounds and duroquinone, which were efficiently reduced by RB√−.     

Oxidation reactions of some natural volatile aromatic compounds: anethole and eugenol

trans-Anethole [1-methoxy-4-(trans-prop-1-en-1-yl)benzene] was isolated from anise seed oil (Pimpinella anisum). Its photochemical oxidation with hydrogen peroxide gave the corresponding epoxy derivative together with 4-methoxybenzaldehyde. The thermal oxidation of trans-anethole with 3-chloroperoxybenzoic acid at room temperature resulted in the formation of dimeric epoxide, 2,5-bis(4-methoxyphenyl)-3,6-dimethyl-1,4-dioxane, as the only product. Photochemical oxygenation of trans-anethole in the presence of tetraphenylporphyrin, Rose Bengal, or chlorophyll as sensitizer led to a mixture of 1-(4-methoxyphenyl)prop-2-en-1-yl hydroperoxide and 4-methoxybenzaldehyde. Eugenol was isolated from clove oil [Eugenia caryophyllus (Spreng.)]. It was converted into 2-methoxy-4-(prop-2-en-1-yl)phenyl hydroperoxide by oxidation with hydrogen peroxide under irradiation. Thermal oxidation of eugenol with 3-chloroperoxypenzoic acid at room temperature produced 2-methoxy-4-(oxiran-2-ylmethyl)phenol, while sensitized photochemical oxygenation (in the presence of Rose Bengal or chlorophyll) gave 4-hydroperoxy-2-methoxy-4-(prop-2-en-1-yl)cyclohexa-2,5-dien-1-one. Published in Russian in Zhurnal Organicheskoi Khimii, 2008, Vol. 44, No. 6, pp. 834–841. The text was submitted by the authors in English.     

Step wise and dissociative mechanisms of the electron transfer in electrochemical reactions involving organosilicon compounds: Molecular-thermodynamic approach1

Electrochemical reduction of organochlorosilanes and oxidation of hexaorganodisilanes may occur via dissociative and stepwise mechanisms, the choice between which is determined by the balance between fundamental structural parameters of elementoorganic molecules. The formation of radical anions of silyl-substituted chloromethane in the conditions of an electrochemical experiment is shown. The formation is due to α-silicon stabilization of the intermediate during the electron transfer. The role of conjugation and hyperconjugation in the organosilicon compounds’ reactivity is analyzed. When employing terms “ stepwise ” and “ dissociative ” mechanisms of the electron transfer, we follow tradition introduced and developed by J.-M. Saveant’s group [1] and recommended by the IUPAC Commission on Electrochemical Nomenclature [2]. The terms refer to an electrochemically reversible transfer of electron whose kinetics does not limit the process rate as a whole and to an electrochemically irreversible transfer which involves a bond cleavage in an elementary act of electron transfer. The term “ activation ” mechanism, which is sometimes applied to electrochemically irreversible processes, is more universal; however, it is somewhat not unique as compared with the term “ dissociative.” Coupled with “ associative,” the latter may be used for indicating processes that involve the formation of a bond in an elementary act of electron transfer.     

EPR evidence for a single electron transfer mechanism in reactions of aromatic ketones with lithium amides

Direct spectroscopic evidence (EPR) supporting a single electron transfer mechanism in the reaction of lithium amides with aromatic ketones is presented.     

Charge transfer absorption for pi-conjugated polymers and oligomers mixed with electron acceptors

Pi-conjugated polymers and oligomers show charge transfer (CT) absorption bands when mixed with electron acceptors in chloroform solution. This is attributed to the formation of (ground state) donor-acceptor complexes in solution. By varying the concentration of the donor and acceptor, the extinction coefficient for the CT absorption and the association constant of donor and acceptor are estimated. The spectral position of the CT bands correlates with the electrochemical oxidation potential of the pi-conjugated donor and the reduction potential of the acceptor.     

Kinetics and mechanisms of the gas-phase reactions of the NO3 radical with aromatic compounds

The kinetics and nitrated products of the gas-phase reactions of the NO₃ radical with methoxybenzene, 1,2-, 1,3-, and 1,4-dimethoxybenzene, dibenzofuran and dibenzo-p-dioxin have been investigated at 297 ± 2 K and in the presence of one atmosphere of air. A relative rate method was used for the kinetic measurements. No reactions of methoxybenzene or dibenzofuran with the NO₃ radical were observed. The dimethoxybenzenes were observed to react by H-atom abstraction and NO₃ radical addition to the aromatic ring, while dibenzo-p-dioxin reacted by NO₃ radical addition to the aromatic rings. For these compounds, the NO₃ radical addition pathways were observed to be reversible. At the NO₂ concentrations employed, the NO₃-aromatic adducts reacted with NO₂ and the observed rate constants increased with increasing NO₂ concentration. However, for dibenzo-p-dioxin the observed rate constant became independent of the NO₂ concentration for concentrations ≥ 4.8 × 10¹³ molecule cm^?3, and under these conditions the rate constant of 6.8 × 10^?14 cm³ molecule^?1 s^?1 was taken to be that for addition of the NO₃ radical to the aromatic rings. The proposed NO₃ radical reaction mechanisms are discussed. © 1994 John Wiley & Sons, Inc.     

Kinetic studies of Cl-atom reactions with selected aromatic compounds using the photochemical reactor-FTIR spectroscopy technique

Relative rate constant measurements have been carried out on the Cl-atom reactions with benzene, chlorobenzene, toluene, xylene, and styrene in 740 torr of air at room temperature (295 K), using the photochemical reactor-FTIR spectroscopy technique. The Cl atoms were generated by the UV photolysis of Cl₂, and the reference compounds were CHF, Cl for benzene and chlorobenzene and isobutane for toluene xylene and styrene. Using the absolute rate constant for these two reference compounds reported in the literature, the following kinetic data were obtained for the study compounds (in units of cm³s⁻¹). \settabs 2 \columns \+Benzene & $(1.3\pm 0.3)\times 10 ^ {-15}$\cr \+Chlorobenzen & $(9.8\pm 2.4)\times 10 ^ {-16}$\cr \+Toluene & $(5.9\pm 0.5)\times 10 ^ {-11}$\cr \+o-Xylene & $(1.5\pm 0.1)\times 10 ^ {-10}$\cr \+m-Xylene & $(1.4\pm 0.1)\times 10 ^ {-10}$\cr \+p-Xylene & $(1.5\pm 0.1)\times 10 ^ {-10}$\cr \+Styrene & $(3.6\pm 0.3)\times 10 ^ {-10}$\cr The quoted error bars are for ± 2σ. The present kinetic results are compared with available literature data to update and expand the kinetics database for Cl-atom reactions of organic compounds. The results are also analyzed to provide insights into the reaction mechanism for the Cl-atom initiated oxidation of benzene under atmospheric conditions. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 349–358, 1997     

Factors controlling electron transfer reactions and stabilisation of uncommon oxidation states of chromium

Some experimental approaches to seek semi-quantitative understanding of factors controlling outer sphere electron transfer reactions of some transition metal complexes have been made. The relative importance of nuclear and electronic factors to outer sphere processes has been examined. By the manipulation of Franck-Condon or nuclear factors, it has now been possible to gain access into the chemistry of chromium in unusual oxidation states. An example of a reorganisation controlled electron transfer reaction involving Cr(IV)-Cr(III) system has been demonstrated. The bimolecular rate of reduction of diperoxoaquaethylenediamine chromium(IV) and diperoxodiethylenetriamine chromium(IV) is independent of the nature of th reductant employed viz. Fe²⁺ or VO²⁺ indicating that the generation of6 coordinate Cr(IV) species from7 coordinate of diperoxochromium(IV) reactant may be rate limiting. Similarly by increasing the barrier for the6 coordinate to4 coordinate structures through equatorial coordination of macrocyclic ligands, it has now been possible to detect through cyclic voltommogram the formation of relatively stable Cr(IV) species in the electrochemical oxidation of Cr(Me₄[14] tetraene)(H₂O) ₂ ³⁺ in aqueous sulphuric acid media. The kinetics and mechanism of the cerium(IV) and iodosyl benzene oxidation of Cr(salen)(H₂O) ₂ ⁺ and Cr(salprn)(H₂O) ₂ ⁺ have been investigated and kinetic and spectroscopic evidence for the formation of Cr(IV) transients and stable Cr(V) products has been presented. The relative importance of Franck-Condon factors in the oxidation of Cr(III) to Cr(IV) and Cr(V) states in different macrocyclic and multidentate ligand environments has been discussed.     

Quantum chemical study of aromatic diamine and dianhydride reactivities in acylation reactions

Semi-empirical calculations of isolated diamines and model anhydrides and of their interaction energy, ΔE, have been made. Reactivity indices of diamines and dianhydrides (atom charges, characteristics of frontier orbitals) have been found. Analysis of the ΔE components has shown that chemical structure of diamines affects mainly electrostatic and charge transfer contributions to ΔE. Chemical structure of diahnydrides influences most significantly charge transfer component. Results of the ΔE calculation have substantiated the choice of reactivity indices.     

Amphiphilic copolymers of some aromatic vinyl compounds and an electrolytic monomer as potential media for photosensitized electron transfer: Fluorescence quenching by an amphiphilic electron acceptor in aqueous media

Amphiphilic polymers were prepared by the copolymerization of 2-acrylamido-2-methylpropanesulfonic acid (AMPS) and aromatic vinyl compounds such as 9-vinylphenanthrene (VPh) and 1-vinylpyrene (VPy) with the expectation that they would serve as potential media for photosensitized electron transfer reactions. AMPS strongly solubilizes the hydrophobic segments into water; i.e., poly(AMPS-co-VPh) with VPh mole fraction (f_Ph) up to about 0.60 and poly(AMPS-co-VPy) with VPy mole fraction (f_Py) up to about 0.35 were found to be soluble in water. Poly(AMPS-co-VPh) in aqueous solution, as compared with that in DMF solution, showed a broad fluorescence spectrum with significant tailing in the longer-wavelength region along with a decrease in the intensity of the structured, monomer fluorescence band. These phenomena seem to imply the presence of an excimerlike interaction of phenanthryl groups in an aqueous solution through which the fluorescence from excited VPh units may be partly self-quenched. A considerable enhancement of the fluorescence from sodium 8-anilino-1-naphthalenesulfonate (ANS) caused by hydrophobic interaction of the probe with poly(AMPS-co-VPh) in aqueous solution indicated that these copolymers assume micellar structures. The fluorescence of these copolymers in aqueous solutions was quenched by bis(2-hydroxyethyl)terephthalate (BHET), an amphiphilic quencher, far more effectively than by fumaric acid, a hydrophilic quencher. This tendency is particularly strong for the copolymers with higher content of hydrophobic units. The second-order rate constants for the quenching of poly(AMPS-co-VPh) (f_Ph = 0.58) by BHET were found to be ca. 3 × 10¹⁰ and 1.5 × 10⁹ M^?1 s^?1 in aqueous and in DMF solution, respectively. The larger value in an aqueous solution is presumably due to an increase of the effective concentration of the amphiphilic quencher around the VPh sequences of the copolymer resulting from hydrophobic interaction.     

Evidence for single electron transfer in the reactions of organometallic compounds with 2,3,3-trimethyl-2-butyl peroxybenzoate

Utilization of a triptoxy radical probe in the reactions of organometallic compounds with 2,3,3-trimethyl-2-butyl peroxybenzoate indicates that the radical character of the reaction decreases in the order $\text{n}$-BuLi > $\text{n}$-BuMgCl > PhMgBr.     

Kinetics and mechanisms of the reactions of OH with some oxygenated compounds of importance in tropospheric chemistry

The kinetics of the reactions have been studied in a discharge flow system under pseudo-first-order conditions. The OH concentration was monitored by laser induced fluorescence and helium was used as the carrier gas. Values of k₁ = (8.1 ± 1.7) × 10^?13, k₂ = (1.31 ± 0.26) × 10^?11, k₃ = (2.6 ± 0.5) × 10^?11, and k₄ = (2.5 ± 0.4) × 10^?11 cm³ molecule^?1 s^?1, at 298 K and 1 torr total pressure, were obtained. To validate the newly constructed system the rate constant for the reaction was determined in a similar manner. The value of k₅ = (6.7 ± 0.9) × 10^?12 cm³ molecule^?1 s^?1 at 298 K and 1 torr total pressure is in very good agreement with other literature values. The mechanisms for the atmospheric degradation of these compounds have been constructed to allow their incorporation in a photochemical trajectory computer model, to assess their impact on photochemical ozone creation in the troposphere. © 1994 John Wiley & Sons, Inc.     

Mechanisms of photoinduced electron transfer reactions of lappaconitine with aromatic amino acids. Time-resolved CIDNP study

CIDNP techniques were applied to the investigation of the elementary mechanism of photoinduced interaction between anti-arrhythmic drug lappaconitine and amino acids tyrosine and tryptophan. It has been shown that the reactions involve the formation of lappaconitine radical anion. Lappaconitine radical anion is unstable and rapidly eliminates N-acetyl anthranilic acid via protonation and ether bond cleavage. The rate constant of ether bond cleavage was estimated to be equal to 4 x 10(5) s(-1). The role of single electron transfer is discussed in the light of the model of drug-receptor interactions.     

The temperature dependence of the OH radical reactions with some aromatic compounds under simulated tropospheric conditions

The temperature dependence of the rate coefficients for the OH radical reactions with toluene, benzene, o-cresol, m-cresol, p-cresol, phenol, and benzaldehyde were measured by the competitive technique under simulated atmospheric conditions over the temperature range 258–373 K. The relative rate coefficients obtained were placed on an absolute basis using evaluated rate coefficients for the corresponding reference compounds. Based on the rate coefficient k(OH + 2,3-dimethylbutane) = 6.2 × 10^?12 cm³ molecule^?1s^?1, independent of temperature, the rate coefficient for toluene k_OH = 0.79 × 10^?12 exp[(614 ± 114)/T] cm³ molecule^?1 s^?1 over the temperature range 284–363 K was determined. The following rate coefficients in units of cm³ molecule^?1 s^?1 were determined relative to the rate coefficient k(OH + 1,3-butadiene) = 1.48 × 10^?11 exp(448/T) cm³ molecule^?1 s^?1: o-cresol; k_OH = 9.8 × 10^?13 exp[(1166 ± 248)/T]; 301–373 K; p-cresol; k_OH = 2.21 × 10^?12 exp[(943 ± 449)/T]; 301–373 K; and phenol, k_OH = 3.7 × 10^?13 exp[(1267 ± 233)/T]; 301–373 K. The rate coefficient for benzaldehyde k_OH = 5.32 × 10^?12 exp[(243 ± 85)/T], 294–343 K was determined relative to the rate coefficient k(OH + diethyl ether) = 7.3 × 10^?12 exp(158/T) cm³ molecule^?1 s^?1. The data have been compared to the available literature data and where possible evaluated rate coefficients have been deduced or updated. Using the evaluated rate coefficient k(OH + toluene) = 1.59 × 10^?12 exp[(396 ± 105)/T] cm³ molecule^?1 s^?1, 213–363 K, the following rate coefficient for benzene has been determined k_OH = 2.58 × 10^?12 exp[(?231 ± 84)/T] cm³ molecule^?1 s^?1 over the temperature range 274–363 K and the rate coefficent for m-cresol, k_OH = 5.17 × 10^?12 exp[(686 ± 231)/T] cm³ molecule^?1 s^?1, 299–373 K was determined relative to the evaluated rate coefficient k(OH + o-cresol) = 2.1 × 10^?12 exp[(881 ± 356)/T] cm³ molecule^?1 s^?1. The tropospheric lifetimes of the aromatic compounds studied were calculated relative to that for 1,1,1-triclorethane = 6.3 years at 277 K. The lifetimes range from 6 h for m-cresol to 15.5 days for benzene. © 1995 John Wiley & Sons, Inc.