Molecular level approaches for investigation of electron transfer in nonpolar solvents

The authors extend their previous work published in Leontyev and TachiyaJ. Chem. Phys. 123, 224502 (2005) and study not only forward but also reverse electron transfer between pyrene and dimethylaniline in a nonpolar solvent, n-hexane. The distribution function methodology and molecular dynamics technique adopted in their previous work are used. Two algorithms (I and II) are formulated for obtaining the reorganization energy and the solvation free energy difference in the linear response approximation. The two algorithms are combined with different cutoff schemes and tested for polarizable and nonpolarizable solvent models. Agreement between the results obtained by the two algorithms was achieved only for simulations employing the particle mesh Ewald treatment. It is concluded that algorithm I provides a reliable scheme for evaluation of the reorganization energy and the solvation free energy difference. Moreover, a new algorithm referred to as the G-function algorithm is formulated which does not assume the linear response approximation, and is tested on evaluation of the solvation free energy difference. Agreement between the results from the G-function algorithm and those from algorithms I and II is fairly good, although it depends on the degree of statistical consistency of the simulations. In the case of nonpolar solvents the G-function method has practical importance because, unlike the conventional thermodynamic integration approach, it requires equilibrium molecular configuration ensembles only for the initial and final states of the system.     

The reorganization energy of electron transfer in nonpolar solvents: molecular level treatment of the solvent

The intermolecular electron transfer in a solute pair consisting of pyrene and dimethylaniline is investigated in a nonpolar solvent, n-hexane. The earlier elaborated approach [M. Tachiya, J. Phys Chem. 97, 5911 (1993)] is used; this method provides a physically relevant background for separating inertial and inertialess polarization responses for both nonpolarizable and polarizable molecular level simulations. The molecular-dynamics technique was implemented for obtaining the equilibrium ensemble of solvent configurations. The nonpolar solvent, n-hexane, was treated in terms of OPLS-AA parametrization. Solute Lennard-Jones parameters were taken from the same parametrization. Solute charge distributions of the initial and final states were determined using ab initio level [HF/6-31G(d,p)] quantum-chemical calculations. Configuration analysis was performed explicitly taking into account the anisotropic polarizability of n-hexane. It is shown that the Gaussian law well describes calculated distribution functions of the solvent coordinate, therefore, the rate constant of the ET reaction can be characterized by the reorganization energy. Evaluated values of the reorganization energies are in a range of 0.03-0.11 eV and significant contribution (more then 40% of magnitude) comes from anisotropic polarizability. Investigation of the reorganization energy lambda dependence on the solute pair separation distance d revealed unexpected behavior. The dependence has a very sharp peak at the distance d=7 A where solvent molecules are able to penetrate into the intermediate space between the solute pair. The reason for such behavior is clarified. This new effect has a purely molecular origin and cannot be described within conventional continuum solvent models.     

Electron transfer in nonpolar solvents in fullerodendrimers with peripheral ferrocene units

Two new fullerodendrimers, with two and four ferrocene units on their periphery, have been synthesized by 1,3-dipolar cycloaddition reactions between the corresponding azomethine ylides and C(60). These new compounds have been studied by using cyclic voltammetry and UV/Vis spectroscopy. Weak intramolecular interactions between the fullerene cage and the ferrocene groups have been found. The photochemical events of both fullerene-ferrocene dendrimers have been probed by means of steady-state and time-resolved techniques. The steady-state emission intensities of the fulleropyrrolidine-ferrocene dendrimers 1 and 2 were found to be quenched relative to the N-methylfulleropyrrolidine without substituents that was used as a model. The nanosecond transient absorption spectral studies revealed efficient charge separation in both systems, even in toluene. The lifetimes of the (C(60))(*-)-(dendron)(*+) are higher for the second-generation fullerodendrimer (with four ferrocene units) and they are of the order of tens of nanoseconds in toluene and hundreds of nanoseconds in polar solvents.     

Charge stabilization in nonpolar solvents

While the important role of electrostatic interactions in aqueous colloidal suspensions is widely known and reasonably well-understood, their relevance to nonpolar suspensions remains mysterious. We measure the interaction potentials of colloidal particles in a nonpolar solvent with reverse micelles. We find surprisingly strong electrostatic interactions characterized by surface potentials, |ezeta|, from 2.0 to 4.4 k(B)T and screening lengths, kappa(-1), from 0.2 to 1.4 microm. Interactions depend on the concentration of reverse micelles and the degree of confinement. Furthermore, when the particles are weakly confined, the values of |ezeta| and kappa extracted from interaction measurements are consistent with bulk measurements of conductivity and electrophoretic mobility. A simple thermodynamic model, relating the structure of the micelles to the equilibrium ionic strength, is in good agreement with both conductivity and interaction measurements. Since dissociated ions are solubilized by reverse micelles, the entropic incentive to charge a particle surface is qualitatively changed from aqueous systems, and surface entropy plays an important role.     

Facile phase transfer of large, water-soluble metal nanoparticles to nonpolar solvents

The facile phase-transfer of large, water-soluble metal nanoparticles to nonpolar solvent is reported here. Thiol-terminated polystyrene (PS-SH) is ligand-exchanged onto water-soluble metal nanoparticles in single-phase acetone/water mixtures, generating a precipitate. The solvent is then removed and the particles are redissolved in nonpolar solvent. This approach is demonstrated for nanoparticles of different metal (Au and Ag), size (3 to >100 nm), shape (spheres, rods, and wires, etc.), and leaving ligand (citrate, cetyltrimethylammonium bromide, poly(vinylpyrrolidone), and 4-dimethylaminopyridine. The resulting PS-SH-stabilized nanoparticles maintain their initial size and shape, and are highly stable. They are soluble in various organic solvents (toluene, benzene, chloroform, dichloromethane, and tetrahydrofuran), and can be readily dried, purified, and re-dissolved. This method makes possible the utilization of a full range of existing nanoparticle cores in nonpolar solvents with a single ligand. It provides access to numerous nanomaterials that cannot be obtained through direct synthesis in nonpolar solvent, and is expected to be of significant value in a number of applications.     

Energetics of electron transfer reactions in polar solvents

The free energy change of an electron transfer reaction in a polar solvent is rigorously analyzed within the framework of the dielectric continuum model. An appropriate expression for the electrostatic energy between the two product ions separated by R is derived. The present result does not support a recent claim by Suppan that, if R is close to the contact distance, the electrostatic energy should be much larger in magnitude than estimated from the usual expression −e²/ε_sR.     

Dynamical arrest of electron transfer in liquid crystalline solvents

We argue that electron transfer reactions in slowly relaxing solvents proceed in the nonergodic regime, making the reaction activation barrier strongly dependent on the solvent dynamics. For typical dielectric relaxation times of polar nematics, electron transfer reactions in the subnanosecond time scale fall into nonergodic regime in which nuclear solvation energies entering the activation barrier are significantly lower than their thermodynamic values. The transition from isotropic to nematic phase results in weak discontinuities of the solvation energies at the transition point and the appearance of solvation anisotropy weakening with increasing solute size. The theory is applied to analyze experimental kinetic data for the electron transfer kinetics in the isotropic phase of 5CB liquid crystalline solvent. We predict that the energy gap law of electron transfer reactions in slowly relaxing solvents is characterized by regions of fast change of the rate at points where the reaction switches between the ergodic and nonergodic regimes. The dependence of the rate on the donor-acceptor separation may also be affected in a way of producing low values for the exponential falloff parameter.     

Solvent reorganization entropy of electron transfer in polar solvents

We report the results of molecular dynamics simulations of the solvent reorganization energy of intramolecular electron transfer in a charge-transfer molecule dissolved in water and acetonitrile at varying temperatures. The simulations confirm the prediction of microscopic solvation theories of a positive reorganization entropy in polar solvents. The results of simulations are analyzed in terms of the splitting of the reorganization entropy into the contributions from the solute-solvent interaction and from the alteration of the solvent structure induced by the solute. These two contributions mutually cancel each other, resulting in the reorganization entropy amounting to only a fraction of each component.     

Femtodynamics reflected in nanoseconds: bimolecular free electron transfer in nonpolar media

The (free) electron transfer (FET) from electron donor molecules to parent solvent radical cations of alkanes and alkyl chlorides exhibits mechanistic peculiarities that are conditioned by the low polarity of these solvents. Because of the negligible solvation of ions in such systems and the almost complete lack of an activation barrier, the electron jump takes place at the very first encounter of the reactants and, as such, in extremely short times of 相似文献   

Organogel from L-leucine-containing surfactant in nonpolar solvents

 Alkyl (S)-2-ammonium-2-isobutylacetate p-toluenesulfonate formed organogel in nonpolar solvents. The gels exhibited thermally reversible sol–gel phase transitions. UV spectroscopic study suggested that dodecyl (S)-2-ammonium-2-isobutylacetate p-toluenesulfonate forms reversed micelle-like aggregate at low concentration in a nonpolar solvent. Circular dichroism spec-troscopy indicated that component molecules of the reversed micelle-like aggregate are cooperatively organized and result in chiral aggregate. The huge fibrous aggregate responsible for gelation was observed with transmission electron microscopy. The accumulation and rearrangement of reversed micelle-like aggregate resulted in the formation of huge fibrous aggregates. A gathering of numerous fibrous aggregates formed the three-dimensional network to immobilize the isotropic liquid. Received: 24 June 1997 Accepted: 3 October 1997     

Local structure of solvation of nonpolar chains in nonpolar solvents at infinite dilution

The SSOZ (site-site Ornstein-Zernike) equation is used to study the local structure of solvation of linear nonpolar molecules in nonpolar solvents. The atom-atomic interaction potentials are described by the Lennard-Jones potential. The chain-solvent atom-atomic pair correlation functions are calculated in relation to the chain length (number of atoms), solvent density, and the ratio of the geometrical parameters of solvent and chain atoms. Institute of Theoretical and Experimental Biophysics, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 4, pp. 742–749, July–August, 1996. Translated by L. Smolina     

Ionization of aromatic sulfides in nonpolar media: free vs reaction-controlled electron transfer

The primary products of the bimolecular free electron transfer (FET) from aromatic sulfides (PhSCH2Ph, PhSCHPh2, PhSCPh3) to n-butyl chloride radical cations are two radical cation conformers: a dissociative and a metastable one. In analogy with formerly studied donor systems, this result seems to reflect femtosecond oscillations in the ground state of the sulfides such as torsion motions around the Ar-S bond. This motion is accompanied by a marked electron fluctuation within the HOMO (or the n) orbitals. The FET products observed in the nanosecond time scale such as the metastable sulfide radical cations (Ar-S-CR3*+), the dissociation products R3C+; and R3C*, and their (experimentally) nondetectable counterparts Ar-S* as well as Ar-S+ can be understood with the simplified assumption of two extreme conformations, namely a planar and a twisted donor molecule. Using mediator radical cations (benzene, butylbenzene, biphenyl), the stepwise reduction of the free energy of the electron transfer from -DeltaH = 2.5 to 相似文献   

Detection of water in organic solvents by photo-induced electron transfer method

A new class of fluorescence sensor for detection of water in organic solvents based on photo-induced electron transfer (PET) of anthracene coupled with an amino acid has been designed and developed.     

Photo-induced intermolecular electron transfer from electron donating solvents to Coumarin dyes in bile salt aggregates: role of diffusion in electron transfer reaction

The photo-induced electron transfer between Coumarin dyes and aromatic amines has been investigated using steady state and time-resolved fluorescence quenching studies. We have observed a Marcus type inversion in the electron transfer rate in correlation of quenching constant to the free energy change occurred during reaction. To justify the "inverted region" obtained in the correlation of quenching constant versus free energy curve, we have performed anisotropy measurement and estimated the several diffusional parameters. The translational diffusion coefficients exhibit a similar picture like electron transfer rate constant when it is plotted against free energy. Thus we argued that the diffusion has played an important role in the electron transfer kinetics.