Sphere Reorganization Energy Research Articles

Unraveling the mechanisms of triplet state formation in a heavy-atom free photosensitizer.

Triplet excited state generation plays a pivotal role in photosensitizers, however the reliance on transition metals and heavy atoms can limit the utility of these systems. In this study, we demonstrate that an interplay of competing quantum effects controls the high triplet quantum yield in a prototypical boron dipyrromethene-anthracene (BD-An) donor-acceptor dyad photosensitizer, which is only captured by an accurate treatment of both inner and outer sphere reorganization energies. Our ab initio-derived model provides excellent agreement with experimentally measured spectra, triplet yields and excited state kinetic data, including the triplet lifetime. We find that rapid triplet state formation occurs primarily via high-energy triplet states through both spin-orbit coupled charge transfer and El-Sayed's rule breaking intersystem crossing, rather than direct spin-orbit coupled charge transfer to the lowest lying triplet state. Our calculations also reveal that competing effects of nuclear tunneling, electronic state recrossing, and electronic polarizability dictate the rate of non-productive ground state recombination. This study sheds light on the quantum effects driving efficient triplet formation in the BD-An system, and offers a promising simulation methodology for diverse photochemical systems.

Acceleration of Nonradiative Charge Recombination Reactions at Larger Distances in Kinked Donor-Bridge-Acceptor Molecules.

Photoinduced electron transfer in donor-bridge-acceptor (D-B-A) molecular systems can occur via tunneling over long distances (rDA) of well over 10 Å. We commonly observe decreasing rates of electron transfer with increasing distances, a result of a decrease in the electronic coupling of the donor and acceptor moiety. In the study of D-B-A molecules with Ru(bpy)32+ as a bridge/core, Kuss-Petermann and Wenger observed the opposite trend (J. Am. Chem. Soc.2016, 138, 1349); a maximum rate constant of electron transfer was observed at an intermediate electron transfer distance. Within the high-temperature limit of the classical Marcus equation, their observation was qualitatively explained by a sharp distance dependence of outer sphere (or solvent) reorganization energy, as predicted by Sutin and co-workers (J. Am. Chem. Soc.1984, 106, 6858), and almost distance-independent electronic couplings. Here, we report another example of such an underexplored behavior with three kinked D-B-A systems of rDA ∼ 10-19 Å, showing increasing rates of nonradiative charge recombination with increasing rDA. The three D-B-A systems are based on boron dipyrromethene and triphenylamine as electron acceptor and donor groups, respectively, with aryl bridges where the donor and acceptor moieties are connected at meso-positions. These D-B-A molecules exhibit radiative electron transfer reactions (or charge-transfer emission), which enables us to experimentally determine the solvent reorganization energy and the electronic couplings. The analysis of charge-transfer emission that explicitly considers electron-vibration coupling, in conjunction with the temperature-dependent analysis and computational method, revealed that the solvent reorganization energy indeed increases with distance, and at the same time, the electronic coupling decreases with distance expectedly. Therefore, under the right conditions for solvent reorganization energy and electronic coupling values, our results show that we can observe the acceleration of electron transfer reactions with increasing distance, even when we have the expected distance dependence of electronic coupling. This work indicates that the acceleration of electron transfer with increasing distance may be achieved with a fine-tuning of molecular design.

What Are We Missing by Not Measuring the Net Charge of Proteins?

The net electrostatic charge (Z) of a folded protein in solution represents a bird's eye view of its surface potentials-including contributions from tightly bound metal, solvent, buffer, and cosolvent ions-and remains one of its most enigmatic properties. Few tools are available to the average biochemist to rapidly and accurately measure Z at pH≠pI. Tools that have been developed more recently seem to go unnoticed. Most scientists are content with this void and estimate the net charge of a protein from its amino acid sequence, using textbook values of pKa . Thus, Z remains unmeasured for nearly all folded proteins at pH≠pI. When marveling at all that has been learned from accurately measuring the other fundamental property of a protein-its mass-one wonders: what are we missing by not measuring the net charge of folded, solvated proteins? A few big questions immediately emerge in bioinorganic chemistry. When a single electron is transferred to a metalloprotein, does the net charge of the protein change by approximately one elementary unit of charge or does charge regulation dominate, that is, do the pKa values of most ionizable residues (or just a few residues) adjust in response to (or in concert with) electron transfer? Would the free energy of charge regulation (ΔΔGz ) account for most of the outer sphere reorganization energy associated with electron transfer? Or would ΔΔGz contribute more to the redox potential? And what about metal binding itself? When an apo-metalloprotein, bearing minimal net negative charge (e.g., Z=-2.0) binds one or more metal cations, is the net charge abolished or inverted to positive? Or do metalloproteins regulate net charge when coordinating metal ions? The author's group has recently dusted off a relatively obscure tool-the "protein charge ladder"-and used it to begin to answer these basic questions.

Heterogeneous Rate Constants of the Electron-Transfer of Iron- and Ruthenium-bipyridine Complexes in Imidazolium-Based Ionic Liquids

The heterogeneous rate constants (ks) for the oxidation of iron- and ruthenium-bipyridine complexes, [Fe(bpy)3]²⁺/³⁺ and [Ru(bpy)3]²⁺/³⁺ have been determined using Nicholson's method in imidazolium-based ionic liquids with five different types of cation/anion over a range of temperature from 298–318 K. The heterogeneous rate constants of these redox reactions range from 10⁻⁴ to 10⁻³ cm s⁻¹, depending on the dynamic viscosity and type of cation/anion of the ILs. Marcus-Hush theory is used to explain the activation energies, Ea. Similar activation energies are found for the Fe-bipyridine and Ru-bipyridine complexes indicating that inner-sphere reorganization energy has only a small influence. For the calculation of the solvent-dependent outer sphere reorganization energy a dipole-free expression is used, adopted to the different dielectric properties of ionic liquids compared to organic solvents. i-Ru drops have been compensated using the corresponding experimental uncompensated Ru-values obtained from temperature dependent Bode-plots.

On the Control of Electron Transfer Rates through the Morphology of Nanoparticles

It will be shown how the morphology and dimension of inorganic nanostructures in a solution should affect the rates of outer sphere electron transfer for nearby redox ions much differently than is predicted for a macroscopic planar interface. Simple Marcus theory and electrostatic evaluations of the self-energy of the ions near these nanostructures are used to calculate this effect, which is controlled by the degree to which the environment about the ion is the high dielectric permittivity of the solid rather than solution. A higher degree results in a lower reorganization energy for the ion and a smaller activation energy for electron transfer. The extreme of interest to this discussion is that of a redox ion within a spherical or cylindrical nanocavity of an inorganic solid.It has long been known how simple Marcus theory predicts electron transfer rates for donor and acceptor couples as a function of the difference in the free energy of the reaction and experimentation1 has confirmed the predicted parabolic form of transfer rates and the “inverted region,” where rates slow down. Variations in the outer sphere reorganization energies of donor-acceptor couples also result in changes of charge transfer rates predicted by this theory and also indicate this decrease in transfer rates.2 Recently, it has been demonstrated how the temperature dependence of electron transfer rates for a specific donor-acceptor couples also describes this parabolic behavior according to theory.3 This presentation on nanostructures and outer sphere electron transfer explores how the dielectric permittivity within Marcus theory may be an additional variable to control rates of electron transfer reactions.Geometric configurations will be examined in this work in the limits where the radius of the nanocavity of the solid varies from large compared to that of the redox ion within it to where it approaches the size of the redox ion. The electron transfer will be calculated as a self-exchange rate for reduced and oxidized forms of a redox couple to avoid considerations of electron exchange with the solid. To illustrate the principles involved, the solution will be taken as acetonitrile and the solid as ZnO. Spherical nanoshells will also be considered to illustrate how the same redox ion can have an accelerated electron transfer rate within the cavity while its rate for ions outside the shell slows down.4

Asn47 and Phe114 modulate the inner sphere reorganization energies of type zero copper proteins.

The geometric structures and electron transfer properties of type 1 Cu proteins are reasonably understood at the molecular level (E. I. Solomon and R. G. Hadt, Coord. Chem. Rev., 2011, 255, 774-789, J. J. Warren, K. M. Lancaster, J. H. Richards and H. B. Gray, J. Inorg. Biochem., 2012, 115, 119-126). Much understanding of type 1 copper electron transfer reactivity has come from site directed mutagenesis studies. For example, artificial "type zero" Cu-centres constructed in cupredoxin-azurin have showcased the capacity of outer-sphere hydrogen bonding networks to enhance Cu II/I electron transfer reactivity. In this paper, we have elaborated on earlier kinetics and electronic structural studies of type zero Cu by calculating the inner sphere reorganization energies of type 1, type 2, and type zero Cu proteins using density functional theory (DFT). Although the choice of density functionals for copper systems is not straightforward, we have benchmarked the density functionals against the recently reported ESI-PES data for two synthetic copper models (S. Niu, D.-L. Huang, P. D. Dau, H.-T. Liu, L.-S. Wang and T. J. Ichiye, Chem. Theory Comput., 2014, 10, 1283). For the Cu proteins, our calculations predict that changes in the coordination number upon metal reduction lead to large inner sphere reorganization energies for type 2 Cu sites, whereas retention in the coordination number is observed for type zero Cu sites. These variations in the coordination number are modulated by the outer-sphere coordinating residues Asn47 and Phe114, which are involved in hydrogen bonding with the Asp112 side chain.

Estimation of charge carrier mobility in amorphous organic materials using percolation corrected random-walk model

A computationally simple yet predictive multi-scale simulation scheme is introduced to estimate zero-field charge mobilities for amorphous OSC materials. A percolating charge model is utilized to describe inhomogeneity of hopping trajectories of carriers in amorphous media. The prediction scheme is composed of the following stages: quantum chemical calculation of Marcus inner sphere reorganization energies, molecular dynamics simulations of the amorphous condensed phase bulk structure, automated quantum chemical calculations of the electronic coupling for dimer pairs in the amorphous solid, and calculation of the Marcus theory charge hopping rates and an estimated bulk mobility using the Einstein relation, corrected for the inhomogeneous hopping network of the solid. Comparisons with independent experimental measurements of hole mobility for ten OSC compounds show that this approach gives good correlation between predictions and measurements suitable for ranking systems, and useful quantitative agreement. This low-cost model with minimal complexity is well-suited for incorporation into a virtual materials discovery framework for advanced OSC solutions.

Influence of a nearby substrate on the reorganization energy of hole exchange between dye molecules.

A numerical method is presented to estimate the influence of a nearby substrate on the polarization energy and outer sphere reorganization energy (λo) for intermolecular hole transfer for a series of dye molecules. The calculation considers the net charge distribution of the oxidised molecule (determined from quantum chemical calculation of the highest occupied molecular orbital of the neutral molecule within the frozen orbital approximation) encapsulated within a conformal cavity, by the molecules total electron density. An analytical point charge approximation was used at longer range. The molecular cavity was either surrounded by a single polarizable continuum, or, to simulate a nearby substrate, embedded at different positions relative to the interface between two semi-infinite slabs with different dielectric constants. The calculated λo values in the single dielectric medium were linearly related to the outer-sphere reorganisation energy calculated from DFT with a polarizable continuum model, validating the approach. In the two phase system, variations in λo was sensitive to the position of the substrate relative to the molecule and differences in the Pekar factor (1/εo - 1/εr) for the media. For dye molecules in ACN positioned touching a TiO2 substrate λo was typically about 20% lower than in pure ACN depending on the molecular configuration. Our approach can be adapted to systems of more than two media.

The reorganization energy of intermolecular hole hopping between dyes anchored to surfaces

We measured the rate of hole hopping between dye molecules on titanium dioxide nanocrystals using cyclic voltammetry. Dyes commonly used in the field of dye sensitized solar cells exhibited efficient intermolecular charge transport, showing apparent diffusion coefficient values between 10−8 up to over 10−7 cm2 s−1 at room temperature. From temperature dependent measurements, we observed that hole transport across dye monolayers is a thermally activated process with Arrhenius activation energies between about 170 and 370 meV depending on the dye. Analysis of the data in terms of non-adiabatic Marcus theory of charge transfer enabled the estimation of the reorganization energy (740–1540 meV) and of an effective electronic coupling for the different systems. The measured reorganization energies show reasonable agreement with values obtained from density functional theory based calculations, validating our computational approach. Finally, we interpret the experimental and calculated data with reference to the chemical structure of the dyes and to the packing of the dyes on the surface of the TiO2 and suggest that delocalization of the HOMO and rigidity of the conjugated molecular structure result respectively in lower outer and inner sphere reorganization energies.

A quantum-chemistry study of novel copper- and cobalt-complex based redox mediators for dye-sensitized solar cells

Novel redox mediators are necessary in order to increase the efficiency and lifetime of dye-sensitized solar cells. In this work, complex copper and cobalt ions with phenanthroline and bipyridine ligands are studied in the quantum chemical aspect. New complexes with cyano bipyridine derivatives are studied. The redox potentials of the complexes and inner sphere reorganization energies are calculated.

Molecular origin of differences in hole and electron mobility in amorphous Alq3—a multiscale simulation study

In order to determine the molecular origin of the difference in electron and hole mobilities of amorphous thin films of Alq(3) (meridional Alq(3) (tris(8-hydroxyquinoline) aluminium)) we performed multiscale simulations covering quantum mechanics, molecular mechanics and lattice models. The study includes realistic disordered morphologies, polarized site energies to describe diagonal disorder, quantum chemically calculated transfer integrals for the off-diagonal disorder, inner sphere reorganization energies and an approximative scheme for outer sphere reorganization energies. Intermolecular transfer rates were calculated via Marcus-theory and mobilities were simulated via kinetic Monte Carlo simulations and by a Master Equation approach. The difference in electron and hole mobility originates from the different localization of charge density in the radical anion (more delocalized) compared to the radical cation (more confined). This results in higher diagonal disorder for holes and less favourable overlap properties for the hole transfer integrals leading to an overall higher electron mobility.

Electronic Structural Effects in Self-Exchange Reactions

We report rate constants for electron self-exchange reactions of a series of trinuclear ruthenium clusters substituted with three pyridines [Ru(3)O(OAc)(6)(L)(3)](+/0) or one carbonyl and two pyridines [Ru(3)O(OAc)(6)(CO)(L)(2)](0/-). For the 0/- couple in the latter series, the observed rate constant is determined by orbital overlap. More electron-withdrawing pyridine ligands increase the donor-acceptor overlap and more effectively reduce a large reorganization barrier (λ ≈ 10,000 cm(-1)), leading to faster exchange. Larger aromatic ligands also increase the rate by increasing the coupling. For the +/0 couple in tris-pyridyl clusters, there is no observable trend based on pyridine electron-withdrawing ability, and we conclude that charge density on peripheral ligands is not a determining factor for this exchanging pair. From structural and vibrational data, the inner sphere reorganization barrier, λ(is), is estimated to be 1520 cm(-1) and outer sphere reorganization energy, λ(os) is estimated to be between 1800 and 3600 cm(-1) in CD(2)Cl(2). The large range is due to uncertainty in the electron transfer distance, r. λ(tot) for the +/0 pair is then estimated to be between 3320 and 5120 cm(-1), well less than the 10,000 cm(-1) estimated for the 0/- pair. The similar rate constants observed despite very different reorganization energies is explained by donor-acceptor orbital overlap in the 0/- pair, which is consistent with greater delocalization allowed by donor-acceptor orbital symmetry.

A density functional theory study on π-nucleophilicity and electron-transfer oxidation of silyl enol ethers and ketene silyl acetals

Conceptual density functional theory based reactivity descriptors are used to study nucleophilic reactivity of silyl enol ethers and ketene silyl acetals having substituents of both electron releasing and electron withdrawing nature at various key positions of the molecules. While intramolecular reactivity towards electrophilic attack is studied on the basis of local softness ( s k - ) and local nucleophilicity index ( N k ), intermolecular reactivity is assessed using group softness ( s g - ) and group nucleophilicity ( N g ). Ionization potential, inner sphere reorganization energy and structural changes associated with single electron-transfer oxidation for the selected trimethylsilyl enolates have been evaluated. Role of β-methyl substituents are found to be crucial for both nucleophilic reactivity and single electron-transfer oxidation of these systems.

Experimental and theoretical spectroscopic studies of ortho derivatives of methyl p-dimethylaminobenzoate

Steady-state spectroscopic studies of two ortho (–OCH 3 and –OH) derivatives of methyl p-dimethylaminobenzoate have been performed. The absorption spectra of molecules under study are analyzed taking into consideration results of quantum chemical semiempirical calculations. The fluorescence spectra of these molecules possess in polar solvents two bands, i.e., the locally excited and intramolecular charge transfer (ICT) fluorescence band. Their intensity ratio as well as the fluorescence/phosphorescence intensity ratio determined at 77 K depends on the solvent polarity. Theoretical studies of the TICT phenomenon have been made in order to explain the dual emission of molecules under study. Calculated electric dipole moments of these molecules in the ground, S 0, and excited, S 1(LE) and S 1(ICT) states, have been compared with experimentally determined data. Theoretically determined transition energies, Δ E i , oscillator strengths, f i , and electric dipole moments μ S 0 , μ S 1 ( LE ) and μ S 1 ( ICT ) of the planar donor–acceptor (D/A) conformer agree with experimental data. According to Marcus theory inner- and outer-solvatation sphere reorganization energies ( λ out, λ in) are calculated using the determined spectroscopic properties of molecules under study.

Kinetic and spectroscopic observations on the azidyl, [formula omitted], radical oxidation of fac-(L spectator)Re I(CO) 3(L acceptor) to fac-(L spectator)Re II(CO) 3(L acceptor), L spectator = 4,4′-bpy; L acceptor = dipyridyl[3,2-a:2′3′-c]phenazine or L spectator = Cl −; L acceptor = bathocuproindisulfonate: A revisitation to the self-exchange rate constants of the [formula omitted] and Re(II)/Re(I) couples and to the redox potential

The oxidation of two triscarbonyl fac-(L spectator)Re I(CO) 3(L acceptor) z complexes (L spectator = 4,4′-bpy; L acceptor = dipyridyl[3,2-a:2′3′-c]phenazine (dppz) and z = + or L spectator = Cl −; L acceptor = bathocuproinedisulfonate (bcds 2−) and z = 2-) by azidyl radicals, N 3 , was investigated by pulse radiolysis. Reaction rate constants were determined for the electron transfer reactions between the Re(II) products and reductants, Ru ( bipy ) 3 3 + and Ni(Me 6-[14]dieneN 4) 2+, and used for the calculation of the self-exchange rate constant of the Re(II)/Re(I) couples. The self-exchange rate constants, k ∼ 10 7 M −1 s −1, were one order of magnitude larger than the constant, k ∼ 10 6 M −1 s −1, communicated in the literature for the [Re(DMPE) 3] +/2+ (DMPE = 1,2-bis(dimethylphosphine)ethane). The larger rate constants of the triscarbonyl complexes are in agreement with the smaller inner sphere reorganization energy of the complexes relative to [Re(DMPE) 3] +/2+. Moreover, the study demonstrated that the redox potential of the azidyl radicals is E N 3 / N 3 - 0 = 1.70 V versus NHE , a value larger than one communicated earlier, and that the self-exchange rate constant of the N 3 / N 3 - couple is k N 3 / N 3 - = 2.7 × 10 6 M - 1 s - 1 . The small value of the N 3 / N 3 - self-exchange rate constant has been related to the large solvent reorganization energy of the reaction.

Effects of friction and asymmetric inner sphere reorganization energy on the electron transfer reaction rate—Two-dimensional simulations

Molecular dynamic simulations are applied to study inner sphere reorganization effects on outer-sphere electron transfer (ET) reactions. The system studied is assumed to be a complex of the form A(H 2O) 6. The Hamiltonian describing the one-electron reduction reaction is based on the Schmickler and Koper model [W. Schmickler, M.T.M. Koper, Electrochem. Commun. 1 (1999) 402]. The resulting potential energy is a function of the generalized solvent coordinate and the change in the A–O distance. The first hydration shell in the oxidized and reduced states is described by two harmonic oscillators with frequencies ω 1 and ω 2. Three cases are studied in the simulations with respect to the frequencies ratio θ = ω 2/ ω 1: 2/3, 1 and 3/2. An effect of different friction parameters applied to the two coordinates and the activation energy on the reaction rate is analyzed and compared for the three cases. A strong decrease in the reaction rate is observed when a very low friction in the inner sphere direction is assumed. The turnover for the three cases is found to increase when θ goes from 2/3 to 3/2, but it corresponds to the friction parameter which is much lower than the barrier frequency. The relation between the saddle point avoidance phenomenon and the reaction rate is qualitatively analyzed.

ESR-Investigations on the Dynamic Solvent Effects of Degenerate Electron Exchange Reactions. Part I: Cyanobenzenes

Summary The rates of degenerate electron exchange (electron self-exchange) of various cyanobenzenes have been measured by EPR line broadening technique in nine different solvents at room temperature. The molecules studied comprise besides benzene-1,2-dicarbonitrile, benzene-1,4-dicarbonitrile and benzene-1,2,4,5-tetracarbonitrile, the two isomeric tricyanobenzenes, benzene-1,2,3-tricarbonitrile and benzene-1,2,4-tricarbonitrile, the anion radicals of which have not been characterized before. The experimentally observed rates vary from 4.5 × 108 to 44.0 × 108 M−1 s−1 and show the pronounced dependence on the longitudinal relaxation times, τL, of the solvents. The solvent dynamical effect so manifested is confirmed with remarkable clarity using solvents spanning a wide range of τL-values, which comprise acetonitrile (0.2 ps) and o-dichlorobenzene (6.0 ps) at its extremes. The rate constants are compared with Marcus theory using the continuum model (CM) and the mean spherical approximation (MSA) for the outer sphere reorganization energies and Nelson’s method for the inner sphere reorganization energies. Furthermore, an estimation of the resonance splitting energies, V RP, is given based on the experimental rates.

The development of a PM3 parameter set to describe iron–sulfur proteins

A PM3 parameter set for iron has been developed, which is appropriate for the active site of iron sulfur proteins having a single iron atom by fitting DFT data obtained for a redox site analogue. These parameters are then tested on a set of such analogues involving a variety of ligands, and show good agreement with both DFT and experimental data for these species. The use of these parameters within a two level ONIOM treatment of the protein rubredoxin, yields accurate predictions of the effect of the enzyme on both Fe–S bond lengths and inner sphere reorganization energies.

Slow electron transfer rates for fluorinated cobalt porphyrins: electronic and conformational factors modulating metalloporphyrin ET.

The electron transfer (ET) properties of a series of closely related cobalt porphyrins, [2,3,7,8,12,13,17,18-octafluoro-5,10,15,20-tetrakis(pentafluorophenyl)porphyrinato]cobalt, CoF(28)TPP, [2,3,7,8,12,13,17,18-octafluoro-5,10,15,20-tetraphenyl)porphyrinato]cobalt, CoF(8)TPP, 5,10,15,20-tetrakis(pentafluorophenyl)porphyrinato]cobalt, CoF(20)TPP, and [5,10,15,20-tetraphenylporphyrinato]cobalt, CoTPP, were investigated by cyclic voltammetry, cyclic voltammetric digital simulation, in situ UV-vis and IR spectroelectrochemistry, kinetic ET studies, bulk electrolysis, (19)F NMR spectroscopy, X-ray crystallography, and molecular modeling. In benzonitrile containing 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF(6)) as supporting electrolyte, the ET rate constants for the Co(2+/3+) redox couples were found to be strongly substituent dependent; the heterogeneous ET rate constant (k(el)) varied by a factor of 10(4), and the ET self-exchange rate constants (k(ex)) varied over 7 orders of magnitude for the compounds studied. The remaining observed ring oxidation and metal and ring reduction events exhibited nearly identical k(el) values for all compounds. UV-vis and IR spectroelectrochemistry, bulk electrolysis, and (19)F NMR spectroscopic studies support attribution of different ET rates to widely varying inner sphere reorganization energies (lambda(i)) for these closely related compounds. Structural and semiempirical (PM3) studies indicate that the divergent kinetic behavior of CoTPP, CoF(8)TPP, CoF(20)TPP, and CoF(28)TPP first oxidations arises mainly from large nuclear reorganization energies primarily associated with core contraction and dilation. Taken together, these studies provide rational design principles for modulating ET rate constants in cobalt porphyrins over an even larger range and provide strategies for similar manipulation of ET rates in other porphyrin-based systems: substituents that lower C-C, C-N, and N-M vibrational frequencies or minimize porphyrin orbital overlap with the metal-centered orbital undergoing a change in electron population will increase k(ET). The heme ruffling apparent in electron transfer proteins such as cytochrome c is interpreted as nature's exploitation of this design strategy.

Electron transfer reactions of iron(III)-polypyridyl complexes with organic sulfoxides

The redox reactions of four iron(III)-polypyridyl complexes with six aryl methyl sulfoxides have been investigated by spectrophotometric technique. The reaction follows clean second order kinetics and proceeds through rate determining electron transfer (ET) from organic sulfoxides to iron(III). The Marcus cross-reaction relation has been applied to obtain the self exchange rate constant for the ArSOR/ArS + (O)R couple as 1.3×10 7 M −1 s −1. The application of Marcus theory to this ET reaction shows that the contribution of inner sphere reorganization energy is 0.4 eV. The rate constant and reaction constant values observed with organic sulfoxides are small compared with organic sulfides towards the same oxidant Fe(NN) 3 3+.