Backward Electron Transfer Rate Research Articles

Photoinduced Electron Transfer between DNA and Water-Soluble Porphyrins.

Photophysical properties of five kinds of porphyrins (H2TMPyP, ZnTMPyP, PdTMPyP, H2TPPS, and ZnTPPS) complexed with model DNAs (ctDNA and dGMP) have been investigated using steady-state absorption, circular dichroism (CD), and femtosecond transient absorption spectroscopy. Upon addition of ctDNA (or dGMP), larger hypochromism and red shifts are observed for H2TMPyP and PdTMPyP compared to the other samples. The steady-state measurements have suggested that the binding modes of H2TMPyP-ctDNA and PdTMPyP-ctDNA are partial intercalation and full intercalation, respectively, while ZnTMPyP-ctDNA shows outside groove binding. No significant interaction was observed between both H2TPPS and ZnTPPS with two kinds of DNA. Upon excitation of the porphyrins into the higher excited state S2 (Soret band), the appearance of the transient absorption from ∼500 to ∼620 nm at about 0.05 ps in H2TMPyP-ctDNA, H2TMPyP-dGMP, and PdTMPyP-dGMP indicates the occurrence of the electron transfer (ET) from guanine to H2TMPyP and PdTMPyP. The forward ET are extremely fast (kf ≥ 1.0 × 1013 s-1), and the backward ET rates are ∼5.6 × 1012 and ∼4.0 × 1012 s-1, respectively. The complexation with DNA may lead to the shorter lifetime of the fluorescence of H2TMPyP and PdTMPyP.

Critical evaluation of electron transfer kinetics in P700–FA/FB, P700–FX, and P700–A1 Photosystem I core complexes in liquid and in trehalose glass

This work aims to fully elucidate the effects of a trehalose glassy matrix on electron transfer reactions in cyanobacterial Photosystem I (PS I). Forward and backward electron transfer rates from A1A− and A1B− to FX, and charge recombination rates from A0−, A1B−, A1A−, FX−, and [FA/FB]− to P700+ were measured in P700–FA/FB complexes, P700–FX cores, and P700–A1 cores, both in liquid and in a trehalose glassy matrix at 11% humidity. By comparing CONTIN-resolved kinetic events over 6 orders of time in increasingly simplified versions of PS I at 480 nm, a wavelength that reports primarily A1A−/A1B− oxidation, and over 9 orders of time at 830 nm, a wavelength that reports P700+ reduction and A0− oxidation, assignments could be made for nearly all of the resolved kinetic phases. Trehalose-embedded PS I samples demonstrated partially arrested forward electron transfer. The fractions of complexes in which electron transfer did not proceed beyond A0, A1 and FX were 53%, 16% and 22%, respectively, with only 10% of electrons reaching the terminal FA/FB clusters. The ~10 μs and ~150 μs components in both liquid and trehalose-embedded PS I were assigned to recombination between A1B− and P700+ and between A1A− and P700+, respectively. The kinetics and amplitudes of these resolved kinetic phases in liquid and trehalose-embedded PS I samples could be well-fitted by a kinetic model that allowed us to calculate the asymmetrical contribution of the A1A− and A1B− quinones to the electrochromic signal at 480 nm. Possible reasons for these effects are discussed.

Proton-Coupled Electron-Transfer Processes in Ultrafast Time Domain: Evidence for Effects of Hydrogen-Bond Stabilization on Photoinduced Electron Transfer.

The proton-coupled electron-transfer (PCET) reaction is investigated for a newly synthesized imidazole-anthraquinone biomimetic model with a photoactive RuII -polypyridyl moiety that is covalently coupled to the imidazole fragment. Intramolecular H-bonding interactions between imidazole and anthraquinone moieties favor the PCET process; this can be correlated to an appreciable positive shift in the one-electron reduction potential of the coordinated anthraquinone moiety functionalized with the imidazole fragment. This can also be attributed to the low luminescence quantum yield of the RuII -polypyridyl complex used. The dynamics of the intramolecular electron-transfer (ET) and PCET processes are studied by using femtosecond transient absorption spectroscopy. The steady-state spectroscopic studies and the results of the time-resolved absorption studies confirm that H-bonded water molecules play a major role in both ET and PCET dynamics as a proton relay in the excited state. The electron-transfer process is followed by a change in the H-bonding equilibrium between AQ and imidazole in acetonitrile solvent, and protonation of AQ.- by water leads to PCET in the presence of water. A slower forward and backward electron-transfer rate is observed in the presence of D2 O compared with that in H2 O. These results provide further experimental support for a detailed understanding of the PCET process.

Probing Driving Force and Electron Accepting State Density Dependent Interfacial Electron Transfer Dynamics: Suppressed Fluorescence Blinking of Single Molecules on Indium Tin Oxide Semiconductor.

Photoinduced, interfacial electron transfer (ET) dynamics between m-ZnTCPP and Sn-doped In2O3 (ITO) film has been studied using single-molecule photon-stamping spectroscopy. The observed ET dynamics of single m-ZnTCPP adsorbed on ITO was compared with that of m-ZnTCPP adsorbed on TiO2 NPs with and without applied electric potential. Compared to m-ZnTCPP on the TiO2 NP surface, m-ZnTCPP on the ITO surface shows a reduced lifetime as well as suppressed blinking and a quasi-continuous distribution of fluorescence intensities, presumably due to higher electron density in ITO. The higher electron density leads to the occupancy of CB acceptor states/trap states, which supports a higher backward electron transfer (BET) rate that results in a quasi-continuous distribution of fluorescence intensities. The dependence of BET rate on electron density and charge trapping is consistent with our previous observations of quasi-continuous distribution of fluorescence intensities of m-ZnTCPP on TiO2 NPs with applied negative potential across the dye-TiO2 interface. The quasi-continuous distribution of fluorescence intensities in both cases of m-ZnTCPP on the ITO surface and m-ZnTCPP on TiO2 NPs with applied negative potential indicates that the electron density in the semiconductor plays a dominant role in dictating the changes in rates of charge transfer, rather than the relative energetics between electrons in the semiconductor and the oxidized sensitizer.

Watching the dynamics of electrons and atoms at work in solar energy conversion.

The photochemical reactions performed by transition metal complexes have been proposed as viable routes towards solar energy conversion and storage into other forms that can be conveniently used in our everyday applications. In order to develop efficient materials, it is necessary to identify, characterize and optimize the elementary steps of the entire process on the atomic scale. To this end, we have studied the photoinduced electronic and structural dynamics in two heterobimetallic ruthenium-cobalt dyads, which belong to the large family of donor-bridge-acceptor systems. Using a combination of ultrafast optical and X-ray absorption spectroscopies, we can clock the light-driven electron transfer processes with element and spin sensitivity. In addition, the changes in local structure around the two metal centers are monitored. These experiments show that the nature of the connecting bridge is decisive for controlling the forward and the backward electron transfer rates, a result supported by quantum chemistry calculations. More generally, this work illustrates how ultrafast optical and X-ray techniques can disentangle the influence of spin, electronic and nuclear factors on the intramolecular electron transfer process. Finally, some implications for further improving the design of bridged sensitizer-catalysts utilizing the presented methodology are outlined.

On the Role of Hydrogen Bonds in Photoinduced Electron‐Transfer Dynamics between 9‐Fluorenone and Amine Solvents

Using ultrafast fluorescence upconversion and mid-infrared spectroscopy, we explore the role of hydrogen bonds in the photoinduced electron transfer (ET) between 9-fluorenone (FLU) and the solvents trimethylamine (TEA) and dimethylamine (DEA). FLU shows hydrogen-bond dynamics in the methanol solvent upon photoexcitation, and similar effects may be anticipated when using DEA, whereas no hydrogen bonds can occur in TEA. Photoexcitation of the electron-acceptor dye molecule FLU with a 400 nm pump pulse induces ultrafast ET from the amine solvents, which is followed by 100 fs IR probe pulses as well as fluorescence upconversion, monitoring the time evolution of marker bands of the FLU S(1) state and the FLU radical anion, and an overtone band of the amine solvent, marking the transient generation of the amine radical cation. A comparison of the experimentally determined forward charge-separation and backward charge-recombination rates for the FLU-TEA and FLU-DEA reaction systems with the driving-force dependencies calculated for the forward and backward ET rates reveals that additional degrees of freedom determine the ET reaction dynamics for the FLU-DEA system. We suggest that hydrogen bonding between the DEA molecules plays a key role in this behaviour.

Ultrafast Forward and Backward Electron Transfer Dynamics of Coumarin 337 in Hydrogen-Bonded Anilines As Studied with Femtosecond UV-Pump/IR-Probe Spectroscopy

Femtosecond infrared spectroscopy is used to study both forward and backward electron transfer (ET) dynamics between coumarin 337 (C337) and the aromatic amine solvents aniline (AN), N-methylaniline (MAN), and N,N-dimethylaniline (DMAN), where all the aniline solvents can donate an electron but only AN and MAN can form hydrogen bonds with C337. The formation of a hydrogen bond with AN and MAN is confirmed with steady state FT-IR spectroscopy, where the C═O stretching vibration is a direct marker mode for hydrogen bond formation. Transient IR absorption measurements in all solvents show an absorption band at 2166 cm(-1), which has been attributed to the C≡N stretching vibration of the C337 radical anion formed after ET. Forward electron transfer dynamics is found to be biexponential with time constants τ(ET)(1) = 500 fs, τ(ET)(2) = 7 ps in all solvents. Despite the presence of hydrogen bonds of C337 with the solvents AN and MAN, no effect has been found on the forward electron transfer step. Because of the absence of an H/D isotope effect on the forward electron transfer reaction of C337 in AN, hydrogen bonds are understood to play a minor role in mediating electron transfer. In contrast, direct π-orbital overlap between C337 and the aromatic amine solvents causes ultrafast forward electron transfer dynamics. Backward electron transfer dynamics, in contrast, is dependent on the solvent used. Standard Marcus theory explains the observed backward electron transfer rates.

Photoinduced electron transfer in tris(2,2′-bipyridine)ruthenium(ii)-viologen dyads with peptide backbones leading to long-lived charge separation and hydrogen evolution

Three 5,5'-disubstituted-2,2'-bipyridine ligands tethered to l-Asp-based peptide backbones having pendant viologen-modified branches, i.e., 5-ethoxycarbonyl-5'-(N-G(1)-carbamoyl)-2,2'-bipyridine ((4+)), 5,5'-bis(N-G(1)-carbamoyl)-2,2'-bipyridine ((8+)), and 5,5'-bis(N-G(2)-carbamoyl)-2,2'-bipyridine ((12+)), were prepared, where G(1) = Asp(NHG(3))-NHG(3), G(2) = Asp(NHG(3))-Asp(NHG(3))-NHG(3), and G(3) = -(CH(2))(2)-(+)NC(5)H(4)-C(5)H(4)N(+)-CH(3), i.e., 2-(1'-methyl-4,4'-bipyridinediium-1-yl)ethyl. These were reacted with cis-Ru(bpy)(2)Cl(2) to give three new dyads [Ru(bpy)(2)()](6+) ((6+)), [Ru(bpy)(2)()](10+) ((10+)), and [Ru(bpy)(2)()](14+) ((14+)), respectively, where bpy = 2,2'-bipyridine. All these dyads undergo extremely efficient intramolecular quenching leading to the formation of charge separated (CS) states (Ru(III)-MV(+) ), and display a triple exponential decay due to the presence of three classes of conformers with each exhibiting the individual rate of electron transfer. The lifetimes (contributions) were determined as 12.5 ps (94.2%), 791 ps (4.5%), and 18.3 ns (1.2%) for , 82.2 ps (79.9%), 1.12 ns (12.4%), and 4.60 ns (7.7%) for , and 43.6 ps (71.6%), 593 ps (20.2%), and 3.75 ns (8.1%) for . The forward electron transfer rate constants (k(ET)) for the major components were calculated as k(ET) = 8.3 x 10(10) s(-1) for , k(ET) = 1.2 x 10(10) s(-1) for , and k(ET) = 2.3 x 10(10) s(-1) for . Further, the lifetimes and quantum yields of charge separated states were determined as tau(CS) = 16 +/- 3 ns and Phi(CS) = 0.81 for , tau(CS) = 20 +/- 3 ns and Phi(CS) = 0.92 for , and tau(CS) = 20 +/- 3 ns and Phi(CS) = 0.64 for . The backward electron transfer rate constants (k(BET)) were also determined as 6.3 x 10(7), 5.0 x 10(7), and 5.0 x 10(7) s(-1) for , , and , respectively. From the analysis of electrical conductivity, the major ion-pair adducts in aqueous media were characterized as (PF(6))(5+) (52%) for , (PF(6))(2)(8+) (29%) and (PF(6))(3)(7+) (32%) for , and (PF(6))(3)(11+) (27%) and (PF(6))(4)(10+) (29%) for , at a total complex concentration of 0.04 mM. The present family is found to be the first example of a Ru(bpy)(3)(2+)-MV(2+) system in which three orders of magnitude of difference is achieved between the forward and backward electron transfer rate constants. These dyads were finally combined with a Pt(ii)-based H(2)-evolving catalyst, i.e., cis-diamminedichloroplatinum(ii), to ascertain the applicability of the system towards the visible light-induced water splitting processes.

Photosensitized Reduction of Water to Hydrogen Using Human Serum Albumin Complexed with Zinc−Protoporphyrin IX

We present the photophysical properties of complexes of recombinant human serum albumin (rHSA) with Zn(II)-protoporphyrin IX (ZnPP) and their activities in the photosensitized reduction of water to hydrogen (H2) using methyl viologen (MV2+) as an electron relay. The ZnPP is bound in subdomain IB of wild-type rHSA [rHSA(wt] by an axial coordination of Tyr-161 and, in the rHSA(I142H/Y161L) mutant [rHSA(His], by a His-142 coordination. Both the rHSA(wt)-ZnPP and rHSA(His)-ZnPP complexes showed a long-lived photoexcited triplet state with lifetimes (tauT) of 11 and 2.5 ms, respectively. The accommodation of ZnPP into the protein matrix efficiently eliminated the collisional triplet self-quenching process. The addition of a water-soluble electron acceptor, MV2+, resulted in a significant decrease in the triplet lifetime. The transition absorption spectrum revealed the oxidative quenching of rHSA-3ZnPP* by MV2+. The quenching rate constant (kq) and backward electron transfer rate constant (kb) were determined to be 1.4 x 10(7) and 4.7 x 10(8) M(-1) s(-1) for rHSA(wt)-ZnPP. In the presence of the colloidal PVA-Pt as a catalyst and triethanolamine (TEOA) as a sacrificial electron donor, the photosensitized reduction of water to H2 takes place. The efficiency of the photoproduction of H2 was greater than that of the system using the well-known organic chromophore, tetrakis(1-methylpyridinium-4-yl)porphinatozinc(II) (ZnTMPyP4+), under the same conditions.

Interference, Fluctuation, and Alternation of Electron Tunneling in Protein Media. 2. Non-Condon Theory for the Energy Gap Dependence of Electron Transfer Rate

Developing the quantum transition rate theory of Prezhdo and Rossky (J. Chem. Phys. 1997, 107, 5863), we produced a new non-Condon theory of the rate of electron transfer (ET) which happens through a protein medium with conformational fluctuation. The new theory is expressed by a convolution form of the power spectrum for the autocorrelation function of the electronic tunneling matrix element T(DA)(t) with quantum correction and the ordinary Franck-Condon factor. The new theory satisfies the detailed balance condition for the forward and backward ET rates. The ET rate formula is divided into two terms of elastic and inelastic tunneling mechanisms on the mathematical basis. The present theory is applied to the ET from Bph(-) to Q(A) in the reaction center of Rhodobacter sphaeroides. Numerical calculations of T(DA)(t) were made by a combined method of molecular dynamics simulations and quantum chemistry calculations. We showed that the normalized autocorrelation function of T(DA)(t) is almost expressed by exponential forms. The calculated energy gap law of the ET rate is nearly Marcus' parabola in most of the normal region and around the maximum region, but it does not decay substantially in the inverted region, which is called the anomalous inverted region. We also showed that the energy gap law at the high uphill energy gap in the normal region is elevated considerably from the Marcus' parabola, which is called the anomalous normal region. Those anomalous energy gap laws are due to the inelastic tunneling mechanism which works actively at the energy gap far from zero. We presented an empirical formula for easily calculating the non-Condon ET rate, which is usable by many researchers. We provided experimental evidence for the anomalous inverted region which was basically reproduced by the present theory. The present theory was extensively compared with the previous non-Condon theories.

Ligand dependence of magnetic spin effects on photooxidation of [Ru(bpy) 3− n(CN) 2 n] (+2−2 n) type complexes

Photoinduced electron transfer reaction from the series of [Ru(bpy) (3− n) (CN) 2 n ] (2+−2 n) complexes ( n=0, 1, 2) to methylviologen has been investigated using external magnetic field modulation in order to obtain relevant kinetic parameters for spin relaxation, backward electron transfer and cage escape of the geminate radical pair. The replacement of one bpy by two cyanide ligands results in an increase of the quenching rate constant and a decrease of the cage escape efficiency. The latter is decreased in a magnetic field and the sensitivity of this effect is weakened by introducing CN − ligands. From the analysis of the magnetic field dependence it can be inferred that with increasing number of CN − ligands, backward electron transfer rate and spin relaxation time in the geminate radical pair exhibit a moderate decrease but that the rate constant of cage escape decreases very strongly.

Spin-orbit coupling controlled spin chemistry of Ru(bpy)32+ photooxidation: Detection of strong viscosity dependence of in-cage backward electron transfer rate

Abstract

Some new perspectives on electron transfer reactions through the intersecting-state model

The intersecting-state model (ISM) is employed to calculate the free-energy barriers for adiabatic self-exchange reactions of transition metal complexes. Systems in which σ *-d electrons are transferred are subject to distance-dependent non-adiabatic factors ranging from χ r = 10 −2 for hexaaquo complexes to χ r = 10 −6 for tris(2,2′-bipyridyl) complexes. In the spin-forbidden electron exchanges of Co 2+/3+ complexes, an additional non-adiabatic factor of χ 0 = 10 −3 is present. Self-exchange rates for NO 2/NO 2 −, ClO 2/ClO 2 − and O 2/O 2 − reactions are calculated in good agreement with experiment. The correlation between log k calc and k exp for the 20 adiabatic and non-adiabatic reactions studied has a slope of 0.97 and a correlation coefficient of 0.98. The ISM is also employed to interpret 25 cross-reaction electron transfer (ET) rates, including those of MN(aq) 2+/3+, Co(aq) 2+/3+, Co(NH 3) 6 2+/3+, NO 2 0/− and O 2 0/−, which yield self-exchange rates spread over several orders of magnitude depending on the systems considered. Consideration of the above-mentioned non-adiabatic factors leads to calculated rates within a factor of 250 of the experimental values, except for the Co(aq) 3+/Mn(aq) 2+ and Ru(bpy) 3 3+/Co(phen) 3 2+ systems, (bpy, bipyridyl; phen, phenanthroline), where a change in mechanism appears to occur. Significant solvent effects on electron self-exchange reactions can be interpreted in terms of variations in the transition state bond order n‡, and some examples are discussed, in particular the O 2/O 2 − couple. The ISM is used to describe the experimental dependence of photoinduced forward and backward ET rate constants on the ΔG 0 values of these reactions. The model can account quantitatively for the occurence of both Rhem—Weller behaviour and a Marcus inverted region in the same systems, depending on the direction of ET.

Homogeneous redox catalysis of electrochemical reactions: Part V. Cyclic voltammetry

Cyclic voltammetry appears as a more convenient tool for studying homogeneous redox catalysis than polarography, due to the recording of the current potential being much faster and the control of the diffusion process more accurate and involving a much more extended range of rates. The theory of homogeneous redox catalysis in the context of cyclic voltammetry is presented for the EC reaction scheme, i.e. the simplest of the mechanisms in which the initial electron-transfer process is followed by chemical reaction. The treatment involves a stationary-state assumption regarding the initial product of the electron transfer The dimensionless voltammograms then depend upon two kinetic parameters and of the ratio of the substrate and catalyst concentrations. Two limiting situations are found for the kinetic control of the catalytic process according to the relative magnitude of the, follow-up chemical reaction and the backward electron-transfer rates. In one of these the kinetic control is by the forward homogeneous electron transfer, while in the other it is by the follow-up chemical reactions. In these two cases the system depends upon only two parameters, allowing a detailed discussion of the shape and characteristics of the voltammograms to be presented, as well a set of working curves giving the relative catalytic increase of the peak current as a function of the pertinent kinetic parameter and of the substrate—catalyst concentration ratio. A numerical procedure is presented for the resolution of the general case involving a mixed kinetic control by the two reactions. Diagnostic criteria for the recognition of the two limiting cases and the transition to mixed kinetic control are given, as well as procedures for determining the characteristic rate constants. The theory is then extended to the more frequently encountered two-electron processes which involves only a slight modification of the results pertaining to the EC scheme. Experimental illustrations of the main theoretical results are given in the context of the reduction of aromatic halides in aprotic media. The results obtained in this study have as their main application the determination, through the redox catalytic method, of the rate and equilibrium parameters for irreversible systems that would be far beyond the possibilities offered by the standard use of electrochemical techniques.