Net Electron Transfer Research Articles

Photoinduced charge separation of methylphenothiazine in microporous titanosilicate M-ETS-10 (M=Na++K+, H+, Li+, Na+, K+, Ni2+, Cu2+, Co2+) and Na+, K+-ETS-4 molecular sieves at room temperature

The photoionization of methylphenothiazine in microporous titanosilicates M-ETS-10 (M=Na++K+, H+, Li+, Na+, K+, Ni+, Cu2+ and Co2+) and Na,K-ETS-4 molecular sieves with UV irradiation at room temperature is studied. Methylphenothiazine cation radicals (PC1+) are produced in M-ETS-10 molecular sieves and are characterized by electron paramagnetic resonance (EPR) and diffuse reflectance (DR) UV-vis spectroscopy. Microporous titanosilicate Na,K-ETS-10 and its ion-exchanged molecular sieves with alkali and transition metal ions are shown to be efficient heterogeneous hosts to accomplish stable net photoinduced electron transfer from methylphenothiazine molecules at room temperature. The photoyield of PC1+ cation radicals was enhanced by incorporation of protons into ion-exchange sites of Na,K-ETS-10. The photoionization efficiency of methylphenothiazine cation radicals decreases in the order H-ETS-10>Na,K-ETS-10>K-ETS-10>Na-ETS-10>Ni-ETS-10>Li-ETS-10>Cu-ETS-10>Co-ETS-10. For Na,K-ETS-4–PC1, no PC1+ radical is detected either by EPR or by DR spectroscopy. As the alkyl chain length of the phenothiazine increases from methyl to hexyl the photoionization yield of H-ETS-10 decreases. Thermogravimetric analysis results support this trend. It is found that the photochemistry in M-ETS-10 and Na,K-ETS-4 molecular sieves is sensitive to the metal ion, pore size and internal void space.

Oxygen Abstraction from N2O with Ground-State Transition Metal Atoms: Density Functional Study on the Mechanism of the Reactions of Sc, Ti, and V + N2O

Density functional calculations have been performed on the ground-state, spin-conserving reactions between N2O and Sc, Ti, and V atom. We have defined a reaction coordinate (N−O bond distance) along which we have investigated the reaction mechanism. Smooth reaction curves and significant exothermicity for each reaction have been obtained. It has been demonstrated that electron transfer from the metal atoms to N2O is an essential ingredient of the mechanism. This facilitates the bending of the N2O molecule, the N−O bond weakening, and an O-(2P) dissociation without surface crossing. Furthermore via 4s−3d hybridization occurring on the metal atoms the 4sβ electron is transferred to the dissociating O atom, thus connecting the reactant and product channels without any energy barrier on a single potential energy surface. It has been found that charge transfer from N2O toward the metal atom compensates the 3dα electron transfer donated by the metal atom, resulting in a net 4sβ electron transfer. We have found that when the number of 3d electrons equals 3 (in the case of vanadium), the reaction exhibits a mechanism different from the reactions with Sc or Ti, and this can be explained by considering the Pauli repulsion between the interacting orbitals. We have shown that the mechanism predicted in this work is in good accordance with the so-called electron-transfer model, whereas only elements of the so-called direct abstraction model can be invoked to account for the predicted mechanism.

Dye Capped Semiconductor Nanoclusters. Role of Back Electron Transfer in the Photosensitization of SnO2 Nanocrystallites with Cresyl Violet Aggregates

Adsorption of a cationic dye, cresyl violet, on SnO2 and SiO2 nanoclusters and nanocrystalline thin films results in the formation of H-aggregates. These dyes are photochemically and electrochemically active and extend the photoresponse of large bandgap semiconductors such as SnO2. Photocurrent generation in dye capped nanocrystalline films of SnO2 has been demonstrated with visible light excitation. A photon-to-photocurrent generation efficiency around 1% has been observed at 510 nm. Back electron transfer between the photoinjected electron and the oxidized sensitizer plays an important role in controlling the efficiency of net electron transfer. Transient absorption and microwave absorption measurements of the dye aggregate capped SnO2 films suggest that the back electron transfer is multiexponential and most is completed within a few hundred nanoseconds. The activation energy of the back electron transfer process is very low (∼1.7 kJ/mol).

Adsoprtion of carbon monoxide at a copper electrode accompanied by electron transfer observed by voltammetry and IR spectroscopy

Copper is known as a unique metal electrode which effectively reduces Co 2 to CH 4, C 2H 4 and alcohols in aqueous electrolytes with intermediate formation of adsorbed CO. Voltammetric measurements revealed that CO is adsorbed on Cu electrodes belwo −7.7V vs. nhe accompanied with quasi-reversible electron transfer. CO is not adsorbed above −0.7V on Cu electrode. Proton is involved with the adsorption process. Other metal electrodes (Au, Ag, Ni, Zn, Cd, Sn and Pb) did not show such activity for CO adsoprtion accompanied with electron transfer. In situ FTIR spectroscopic measurements indicated that adsorbed Co is present with CO stretching adsoprtion band at 2087 cm −1 on Cu electrode below −0.7V. Analysis of experimental results suggest that CO is likely to be adsorbed in a hydridocarbonly complex on Cu electrode without net electron transfer to CO molecule. The surface complex formation may be a preceding step to electrochemical reduction of CO to hydrocarbons and alcohols.

Variation of Tc and resistivity in charge-compensated (CaxLa1-x)(Ba1.75-xLa0.25+x)Cu3Oy.

We report careful measurements of the transition temperature ${\mathit{T}}_{\mathit{c}}$ of (${\mathrm{Ca}}_{\mathit{x}}$${\mathrm{La}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$) (${\mathrm{Ba}}_{1.75\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{La}}_{0.25+\mathit{x}}$)${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathit{y}}$ as a function of oxygen content y, mainly for compositions x=0.1 and 0.4. This 1:2:3 superconductor family, that exists in the composition range 0\ensuremath{\le}x\ensuremath{\le}\ensuremath{\sim}0.5, is tetragonal and has a maximal ${\mathit{T}}_{\mathit{c}}$ of \ensuremath{\sim}82 K. Through mutual charge-compensating cosubstitutions, it preserves a constant charge on the noncopper cations (Q=7.25), independent of x. We observed a monotonic decrease in ${\mathit{T}}_{\mathit{c}}$ as y varies from the initial value \ensuremath{\sim}7.15 to the insulator state roughly at y\ensuremath{\simeq}6.9. At the metal-insulator (MI) transition, the value of ${\mathit{y}}_{\mathrm{MI}}$ decreases with increasing x. Most unusual is the substantial increase in ${\mathit{T}}_{\mathit{c}}$ by 21 to 24 K when x changes from 0.1 to 0.4, under conditions where the charge density remains constant. It is accompanied by a twofold decrease in resistivity \ensuremath{\rho}. We attribute this to the change in band structure with composition x, which occurs due to increase in Ca content and to oxygen redistribution near La that substitutes for Ba. This induces a net electron transfer from the planes to the disordered chains. This assumption is supported by the decrease in ${\mathit{y}}_{\mathrm{MI}}$ and in lattice parameters with increasing x. It implies that changes in mobile hole density, which are responsible for the increase in ${\mathit{T}}_{\mathit{c}}$ and decrease in \ensuremath{\rho}, take place despite the total charge density being constant.

Performance characteristics of a certain class of electrolysers

The performance properties of a continuous-flow electrolyser are analysed under plug-flow and parabolic-flow conditions. The duty of the electrolyser is to produce a species via anodic oxidation or cathodic reduction, upon which the product undergoes a first-order irreversible chemical decomposition (without net electron transfer). The effect of various parameters on electrolyser performance is also discussed.

VECTORIAL ELECTRON TRANSPORT ACROSS LIPID VESICLE MEMBRANES DRIVE BY TWO PHOTOREACTIONS. II. PHOTOCHEMICAL REGENERATION OF REDUCED CYTOCHROME c BY FLAVIN TRIPLET STATE AND ETHYLENEDIAMINETETRAACETIC ACID IN THE INNER COMPARTMENT AND REDUCTION OF FERREDOXIN BY CHLOROPHYLL TRIPLET STATE IN THE OUTER COMPARTMENT

AbstractWe have attempted to mimic natural photosynthesis with regard to the photogeneration of a powerful reductant, using a negatively charged lipid bilayer vesicle system incorporating two photoreactions sensitized by a flavin analog (flavin mononucleotide [FMN]) and chlorophyll (chl) in their respective triplet states. Ethylenediamine‐tetraacetic acid (EDTA) in the inner aqueous compartment was used as a sacrificial electron donor to the FMN triplet, and ferredoxin in the outer aqueous compartment served as the final electron acceptor (mediated via triplet electron transfer chain in this multicomponent system to be elucidated. By itself, EDTA does not function as an effective donor to membran‐bound oxidized chl (chl+.), which is formed by electron transfer from triplet chl to the viologen follwed by transbilayer electron migration. This is a consequence of electrostatic repulsive interactions with the negatively charged membrane. This limitation is avoided when FMN is used as a photomediator between EDTA and chl+.. The overall reaction is dramatically increased in rate by enclosing cytochrom c together with EDTA and FMN in the inner compartment. The rate constant of the key step in the reaction, i.e. elctron transfer from reduced cytochrome c, generated via photoreduction by the FMN/EDTA system, to chl+. is increased 20‐fold over that obtained with cytochrome c alone as the elctron donor. One of the important constraints that limited the net electron transfer across the bilayer to 50% of the added cytochrome, i.e. inhibition by oxidized cytochrome c formed in the inner compartment, is avoided by the inclusion of the second photoreaction in this system, thus allowing photoreduction of all of the added ferredoxin to be achieved. This system provides a model for a photochemical energy storage process that utilizes two photorections operating in series resulting in electron flow across a lipid bilayer membrane.

C60.bul.- with coordination compounds. (Tetraphenylporphinato)chromium(III) fulleride

C60.bul.- with coordination compounds. (Tetraphenylporphinato)chromium(III) fulleride

Aspects of Ligand and Electron-Acceptor Dependence of Magnetic Field Effects on Net Electron Transfer Efficiencies in Photooxidation of Ru(II)-trisbipyridyl Type Complexes

The heteroleptic series of Ru(bpY)n(dce)~:::n,with bpy = 2,2'-bipyridine, and dce = 4,4'­ diethoxycarbonyl-2,2'-bipyridine, has been investigated for the kinetic parameters of photo electron transfer, and for the magnetic field dependence of the net electron transfer efficiency (rIce) in photooxidations with methylviologen and propylviologen sulfonate (the latter only in the case n = 3) using ns-laser-flash spectroscopy and stationary illumination methods. Whereas the electric charge of the electron acceptor had no influence on the magnetic field effect on !Jceo the variation of ligands was found to result in a strictly linear (with n) increase ofthe magnetic field effect on kbet , the effective rate constant ofbackward electron transfer in the primary redox pair. The physical implications ofthese observations with respect to a previously developed magnetokinetic model are discussed.

19th Sir Hans Krebs lecture. Engineering of protein bound iron-sulfur clusters. A tool for the study of protein and cluster chemistry and mechanism of iron-sulfur enzymes.

An increasing number of iron-sulfur (Fe-S) proteins are found in which the Fe-S cluster is not involved in net electron transfer, as it is in the majority of Fe-S proteins. Most of the former are (de)hydratases, of which the most extensively studied is aconitase. Approaches are described and discussed by which the Fe-S cluster of this enzyme could be brought into states of different structure, ligation, oxidation and isotope composition. The species, so obtained, provided the basis for spectroscopic and chemical investigations. Results from studies by protein chemistry, EPR, Mössbauer, 1H, 2H and 57Fe electron-nuclear double resonance spectroscopy are described. Conclusions, which bear on the electronic structure of the Fe-S cluster, enzyme-substrate interaction and the enzymatic mechanism, were derived from a synopsis of the recent work described here and of previous contributions from several laboratories. These conclusions are discussed and summarized in a final section.

Spin-orbit coupling and magnetic field effects in photoredox reactions of ruthenium(II) complexes

Spin-orbit coupling (SOC) and magnetic field effects (MFEs) on the efficiency {eta}{sub net} of net electron transfer in the reductive and oxidative quenching of excited Ru{sup II} trisbipyridyl complexes have been investigated by nanosecond laser flash spectroscopy and by a special continuous photolysis technique. The reductive process was studied with tris(4,4{prime}-bis(ethoxycarbonyl)-2,2{prime}-bipyridine)ruthenium(2+) quenched by anilines X-An (X = H, 4-Cl, 4-Br, 2-I, 4-I). Increasing SOC of the substituent X leads to a decrease of relative {eta}{sub net} values (1.0, 0.79, 0.45, 0.22, 0.12). With X = 4-I {eta}{sub net} is decreased by 12.5 {plus minus} 2.5% in a magnetic field of 1 T. These effects are interpreted as SOC effects of the X center on the spin-forbidden back electron transfer in triplet radical pairs. The oxidative process was studied with (Ru{sup II}(2,2{prime}-bipyridine){sub 3}){sup 2+} and methylviologen (MV{sup 2+}) as a quencher. Here a MFE of {minus}10% at 1 T was observed on the yield of radicals detected on the nanosecond time scale. A detailed MF dependence between 0 and 1 T of the yield of MV{sup {center dot}+} radicals was measured by continuous photolysis using EDTA as a sacrificial electron donor to reduce Ru{sup III}. The MFE shows a gradual onsetmore » followed by a linear increase above 50-100 mT. The onset field decreases and the (negative) slope of the MFE increases as the solvent viscosity is increased in water/ethylene glycol mixtures. On the basis of measurements of lifetime and radical yield at various quencher concentrations, it is shown that the magnetic field mainly affects {eta}{sub net} and not the rate of quenching k{sub q}, as recently asserted by other authors. The MFE is attributed to SOC effects in the Ru{sup III} complex constituent of the primary radical pair.« less

Absorption and static emission properties of monometalated quinone-substituted porphyrin dimers: evidence for “superexchange” mediated electron transfer in multicomponent photosynthetic model systems

The absorption and static emission properties of several selectively metalated quinone-substituted porphyrin dimers and control compounds are described. Whereas the unsubstituted metal-free and bis-zinc dimers 16, 19, 17, and 20 give rise to fluorescence emission spectra characteristic of typical monomeric free-base and zinc-containing porphyrins (e.g. 13 and 14), the corresponding monometalated dimers ( 15 and 18) display emission spectra characteristic only of free-base porphyrins, indicating that rapid exothermic energy transfer takes place between the metalated and free-base subunits in these latter systems. In the case of the control monomers, direct covalent attachment of a quinone subunit (to give 9–12) serves to quench all detectable fluorescence emission. Similar results are obtained in the case of the metal-free ( 3), bis-zinc ( 4), and “distal” monometalated materials ( 2) in either 2-methyl THF or toluene at room temperature. This suggests that in these systems net electron transfer from the porphyrin dimer ensemble to the quinone is fast compared to the rate of fluorescence emission. In the case of the “proximal” monometalated complex, 1, however, a weak but detectable fluorescence signal is observed when the sample is irradiated in toluene at room temperature, indicating that the built-in energy barrier provided by the “proximal” zinc porphyrin subunit is slowing the rate of net electron transfer. Nonetheless, even in the case of this system, the rate of net electron transfer remains exceedingly high, suggesting that the central or “proximal” metalated porphyrin serves to mediate the electron transfer process. The quantum yield for fluorescence for this material in 2-methyl THF is essentially temperature independent, increasingly by only a factor of 3 upon cooling from room temperature to 77 K. This suggests that that net electron transfer from the “distal” free-base subunit to the quinone is not thermally activated but takes place by a direct superexchange mediated process. A similar conclusion is derived from analogous studies of the “flat” compounds 5–8. Here, however, the quantum yields for fluorescence are higher through-out the series suggesting that net electron transfer is slower for these more open photosynthetic models.

The photochemical reduction of 2,1,3-benzothiadiazole-4, 7-dicarbonitrile in the presence of cationic micelles, and onward electron-transfer reactions

Photolysis of solutions containing 2,1,3-benzothiadiazole-4,7-dicarbonitrile (BTDN) ethylenediaminetetra-acetic acid disodium salt (EDTA) and cetyltrimethylammonium bromide (CTAB) micelles at pH 5.5 produces the radical anion, BTDN˙–, with BTDN acting both as an electron acceptor and as a chromophore. Variations in the efficiency of this reaction with pH, [CTAB] and [EDTA] are interpreted in terms of the formation of a complex between EDTA and BTDN which is broken down by the micelles. Net electron transfer does not occur within the complex. Alternative electron donors such as triethanolamine (TEOA) or 4-morpholine-ethanesulphonic acid (MES) can be employed, but not ascorbic acid or sulphite ion. The use of BTDN˙– to reduce anthraquinones in stoichiometric or catalytic (with continuous photolysis) reactions is described. This allows an estimate of the redox potential of BTDN/BTDN˙– when incorporated into micelles.

Transient Raman detection of excited state electron, transfer to methyl viologen from photoexcited molecular reagents in fluid solution and anchored to SiO 2

The lowest electronic excited state of the complexes [Ru(2,2′-bipyridine) 3] 2+, fac-[ClRe (CO) 3(2,2′-bipyridine)], and fac-[(pyridine) Re (CO) 3(2,2′-bipyridine)] + can be quenched by methyl viologen, MV 2+, N, N′-dimethyl-4,4′-bipyridinium, in fluid solutions. The quenching obeys Stern—Volmer kinetics as deduced from plots of relative luminescence quantum yield vs [MV 2+], and the data are consistent with a quenching process that is essentially diffusion controlled. Pulsed laser excitation (18 ns, 354.7 nm frequency tripled Nd: YAG) of the metal complexes in the presence of MV 2+ shows that a detectable fraction of the quenching results in net electron transfer to form MV +. The MV + is detectable by resonance Raman scattering from the trailing portion of the excitation pulse. Excited state electron transfer to MV 2+ from a photo-excited complex anchored to SiO 2 has also been detected by transient Raman spectroscopy. High surface area SiO 2 was functionalized by reaction with 4-[2-(trimethoxysilyl)ethyl]pyridine to give [SiO 2]-SiEtpyr. Reaction of [SiO 2]-SiEtpyr with [(CH 3CN)Re(CO) 3(2,2′-bipyridine)] + then yields [SiO 2]-[(SiEtpyr) Re (CO) 3 (2,2′-bipyridine)] +. Electron transfer quenching of the photo-excited immobilized Re complex occurs when suspended in CH 3CN solutions of MV 2+ to yield MV + as detected by resonance Raman scattering and by lifetime attenuation in the presence of MV 2+.

Synthesis and characterization of a directly linked porphyrin-anthraquinone molecule

We have synthesized a porphyrin-anthraquinone molecule (PAQ) in which the AQ moiety is attached directly at a meso-position of tritolylporphyrin. The absorption spectrum shows pronounced perturbations in the bands of both the P and AQ groups that are independent of solvent polarity; a charge-transfer band was not observed. In contrast, the spectrum of the porphyrin fluorescence in PAQ is only moderately red-shifted, but the lifetimes and intensities depend markedly on solvent dielectric constant: the fluorescence is quenched negligibly in solvents with ϵ s ≤ 4, moderately in solvents with 4 < ϵ s ≤ 6 and strongly in solvents with ϵ s ≥ 7. In methylene chloride, the major emitting component has a lifetime of ∼30 ps as compared to ∼9.0 ns for both tetratolylporphyrin (TTP) and an ester-linked TTP-AQ molecule. An electron-transfer mechanism is implicated even though the energetics for net electron transfer do not appear to be favorable (the sum of the redox potentials being essentially isoenergetic with the porphyrin S 1 state in benzonitrile). We infer that the short distance between the P and AQ moieties (∼1.4 Å edge-to-edge) compensates for the otherwise marginal energetics in accord with Marcus theory. However, calculations of the reorganization energy, based on a two-sphere dielectric continuum model, and estimates of the solvent-dependent reaction energetics, using the Weller equation, do not yield a meaningful correlation with the fluorescence data measured in 19 solvents and binary solvent mixtures. Electron transfer in this intimately linked donor-acceptor molecule may involve an inner-sphere and/or adiabatic mechanism.

Interaction of triplet-state nucleic acid bases with electroaffinic molecules in solution by laser flash photolysis

Triplet-state thymine, thymidine, and uracil, generated by 249.1 nm laser flash photolysis, are quenched by a range of electroaffinic molecules, including the radiosensitising drugs metronidazole and misonidazole, at rates which accord satisfactorily with the predictions of the Rehm–Weller equation for photoelectron transfer. Such a mechanism is confirmed for two of the selected solutes, tetranitromethane and galvinoxyl, when the spectrum of C(NO2)3– and of galvinoxyl anion are observed, indicating net electron transfer with yields of 0.69 ± 0.03 (thymine) and 0.40 ± 0.03 (uracil) for C(NO2)3– and of 0.024 ± 0.002 (thymine) and 0.0135 ± 0.0005 (uracil) for galvinoxyl anion. The mechanistic implications of these results for photo- and radiation-damage to DNA and cellular systems are discussed.

Polymer viologen as a stabilizing agent of colloidal platinum for photoinduced electron transfer reactions

Photoinduced electron transfer and hydrogen evolution were investigated using colloidal platinum stabilized by viologen-pendant Nylon (Pt—PV 2+). From laser flash photolysis experiments, the Pt—PV 2+ was found to accept directly an electron from excited ZnTMPyP 4+, and the reverse electron transfer was retarded by the positive charge of the protecting polymer, while no net electron transfer was observed for colloidal platinum stabilized by poly(vinyl alcohol) (Pt—PVA). The rate of hydrogen evolution was measured via steady-state irradiation experiments in the presence of a sacrificial electron donor. The quantum yield of hydrogen production with a Pt—PV 2+ catalyst was higher than that with Pt—PVA by a factor of 7 and reached 0.38.

Photoinduced electron-transfer reactions of 2(3H)-furanones and bis(benzofuranones). Spectral and kinetic behavior of radicals and radical cations

The spectral and kinetic behaviors of radical cations produced under efficient electron-transfer quenching of 1,4-dicyanonaphthalene or 9,10-dicyanoanthracene singlet by a number of 2(3H)-furanones and bis(benzo-furanones) have been examined by nanosecond laser flash photolysis (337.1 and 425 nm). The efficiencies of net electron transfer in the course of quenching processes are moderately high (0.2-0.6) in acetonitrile. The radical cations from bis(benzofuranones) several 2(3H)-furanones containing a benzyl group at the 3-position undergo fragmentation to benzofuranoxy and furanoxy radicals (+ benzyl or benzofuranoyl carbocations) with rate constants of 0.3 x 10W s . The long-lived furanoxy radicals, independently generated via hydrogen abstraction by tert-butoxy radicals from 2(5H)-furanones and 3-phenyl-2(3H)-benzofuranone, as well as via direct photolysis of 3-benzoyl-3,5-diphenyl-2(3H)-furanone, are characterized by sharply structured absorption spectra and relatively slow second-order decay kinetics (6-8 x 10Y M s in 1:2 benzene-di-tert butyl peroxide, v/v). 28 references, 7 figures, 3 tables.

Ab initio cluster-model study of the electronic ground-state and photoemission properties of NiN2 and NiCO: Models for chemisorption.

The ground state of the linear ${\mathrm{NiN}}_{2}$ cluster is found, from generalized valence-bond configuration-interaction calculations, to be a $^{1}\mathrm{\ensuremath{\Sigma}}^{+}$ state with a short Ni--N distance of 1.64 A\r{} and a Ni--${\mathrm{N}}_{2}$ dissociation energy of 0.8 eV. In this $^{1}\mathrm{\ensuremath{\Sigma}}^{+}$ state near the calculated equilibrium internuclear distance, the electronic wave function may be characterized as arising from a significant admixture of the Ni 3${d}^{10}$ configuration, and the bonding of the ${\mathrm{N}}_{2}$ molecule to the Ni atom may be characterized as ${\mathrm{N}}_{2}$ \ensuremath{\sigma} donation and Ni 3d\ensuremath{\pi} backward donation. Low-lying excited states ${(\mathrm{}}^{3}$${\ensuremath{\Sigma}}^{+}$, $^{3}\mathrm{\ensuremath{\Delta}}$, and $^{3})$ arising from the Ni 3${d}^{9}$4${s}^{1}$ configuration are also studied. Mulliken population analyses, dipole moments, and vibrational and photoemission properties are calculated. Parallel calculations are also performed for the linear NiCO molecule. The $^{1}\mathrm{\ensuremath{\Sigma}}^{+}$ states of both ${\mathrm{NiN}}_{2}$ and NiCO are good models for the chemisorption of ${\mathrm{N}}_{2}$ and CO on a Ni surface, with regard to the downward shift of the molecular stretching frequency in the interacting systems, the photoemission satellite structure, and the orientation of the dipole moments (with a net electron transfer from Ni to ${\mathrm{N}}_{2}$ or CO).

Electron donor-acceptor quenching and photoinduced electron transfer for coumarin dyes

Abstract : The fluorescence of 7-aminocoumarins is quenched by a variety of organic electron donors or acceptors in acetonitrile. In general, donors with half-wave oxidation potentials less positive than 1.0 V vs SCE and acceptors with reduction potentials less negative than -1.5 V vs SCE are candidates for diffusion limited quenching of coumarin singlet states. Profiles of quenching rates are consistent with calculated free energies for electron transfer between excited coumarins and donors or acceptors. In flash photolysis experiments electron transfer for several dyes and quenchers (e.g., methyl viologen) is demonstrated. Relatively low yields of net electron transfer are consistently obtained due to inefficient ionic photodissociation via singlet quenching or a low yield of more photoactive coumarin triplets. Electrochemical properties of the coumarins have been investigated by cyclic voltammetry with the indications of reversible oxidation and irreversible reduction as important processes. (Author)