Donor-acceptor Coupling Research Articles

Nonadiabaticity in the photoinduced electron transfer reactions of metal complexes

The rate of electron transfer (ET) in a variety of chemical and biological processes is influenced by factors like the free energy change (†G), the donor-acceptor electronic coupling and the medium. The effect of donor-acceptor electronic coupling on the rate of photoinduced intermolecular electron transfer is considered by taking Ru(II) and Cr(III) metal complexes in the excited state as electron acceptors and organic compounds as electron donors. The electronic coupling between the donor and acceptors depends strongly on donor-acceptor distance. The electron transfer distance is varied by introducing alkyl groups of different sizes either on the bipyridine ligand of the metal complex or on the quencher. The semiclassical theory of electron transfer expresses kET as the product of a nuclear and an electronic transmission coefficient (K n andK el respectively) and an effective nuclear-vibration frequency (v n),k ET =v nKel, Kn. The electron transfer reaction becomes nonadiabatic if the donor-acceptor distance is long. The change of electron transfer mechanism from adiabatic to nonadiabatic due to the introduction of bulky groups is explained in terms of semiclassical theory and from the temperature-dependence study of photoinduced electron transfer reactions of metal complexes.

Electron transfer in cytochrome c depends upon the structure of the intervening medium.

Long-distance electron-transfer (ET) reactions through proteins are involved in a great many biochemical processes; however, the way in which the protein structure influences the rates of these reactions is not well understood. We have therefore measured the rates of intramolecular ET from the ferroheme to a bis(2,2'-bipyridine)imidazoleruthenium(III) acceptor at histidine 39 or 54 in derivatives of yeast iso-1-cytochrome c, and studied the effect of an asparagine to isoleucine mutation at position 52, a residue situated between the heme and the electron acceptor. The Fe2+-->Ru3+ rate constants demonstrate that residue 52 affects ET from the heme to His54 (Ile52 > Asn52), but not to His39 (Ile52 = Asn52). The enhanced Fe(2+)-Ru3+(His54) electronic coupling for the N52I/K54H protein is in good agreement with sigma-tunneling calculations, which predict the length of the ET pathways between the heme and His54. The structure of the intervening medium between the heme and electron acceptors at the protein surface influences the donor-acceptor couplings in cytochrome c.

Electron transfer in proteins: a novel approach for the description of donor-acceptor coupling

Electron transfer in proteins: a novel approach for the description of donor-acceptor coupling

Electrochemical and spectroscopic manifestations of donor-acceptor coupling in cyanide bridged transition metal complexes: contrasts between RuCNRu, CoCNRu and RhCNRu systems

An issue of contrasting donor-acceptor electronic coupling in cyano-bridged transition metal complexes containing ruthenium, rhodium or cobalt acceptors has been addressed by means of spectroscopic and electrochemical comparisons of several series of M(CNRu(NH 3) 5) 2 complexes where M is Ru II(bpy) 2, Co III(MCL), Rh III(bpy) 2 or Rh III(MCL) and MCL is a tetraazamacrocyclic ligand. All the complexes exhibit metal-to-metal (M II→Ru III(NH 3) 5 or Ru II(NH 3) 5→M III) charge transfer (MMCT) absorptions. These are highest in energy for M=Rh(III) and lowest for M=Ru(II). The electrochemical half-wave potentials of the Ru(NH 3) 3+,2+ 5 couples are strongly correlated with the energies of the MMCT transitions and span a 300 mV range. The M=Co III(MCL), Rh III(bpy) 2 and Rh III(MCL) complexes, in which Ru II(NH 3) 5→M IIIMMCT transitions are observed, have E 1 2 values greater than 0.22 V versus SCE, while the M=Ru II(bpy) 2 complexes have values of E 1 2 which are less than 0.0 V. The results suggest that common perturbational treatments of the MMCT transitions in RuCNRu complexes are inadequate owing to the exceptionally strong electronic coupling, and that the perturbational MMCT band shape analysis underestimates the electronic coupling. This work indicates that the donor-acceptor electronic coupling is greater in RuCNRu complexes than in RuCNCo complexes, qualitatively in accord with reports of contrasting MMCT excited state back electron transfer behavior.

Donor-acceptor electronic coupling as a function of bridging group: mixed-valence diruthenium(II,III) complexes bridged by isonicotinato and isonicotinamido ligands

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTDonor-acceptor electronic coupling as a function of bridging group: mixed-valence diruthenium(II,III) complexes bridged by isonicotinato and isonicotinamido ligandsMei H. Chou, Carol Creutz, and Norman SutinCite this: Inorg. Chem. 1992, 31, 12, 2318–2327Publication Date (Print):June 1, 1992Publication History Published online1 May 2002Published inissue 1 June 1992https://doi.org/10.1021/ic00038a007RIGHTS & PERMISSIONSArticle Views208Altmetric-Citations42LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (1 MB) Get e-Alerts Get e-Alerts

Electron-transfer reactions of cytochrome f with flavin semiquinones and with plastocyanin. Importance of protein-protein electrostatic interactions and of donor-acceptor coupling.

Reduction of turnip ferricytochrome f by flavin semiquinones and oxidation of this ferrocytochrome f by French bean cupriplastocyanin are studied by laser flash photolysis over a wide range of ionic strengths. Second-order rate constants (+/- 15%) at extreme values of ionic strength, all at pH 7.0 and 22 degrees C, are as follows: with FMN semiquinone at 1.00 and 0.0040 M, 5.0 x 10(7) and 3.9 x 10(8) M-1 s-1; with riboflavin semiquinone at 1.00 and 0.0040 m, 1.7 x 10(8) and 1.9 x 10(8) M-1 s-1; with lumiflavin semiquinone at 1.00 and 0.0045 M, 1.8 x 10(8) and 4.5 x 10(8) M-1 s-1; with cupriplastocyanin at 1.00 and 0.100 M, 1.4 x 10(6) and 2.0 x 10(8) M-1 s-1. These reactions of cytochrome f are governed by the local positive charge of the interaction domain (the exposed heme edge), not by the overall negative charge of the protein. Lumiflavin semiquinone behaves as if it carried a small negative charge, probably because partial localization of the odd electron gives this electroneutral molecule some polarity; local charge seems to be more important than overall charge even for relatively small redox agents. The dependence of the rate constants on ionic strength was fitted to the equation of Watkins; this model recognizes the importance of local charges of the domains through which redox partners interact. There is kinetic evidence that a noncovalent complex between cytochrome f and plastocyanin exists at low ionic strength.(ABSTRACT TRUNCATED AT 250 WORDS)

Spin polarization and ferromagnetic coupling in metallocenium charge transfer complexes

Spin polarization effects in metallocenium complexes across the transition metal series Cr, Mn, Fe, Co, Ni are discussed in a qualitative fashion using a restricted open-shell Hartree–Fock approach. The spin polarization is taken into account by a limited configuration interaction (CI) with singly excited configurations. In Fe(C5Me5)+2, Mn(C5Me5)+2, and Cr(C5Me5)+2, the unpaired electrons are localized in molecular orbitals with predominant metal 3d character. The polarization of the metal–cyclopentadiene π bonds then leads to a negative spin density on the cyclopentadiene (Cp) rings. The amount of negative spin density on Cp increases with increasing spin on the metal (S=1/2, S=1, and S=3/2 for ferrocenium, manganocenium, and chromocenium, respectively). The Cp spin density of nickelocenium on the other hand is positive because there is a direct delocalization of spin density to the Cp rings due to the considerable Cp contributions to the e*1g molecular orbitals in which the unpaired electron of nickelocenium is located. Correspondingly, the charge transfer complexes of these compounds with the acceptors tetracyanoethylene (TCNE) or tetracyanoquinodimethane (TCNQ) in an alternating arrangement ...D+A−D+A−... are expected to exhibit ferromagnetic donor–acceptor coupling for Fe(C5Me5)+2, Mn(C5Me5)+2, and Cr(C5Me5)+2 and antiferromagnetic coupling for Ni(C5Me5)+2 according to a model suggested previously by the authors and based on an idea of McConnell. This is in agreement with the experimental results. The observed nuclear magnetic resonance (NMR) shifts of some metallocenes in relation to these findings are discussed.

Electron tunneling pathways in proteins: A method to compute tunneling matrix elements in very large systems

A tight-binding Hamiltonian and Dyson’s equation method are described that allow the computation of the tunneling matrix elements between electron donor and acceptor sites in a protein. The method is exact and computationally tractable. The Green’s function matrix elements of the bridge are computed using a strategy that builds up the bridge one orbital at a time, allowing inclusion of all orbitals on proposed tunneling pathways and elsewhere. The tunneling matrix element is determined directly from the bridge Green’s function. A simple representation of a helical protein segment is used to illustrate the method and its ability to include contributions from high-order backscattering and multiple pathway interference in the donor–acceptor coupling.

Effects of donor-acceptor electronic interactions on the rates of gas-phase metallocene electron-exchange reactions

Rate constants for electron self-exchange, k{sub ex}, of five cobaltocenium-cobaltocene and ferrocenium-ferrocene couples in the gas phase have been measured by means of Fourier transform ion cyclotron resonance mass spectrometry in order to explore the possible effects of donor-acceptor electronic coupling on gas-phase redox reactivity. The systems studied, Cp{sub 2}Co{sup +/0}, Cp{sub 2}Fe{sup +/0} (Cp = cyclopentadienyl), the decamethyl derivative Cp{prime}{sub 2}Fe{sup +/0}, carboxymethyl(cobaltocenium-cobaltocene) (Cp{sub 2}{sup e}Co{sup +/0}), and hydroxymethyl(ferrocenium-ferrocene) (HMFc{sup +/0}), were selected in view of the substantial variations in electronic coupling inferred on the basis of their solvent-dependent reactivities and theoretical grounds. The sequence of k{sub ex} values determined in the gas phase, Cp{sub 2}{sup e}Co{sup +/0} {approx} Cp{sub 2}Co{sup +/0} > Cp{prime}{sub 2}Fe{sup +/0} > HMFc{sup +/0} > Cp{sub 2}Fe{sup +/0}, is roughly similar to that observed in solution, although the magnitude (up to 5-fold) of the k{sub ex} variations is smaller in the former case. The likely origins of these differences in gas-phase reactivity are discussed in light of the known variations in the electronic coupling matrix element H{sub 12}, inner-shell reorganization energy {Delta}E*, and gas-phase ion-molecule interaction energy {Delta}E{sub w} extracted from solution-phase rates, structural data, and theoretical calculations. It is concluded that the observedmore » variations in gas-phase k{sub ex} values, especially for Cp{sub 2}Fe{sup +/0} versus Cp{sub 2}Co{sup +/0}, arise predominantly from the presence of weaker donor-acceptor orbital overlap for the ferrocene couples, yielding inefficient electron tunneling for a substantial fraction of the gas-phase ion-molecule encounters. The anticipated differences as well as similarities of such nonadiabatic effects for gas-phase and solution electron-transfer processes are briefly outlined.« less

Intramolecular Electron Transfer in Iridium Dimers

Abstract The kinetics of photoinduced electron transfer and thermal recombination have been measured in a series of covalently linked donor-acceptor complexes. The molecules are based on pyrazolate-bridged iridium(I) dimers with pyridinium groups covalently bound to terminal phosphinite ligands. The driving-force dependence of the electron-transfer kinetics in one series agrees well with the predictions of the classical theory of electron transfer. The donor-acceptor electronic coupling, however, exhibits a sensitive and unexpected dependence on the nature of the bridge between the redox partners.

Controlled Electron Transfer for Molecular Electronics

The ability to flexibly initiate and control electron transfer reactions in molecules contributes to their central role in molecular electronics. Conventional applications of electron transfer reactions discussed in this volume center on organic conductors and chromophores for nonlinear optics. However, molecules that undergo thermal or photoinduced electron transfer might serve as the basis for very small scale electronic devices. We will present a summary of our proposal for a shift register memory based on electron transfer reactions, and the synthetic tools that are needed to implement the proposal. A unique function of biomolecules is derived from their ability to deliver electrons to specific sites in proteins at prescribed rates. An understanding of how biology solves the rate control and selectivity problem may aid the design of semi-synthetic systems for molecular electronic applications. We will describe a scheme for mapping the specific bonds within proteins that mediate the donor-acceptor coupling responsible for distant (>10 A) electron transfer

Long-range electron transfer in ruthenium-modified cytochrome c: evaluation of porphyrin-ruthenium electronic couplings in the Candida krusei and horse heart proteins

Experiments in several laboratories have shown that electron transfer (ET) can take place at appreciable rates over long distances (> 10 {angstrom}) in organic and inorganic molecules and in proteins. There is a bewildering array of potential ET pathways in proteins; interestingly, the through-peptide routes (if there are any) generally involve so many bonds that they cannot possibly account for the observed rates. In searching for good pathways through cytochrome c, the authors discovered a relatively short route from His-39 to the heme in the Candida krusei (C.k.) protein. Along this route, the crucial shortcut is a hydrogen bond that bridges Gly-41 and the heme. Since experimental information relevant to protein pathway models is lacking, they have extracted donor-acceptor electronic coupling constants from an analysis of the driving-force dependence of ET rates in Ru(His-39)-modified C.k. zinc cytochrome c.

A unified theory of electron transfer and internal conversion based on solitary electronic states

A description of electron transfer processes is presented which makes use of the concept of solitary states for the construction of the initial and the final state wave functions. Even though this concept is closely related to an adiabatic description in the sense that it provides partial delocalization of these states it also assures localization for a symmetric donor–acceptor system. The theory can be applied to arbitrary values of the internal parameters characterizing the system, that is the electronic donor–acceptor coupling V, the free energy change ΔE, and the coupling of the electron to nuclear motions represented by a Stokes shift S. This way the theory of nonadiabatic electron transfer processes is put on equal footing with the theory of adiabatic transitions and even more interestingly the same concept unifies the common electron transfer theory and the theory of internal conversion processes if ‖ΔE‖ and (or) ‖V‖ exceed S. It is shown analytically how the results of the conventional treatments for adiabatic, nonadiabatic, and internal conversion processes fall out of this theory as special cases. It is further examplified how the system develops from the so-called normal regime with ‖ΔE‖<S into the inverted regime with ‖ΔE‖>S as function of ΔE for fixed parameters of V and S and as function of V for fixed parameters of ΔE and S.

Charge-transfer perturbations of the electronic contributions to electron-transfer reactions. Enhanced donor-acceptor couplings mediated by coordinated ligands

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTCharge-transfer perturbations of the electronic contributions to electron-transfer reactions. Enhanced donor-acceptor couplings mediated by coordinated ligandsJohn F. Endicott and T. RamasamiCite this: J. Phys. Chem. 1986, 90, 16, 3740–3747Publication Date (Print):July 1, 1986Publication History Published online1 May 2002Published inissue 1 July 1986https://doi.org/10.1021/j100407a048RIGHTS & PERMISSIONSArticle Views144Altmetric-Citations31LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (1 MB) Supporting Info (1)»Supporting Information Supporting Information Get e-Alerts