Marcus Theory Of Electron Transfer Research Articles

Charge Transport Properties of Tris(8-hydroxyquinolinato)aluminum(III): Why It Is an Electron Transporter

The charge transport properties of mer-tris(8-hydroxyquinolinato)aluminum(III) (mer-Alq), which is the most widely used electron transport material in OLED, were investigated by quantum chemistry calculations within the framework of the charge hopping model and Marcus electron transfer theory. Internal reorganization energies of 0.276 and 0.242 eV were calculated by the DFT-B3LYP method employing a 6-31 G* basis set for the electrons lambdai(e) and holes lambdai(h), respectively. The relative distances and orientations of Alq molecules in amorphous film were simulated by those in the beta-phase. The intermolecular charge-transfer integrals, Hda(h) and Hda(e), along all 14 hopping pathways were then calculated by the Koopmans Theorem in conjunction with the Hartree-Fock method employing a 6-31 G* basis set as well as by the direct coupling method. The results showed that there were some Hda(e) that were 1 order of magnitude larger than any Hda(h), because hopping pathways with effective overlaps of LUMOs can occur and, thus, large Hda(e). On the other hand, effective overlap of HOMO was absent in all pathways, resulting in a relatively small Hda(h). This difference in the magnitudes of Hda(e) and Hda(h) would predict a 2 orders of magnitude difference in the electron-transfer rate constants and account for the observed 2 orders of magnitude difference in the mobilities of electrons and holes.

Remarkable Accelerating Effects of Ammonium Cations on Electron-Transfer Reactions of Quinones by Hydrogen Bonding with Semiquinone Radical Anions

Remarkable accelerating effects of the ammonium cation (NH4+) have been observed on photoinduced electron-transfer reactions from the triplet excited state of tetraphenylporphyin (H2P) to quinones [p-benzoquinone (Q) and naphthoquinone (NQ)] in dimethyl sulfoxide (DMSO). The tetrabutylammonium cation (NBu4+) is also effective to accelerate the electron-transfer reduction of Q and NQ in dichloromethane (CH2Cl2). The hydrogen bonding interaction between the semiquinone radical anion (Q•- or NQ•-) and NH4+ was confirmed by the ESR spectra of the Q•-/NH4+ and Q•-/(NH4+)2 [NQ•-/NH4+ and NQ•-/(NH4+)2] complexes in DMSO. Accelerating effects of NH4+ in DMSO and NBu4+ in CH2Cl2 on the rates of photoinduced electron-transfer reduction of Q by H2P result from the positive shift of the Ered value of quinones together with a constant Eox value of H2P determined from cyclic voltammetry measurements. The driving force dependence of the rate constants for photoinduced electron-transfer reduction of quinones in the presence of various concentrations of ammonium cations was evaluated in light of the Marcus theory of electron transfer.

Effect of Molecular Structure on Interfacial Electron Transfer Dynamics of 7-N,N-Dimethyl Coumarin 4-Acetic Acid (DMACA) and 7-Hydroxy Coumarin 4-Acetic Acid (HCA) Sensitized TiO2 and ZrO2 Nanoparticles

Ultrafast transient absorption spectroscopy has been employed to understand the effect of molecular structure on interfacial electron transfer (ET) dynamics of 7-N,N-dimethyl amino coumarin 4-acetic acid (DMACA) and 7-hydroxy coumarin 4-acetic acid (HCA) sensitized TiO 2 and ZrO 2 nanoparticles. Electron injection is confirmed by observing the cation radical of the dye molecules as well as the conduction band electron in the visible and near-IR regions. Electron injection efficiency has been found to be higher for the HCA/TiO 2 system as compared to the DMACA/TiO 2 system. Both dyes are structurally similar except that HCA has a hydroxyl group at the 7-position while DMACA has a dimethyl amino group at the 7-position. Steady-state and time-resolved fluorescence measurements confirmed that, in highly polar solvent, the excited state of DMACA dye exists both in twisted intramolecular charge transfer (TICT) and intramolecular charge transfer (ICT) states, whereas excited HCA exists only in the ICT state. Because the charge in the case of the TICT state of DMACA is localized away from the nanoparticle surface, electron injection from that state is not efficient. However, ICT states of both DMACA and HCA can inject electrons efficiently as the charge is delocalized. Hence, the quantum yield of electron injection is high in the case of HCA compared to DMACA. We have also observed that photoexcited DMACA whose energy level lies above the conduction band of ZrO 2 nanoparticle can inject electrons after exciting with 400 nm laser light. Back electron transfer (BET) rates have been determined by following the decay kinetics of the cation radical and conduction band electron in different systems. The BET rate is found to be slower for HCA/TiO 2 compared to DMACA/TiO 2 , as they fall in the inverted region of Marcus electron transfer theory. The BET is faster in the DMACA/TiO 2 system as compared to that of the DMACA/ZrO 2 system. However, the residual absorption after 460 ps is less in the case of the DMACA/ZrO 2 system.

Optimal energy offsets for organic solar cells containing a donor/acceptor pair

Optimal energy levels and offsets of an organic donor/acceptor binary-type solar cell have been analyzed using the classic Marcus electron transfer theory to identify the most efficient photo-induced charge separation. This study reveals that, an exciton quenching parameter (Yeq) yields one optimal donor/acceptor energy offset where the photo-induced exciton is converted to charges most efficiently, and a recombination quenching parameter (Yrq) yields a second optimal donor/acceptor energy offset where the ratio of charge separation rate constant over charge recombination rate constant becomes maximum. Another energy offset is also identified where charge recombination becomes most severe. This information would be useful for evaluating and fine tuning frontier energy levels of a donor/acceptor pair for optimum solar cell applications.

Photoelectric Properties Based on Electric Field Modulation of Photoinduced Electron Transfer Processes in Flavin-Porphyrin Hetero-type Langmuir–Blodgett Films

Metal–insulator–meal devices composed of flavin-porphyrin hetero-type Langmuir–Blodgett films showed highly efficient photoelectric properties mainly attributable to the fast charge separation process at a molecular heterojunction (MHJ) between flavin and porphyrin. The photoelectric properties of the MHJ devices showed different characteristics depending on the redox state of the central metal of porphyrin, i.e., Ru(III) or Ru(II). The rectifying behavior of the photocurrent was observed for the Ru(III)-MHJ device, whereas the Ru(II)-MHJ device did not show the rectifying behavior. We concluded that the rectifying behavior was mainly controlled by the electric field dependence of the charge recombination process. Furthermore, a bell-shaped photocurrent-voltage curve was observed for the Ru(II)-MHJ device. The mechanism underlying the negative resistance might be based on the electric field dependence of the charge shift process in flavin monolayers controlled by the inverted region mechanism of the Marcus electron transfer theory.

New Paradigm of Electron Transfer Chemistry in Organic Reactions

AbstractFor Abstract see ChemInform Abstract in Full Text.

Substituent Effects on the Site of Electron Transfer during the First Reduction for Gold(III) Porphyrins

Gold(III) porphyrins of the type (P-R)AuPF(6), where P = 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrin and R is equal to H (1), NO(2) (2), or NH(2) (3) which is substituted at one of the eight beta-pyrrolic positions of the macrocycle, were investigated as to their electrochemistry and spectroelectrochemistry in nonaqueous media. Each compound undergoes three reductions, the first of which involves the central metal ion to give a Au(II) porphyrin or a Au(III) porphyrin pi-anion radical depending upon the nature of the porphyrin ring substituent. A similar metal-centered reduction also occurs for compounds 1, 3, and Au(III) quinoxalinoporphyrin, (PQ)AuPF(6) (4), where PQ = 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)quinoxalino[2,3-b]porphyrin, and these results on the three Au(III) porphyrins overturn the long held assumption that reductions of such complexes only occur at the macrocycle. In contrast, when a NO(2) group is introduced on the porphyrin ring to give (P-NO(2))AuPF(6) (2), the site of electron transfer is changed from the gold metal to the macrocycle to give a porphyrin pi-anion radical in the first reduction step. This change in the site of electron transfer was examined by electrochemistry combined with thin-layer UV-vis spectroelectrochemistry and ESR spectroscopy of the singly reduced compound produced by chemical reduction. The reorganization energy (lambda) of the metal-centered electron transfer reduction for (P-H)AuPF(6) (1) in benzonitrile was determined as lambda = 1.23 eV by analyzing the rates of photoinduced electron transfer from the triplet excited states of an organic electron donor to 1 in light of the Marcus theory of electron transfer. The lambda value of the metal-centered electron transfer of gold porphyrin (1) is significantly larger than lambda values of ligand-centered electron transfer reactions of metalloporphyrins.

Effects of metal ions on photoinduced electron transfer in zinc porphyrin-naphthalenediimide linked systems.

Zinc porphyrin-naphthalenediimide (ZnP-NIm) dyads and zinc porphyrin-pyromellitdiimide-naphthalenediimide (ZnP-Im-NIm) triad have been employed to examine the effects of metal ions on photoinduced charge-separation (CS) and charge-recombination (CR) processes in the presence of metal ions (scandium triflate (Sc(OTf)(3)) or lutetium triflate (Lu(OTf)(3)), both of which can bind with the radical anion of NIm). Formation of the charge-separated states in the absence and in the presence of Sc(3+) was confirmed by the appearance of absorption bands due to ZnP(.) (+) and NIm(.) (-) in the absence of metal ions and of those due to ZnP(.) (+) and the NIm(.) (-)/Sc(3+) complex in the presence of Sc(3+) in the time-resolved transient absorption spectra of dyads and triad. The lifetimes of the charge-separated states in the presence of 1.0 x 10(-3) M Sc(3+) (14 micros for ZnP-NIm, 8.3 micros for ZnP-Im-NIm) are more than ten times longer than those in the absence of metal ions (1.3 micros for ZnP-NIm, 0.33 micros for ZnP-Im-NIm). In contrast, the rate constants of the CS step determined by the fluorescence lifetime measurements are the same, irrespective of the presence or absence of metal ions. This indicates that photoinduced electron transfer from (1)ZnP(*) to NIm in the presence of Sc(3+) occurs without involvement of the metal ion to produce ZnP(.) (+)-NIm(.) (-), followed by complexation with Sc(3+) to afford the ZnP(.) (+)-NIm(.) (-)/Sc(3+) complex. The one-electron reduction potential (E(red)) of the NIm moiety in the presence of a metal ion is shifted in a positive direction with increasing metal ion concentration, obeying the Nernst equation, whereas the one-electron oxidation potential of the ZnP moiety remains the same. The driving force dependence of the observed rate constants (k(ET)) of CS and CR processes in the absence and in the presence of metal ions is well evaluated in terms of the Marcus theory of electron transfer. In the presence of metal ions, the driving force of the CS process is the same as that in the absence of metal ions, whereas the driving force of the CR process decreases with increasing metal ion concentration. The reorganization energy of the CR process also decreases with increasing metal ion concentration, when the CR rate constant becomes independent of the metal ion concentration.

Some recent trends in computer simulations of aqueous double layers

Recent molecular simulations of the electric double layer between an aqueous and a metallic phase are reviewed. Several trends in the field can be identified: (i) the increasing use of ab initio simulation methods, most notably the Car-Parrinello method, allows to combine a statistical mechanical description of the double layer with a description of elementary chemical processes on the electronic structure level; (ii) the application of free-energy methods in one and (recently) two dimensions to describe chemical reactivity within and beyond the framework of the Marcus theory of electron transfer; and (iii) at high concentrations, direct simulations of two-phase systems with an aqueous solution and a charged or uncharged solid phase or surface can model the entire double layer region.

Charge transport in micas: The kinetics of FeII/III electron transfer in the octahedral sheet

The two principal FeII/III electron exchange reactions underlying charge transport in the octahedral sheet of ideal end-member annite were modeled using a combination of ab initio calculations and Marcus electron transfer theory. A small polaron model was applied which yielded electron hopping activation energies that agree well with the limited available experimental data. A small ab initio cluster model successfully reproduced several important structural, energetic, and magnetic characteristics of the M1 and M2 Fe sites in the annite octahedral sheet. The cluster enabled calculation of the internal reorganization energy and electronic coupling matrix elements for the M2–M2 and M1–M2 electron transfer reactions. The M2–M2 electron transfer is symmetric with a predicted forward/reverse electron hopping rate of 106 s−1. The M1–M2 electron transfers are asymmetric due to the higher ionization potential by 0.46 eV of FeII in the M1 site. The electronic coupling matrix elements for these reactions are predicted to be small and of similar magnitude, suggesting the possibility that the coupling is essentially direction independent amongst hopping directions in the octahedral sheet. M1 Fe sites are predicted to be efficient electron traps and charge transport should occur by nearest-neighbor electron hops along the M2 Fe sublattice.

Solvent reorganization in electron and ion transfer reactions near a smooth electrified surface: a molecular dynamics study.

Molecular dynamics simulations of electron and ion transfer reactions near a smooth surface are presented, analyzing the effect of the geometrical constraint of the surface and the interfacial electric field on the relevant solvation properties of both a monovalent negative ion and a neutral atom. The simulations show that, from the solvation point of view, ion adsorption is an uphill process due to the need to shed off the ion's solvation shell and displace water from the surface. Atom adsorption, on the other hand, has only a small barrier, related to the molecularity of the solvent. Both the electrostatic interaction of the ion with the solvent and the ion's solvent reorganization energy (the relevant parameter in the Marcus electron transfer theory) decrease as the surface is approached, whereas these parameters are not sensitive to the distance from the surface for the atom. This is a consequence of the importance of long-range electrostatic interactions for ion solvation and the importance of short-range interactions for atom solvation. The electric field either attracts or repels an ion to or from the surface, but the field has no influence on the solvent reorganization energy. By including the quantum-mechanical electron transfer between the metal surface and the ion/atom in solution in the MD simulation by using a model Hamiltonian, we calculated two-dimensional free energy surfaces for ion adsorption allowing for partial charge transfer, based on a fully molecular picture of ion solvation near the surface.

Reorganization Energies in the Transports of Holes and Electrons in Organic Amines in Organic Electroluminescence Studied by Density Functional Theory

To enable the design of efficient organic electroluminescence (OLED) devices with desirable charge carrier transport properties, the mobilities of hole and electron in a series of compounds were studied computationally based on the Marcus electron transfer theory. MO calculations were performed, using the DFT B3LYP/6-31G* method in the Gaussian 98 program suite, on the following compounds: biphenyl (Bp), 4,4‘-biphenyldiamine (BA), triphenylamine (TPA), tri-p-tolylamine (TTA), 4-biphenylphenyl-m-tolylamine (BPTA), 4,4‘-bis(phenyl-m-tolylamino)biphenyl (TPD), naphthalene (Np), 1-naphthyldiphenylamine (NDPA), 1-biphenylnaphthylphenylamine (BNPA), and 4,4‘-bis(1-naphthylphenylamino)biphenyl (NPB). The geometries of these compounds in their neutral, cationic, and anionic states were optimized. The optimized geometries were then used to calculate the ionization potential, electron affinity, and reorganization energies. For compounds containing a biphenyl moiety (Bp, BA, BPTA, TPD, BNPA, and NPB), the inter-rin...

New Perspective of Electron Transfer Chemistry

AbstractFor Abstract see ChemInform Abstract in Full Text.

Metal ion-promoted intramolecular electron transfer in a ferrocene-naphthoquinone linked dyad. Continuous change in driving force and reorganization energy with metal ion concentration.

Thermal intramolecular electron transfer from the ferrocene (Fc) to naphthoquinone (NQ) moiety occurs efficiently by the addition of metal triflates (M(n)()(+): Sc(OTf)(3), Y(OTf)(3), Eu(OTf)(3)) to an acetonitrile solution of a ferrocene-naphthoquinone (Fc-NQ) linked dyad with a flexible methylene and an amide spacer, although no electron transfer takes place in the absence of M(n)()(+). The resulting semiquinone radical anion (NQ(*)(-)) is stabilized by the strong binding of M(n)()(+) with one carbonyl oxygen of NQ(*)(-)( )()as well as hydrogen bonding between the amide proton and the other carbonyl oxygen of NQ(*)(-). The high stability of the Fc(+)()-NQ(*)(-)/M(n)()(+)() complex allows us to determine the driving force of electron transfer by the conventional electrochemical method. The one-electron reduction potential of the NQ moiety of Fc-NQ is shifted to a positive direction with increasing concentration of M(n)()(+), obeying the Nernst equation, whereas the one-electron oxidation potential of the Fc moiety remains the same. The driving force dependence of the observed rate constant (k(ET)) of M(n)()(+)-promoted intramolecular electron transfer is well evaluated in light of the Marcus theory of electron transfer. The driving force of electron transfer increases with increasing concentration of M(n)()(+) [M(n)()(+)], whereas the reorganization energy of electron transfer decreases with increasing [M(n)()(+)] from a large value which results from the strong binding between NQ(*)(-) and M(n)()(+).

New perspective of electron transfer chemistry.

A new perspective of electron transfer chemistry is described for fine control of electron transfer reactions including back electron transfer in the charge separated state of artificial photosynthetic compounds and its synthetic application. Fundamental electron transfer properties of suitable components of efficient electron transfer systems are described in light of the Marcus theory of electron transfer, in particular focusing on the Marcus inverted region, and they are applied to design multi-step electron transfer systems which can well mimic the function of a photosynthetic reaction center. Both intermolecular and intramolecular electron transfer processes are finely controlled by complexation of radical anions, produced in the electron transfer, with metal ions which act as Lewis acids. Quantitative measures to determine the Lewis acidity of a variety of metal ions are given in relation to the promoting effects of metal ions on the electron transfer reactions. The mechanistic viability of metal ion catalysis in electron transfer reactions is demonstrated by a variety of examples of chemical transformations involving metal ion-promoted electron transfer processes as the rate-determining steps, which are made possible by complexation of radical anions with metal ions.

New Paradigm of Electron Transfer Chemistry in Organic Reactions

New paradigm of electron transfer chemistry in organic reactions is described, making it possible to finely control electron transfer reactions of organic molecules as nature has developed to a high degree of perfection in biological systems raging from photosynthesis to respiration. Important approaches to design and to control organic electron transfer reactions are based on the Marcus theory of electron transfer with weak interactions. New approaches also include electron transfer reactions accompanied by transient chemical bonding with third components such as Lewis acids which can act as promoters to accelerate organic electron transfer reactions. Synthetic application of such new paradigm of electron transfer chemistry in organic reactions is demonstrated by a variety of examples of overall organic chemical transformations which involve Lewis acid-promoted electron transfer processes as the rate-determining step.

Dissociative Electron Transfer

Predicting the rate of chemical reactions on the basis of reorganization energies and driving force is a fundamental challenge in all areas of chemistry and biochemistry. Considerable achievements in this regard have been reached in the area of electron transfer (ET) processes, thanks to the Marcus theory of ET and related treatments.

Electron transfer from plastocyanin to the photosystem I reaction center in mutants with increased potential of the primary donor in Chlamydomonas reinhardtii.

The dependence of the P(700)(+)/P(700) midpoint potential on kinetics of reduction of P(700)(+) in vivo has been examined in a series of site-directed mutants of Chlamydomonas reinhardtii in which the histidyl axial ligand to the Mg(2+) of the P(700) chlorophyll a has been changed to several different amino acids. In wild-type photosystem I, the potential of P(700)(+)/P(700) is 447 mV and the in vivo half-time of P(700)(+) reduction by its natural donor, plastocyanin, is 4 micros. Substitution of the axial histidine ligand with cysteine increases the potential of P(700)(+)/P(700) to 583 mV and changes the rate of P(700)(+) reduction to 0.8 micros. Mutants with a range of potentials between 447 and 583 mV show a strong correlation of the P(700)(+)/P(700) potential to the rate of reduction of P(700)(+) by plastocyanin. There is also an increase in the rate of photosystem I-mediated electron transfer from the artificial electron donor DCPIP to methyl viologen in thylakoid membranes. The results indicate that the overall rate constant of P(700)(+) reduction is determined by the rate of electron transfer between the copper and P(700)(+) and confirmed that in vivo there is a preformed complex between plastocyanin and photosystem I. Using approximations of the Marcus electron transfer theory, it is possible to estimate that the distance between the copper of plastocyanin and P(700)(+) is approximately 15 A. On the basis of this distance, the plastocyanin docking site should lie in a 10 A hollow formed by the lumenal exposed loops between transmembrane helices i and j of PsaA and PsaB.

Molecular dynamics simulation of the first electron transfer step in the oxygen reduction reaction

We present a molecular dynamics simulation of solvent reorganization in the first electron transfer step in the oxygen reduction reaction, i.e. O 2+e −→O 2 −, modeled as taking place in the outer Helmholtz plane. The first electron transfer step is usually considered the rate-determining step from many experimental studies. From our results, we conclude that solvent reorganization rather than inner-sphere reorganization provides the dominant contribution to the activation barrier, if no specific interaction of O 2 or O 2 − with the electrode is included. The free energy surfaces for solvent reorganization are strongly non-linear owing to the effect of electrostriction, which is not incorporated in the classical Marcus electron transfer theory. In spite of the non-symmetric free energy barrier, the transfer coefficient at equilibrium is still close to 0.5, whereas it would be estimated to be much lower from harmonic model considerations.

Modification phenomena of solid-state lignin caused by electron-abstracting oxidative systems

Oxidative treatments of wood pulp lignin by one-electron-abstracting enzymatic or chemical systems result in modification phenomena which are not fully described in terms of those known from lignin model compound studies. The generation of, e.g., long-lived radicals necessitates nondestructive spectroscopic analysis of the lignin polymer for a proper characterization of these. The present work exposes a complexity of spectroscopic modification phenomena, which has not previously been realized. This is achieved by a laccase–mediator system, where the mediator is an aromatic low-molecular-weight compound, which mediates the electron abstraction between the lignin and the enzyme laccase. It is demonstrated that the modification generated exhibits qualitatively different temporal phases. The mechanisms are partly explained in terms of Marcus electron transfer theory, and it is suggested that these may play a role in the in vivo synthesis and degradation of lignin.