Electronic Coupling Factor Research Articles

Effective electronic properties and coupling for two-temperature model-molecular dynamics simulation of ultrafast laser ablation of nickel

ABSTRACT The accuracy of hybrid two temperature model-molecular dynamics (TTM-MD) simulations in predicting the ultrafast laser ablation phenomena in metals is influenced by the functional definition of subsystem properties and electronic-lattice coupling. In this work, laser ablation of nickel is simulated using hybrid TTM-MD with empirical and density function theory (DFT) derived electronic and coupling parameter sets. The investigation revealed that the empirically derived temperature-dependent parameterisation of electronic specific heat, thermal conductivity, and coupling factor enhanced the fidelity of electron–phonon nonequilibrium dynamics when used in conjunction with simple optical absorption models, aligning closely with prior experimental observations for nickel specimens. The TTM-MD setup is then coupled utilising two widely used coupling techniques – the temperature difference scaled and Langevin thermostat. It is found that the former exhibits increased tensile stress due to artificial enhancement of the collision cascade owing to deterministic nature of the coupling force. While the probabilistic nature of the Langevin thermostat offers more accurate predictions of the ablation threshold/yield. Under these conditions, the TTM-MD scheme is tested for a range of absorbed laser fluence, with ablation threshold, and the onset of phase explosion is determined as 115 and 270 mJ/cm2, respectively for a 1 picosecond (ps-) laser pulse.

Long-Range Electron Tunneling from the Primary to Secondary Quinones in Photosystem II Enhanced by Hydrogen Bonds with a Nonheme Fe Complex.

The mechanisms governing the long-range electron tunneling from the primary (QA) to secondary (QB) quinones in photosystem II are clarified by analyzing superexchange pathways through a nonheme Fe complex, using a quantum mechanics/molecular mechanics/polarizable continuum model approach. The electron tunneling rate is evaluated using the Marcus-Levich-Jortner theory considering electronic coupling, energy difference, and Franck-Condon factor. The superexchange QA → QB electron tunneling is enhanced by hybridized σ/σ* orbitals of histidines (D2-His214 and D1-His215) via penetration of the wave function into hydrogen bonds with both QA and QB. Despite a large energy gap to the intermediate states, the contributions of the histidine σ/σ* orbitals to the superexchange coupling are larger than those of π/π* orbitals. Fe2+ is not an essential component for the QA → QB electron tunneling because hybridized histidine molecular orbitals can be coupled with both QA and QB simultaneously in the absence of Fe d orbitals.

The Significant Effect of Electronic Coupling on Electron Transfer between Surface-Bound Porphyrins and Co2+/3+ Complex Electrolytes

The coordination of metal ions such as Zn2+ with the π-conjugated porphyrin ring is one of the simplest and the most often used structural modification to alter the electronic nature of a molecule without significantly affecting its size or geometry. The question answered in this work is to what extent electron transfer rates between a surface bound Zn2+porphyrin and a cationic electron donor (Co2+ polypyridyl complexes) is affected by the altered electrostatic interaction between the two molecules at the same electrochemical driving force (DG°). To date, very few studies addressed the effect of electrostatic intermolecular interactions on electron transfers explicitly.We measured electron transfer rates between 4 porphyrin (2 freebase and 2 Zn porphyrin) molecules attached to TiO2 electrodes and 5 Co2+/3+ complexes dissolved in electrolytes covering a DG° range of close to 1 V. The porphyrin molecules were photo-excited at 532 nm leading to an oxidized porphyrin and an electron injected into the conduction band of the TiO2 electrode. The rate of electron transfer was been determined using transient absorption spectroscopy (TAS) by monitoring the absorption change of the oxidized porphyrin molecules in the presence of the redox mediators in various concentrations. The measured rate constants were fitted to Marcus theory. We have found that even the simplest dimethyl substitution on the bipyridine ligands of the Co2+/3+ complexes resulted in a significant decrease in electron coupling. To account for the variation in electron coupling among the Co2+/3+ complexes, the electron rate constants were normalized by the square of the ratio of the electronic coupling factors. The normalized rate constants showed a 30% increase in electron coupling of the free base porphyrins with Co2+/3+ compared to the electronic coupling between Zn porphyrins and Co2+/3+. Increased coupling suggests shorter donor – acceptor distance or better wavefunction overlap, which could be due to electrostatic interaction of the Co2+ with the lone pairs of the free base porphyrin, which interaction is blocked by Zn2+ coordination. The measurements were also performed in 1,2-dimethoxyethane instead of acetonitrile with a five times lower dielectric constant, resulting in even larger difference in electron transfer rates between free base and Zn porphyrins. This suggests that the origin of the different coupling is indeed electrostatic in nature.The key finding of this work is that electronic coupling can be as significant contributor to electron transfer rates as the driving force even between similar molecular structures, hence redox active molecules should not be selected based on their free energy difference alone.

Plasmon-Coupled Resonance Energy Transfer II: Exploring the Peaks and Dips in the Electromagnetic Coupling Factor

We study resonance energy transfer between a donor–acceptor pair located on opposite sides of a spherical silver nanoparticle and explore the dependence of energy-transfer rate on nanoparticle size using a quantum electrodynamics theory we developed previously. This theory indicates that the rate is determined by the product of donor emission spectra, acceptor absorption spectra, and an electronic coupling factor (CF) that is determined by electrodynamics associated with the donor as a dipole emitter near the nanoparticle. We find that the CF spectra show peaks that are associated with localized surface plasmon resonances, but the locations of the most significant peaks are less correlated to the size of the nanoparticle than is found for extinction spectra for the same particle. For small nanoparticles (≲30 nm), where dipole plasmon excitation dominates, a quasi-static analysis leads to an analytical formula, in which the CF peaks and dips involve interference between donor electric field and the scattered dipolar field of the nanoparticle. For larger nanoparticles (60–210 nm), the CF maximizes at a wavelength near 355 nm independent of particle size that is determined by the highest multipole plasmon that contributes significantly to the extinction spectrum, with only small contributions arising from lower multipole plasmons, such as the dipole plasmon. Also, for wavelengths near 325 nm where the bulk plasmon resonance of silver can be excited, surface plasmons cannot be excited, so excitation from the donor cannot be transmitted by surface plasmons to the acceptor, leading to a pronounced dip in the CF. This work provides new concepts concerning plasmon-mediated energy transfer that are quite different from conventional (Förster) theory, but which should dominate energy-transfer behavior when donor and acceptor are sufficiently separated.

Electron-Transfer Dynamics in a Zn-Porphyrin-Quinone Cyclophane: Effects of Solvent, Vibrational Relaxations, and Conical Intersections.

Rate constants for photochemical charge separation and recombination in a zinc-porphyrin-benzoquinone cyclophane are calculated by an approach that was developed recently to include effects of vibrational dephasing and relaxation and to reduce the dependence on freely adjustable parameters. The theory is extended to treat the rate of vibrational relaxation individually for each vibrational sublevel of the initial charge-transfer product. Quantum-mechanical/molecular-mechanical simulations of the reactions in iso-octane, toluene, dichloromethane, and acetonitrile suggest that charge separation occurs at conical intersections in the two more polar solvents, but at avoided crossings in the nonpolar solvents. In agreement with experimental measurements, however, the calculated rate constants for charge separation are similar in polar and nonpolar solvents. Charge recombination to the ground state is found to have electronic coupling factors smaller than that of charge separation and to be affected more strongly by interactions with the solvent.

Charge-transfer excited state in pyrene-1-carboxylic acids adsorbed on titanium dioxide nanoparticles

The electronic structure of excited photosensitizer adsorbed at the surface of a solid is the key factor in the electron transfer processes that underlie the efficiency of dye-sensitized solar cells and photocatalysts. In this work, Stark effect (electroabsorption) spectroscopy has been used to measure the polarizability and dipole moment changes in electronic transitions of pyrene-1-carboxylic (PCA), -acetic (PAA) and -butyric (PBA) acids in ethanol, both free and adsorbed on colloidal TiO2, in glassy ethanol at low temperature. The lack of appreciable increase of dipole moment in the excited state of free and adsorbed PAA and PBA points that two or more single bonds completely prevent the expansion of π-electrons from the aromatic ring towards the carboxylic group, thus excluding the possibility of direct electron injection into TiO2. In free PCA, the pyrene's forbidden S0→S1 transition has increased intensity, exhibits a long progression in 1400cm−1 Ag mode and is associated with |∆μ| of 2 D. Adsorption of PCA on TiO2 causes a broadening and red shift of the S0→S1 absorption band and an increase in dipole moment change on electronic excitation to |∆μ|=6.5 D. This value increased further to about 15 D when the content of acetic acid in the colloid was changed from 0.2% to 2%, and this effect is ascribed to the surface electric field. The large increase of |∆μ| points that the electric field effect can not only change the energetics of electron transfer from the excited sensitizer into the solid, but can also shift the molecular electronic density, thus directly influencing the electronic coupling factor relevant for electron transfer at the molecule-solid interface.

Guest to framework photoinduced electron transfer in a cobalt substituted RWLC-2 metal organic framework.

Photoinduced electron transfer (PET) between donors and acceptors in porous materials is a key element in the development of light harvesting applications. Metal organic frameworks (MOFs) are ideal materials for PET processes due to their tunable pore size and diversity in framework building units. Here, PET between excited [Ru(2,2'-bipyridine)3]2+ (RuBpy) and Co-carboxylate clusters composing the metal building blocks of a RWLC-2 metal organic framework is described. The lifetime of the RuBpy decreases from ∼600 ns in deaerated solution to 9.5 ns (kET ∼ 1 × 108 s-1) when encapsulated within CoRWLC-2. The decrease in lifetime is attributed to PET between the 3MLCT of RuBpy to the Co ion cluster composing the MOF building blocks. A fit of the lifetime vs. temperature data to the semi-empirical Marcus equation gives a reorganizational energy of 1.6 eV and an electronic coupling factor (HAB) of 0.006 eV.

Precise tuning of single molecule conductance in an electrochemical environment.

Tuning of molecular conductance in a liquid environment is a hot topic in molecular electronics. In this article, we explore a new concept where the Fermi level positions of the metallic ends are varied simply by modifying the electroactive salt concentration in solution. We rely on the electrochemical scanning tunneling microscope break junction method that allows the construction in solution of copper atomic contacts that can be then bridged by single molecules. The experimental conductance evolution is first confronted with an analytical formulation that allows the deduction of the molecule's LUMO position and electronic coupling factors. These parameters are in close agreement with those obtained by independent DFT calculations.

Theory and calculation for the electronic coupling in excitation energy transfer

Excitation energy transfer (EET) is a process where the electronically excitation is transferred from a donor to an acceptor. EET is widely seen in both natural and in artificial systems, such as light-harvesting in photosynthesis, the fluorescence resonance energy transfer technique, and the design of light-emitting molecular devices. In this work, we outline the theories describing both singlet and triplet EET (SEET and TEET) rates, with a focus on the physical nature and computational methods for the electronic coupling factor, an important parameter in predicting EET rates. The SEET coupling is dominated by the Coulomb coupling, and the remaining short-range coupling is very similar to the TEET coupling. The magnitude of the Coulomb coupling in SEET can vary much, but the contribution of short-range coupling has been found to be similar across different excited states in naphthalene. The exchange coupling has been believed to be the major physical contribution to the short-range coupling, but it has been pointed out that other contribution, such as the orbital overlap effect is similar or even larger in strength. The computational aspects and the subsequent physical implication for both SEET and TEET coupling values are summarized in this work. © 2013 The Authors. International Journal of Quantum Chemistry Published by Wiley Periodicals, Inc.

Looking for the peptide 2.05‐helix: A solvent‐ and main‐chain length‐dependent conformational switch probed by electron transfer across cα,α‐diethylglycine homo‐oligomers

The elusive, multiple fully extended (2.0(5) -helix) peptide conformation was searched with a series of C(α,α) -diethylglycine homo-oligomers (n = 1 to 5) functionalized by an electron transfer (ET) donor···acceptor (D···A) pair in acetonitrile and chloroform solutions. In the former solvent, all peptides investigated were shown to populate the 3(10) -helix conformation, whereas in chloroform the two shortest members of the series (n = 1 and n = 2) adopt predominantly the 2.0(5) -helix. Interestingly, for the longest components (n = 3 to n = 5) in this latter solvent, an equilibrium between the 2.0(5) - and 3(10) -helices takes place, the latter conformation becoming progressively more populated as the peptide main-chain length increases. Time-resolved fluorescence (TRF) experiments and molecular mechanics (MM) calculations were used in a combined approach to analyze the ET efficiencies and to associate a specific conformer (from MM) to an experimentally determined ET rate constant (from TRF). Therefore, because of the high sensitivity of the ET process to the D···A distance, ET can be used as a kinetic spectroscopic ruler, allowing for the characterization of the transition from a pure 3(10) -helix conformation to a 2.0(5) -/3(10) -helix equilibrium for the longest Deg homo-peptides of this series upon changing the solvent from acetonitrile to chloroform. To our knowledge, this is the first time that the electronic coupling factor β for ET across a peptide chain in the 2.0(5) -helix conformation is provided.

Do Molecular Conductances Correlate with Electrochemical Rate Constants? Experimental Insights

We measured single-molecule conductances for three different redox systems self-assembled onto gold by the STMBJ method and compared them with electrochemical heterogeneous rate constants determined by ultrafast voltammetry. It was observed that fast systems indeed give higher conductance. Monotonous dependency of conductance on potential reveals that large molecular fluctuations prevent the molecular redox levels to lie in between the Fermi levels of the electrodes in the nanogap configuration. Electronic coupling factors for both experimental approaches were therefore evaluated based on the superexchange mechanism theory. The results suggest that coupling is surprisingly on the same order of magnitude or even larger in conductance measurements whereas electron transfer occurs on larger distances than in transient electrochemistry.

Ab Inito Study on Triplet Excitation Energy Transfer in Photosynthetic Light-Harvesting Complexes

We have studied the triplet energy transfer (TET) for photosynthetic light-harvesting complexes, the bacterial light-harvesting complex II (LH2) of Rhodospirillum molischianum and Rhodopseudomonas acidophila, and the peridinin-chlorophyll a protein (PCP) from Amphidinium carterae. The electronic coupling factor was calculated with the recently developed fragment spin difference scheme (You and Hsu, J. Chem. Phys. 2010, 133, 074105), which is a general computational scheme that yields the overall coupling under the Hamiltonian employed. The TET rates were estimated based on the couplings obtained. For all light-harvesting complexes studied, there exist nanosecond triplet energy transfer from the chlorophylls to the carotenoids. This result supports a direct triplet quenching mechanism for the photoprotection function of carotenoids. The TET rates are similar for a broad range of carotenoid triplet state energy, which implies a general and robust TET quenching role for carotenoids in photosynthesis. This result is also consistent with the weak dependence of TET kinetics on the type or the number of π conjugation lengths in the carotenoids and their analogues reported in the literature. We have also explored the possibility of forming triplet excitons in these complexes. In B850 of LH2 or the peridinin cluster in PCP, it is unlikely to have triplet exciton since the energy differences of any two neighboring molecules are likely to be much larger than their TET couplings. Our results provide theoretical limits to the possible photophysics in the light-harvesting complexes.

Theoretical characterization of photoinduced electron transfer in rigidly linked donor–acceptor molecules: the fragment charge difference and the generalized Mulliken–Hush schemes

We calculate the electron transfer (ET) rates for a series of heptacyclo[6.6.0.02,6.03,13.014,11.05,9.010,14]-tetradecane (HCTD) linked donor–acceptor molecules. The electronic coupling factor was calculated by the fragment charge difference (FCD) [19] and the generalized Mulliken–Hush (GMH) schemes [20]. We found that the FCD is less prone to problems commonly seen in the GMH scheme, especially when the coupling values are small. For a 3-state case where the charge transfer (CT) state is coupled with two different locally excited (LE) states, we tested with the 3-state approach for the GMH scheme [30], and found that it works well with the FCD scheme. A simplified direct diagonalization based on Rust's 3-state scheme was also proposed and tested. This simplified scheme does not require a manual assignment of the states, and it yields coupling values that are largely similar to those from the full Rust's approach. The overall electron transfer (ET) coupling rates were also calculated.

Beyond Förster Resonance Energy Transfer in Biological and Nanoscale Systems

After photoexcitation, energy absorbed by a molecule can be transferred efficiently over a distance of up to several tens of angstroms to another molecule by the process of resonance energy transfer, RET (also commonly known as electronic energy transfer, EET). Examples of where RET is observed include natural and artificial antennae for the capture and energy conversion of light, amplification of fluorescence-based sensors, optimization of organic light-emitting diodes, and the measurement of structure in biological systems (FRET). Forster theory has proven to be very successful at estimating the rate of RET in many donor-acceptor systems, but it has also been of interest to discover when this theory does not work. By identifying these cases, researchers have been able to obtain, sometimes surprising, insights into excited-state dynamics in complex systems. In this article, we consider various ways that electronic energy transfer is promoted by mechanisms beyond those explicitly considered in Forster RET theory. First, we recount the important situations when the electronic coupling is not accurately calculated by the dipole-dipole approximation. Second, we examine the related problem of how to describe solvent screening when the dipole approximation fails. Third, there are situations where we need to be careful about the separability of electronic coupling and spectral overlap factors. For example, when the donors and/or acceptors are molecular aggregates rather than individual molecules, then RET occurs between molecular exciton states and we must invoke generalized Forster theory (GFT). In even more complicated cases, involving the intermediate regime of electronic energy transfer, we should consider carefully nonequilibrium processes and coherences and how bath modes can be shared. Lastly, we discuss how information is obscured by various forms of energetic disorder in ensemble measurements and we outline how single molecule experiments continue to be important in these instances.

The Electronic Couplings in Electron Transfer and Excitation Energy Transfer

The transport of charge via electrons and the transport of excitation energy via excitons are two processes of fundamental importance in diverse areas of research. Characterization of electron transfer (ET) and excitation energy transfer (EET) rates are essential for a full understanding of, for instance, biological systems (such as respiration and photosynthesis) and opto-electronic devices (which interconvert electric and light energy). In this Account, we examine one of the parameters, the electronic coupling factor, for which reliable values are critical in determining transfer rates. Although ET and EET are different processes, many strategies for calculating the couplings share common themes. We emphasize the similarities in basic assumptions between the computational methods for the ET and EET couplings, examine the differences, and summarize the properties, advantages, and limits of the different computational methods. The electronic coupling factor is an off-diagonal Hamiltonian matrix element between the initial and final diabatic states in the transport processes. ET coupling is essentially the interaction of the two molecular orbitals (MOs) where the electron occupancy is changed. Singlet excitation energy transfer (SEET), however, contains a Frster dipole-dipole coupling as its most important constituent. Triplet excitation energy transfer (TEET) involves an exchange of two electrons of different spin and energy; thus, it is like an overlap interaction of two pairs of MOs. Strategies for calculating ET and EET couplings can be classified as (1) energy-gap-based approaches, (2) direct calculation of the off-diagonal matrix elements, or (3) use of an additional operator to describe the extent of charge or excitation localization and to calculate the coupling value. Some of the difficulties in calculating the couplings were recently resolved. Methods were developed to remove the nondynamical correlation problem from the highly precise coupled cluster models for ET coupling. It is now possible to obtain reliable ET couplings from entry-level excited-state Hamiltonians. A scheme to calculate the EET coupling in a general class of systems, regardless of the contributing terms, was also developed. In the past, empirically derived parameters were heavily invoked in model description of charge and excitation energy drifts in a solid-state device. Recent advances, including the methods described in this Account, permit the first-principle quantum mechanical characterization of one class of the parameters in such descriptions, enhancing the predictive power and allowing a deeper understanding of the systems involved.

Tuning of the [Cu3(μ-O)]4+/5+Redox Couple: Spectroscopic Evidence of Charge Delocalization in the Mixed-Valent [Cu3(μ-O)]5+Species

Trinuclear Cu (II)-complexes of formula [Cu (II) 3(mu 3-E)(mu-4-R-pz) 3X 3] (+/- n ), E = O and OH; R = H, Cl, Br, CH(O) and NO 2; X = Cl, NCS, CH 3COO, and py, have been synthesized and characterized and the effect of substitution of terminal ligands, as well as 4-R-groups, in the one-electron oxidation process has been investigated by cyclic voltammetry. In situ UV-vis-NIR spectroelectrochemical characterization of the mixed valence Cu 3 (7+)-complex [Cu 3(mu 3-O)(mu-pz) 3Cl 3] (-) revealed an intervalence charge transfer band at 9550 cm (-1) (epsilon = 2600 cm (-1) M (-1)), whose analysis identifies this species as a delocalized, Robin-Day class-III system, with an electronic coupling factor, H ab, of 4775 cm (-1).

An Anderson impurity model for efficient sampling of adiabatic potential energy surfaces of transition metal complexes

We present a model intended for rapid sampling of ground and excited state potential energy surfaces for first-row transition metal active sites. The method is computationally inexpensive and is suited for dynamics simulations where (1) adiabatic states are required "on-the-fly" and (2) the primary source of the electronic coupling between the diabatic states is the perturbative spin-orbit interaction among the 3d electrons. The model Hamiltonian we develop is a variant of the Anderson impurity model and achieves efficiency through a physically motivated basis set reduction based on the large value of the d-d Coulomb interaction U(d) and a Lanczos matrix diagonalization routine to solve for eigenvalues. The model parameters are constrained by fits to the partial density of states obtained from ab initio density functional theory calculations. For a particular application of our model we focus on electron transfer occurring between cobalt ions solvated by ammonium, incorporating configuration interaction between multiplet states for both metal ions. We demonstrate the capability of the method to efficiently calculate adiabatic potential energy surfaces and the electronic coupling factor we have calculated compares well to previous calculations and experiment. (

Distance-dependent electron hopping conductivity and nanoscale lithography of chemically-linked gold monolayer protected cluster films

Films of monolayer protected Au clusters (MPCs) with mixed alkanethiolate and ω-carboxylate alkanethiolate monolayers, linked together by carboxylate–Cu 2+–carboxylate bridges, exhibit average edge-to-edge cluster spacings that vary with the numbers of methylene segments in the alkanethiolate ligand as determined by a combined atomic force microscopy (AFM)/UV-Vis spectroscopy method. The electronic conductivity ( σ EL) of dry films is exponentially dependent on the cluster spacing, consistent with electron tunneling through the alkanethiolate chains and non-bonded contacts between those chains on individual, adjacent MPCs. The calculated electronic coupling factor ( β) for tunneling between MPCs is 1.2 Å −1, which is similar to other values obtained for tunneling through hydrocarbon chains. Electron transfer rate constants measured on the films reflect the increased cluster–cluster tunneling distance with increasing chainlength. The MPC films are patterned by scanning the surface with an AFM or scanning tunneling microscopy (STM) tip under appropriate conditions. The patterning mechanism is physical in nature, where the tip scrapes away the film in the scanned region. Large forces are required to pattern films with AFM while normal imaging conditions are sufficient to produce patterns with STM. Patterns with dimensions as small as 100 nm are shown. Subsequent heating (300 °C) of the patterned surfaces leads to a metallic Au film that decreases in thickness and is smoother compared to the MPC film, but retains the initial shape and dimensions of the original pattern.

Kinetics of Intramolecular Electron Transfer in Cytochrome bo3 from Escherichia coli

We have examined the temperature dependence of the intramolecular electron transfer (ET) between heme b and heme o 3 in CO-mixed valence cytochrome bo 3 (C bo) from Escherichia coli. Upon photolysis of CO-mixed valence C bo rapid ET occurs between heme o 3 and heme b with a rate constant of 2.2 × 10 5 s −1 at room temperature. The corresponding rate of CO recombination is found to be 86 s −1. From Eyring plots the activation energies for these two processes are found to be 3.4 kcal/mol and 6.7 kcal/mol for the ligand binding and ET reactions, respectively. Using variants of the Marcus equation the reorganization energy ( λ), electronic coupling factor (H AB), and the ET distance were found to be 1.4 ± 0.2 eV, (2 ± 1) × 10 −3 eV, and 9 ± 1 Å, respectively. These values are quite distinct from the analogous values previously obtained for bovine heart cytochrome c oxidase (C cO) (0.76 eV, 9.9 × 10 −5 eV, 13.2 Å). The differences in mechanisms/pathways for heme b/heme o 3 and heme a/heme a 3 ET suggested by the Marcus parameters can be attributed to structural changes at the Cu B site upon change in oxidation state as well as differences in electronic coupling pathways between Heme b and heme o 3.

Temperature dependence of photoinduced electron transfer within self-associated porphyrin: guanine monophosphate complexes

In the current report, the temperature dependence of photoinduced electron transfer between tetrakis-(4-tetramethylpyridyl)porphine (T4MPyP) and guanine monophosphate (GMP) has been examined. In the presence of GMP the fluorescence lifetime analysis reveals a Lorentzian distribution of lifetimes centered at 0.7 ns with a width of 0.9 ns displaying significant temperature dependence. Fitting temperature dependent data to the Marcus equation gives a reorganizational energy ( λ) for the electron transfer reaction of 0.6 eV and an electronic coupling factor ( H AB) of 3×10 −3 eV . These results suggest conformational regulation of electron transfer within the non-covalent porphyrin:nucleotide complex.