Photostimulated electron detrapping and the two-state model for electron transport in nonpolar liquids

In common nonpolar liquids, such as saturated hydrocarbons, there is a dynamic equilibrium between trapped (localized) and quasifree (extended) states of the excess electron (the two-state model). Using time-resolved dc conductivity, the effect of 1064 nm laser photoexcitation of trapped electrons on the charge transport has been observed in liquid n-hexane and methylcyclohexane. The light promotes the electron from the trap into the conduction band of the liquid. From the analysis of the two-pulse, two-color photoconductivity data, the residence time of the electrons in traps has been estimated as ca. 8.3 ps for n-hexane and ca. 13 ps for methylcyclohexane (at 295 K). The rate of detrapping decreases at lower temperature with an activation energy of ca. 200 meV (280-320 K); the lifetime-mobility product for quasifree electrons scales linearly with the temperature. We suggest that the properties of trapped electrons in hydrocarbon liquids can be well accounted for using the simple spherical cavity model. The estimated localization time of the quasifree electron is 20-50 fs; both time estimates are in agreement with the "quasiballistic" model. This localization time is significantly lower than the value of 310+/-100 fs obtained using time-domain terahertz (THz) spectroscopy for the same system [E. Knoesel, M. Bonn, J. Shan, F. Wang, and T. F. Heinz, J. Chem. Phys. 121, 394 (2004)]. We suggest that the THz signal originates from the oscillations of electron bubbles rather than the free-electron plasma; vibrations of these bubbles may be responsible for the deviations from the Drude behavior observed below 0.4 THz. Various implications of these results are discussed.     

Photoinduced electron transfer in arylacridinium conjugates in a solid glass matrix

The photophysical properties of a series of 9-arylacridinium conjugates in solid glass matrices composed of sucrose octaacetate have been determined. The fluorescence of the charge-shift states is significantly enhanced because of the retardation of nonradiative pathways for back-electron transfer. Changes of more than 3 orders of magnitude in back-electron-transfer rates (sucrose octaacetate glass vs conventional solvents at room temperature) were observed. Transient spectra displayed long-lived charge-shift species in the microsecond time regime for thianthrene acridinium conjugates. The rate retardation is associated with slow solvation times for surrounding solvent layers in the solid matrix. The red-edge effect (excitation wavelength-dependent fluorescence) for the arylacridinium ions in solid glass confirms the microheterogeneity of the sucrose octaacetate medium.     

Metal speciation in solid matrices

The literature on metal ion speciation in solid matrices is reviewed, taking into account its applications in the analysis of soil, sediment, biological materials, foodstuff and other solid samples. The pretreatment methods of various solid materials required for carrying out speciation studies have been highlighted. The basis of the methods of separation of different species from matrices, such as sequential extraction, selective extraction, etc. is discussed. The instrumental techniques used for the characterization of different chemical species in solid matrices have been mentioned. The literature survey reveals the analytical details of the developed methodologies, and these have been examined in terms of the limit of detection, precision and accuracy.     

Chronoamperometric study of proton transfer/electron transfer in solid state electrochemistry of organic dyes

Chronoamperometric data for proton-assisted reduction/oxidation of a series of dyes, namely, alizarin, purpurin, luteolin, morin, and indigo are compared with the predictions of the model of Lovric and Scholz for redox conductive microcrystals assuming electron and proton hopping in mutually perpendicular directions. Upon the attachment of solid dye microparticles to paraffin-impregnated graphite electrodes in contact with aqueous electrolytes in the pH range between 4 and 7, an excellent agreement between theory and experimental data was obtained. The diffusion coefficients of electrons and protons across dye microcrystals were estimated as 5×10⁻⁷ cm² s⁻¹ and 1×10⁻¹⁰ cm² s⁻¹, respectively.     

Superheated water extraction and phase transfer methylation of phenoxy acid herbicides from solid matrices

Phase transfer catalytic methylation was applied to directly derivatise chlorophenoxy acid herbicides in superheated water extracts from sand and soil samples. The extractions were carried out at 120 degrees C statically for 5 min and then dynamically for 10 min at 1.0 mL min(-1) using water at pH 11.0 for a sand matrix and a flow rate of 0.5 mL min(-1) at pH 7.0 for soil samples. The methylation was carried out on-line on the extraction solution with ultrasonication at 80 degrees C, using either 0.05 mmol tetrabutylammonium bromide (TBAB) or 0.0125 mmol cetyltrimethylammonium bromide (CTAB) as phase transfer catalysts with 0.20 mmol methyl iodide in 2.0 mL dichloromethane trapping solvent. The former catalyst provided a higher yield but the latter gave fewer interfering peaks. The recoveries of most chlorophenoxy acids using the TBAB catalyst ranged from 67 to 105% for sand and from 82 to 114% for soil sample, except phenoxyacetic acid, 2-(2, 4-dichlorophenoxy)propanoic acid and 1-naphthaleneacetic acid, while those by using CTAB were slightly lower. Detection limits of all the analytes extracted from sand using TBAB catalyst were in a range of 5.3-16 microg g(-1) analysed by using gas chromatography with flame ionization detection (GC-FID).     

Photoinduced intra- and intermolecular electron transfer in solutions and in solid organized molecular assemblies

The present paper highlights results of a systematic study of photoinduced electron transfer, where the fundamental aspects of the photochemistry occurring in solutions and in artificially or self-assembled molecular systems are combined and compared. In photochemical electron transfer (ET) reactions in solutions the electron donor, D, and acceptor, A, have to be or to diffuse to a short distance, which requires a high concentration of quencher molecules and/or long lifetimes of the excited donor or acceptor, which cannot always be arranged. The problem can partly be avoided by linking the donor and acceptor moieties covalently by a single bond, molecular chain or chains, or rigid bridge, forming D-A dyads. The covalent combination of porphyrin or phthalocyanine donors with an efficient electron acceptor, e.g. fullerene, has a two-fold effect on the electron transfer properties. Firstly, the electronic systems of the D-A pair result in a formation of an exciplex intermediate upon excitation both in solutions and in solid phases. The formation of the exciplex accelerates the ET rate, which was found to be as fast as >10(12) s(-1). Secondly, the total reorganization energy can be as small as 0.3 eV, even in polar solvents, which allows nanosecond lifetimes for the charge separated (CS) state. Molecular assemblies can form solid heterogeneous, but organized systems, e.g. molecular layers. This results in more complex charge separation and recombination dynamics. A distinct feature of the ET in organized assemblies is intermolecular interactions, which open a possibility for a charge migration both in the acceptor and in the donor layers, after the primary intramolecular exciplex formation and charge separation in the D-A dyad. The intramolecular ET is fast (35 ps) and efficient, but the formed interlayer CS states have lifetimes in microsecond or even second time domain. This is an important result considering possible applications.     

Dielectric-dependent electron transfer behaviour of cobalt hexacyanides in a solid solution of sodium chloride

Here we emphasise the importance of the dielectric environment on the electron transfer behavior in interfacial electrochemical systems. Through doping cobalt hexacyanide (Co(CN)₆^3–) into single microcrystals of sodium chloride (NaCl), for the first time, we obtained the direct electrochemical behavior of Co(CN)₆^3– which is hardly ever obtained in either aqueous or conventional nonaqueous solutions. DFT calculations elucidate that, as the Co(CN)₆^3– anions occupy the lattice units of NaCl₆^5– in the NaCl microcrystal, the redox energy barrier of Co(CN)₆^3–/4– is decreased dramatically due to the low dielectric constant of NaCl. Meanwhile, the low-spin Co(CN)₆^4– anions are stabilized in the lattices of the NaCl microcrystal. The results also show that the NaCl microcrystal is a potential solvent for solid-state electrochemistry at ambient temperature.     

Isotropic reorientations of fullerene C70 triplet molecules in solid glassy matrices revealed by light-induced electron paramagnetic resonance

Continuous-wave X-band electron paramagnetic resonance (EPR) of fullerene C(70) molecules excited to a triplet state by continuous light illumination was studied in molecular glasses of o-terphenyl and cis/trans-decaline and in the glassy polymers polymethylmethacrylate (PMMA) and polystyrene (PS). Above ～100 K, a distinct narrowing of EPR lineshape of the triplet was observed, which was very similar for all systems studied. EPR lineshape was simulated reasonably well within a framework of a simple model of random jumps, which implies that the C(70) molecule performs isotropic orientational motion by sudden jumps of arbitrary angles. In simulations, a single correlation time τ(c) was used, varying in the range of 10(-7)-10(-8) s. Near and below 100 K electron spin echo (ESE) signals were also obtained which were found to decay exponentially. Correlation times τ(c) obtained from simulation of the EPR spectra in the slow-motion limit (τ(c) close to 10(-7) s) turned out to be in good agreement with the phase memory times T(M) of the ESE decay, which additionally supports the employed simple model. The observed motional effects provide evidence that the nanostructure of the solid glassy media of different origins is soft enough to allow a large asymmetric C(70) molecule to reorient rapidly. Except for the EPR spectra of the triplet, in the center of the spectra, a small admixture of a narrow line was also observed; its possible nature is briefly discussed.     

Coherence in electron transfer pathways

Central to the view of electron-transfer reactions is the idea that nuclear motion generates a transition state geometry at which the electron/hole amplitude propagates coherently from the electron donor to the electron acceptor. In the weakly coupled or nonadiabatic regime, the electron amplitude tunnels through an electronic barrier between the donor and acceptor. The structure of the barrier is determined by the covalent and noncovalent interactions of the bridge. Because the tunneling barrier depends on the nuclear coordinates of the reactants (and on the surrounding medium), the tunneling barrier is highly anisotropic, and it is useful to identify particular routes, or pathways, along which the transmission amplitude propagates. Moreover, when more than one such pathway exists, and the paths give rise to comparable transmission amplitude magnitudes, one may expect to observe quantum interferences among pathways if the propagation remains coherent. Given that the effective tunneling barrier height and width are affected by the nuclear positions, the modulation of the nuclear coordinates will lead to a modulation of the tunneling barrier and hence of the electron flow. For long distance electron transfer in biological and biomimetic systems, nuclear fluctuations, arising from flexible protein moieties and mobile water bridges, can become quite significant. We discuss experimental and theoretical results that explore the quantum interferences among coupling pathways in electron-transfer kinetics; we emphasize recent data and theories associated with the signatures of chirality and inelastic processes, which are manifested in the tunneling pathway coherence (or absence of coherence).     

Peculiarities in free electron transfer

Ionizing irradiation of non-polar solvents generates radical cations which in the case of alkanes and alkyl chlorides are metastable and can be detected in the nanosecond time range. In the presence of solutes, free electron transfer (FET) to the parent radical cations can occur. In most cases, this reaction enables the easy generation of a lot of scavenger radical cations. From the unusual product distribution of the FET involving phenols, general peculiarities of the free electron transfer compared with the commonly known photosensitized process are derived and discussed. They are presented here in a minireview manner.     

Excess electron transfer in G-quadruplex

The excess electron transfer in a G-quadruplex is successfully probed by using the reaction of hydrated electrons with quadruplex complex of pentamers and the 8-bromoguanine moieties as the detection system.     

Encapsulation of small metal particles in solid organic matrices

The methods of synthesis of macroscopic amounts of size-selected clusters with desired properties, and most importantly, the possibility of their controlled assembly into new materials with novel properties are currently of great interest. The interaction of metal atoms and small clusters with solid organic matrices may lead to the stabilization of reactive particles up to room temperatures. Thus obtained nanoscale heterogeneous materials offer an area of intriguing technological promise. In this review, we discuss recent developments in the encapsulation of small metallic particles in different organic solid matrices: organic monomeric compounds, polymers and carbon derivatives.     

Enhanced electron transfer by dendritic architecture: energy transfer and electron transfer in snowflake-shaped Zn porphyrin dendrimers

[structure: see text] Photoinduced electron transfer was observed for the snowflake-shaped dendrimer with the Zn porphyrin core and anthraquinonyl terminals. Comparison of the electron-transfer efficiency of the dendrimer with the linear analogues indicates the advantage of the dendritic structure for the electron-transfer process.     

Dissociative electron transfer reactions

We consider recent data on dissociative electron transfer reactions in which the electron transfer causes practically concerted dissociation of the chemical bond in the reagent. We discuss considerable experimental data on reactions in the gas phase and in solutions, and also existing theoretical models for describing the kinetics of these complex processes. Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 34, No. 2, pp. 67–78, March–April, 1998.     

Intramolecular dissociative electron transfer

Dissociative electron transfers (ET) are reactions in which the ET is associated with the cleavage of a sigma bond. Although a rather satisfactory amount of information is currently available on the intermolecular and heterogeneous dissociative ET reactions, less is known for the corresponding intramolecular processes, despite the relevance of these reactions in both chemistry and biochemistry. This tutorial review focuses on the most recent developments in this area, with particular emphasis on the reactions occurring in well-defined Donor-Spacer-Acceptor molecular systems. The goal is to provide the reader with the essential background to understand and possibly predict the feasibility and rates of these reactions, as well as to stimulate the application of the intramolecular dissociative ET concepts and related issues to still unexplored molecular systems.     

Light-induced electron transfer reactions

The reactions of the luminescent excited states of the polypyridine-ruthenium(II) complexes (^*RuL₃²⁺) with electron acceptors and donors are discussed. These electron transfer reactions convert the excited state into RuL₃³⁺ and RuL₃⁺, respectively. The former ruthenium complex is a more powerful oxidant and the latter is a more powerful reductant than the excited state itself. Some applications of these complexes in the conversion and storage of solar energy are presented. Theoretical models for electron transfer reactions are described and the implications of these models for the quenching and back electron transfer reactions are discussed. It is pointed out that the exploitation of the inverted region may provide a useful means of slowing down back electron transfer reactions.