Complete Electron Transfer Research Articles

Finite supercluster model of vanadium oxide catalysts. Part II. Adsorbate—adsorbate interactions

The recently proposed supercluster model is applied to study the interaction between two molecules (atoms) adsorbed at the surface of a vanadium oxide catalyst. The results suggest that the interaction between adsorbed molecules via the catalyst may be particularly important for weakly chemisorbed species. For a pair of identical, weakly adsorbed molecules this interaction may (for some relative orientations) affect to some extent the conclusions obtained for a single admolecule, which may be of some relevance at high coverages. The interaction between unlike admolecules is normally expected to result in complete electron transfer to the molecule which is more strongly chemisorbed, but for some relative orientations of the two molecules substantial back donation may take place. This fact may have some consequences for the dependence of the reaction yield on reactant concentration.

Dioxygen adducts of cobalt(II) complexes of 6,6′-bis(p-substituted benzoylamino)-2,2′-bipyridines and their catalytic activities in oxygenation of 2,6-di-tert-butylphenol

The oxygen affinities and properties of oxygen adducts of complexes [CoL][H2L = 6,6′-bis(benzoyl-amino)-2,2′-bipyridine and its 4-methyl-, -tert-butyl or -chloro-benzoylamino analogues] has been studied by cyclic voltammetry and IR, electronic and ESR spectroscopy. The complexes exhibited oxygen-binding ability in the presence of an appropriate axial base ligand (B), and the oxygen adducts were of the end-on bonded CoIII(O2–) type [CoL(B)(O2)]. More complete electron transfer from the metal centre to the oxygen was indicated compared to that with [Co(salen)][H2salen =N,N′-ethylenebis(sali-cylideneimine)]. Electron-donating substituents gave higher concentrations of the oxygen adducts. The superior catalytic activity of these complexes, especially the Me- and But-substituted derivatives, in the oxygenation of 2,6-di-tert-butylphenol to the corresponding quinone is interpreted in terms of the reactivity of the co-ordinated oxygen and the oxygen affinity of the complexes.

Polyanionic clustering in crystalline and liquid alkali-group-III metal alloys

A theoretical investigation of the structural and electronic properties of crystalline and liquid alloys of alkali metals with group-III metals is presented. The results suggest the validity of a generalized Zintl principle assuming at least formally complete electron transfer, so that the local coordination and the electronic DOS in, e.g., LiGa alloys becomes Ge-, As-, and finally Se-like with increasing Li-concentration and hence an increasing number of electrons per Ga atom.

Ballistic-electron transport through a coupled-quantum-wire system.

We study the ballistic-transport properties of a coupled-quantum-wire system where two wires are connected by a coupling region. The region is gated and permits, therefore, a coupling which can be varied. In particular, we evaluate the transmission and reflection coefficients for a variety of system parameters. By considering the scattering at the interaction region between the quantum wires, we find that this system behaves as an electron-waveguide coupler, similar to that of an optical coupler. Periodic switching of the electron wave between the quantum wires is observed. Complete electron transfer occurs at a transfer length as short as 400 \AA{}, which is much smaller than the phase-breaking length, implying that this system is experimentally realizable. We also present a detailed study of the quantum bound states in such an open system. Finally, the B\"uttiker resistance of this four-terminal system is presented.

Photosynthetic electron-transfer reactions in the green sulfur bacterium Chlorobium vibrioforme: evidence for the functional involvement of iron-sulfur redox centers on the acceptor side of the reaction center.

The green sulfur bacterium Chlorobium vibrioforme was cultured in the presence of ethylene to selectively inhibit the synthesis of the chlorosome antenna BChl d. Use of these cells as starting material simplified the isolation of a photoactive antenna-depleted membrane fraction without the use of high concentrations of detergents. The preparation had a BChl alpha/P840 of 50, and the spectral properties were similar to those of preparations isolated from cells grown with a normal complement of chlorosomes. The membrane preparation was active in NADP+ photoreduction. This indicated that the fraction contained reaction centers with complete electron-transfer sequences which were then characterized further by flash kinetic spectrophotometry and EPR. We confirmed that cytochrome c553 is the endogenous donor to P840+, and at room temperature we observed a recombination reaction between the reduced terminal acceptor and P840+ with a t1/2 = 7 ms. Oxidative degradation of iron-sulfur centers using low concentrations of chaotropic salts introduced a faster recombination reaction of t1/2 = 50 microseconds which was lost at higher concentrations of chaotrope, indicating the participation of another iron-sulfur redox center earlier than the terminal acceptor. Cluster insertion using ferric chloride and sodium sulfide in the presence of 2-mercaptoethanol restored both the 50-microseconds and 7-ms recombination reactions, allowing definitive assignments of these centers as iron-sulfur centers. Following the suggestion of Nitschke et al. [(1990) Biochemistry 29, 3834-3842], we associate these two kinetic phases to back-reactions between P840+ and iron-sulfur centers FX and FAFB, respectively. The iron-sulfur cluster degradation and reconstitution protocols also led to inhibition and restoration of NADP+ photoreduction by the membrane preparation, providing unequivocal evidence for the function of the centers FX and FAFB in the physiological electron-transfer sequence on the acceptor side of the Chlorobium reaction center. At 77 K we observed a recombination reaction of t1/2 = 20 ms that we suggest occurs between Fx- and P840+. Degradation of the iron-sulfur clusters resulted in replacement of the 20-ms phase with a faster reaction of t1/2 = 80 microseconds that was most likely a recombination between the early acceptor A1- and P840+ or decay of 3P840. Analysis of the iron-sulfur centers in the preparation by EPR at cryogenic temperature supports the optical measurements. EPR signals originating from the terminal acceptor(s) were not observed following treatment of the membrane preparation by chaotropes, and a modified signal was restored following cluster reinsertion.(ABSTRACT TRUNCATED AT 400 WORDS)

Direct redox intercalation of ferrocene in α-VOPO 4

A direct redox intercalation of iron-biscyclopentadiene (ferrocene) in α-VOPO 4 has been carried out for the first time without help of any extra-reducing agent. The reaction only involves gaseous ferrocene Fecp 2 and solid α-VOPO 4. By chemical analysis, the composition of the obtained intercalation compound is found to be VOPO 4, O.22 Fecp 2. Mössbauer spectroscopy shows that a complete electron transfer occurs between the guest molecule Fecp 2 and the vanadium atoms of the host lattice. The magnetic properties, investigated by ESR and by magnetic susceptibility measurements, confirm the presence of the resulting Fecp 2 + and VO 2+ paramagnetic ions interacting only via dipolar couplings.

Electron waveguide coupler: A four-terminal device

We model the electron waveguide coupler as a four-terminal device, by considering the scattering at the interaction region between the electron waveguides. Transmission and reflection coefficients are evaluated for a variety of system parameters. Periodic switching of the electron wave between the waveguides are observed. Complete electron transfer occurs at a transfer length as short as 400 Å. This value is much smaller than that predicted by coupled-mode theory and the device is thus experimentally realizable.

Oxidation of cobalt(II) tetrapyrroles in the presence of an electron acceptor

The interaction of cobalt(II) porphyrins and chlorin with 4,6-dinitrobenzofurazan 1-oxide (dnbfo) has been studied by spectroscopic methods. Initially all the cobalt(II) porphyrins form molecular complexes with the electron acceptor dnbfo, but with time a complete electron transfer is observed. This results in the formation of cobalt(III) porphyrins and the anion radical of dnbfo. The electron transfer is facilitated in the presence of molecular oxygen. The rate of oxidation is dependent on the porphyrin ring substituents as well as on the solvent mixture used. In the case of cobalt(II) 5,10,15,20-tetraphenylchlorin first oxidation occurs to cobalt(II) 5,10,15,20-tetraphenylporphyrin and this is followed by metal oxidation to give the cobalt(III) porphyrin. The oxidation of the cobalt(II) porphyrins and formation of the anion radical of dnbfo has been monitored by ESR spectroscopy. The g value and the linewidth of the radical compares well with that of the anion formed by chemical reduction. Proton and 19F NMR data confirm the occurrence of the electron-transfer reaction.

Evaluation of selected benzoquinones, naphthoquinones, and anthraquinones as replacements for phylloquinone in the A1 acceptor site of the photosystem I reaction center

Selected substituted 1,4-benzoquinones, 1,4-naphthoquinones, and 9,10-anthraquinones were investigated as possible replacement quinones in spinach photosystem I (PSI) preparations that had been depleted of endogenous phylloquinone by extraction with hexane/methanol. As a criterion for successful biochemical reconstitution, the restoration of electron transfer was determined by measuring P-430 turnover at room temperature from flash-induced absorbance transients. Restoration of complete electron transfer between A0- and P-430 (terminal iron-sulfur centers, FAFB) was demonstrated by using phylloquinone, 2-methyl-3-decyl-1,4-naphthoquinone, 2-methyl-3-(isoprenyl)2-1,4-naphthoquinone, and 2-methyl-3-(isoprenyl)4-1,4-naphthoquinone. All other quinones tested did not restore P-430 turnover but acted as electron acceptors and oxidized A0-. It is concluded that the specificity of the replacement quinone for interaction with the primary acceptor, A0-, is low but additional structural constraints are required for the quinone occupying the A1 site to donate to the iron-sulfur center, Fx. It is suggested that the 3-phytyl side chain of phylloquinone and the 3-alkyl tails of the three naphthoquinones that restored P-430 turnover may be required for interaction with a hydrophobic domain of the A1 site in the PSI core to promote electron transfer to Fx and then to FAFB.

Multistep Photoinduced Electron Transfer in a Photoacceptor Terminated Molecular Triode

Abstract A molecular “triode” (3) is described containing a nonconjugatively interconnected array (D2-D1-Pa) of two potetial one-electron donor (D) moieties and a powerful photoacceptor (Pa). In a related Ldquo;diode” (2) consisting of the array D1-Pa excitation of the photoacceptor, Pa, induces charge-separation as detected by time resolved microwave conductivity (TRMC) measurements and as evidenced from the observation of charge transfer emission resulting from radiative recombination. The latter emission is quenched in the triode 3 indicating occurrence of virtually complete consecutive electron transfer from D2 to D1. Instead 3 displays a much weaker charge transfer emission at longer wavelength attributed to direct radiative recombination from D2 to Pa. Fluorescence lifetime measurements and TRMC data show that the recombination in 3 is significantly slowed down as compared to the shorter range recombination in 2.

Photochemical transformations. 46. Photophysics and photochemistry of some compounds undergoing light-induced solvolysis and Wagner-Meerwein rearrangement

Absorption maxima, fluorescence maxima, and fluorescence quantum yields are reported for a variety of 7-substituted and 7,8-disubstituted dibenzobicyclo(2.2.2)octadienes and veratrolobenzobicyclo(2.2.2)octadienes in cyclohexane and/or acetonitrile. Singlet lifetimes have been computed from these data. Those compounds that are reactive toward photosolvolysis and/or Wagner-Meerwin photorearrangement have fluorescence yields (and lifetimes) 10/sup -2/-10/sup -4/ times those of the corresponding compounds containing only hydrogens on the saturated bridges. Compounds that are photoinert but have an electron-attracting group on the bridge have lifetimes about one-tenth those of unsubstituted compounds. These results and the variations in fluorescence maximum wavelengths are rationalized by assumptions that there is some degree of charge transfer from aromatic ring to C-X bond in the photoexcited state and that this favors decay to the ground state. When more or less complete intramolecular electron transfer to a zwitterionic biradical intermediate is exergonic, lifetimes in the 4-80-ps range are completed.

Polyimides: Versatile Specialty Polymers

Polyimide specialty polymers are finding growing usage in applications such as high temperature, thermally stable insulators and dielectrics, coatings, adhesives, and high performance composites. To add to this already rich field of polyimide functions, we wish to review recent research from our laboratory which features the photoconductive and electrochemical behavior of polyimides, areas which have not as yet been widely explored. The first area of research, which illustrates polyimides as electrochemically active materials, is the recent work of S. Mazur and S. Reich. It deals with the specially controlled deposition of very thin silver layers, or certain other metal layers, into a polyimide film prepared from 4,4′-oxydianiline and pyromellitic dianhydride. This is achieved by coating the polyimide film, ~10μm thick, onto a cathode. The cathode is placed in an electrolyte solution containing silver ion. A potential is applied between the solution and the cathode, resulting in the diffusion of silver ions towards the cathode. At steady state, when the flux of electrons (moving away from the cathode) equals that of the silver ions, a well defined, particulate, dense silver layer, ~0.1μm thick, is deposited within the polyimide film. It is shown that the position and thickness of the metal layer as well as its geometrical shape can be controlled very precisely. The second area of research, photoconductivity in polyimides, is the work of S. C. Freilich. It is found that addition of electron donors, such as dimethylaniline, to Kapton® polyimide film results in an enhancement of the photocurrent generated by as much as five orders of magnitude when compared to the undoped polymer. The mechanism of the enhancement is shown to be the result of radiation absorption by a charge transfer complex formed between the added electron donor and the imide portion of the polymer backbone. Excitation is followed by rapid and complete electron transfer from the donor to pyromellitimide to yield the radical anion of the polymer and the radical cation of the donor. These ion pairs are demonstrated to be the carriers of the photocurrent. They also undergo rapid back electron transfer, and the geminate recombination process is consistent with an Onsager model analysis.

Donor acceptor interactions occurring in solutions of polymeric viologenes

In solutions of polyepichlorohydrin modified by N-butylpyridinepyridinium bromide in DMF there were found to be charge transfer complexes of three types between dipyridilium dications and halide anions. These three types differ as to the position of the charge transfer bands in the absorption spectra. Complexes are formed more rapidly in air in view of the coordinating action of atmospheric oxygen. The presence of oxygen coordinated with the complexes results in a higher quantum yield of formation of products of complete electron transfer under the action of light in charge transfer bands. Differences were detected in the polymeric viologenes in the case of complexation of halide-anions with dipyridilium dications in DMF compared with the low molecular analogoues.

ChemInform Abstract: The Triplet State of N‐(n‐Butyl)‐5‐nitro‐2‐furamide by Laser Flash Photolysis. Spectrum, Lifetime, Energy and Electron‐transfer Reactions.

Upon nanosecond laser flash photolysis (347.1 nm) of N-(n-butyl)-5-nitro-2-furamide (BNFA) in solution (at 298 K), a short-lived transient absorption has been observed with λmax= 500 ± 5 nm, which shows a small bathochromic shift on going to hydroxylic solvents (irrespective of polarity) and which is attributed to its lowest excited triplet state (3BNFA). A long-lived species, formed in high yield in alkanols, is assigned to the radical anion (BNFA˙–) and has λmax⩽ 360 nm with a weaker band peaking ca. 510 ± 5 nm in propan-2-ol. The lifetime of 3BNFA depends on both the solvent and the ground-state concentration of BNFA, having its smallest value in water (ca. 19 ns). Sensitization and quenching experiments enabled the triplet energy, ET, to be estimated as 238 ± 4 kJ mol–1; this value, together with the ground-state reduction potential, E17, yields E17(3BNFA/BNFA˙–)= 2.23 V vs NHE for the one-electron reduction potential of 3BNFA in neutral aqueous solution. 3BNFA is rather oxidizing, thus it abstracts a hydrogen atom from diphenylmethanol with k2=(3.01 ± 0.12)× 107 dm3 mol–1 s–1 in acetone solution, and the second-order rate constants for the one-electron oxidation of a number of substrates by 3BNFA have been measured in 1 : 4 (v/v) water + acetonitrile mixtures and range from ca. 2 × 108 for CH3CO–2 to ca. 6 × 109 dm3 mol–1 s–1 for I–. Evidence for complete electron transfer arises, in several cases, from the observation of the semi-oxidized substrate. The correlation of the logarithm of k2 with the standard free-energy change, obtained from the estimated thermodynamic potentials of the reactants, are discussed in terms of the Rehm–Weller and Marcus free-energy relationships. Absolute rate constants for the one-electron reduction of the lowest triplet states of duroquinone, 2-nitrothiophen and 5-nitro-2-furoic acid are also included in this correlation. Pulse photolysis of a solution of 2-nitropyrrole showed no absorbing transient in the spectral region 390–700 nm. The implications of some of these findings for nucleophilic photosubstitution reactions of 5-membered nitroheterocyclic compounds are discussed.

LASER FLASH PHOTOLYSIS OF INDOLE IN THE PRESENCE OF AMINO ACIDS

AbstractThe laser flash photolysis of indole at 265 nm in the presence of glycine, proline and hydroxy proline was studied. The relative yields of c aq, triplet state, and indole cation radical were determined in the absence and in the presence of the amino acids. The yields were determined as a function of laser intensity and the values at very low intensity were compared with the fluorescence quenching results. It was concluded that in these conditions the photoionization of indole occurs via the fluorescent state. From the curves of triplet yield vs laser intensity, the triplet quantum yield extrapolated at low laser intensity was obtained, φr = 0.55 φ 0.05, relative to the literature value of 0.15 for φeag. This gives φF+φeaq= 1.0 ± 0.1 at room temperature. When proline and hydroxy proline were used as singlet quenchers, the yield of In was greater than the yield of caq. This was considered as evidence that a fraction of the quenching processes leads to complete electron transfer from indole to the amino acids.

Exciplex formation between aromatic amines and 4-dimethylamino-2-pyridyl 3-pyridyl ketone

The fluorescence of 4-dimethylamino-2-pyridyl 3-pyridyl ketone in 2-propanol is quenched by aromatic amines. Exciplex formation appears to be involved in the quenching process. Evaluation of the rate constants for the decay of the exciplex suggests that complete electron transfer from the amine to the ketone may take place.

The triplet state of N-(n-butyl)-5-nitro-2-furamide by laser flash photolysis. Spectrum, lifetime, energy and electron-transfer reactions

Upon nanosecond laser flash photolysis (347.1 nm) of N-(n-butyl)-5-nitro-2-furamide (BNFA) in solution (at 298 K), a short-lived transient absorption has been observed with λmax= 500 ± 5 nm, which shows a small bathochromic shift on going to hydroxylic solvents (irrespective of polarity) and which is attributed to its lowest excited triplet state (3BNFA). A long-lived species, formed in high yield in alkanols, is assigned to the radical anion (BNFA˙–) and has λmax⩽ 360 nm with a weaker band peaking ca. 510 ± 5 nm in propan-2-ol. The lifetime of 3BNFA depends on both the solvent and the ground-state concentration of BNFA, having its smallest value in water (ca. 19 ns). Sensitization and quenching experiments enabled the triplet energy, ET, to be estimated as 238 ± 4 kJ mol–1; this value, together with the ground-state reduction potential, E17, yields E17(3BNFA/BNFA˙–)= 2.23 V vs NHE for the one-electron reduction potential of 3BNFA in neutral aqueous solution. 3BNFA is rather oxidizing, thus it abstracts a hydrogen atom from diphenylmethanol with k2=(3.01 ± 0.12)× 107 dm3 mol–1 s–1 in acetone solution, and the second-order rate constants for the one-electron oxidation of a number of substrates by 3BNFA have been measured in 1 : 4 (v/v) water + acetonitrile mixtures and range from ca. 2 × 108 for CH3CO–2 to ca. 6 × 109 dm3 mol–1 s–1 for I–. Evidence for complete electron transfer arises, in several cases, from the observation of the semi-oxidized substrate. The correlation of the logarithm of k2 with the standard free-energy change, obtained from the estimated thermodynamic potentials of the reactants, are discussed in terms of the Rehm–Weller and Marcus free-energy relationships. Absolute rate constants for the one-electron reduction of the lowest triplet states of duroquinone, 2-nitrothiophen and 5-nitro-2-furoic acid are also included in this correlation. Pulse photolysis of a solution of 2-nitropyrrole showed no absorbing transient in the spectral region 390–700 nm. The implications of some of these findings for nucleophilic photosubstitution reactions of 5-membered nitroheterocyclic compounds are discussed.

Further studies on electronic guest/host interactions of FeCl 3-graphite from Mössbauer spectroscopy — A comparative investigation with FeOCl

Mössbauer spectra of first stage FeCl 3-graphite and of the inverse Fe(C 5H 5) 2-iron oxide chloride intercalation compounds have been measured between room temperature and 4 K. The spectra at 295 K consist of resonance lines which are characteristic of ferric ions only. In addition, the spectra showed high-spin Fe 2+-sits at low temperatures. This result may be accounted for by an incomplete redox reaction caused by partial transfer of the π-electrons of the graphite (host layer), or of the ferricinium ions (guest layers), to the adjacent layers. Contrary to what is found in the literature, at low temperatures only these delocated electrons are fixed in the d-orbitals of some Fe 3+ atoms resulting in Fe 2+-sites. This result suggests that a shielding effect of the chlorine layers exists in these intercalation compounds which hinders a complete electron transfer to the Fe 3+-centers at room temperature. The ratio Fe 2+/Fe 3+ = 1:3.6 at 4 K is in agreement with Hennig's formula Cl −FeCl 2·3FeCl 3(C n ) +.

Interatomic Auger analysis of the oxidation of thin Ba films: II. Applications to impregnated cathodes

The oxidation and thermal desorption of evaporated Ba films on different substrates have been characterized using low energy interatomic AES techniques. The films, which were deposited on W〈100〉 (as well as Ir and Os-W) varied from a fractional layer to ∼ 10 layers. Oxidation states, i.e. Ba: O x , ranged from x=0 to x≈2. Comparison of these data to those from actual impregnated cathodes allows one to infer the thickness and state of oxidation of the cathodes. The results indicate that the active state for “B-type” impregnated cathodes can be reproduced by a near monolayer of the stoichiometric BaO on the W surface. The density of Ba atoms (or O atoms) in this “monolayer” should be ∼ one half that of the W atoms based on size consideration of the Ba compared to the W. This was confirmed by LEED. Measurements of the effusion of Ba compounds from the pores, the substrate interaction and the thermal evaporation indicate that on the W substrate, the monolayer BaO stoichiometric ratio is not only a stable configuration, but also is the one that has the lowest work function (i.e. ∼ 2 eV). The substrate work function is lowered because of the dipole formed from the adsorbed BaO. Studies of the electron interactions between the Ba and O indicate that (in agreement also with surface plasmon results) a more complete electron transfer exists between the Ba and O for the BaO layer on the Ir substrate than on the W substrate. These results are based on the observation that O 2p states on the W substrate appear to be filled not only by Ba 6s but also by W valence electrons.

Solid-state properties of TCNQ, perchlorate, and bromide salts of a phenothiazine derivative

Three different salts, perchlorate, bromide, and TCNQ simple salts of the phenothiazine derivative, were prepared. Magnetic susceptibility, diffuse reflectance, and EPR were used to study the electronic properties of these compounds in the solid state. The most paramagnetic is the perchlorate, and bromide is less paramagnetic; this is attributed to the minor steric effect produced by the Br − within a column formed by stacked radicals of promazine +. CT bands of the ClO − 4 are those of a CT between ion radicals stacked flat upon each other where the unpaired π electrons form an overlap which produces a more than pairwise interaction. The stronger interaction among Br − radicals is made evident mainly by a splitting of the CT band. From the ir spectra it can be concluded that the TCNQ-promazine salt is mainly ionic: TCNQ ⨪ Pro ⨥ is diamagnetic as expected in compounds where an almost complete electron transfer is produced as shown by the EPR. TCNQ-promazine shows an absorption spectrum in the solid state with at least three maxima, which can be attributed to three transitions: 1. (a) the lowest-energy transition to TCNQ ⨪: promazine ⨥, 2. (b) the highest-energy transition to a CT among the cation radicals Pr ⨥ : Pr- ⨥, and 3. (c) the intermediate energy band to a CT between the anion radicals TCNQ ⨪ : TCNQ ⨪.