Electron Exchange Mechanism Research Articles

Photoinduced energy and electron transfer processes in heteropolynuclear polypyridyl complexes of Ru(ii) and Fe(ii)

Photophysical, photochemical and electrochemical studies of a new series of heterobinuclear complexes of ruthenium and iron, [RuII(bpy)2(Ln)FeII(bpy)2]4+ (bpy = 2,2′-bipyridine, n = 2 ([RuII(L2)FeII]4+), n = 4 ([RuII(L4)FeII]4+), n = 6 ([RuII(L6)FeII]4+), issued from the linkage of the Ru(bpy)32+ and Fe(bpy)32+ subunits by covalently bridging bis-bipyridine Ln ligands have been investigated. The cyclic voltammetry of the three complexes exhibits two successive metal-centered electron reversible oxidation processes, clearly separated, in the positive region. In the negative area, three successive reversible two electron waves are observed (the second and the third being strongly distorted by adsorption phenomena) corresponding to the ligand-based reduction processes. In addition, the two oxidized forms of these complexes [RuII(Ln)FeIII]5+ and [RuIII(Ln)FeIII]6+ obtained by two successive exhaustive electrolyses exhibit high stability, which is crucial for an efficient photocatalytic operating system. Luminescence of the complexes [RuII(Ln)FeII]4+ has been observed indicating that the covalently linkage between the Ru(bpy)32+ and Fe(bpy)32+ units leads to an only partial quenching of the RuII* excited state by energy transfer to FeII. The nature of the energy transfer process involved in those heterobinuclear complexes is studied and an intermolecular electron exchange mechanism is proposed as a preferably deactivation route. On the other hand, the photoxidation of the RuII subunit into the RuIII one could be easily obtained in the presence of a diazonium salt, playing the role of sacrificial oxidant. Finally photocatalytic oxidation of the complexes has been performed by continuous photolysis experiments. For each heteronuclear complexes, the multi-step oxidation process (FeII → FeIII and RuII → RuIII) has been observed. The comparison with an isoconcentrated mixture of the corresponding homonuclear parent complexes has been made.

Electronic Energy Transfer from Excitons Confined in Silicon Nanocrystals to Molecular Oxygen

AbstractWe report on efficient electronic energy transfer from excitons confined in silicon (Si) nanocrystals to molecular oxygen (MO). The remarkable photosensitizing properties of Si nanocrystal assemblies result from a broad energy spectrum of photoexcited excitons, a long triplet exciton lifetime and a highly developed surface area. Quenching of photoluminescence (PL) of Si nanocrystals by MO physisorbed on their surface is found to be most efficient when the energy of excitons coincides with the triplet-singlet splitting energy of oxygen molecules. Spectroscopic analysis of the quenched PL spectrum evidences that energy transfer is accompanied by multi-phonon emission. From time-resolved measurements the characteristic time of energy transfer is found to be in the range of microseconds. The dependence of PL quenching efficiency on the surface termination of nanocrystals is consistent with short-range resonant electron exchange mechanism of energy transfer. The energy transfer to oxygen molecules in the gaseous phase at elevated temperatures is demonstrated.

Donor/acceptor interactions in systematically modified Ru(II)-Os(II) oligonucleotides.

Donor/acceptor (D/A) interactions are studied in a series of doubly modified 19-mer DNA duplexes. An ethynyl-linked Ru(II) donor nucleoside is maintained at the 5' terminus of each duplex, while an ethynyl-linked Os(II) nucleoside, placed on the complementary strands, is systematically moved toward the other terminus in three base pair increments. The steady-state Ru(II)-based luminescence quenching decreases from 90% at the shortest separation of 16 A (3 base pairs) to approximately 11% at the largest separation of 61 A (18 base pairs). Time-resolved experiments show a similar trend for the Ru(II) excited-state lifetime, and the decrease in the averaged excited-state lifetime for each duplex is linearly correlated with the fraction quenched obtained by steady-state measurements. Analysis according to the Förster dipole-dipole energy transfer mechanism shows a reasonable agreement. Deviation from idealized behavior is primarily attributed to uncertainty in the orientation factor, kappa(2). Analyzing D/A interactions in an analogous series of doubly modified oligonucleotides, where the ethynyl-linked Ru(II) center is replaced with a saturated two-carbon linked complex, yields an excellent correlation with the Förster mechanism. As this simple change partially relaxes the rigid geometry of the donor chromophore, these results suggest that the deviation from idealized Förster behavior observed for the duplexes containing the rigidly held Ru(II) center originates, at least partially, from ambiguities in the orientation factor. Surprisingly, analyzing both quenching data sets according to the Dexter mechanism also shows an excellent correlation. Although this can be interpreted as strong evidence for a Dexter triplet energy transfer mechanism, it does not imply that this electron exchange mechanism is operative in these D/A duplexes. Rather, it suggests that systems that transfer energy via the Förster mechanism can under certain circumstances exhibit Dexter-like "behavior", thus illustrating the danger of imposing a single physical model to describe D/A interactions in such complex systems. While we conclude that the Förster dipole-dipole energy transfer mechanism is the dominant pathway for D/A interactions in these modified oligonucleotides, a minor contribution from the Dexter electron exchange mechanism at short distances is likely. This complex behavior distinguishes DNA-bridged Ru(II)/Os(II) dyads from their corresponding low molecular-weight and covalently attached counterparts.

Resonant electronic energy transfer from excitons confined in silicon nanocrystals to oxygen molecules.

We demonstrate efficient resonant energy transfer from excitons confined in silicon nanocrystals to molecular oxygen (MO). Quenching of photoluminescence (PL) of silicon nanocrystals by MO physisorbed on their surface is found to be most efficient when the energy of excitons coincides with triplet-singlet splitting energy of oxygen molecules. The dependence of PL quenching efficiency on nanocrystal surface termination is consistent with short-range resonant electron exchange mechanism of energy transfer. A highly developed surface of silicon nanocrystal assemblies and a long radiative lifetime of excitons are favorable for achieving a high efficiency of this process.

Excitation process simulation for atoms leaving a metal surface

Recently, experimental data about the dependences of secondary atom excitation probability on atom velocity were obtained. In the present work an attempt is made to give a theoretical explanation. We used a model with the Anderson-type Hamiltonian, where all possible resonant and non-resonant single-electron processes, taking part in “metal–emitted multi-level particle” system, are taken into account. Calculations which are carried out in the frame of the proposed model are found to be in good quantitative agreement with the experimental data and explain facts such as: (1) the high efficiency of the electron-exchange mechanism in secondary atom excitation, (2) the relatively slow dependence of the probability of excitation of secondary atoms on their velocity, (3) the non-trivial characteristic type of such dependence for the secondary atom excitation and ionization probabilities.

Stereo-selectivity in the Penning ionization reaction of [formula omitted

The steric opacity functions for the title Penning ionization reactions were determined. It was found that the most reactive site shifts from the collinear X-end toward the sideways direction, as the X change from Cl to I. The main feature in the experimental steric opacity function can be well explained by the electron exchange mechanism, which is controlled by the shift of center-of-mass and the electron density distributions of the HOMO of CH 3X molecule.

Temperature and viscosity dependence of the triplet energy transfer process in porphyrin dimers.

The temperature and viscosity dependence of the triplet energy transfer (TET) process in porphyrin dimers has been studied. A zinc porphyrin (donor) and a free base porphyrin (acceptor) are covalently linked together by rigid bridging chromophores at a center-center distance of 25 A. Due to the large donor-acceptor distance and the weakness of the spin forbidden transitions involved, neither direct (through space) electron exchange nor Coulombic mechanisms are expected to contribute to the observed TET process. The results from transient absorption measurements at temperatures between room temperature and 80 K show that TET occurs with unexpectedly high efficiency in the systems connected by fully conjugated bridges and a pronounced temperature dependence of the process is observed. Comparison of the TET efficiencies in dimers connected by different bridging chromophores correlates well with a transfer reaction governed by a through bond exchange (superexchange) interaction. However, in high viscosity media the TET process is dramatically slowed down. This is attributed to a conformational gating of the TET process where the electronic coupling varies strongly with the relative orientation of the donor and the bridging chromophore. Further, the zinc porphyrin donor offers two distinct donor species, T(1A) and T(1B). At room temperature, the TET rate constant of the T(1A) species is about two orders of magnitude larger than for the T(1B) species. The dimers studied are well suited model systems for materials where the rate of the transfer reactions can be changed by external stimuli.

Quasi-Solid-State Dye-Sensitized TiO2 Solar Cells: Effective Charge Transport in Mesoporous Space Filled with Gel Electrolytes Containing Iodide and Iodine

Quasi-solid-state dye-sensitized solar cells were fabricated using low-molecular-weight gelators. They showed comparable photoenergy conversion efficiencies to the liquid cell at high illumination intensity up to AM 1.5 (1 sun). Conductivity measurements of the electrolyte phases revealed that the gelation does not affect the conductivity of the electrolyte and that the conductivity increased with an increase of iodine in both gel electrolytes and liquid electrolyte. The formation of polyiodide ions, such as I3- and I5-, caused by addition of iodine was confirmed by Raman spectroscopic measurement. The self-diffusion of iodide species in the gel electrolyte was found about a quarter of that of I- in acetonitrile. The formation of less-mobile polyiodide ions in electrolyte increased the conductivity in the mesoporous phase, which should be rationalized as due to the Grotthuss-type electron exchange mechanism caused by rather packed polyiodide species in the electrolytes. The optimized quasi-solid-state cell showed the values of 0.67 V for open-circuit voltage, 12.8 mA cm-2 for short-circuit photocurrent density, and 5.91% for photoenergy conversion efficiency under AM 1.5 irradiation with higher durability.

Kinetic study of thermal dehydrochlorination of poly(vinyl chloride) in the presence of oxygen: III. Statistical thermodynamic interpretation of the oxygen catalytic activity

The mechanism of the hydrogen chloride elimination from poly(vinyl chloride) (PVC) catalyzed by molecular oxygen has been proposed and analyzed. The oxygen catalytic action is ascribed to the excited singlet 1O 2 ( 1Δ g) which is generated by an interaction of ground triplet 3O 2 ( 3 Σ g −) through the charge-transfer and electron exchange mechanisms with either biradical or ionic (zwitterionic) structure of the cyclic, four center polar transition state. An alone oxygen catalytic action is interpreted through Langmuir nondissociative quasi-equilibrium chemisorption process described in terms of the statistical thermodynamics. Assuming the catalyzed thermal dehydrochlorination (DHC) occurring independently from the noncatalyzed one, obtained kinetic parameters, e.g. the activation energy and frequency factor, describe satisfactorily Talamini and Pezzin experimental data. The most important result is that the average decrease of the activation energy in comparison with the noncatalyzed DHC process, determined on the two independent kinetic data sets, is 91 kJ mol −1, this is just the effective transfer excitation energy to forming the singlet molecular oxygen. Moreover, the inherent role of the singlet oxygen in the oxidative degradation of PVC is suggested.

Direct observation of the steric effect in Penning ionization reaction of Ar*+CHCl3→CHCl2++Cl+e−+Ar

Steric effect in the Penning ionization reaction of Ar*(3P2,0)+CHCl3→Ar+CHCl2++Cl+e− was directly observed at an average collision energy of 0.13 eV using the oriented CHCl3 molecular beam. The product CHCl2+ ions are measured for the H-end, the CCl3-end, and sideways orientations. The obtained steric opacity function reveals that the CCl3-end orientation is more favorable than the H-end orientation, and the sideways approach is found to be more favorable than the collinear approaches from both ends of the molecule. Furthermore, we confirm the good correlation between Penning ionization anisotropy and the electron density distribution of the 2a2 HOMO orbital of the CHCl3 molecule, whose electron cloud is mostly localized around the sideways. These results substantiate the electron exchange mechanism which is commonly accepted for the Penning ionization reaction, where the overlap of projectile atomic and target molecular orbital plays a key role in Penning ionization efficiency.

Kinetics of Two Pathways in Peroxyoxalate Chemiluminescence

It has been shown that 1,1'-oxalyldiimidazole (ODI) is formed as an intermediate in the imidazole-catalyzed reaction of oxalate esters with hydrogen peroxide. Therefore, the kinetics of the chemiluminescence reaction of 1,1'-oxalyldiimidazole (ODI) with hydrogen peroxide in the presence of a fluorophore was investigated in order to further elucidate the mechanism of the peroxyoxalate chemiluminescence reaction. The effects of concentrations of ODI, hydrogen peroxide, imidazole (ImH), the general-base catalysts lutidine and collidine, and temperature on the chemiluminescence profile and relative quantum efficiency in the solvent acetonitrile were determined using the stopped-flow technique. Pseudo-first-order rate constant measurements were made for concentrations of either H2O2 or ODI in large excess. All of the reaction kinetics are consistent with a mechanism in which the reaction is initiated by a base-catalyzed substitution of hydrogen peroxide for imidazole in ODI to form an imidazoyl peracid (Im(CO)2OOH). In the presence of a large excess of H2O2, this intermediate rapidly decays with both a zero- and first-order dependence on the H2O2 concentration. It is proposed that the zero-order process reflects a cyclization of this intermediate to form a species capable of exciting a fluorophore via the "chemically initiated electron exchange mechanism" (CIEEL), while the first-order process results from the substitution of an additional molecule of hydrogen peroxide to the imidazoyl peracid to form dihydroperoxyoxalate, reducing the observed quantum yield. Under conditions of a large excess of ODI, the reaction is more than 1 order of magnitude more efficient at producing light, and the quantum yield increases linearly with increasing ODI concentration. Again, it is proposed that the slow initiating step of the reaction involves the substitution of H2O2 for imidazole to form the imidazoyl peracid. This intermediate may decay by either cyclization or by reaction with another ODI molecule to form a cyclic peroxide that is much more efficient at energy transfer with the fluorophore. The reaction kinetics clearly distinguishes two separate pathways for the chemiluminescent reaction.

Electrochemistry of homoepitaxial CVD diamond: energetics and electrode kinetics in aqueous electrolytes

The electrochemical properties of highly doped p -type single crystalline diamond electrodes (100 and 110 oriented) in aqueous electrolytes were investigated. The interfacial capacitance obeys the Mott–Schottky relationship in a considerable potential range and can be assigned to a depletion layer in the diamond. The energetic position of the valence band edge is about 4 and 2 V versus SHE for (100) and (110) oriented diamond respectively. Oxygen and hydrogen evolution occur at large overpotentials (1 V) in agreement with previous results reported for polycrystalline diamond. Interestingly, with reversible redox systems, metal-type redox kinetics around the equilibrium potential are observed. The mechanism of electron exchange between the valence band of diamond and simple redox systems was investigated in detail using electrochemical impedance spectroscopy. A quantitative model is proposed, that assumes that electron exchange is mediated by bandgap states.

Highly Efficient Through-Bond-Mediated Electronic Excitation Energy Transfer Taking Place over 12 Å

There is ample documentary evidence that both triplet and singlet electronic excitation energy transfer (EET) can take place over substantial distances in rigid donor-{saturated hydrocarbon bridge}-acceptor (D-B-A) systems by a type of electron exchange mechanism involving through-bond (TB) coupling with the bridge orbitals (hereafter referred to as the TB mechanism).1-4 As expected for the TB mechanism, the EET rates display a strong exponential distance dependence of the form shown by eq 1, in which n is the number of bonds in the bridge relay, and â is an attenuation coefficient with experimentally determined values in the range of 2-2.5 per bond.2,3

Spectator- and participant-like behavior of a Rydberg electron on predissociation of superexcited states of OCS

Predissociation of superexcited states of OCS is studied by two-dimensional photoelectron spectroscopy using synchrotron radiation in the photon energy range of 15–16.5 eV. A two-dimensional photoelectron spectrum exhibits two kinds of characteristic patterns both of which are ascribed to autoionization of sulfur atoms. This superexcited atom S* is produced by predissociation of a Rydberg state OCS*(RB) converging to OCS+(B̃ 2Σ+). The pattern of the first kind results from predissociation processes in which the effective principal quantum number n of the Rydberg electron is almost conserved. This suggests that the Rydberg electron behaves as a spectator because of its negligibly weak interaction with the ion core (spectator predissociation). On the contrary, n of S* does not accord with that of OCS*(RB) in the pattern of the second kind, indicating that the Rydberg electron participates directly in the electron exchange mechanism controlling conversion from OCS*(RB) to a predissociating state (participant predissociation). With increasing n, OCS*(RB) decays more preferentially by the spectator than by the participant predissociation. The spectator predissociation of OCS*(RB) proceeds through a two-step conversion which involves Rydberg states converging to OCS+(Ã 2Π and X̃ 2Π) and a dissociative multiple-electron-excited satellite state OCS* (SAT) asymptotically correlating with S*+CO(X̃ 1Σ+). In contrast, the participant predissociation may be accounted for by a direct conversion from OCS*(RB) to OCS*(SAT). The quantum yields are estimated to be 0.06 and 0.02 for the spectator and participant predissociation, respectively, at the incident photon energy of 15.95 eV where OCS*(RB) states with n∼12 lie. A simulation is performed to reproduce the partial cross section curve for the spectator predissociation by using a model in which the decay rates for the participant and spectator predissociation are assumed to be proportional to n−3 and n0, respectively. The simulated and experimental cross section curves are in good agreement with each other in the photon energy range of 15.8–16.04 eV.

Chemiluminescence and catalysis of decomposition of dimethyldioxirane adsorbed from the gas phase on silipore containing tris(bipyridyl)ruthenium complex Ru(bpy)3Cl2 and 9,10-diphenylanthracene

The kinetics of decomposition of dimethyldioxirane (DMD) adsorbed from the gas phase on the silipore surface was studied by the chemiluminescence (CL) technique. The lower boundary of the CL yield in the reaction of DMD decomposition (4·10−9 Einstein mol−1) and chemiexcitation yield of methyl acetate (4·10−4) were estimated. Chemiluminescence upon decomposition of DMD on the silipore surface in the presence of activators of CL such as tris(bipyridyl)ruthenium complex Ru(bpy)3Cl2 and 9,10-diphenylanthracene was revealed. Ru(bpy)3Cl2 activates the luminescence according to chemically induced electron exchange mechanism.

Electron-exchange mechanisms of the secondary ion emission of metals and some incompatible experimental data

Two electron-exchange theories of secondary ion emission are known. In the first one the ionization of the ejected atom occurs as a result of electron resonant tunneling through the potential barrier separating an atom and a metal. In the second the ionization occurs by the action of strong time-dependent perturbation due to the wave function of an atom and metal overlapping. Both theories predict different dependences that can be verified by experiments. Experimental data concerning the investigation of the secondary ion emission of metals have been analyzed to show (i) the form of the dependence of the probability of secondary particle ionization on its energy — it has been shown that this agrees with the first theory but does not agree with the second one; (ii) the dependence of the secondary ion emission yield on the metal work function — it has been found that this dependence can be described as incompletely by the second theory as by the first one united with the known thermodynamic non-equilibrium theory of Saha-Langmuir-Dobretsov.

Solvent dependence of triplet energy transfer from triplet benzophenone to naphthols and methoxynaphthalenes

Solvent effects on triplet–triplet energy transfer (TET) from triplet benzophenone (3BP*) to naphthol (NpOH) competing with hydrogen atom abstraction (HA) of 3BP* from NpOH and methoxynaphthalene (NpOMe) without HA have been studied in fluid media by 355 nm laser flash photolysis at 295 K. The efficiency (ψTET) and rate constant (kTET) of TET in these systems were obtained. It was shown that the value of kTET was dependent not only on the solvent viscosity (η) but also on the dielectric constant (κ) of the solvents, and ψTET and kTET increased with increasing κ, contrary to the Dexter prediction. An increase in kTET with increasing κ may be caused by the contribution of the dipole–dipole interaction (by the Forster theory) due to perturbation of the 1(π, π*) state to the lowest triplet state 3(n, π*) of benzophenone, in addition to the electron-exchange mechanism (by the Dexter theory).

Sensitized dissociation of peroxides and peresters in the presence of thiopyrylium salts

The photosensitized dissociation of peroxides and peresters in the presence of thiopyrylium salts has been studied. The quenching rate constants of the triplet state of the salts under consideration were (3–5) × 107 l mol−1 s−1 with benzoyl peroxide (BPO) and (3–11) × 107 l mol−1 s−1 with 3,3′,4,4′-tetra(tert-butylperoxycarbonyl)benzophenone (BTTB) in acetonitrile, the thiopyranyl radical is formed in the presence of BTTB. The mechanism of dissociation photosensitization of these peroxides by pyrylium and thiopyrylium salts is discussed and interpreted in terms of an energy transfer through an electron-exchange mechanism in the case of BTTB and classical energy transfer when BPO is used.

Photoinduced intercomponent energy transfer in covalently-linked dinuclear complexes containing Ru(II)-bipyridine and Ru(II)-biquinoline chromophores and aromatic and aliphatic spacers

We have synthesized a number of dinuclear species containing both identical or different metal-based components by employing new bridging ligands having either aliphatic or aromatic spacers and taking advantage of the “complexes as metals and complexes as ligands” synthetic strategy. The bridging ligands are dpt-S-dpt (S is 1,4-cyclohexyl, 1,4-phenyl, 4,4′-biphenyl; dpt is 4-amino-3,5-bis(2-pyridyl)- 1,2,4-triazole; the connections between S and dpt are provided by amide links). The complexes synthesized are: [(bpy) 2Ru(dpt-S-dpt)Ru(bpy) 2](PF 6) 4 (bpy=2,2′-bipyridine; biq=2,2′-biquinoline; S=1,4-cyclohexyl ( 1), 1,4-phenyl ( 4), 4,4′-biphenyl ( 7)); [(biq) 2Ru(dpt-S-dpt)Ru(biq) 2](PF 6) 4 (S=1,4-cyclohexyl ( 2), 1,4-phenyl ( 5), 4,4′-biphenyl ( 8); [(bpy) 2Ru(dpt-S-dpt)-Ru(biq) 2](PF 6) 4 (S=1,4-cyclohexyl ( 3), 1,4-phenyl ( 6), 4,4′-biphenyl 3 9)). The absorption spectra, luminescence properties and redox behavior of all the compounds have been studied. In the complexes containing different metal-based components, photoinduced energy transfer occurs from the higher-lying Ru→bpy CT level, centered on a metal subunit, to the lower-lying Ru→biq CT excited state, centered on the other metal component. In fluid solution at room temperature, the energy transfer is suggested to be mediated by a two-step electron transfer mechanism, whereas direct energy transfer between the chromophores most likely occurs at 77 K in rigid matrix. At the moment we arc not able to say if the energy transfer at 77 K takes place via electron exchange or coulombic mechanisms. The results obtained indicate that the efficiency of the processes depends on the donor-acceptor distance, as expected, and that occasional π bonds which are present within the bridging ligands cannot be used for speeding up electron transfer in multicomponent systems if the main skeleton of the bridge is made by σ bonds.

The study of E- E energy transfer between CO(a 3Π, ν′) and NO(X 2Π, ν″ = 0) in beams

The formation of NO(A,ν) and NO(B,ν) was investigated from the reaction CO(a, ν′) + NO(X, ν″ = 0) in a beam-beam configuration. The nascent rovibrational distributions of NO(A, ν = 0–2) and NO(B, ν = 0–2) were determined from NO(A → X) and NO(B → X) emissions by means of spectral simulation. The branching rate of NO(A) to NO(B) was determined to be γ/ β = 1.3 ± 0.3 by using the total emission intensities of NO(A) and NO(B). Experiments also show that the vibrational distribution of NO(A) is sensitive to the vibrational distribution of CO(a,ν′). An electron exchange mechanism is proposed for the reaction.