Rate Of Photoinduced Electron Transfer Research Articles

Nuclear quantum effects on electron transfer reactions in DNA hairpins.

The driving force dependence of photoinduced electron transfer rate constant in synthetic DNA hairpins in aqueous solutions has been analyzed by means of molecular dynamics simulations. The quantum energy gap law has thus been investigated from a fully atomistic point of view, well reproducing the experimental results with reduced ambiguities in the parameter fitting. Although the contribution from the high-frequency vibrational modes of DNA and water solvent to the reorganization energy is fairly small, their quantum effect on the electron transfer rate constant is significant, well accounting for the deviation from the Marcus parabola observed in the experiments.

Diels−Alder Reactions of Anthracenes with Dienophiles via Photoinduced Electron Transfer

Photochemical Diels−Alder reaction of anthracene and its derivatives with dienophiles (p-benzoquinone and fumaronitrile) occurs in competition with the dimerization of anthracenes in chloroform at 298 K. The dependence of the quantum yields on the concentrations of dienophiles has revealed that the photochemical Diels−Alder reaction proceeds via electron transfer from the singlet excited states of anthracenes to dienophiles. The rates of photoinduced electron transfer are diffusion limited, agreeing with the largely negative free energy change of electron-transfer judging from the more negative one-electron oxidation potentials of the singlet excited states of anthracenes than the one-electron reduction potentials of dienophiles. The radical ion pair produced in the photoinduced Diels−Alder from the singlet excited states of anthracenes to p-benzoquinone and fumaronitrile has been detected as the transient absorption spectrum at 298 K with use of laser flash photolysis. The diradical intermediates prior t...

Rational design principle for modulating fluorescence properties of fluorescein-based probes by photoinduced electron transfer.

Fluorescence properties of fluorescein-based probes are shown to be finely controlled by the rate of photoinduced electron transfer from the benzoic acid moiety (electron donor) to the singlet excited state of the xanthene moiety (electron acceptor fluorophore). The occurrence of photoinduced electron transfer is clearly evidenced by transient absorption spectra showing bands due to the radical cation of the electron donor moiety and the radical anion of the xanthene moiety, observed in laser flash photolysis experiments. The photoinduced electron transfer rates and the rates of back electron transfer follow the Marcus parabolic dependence of electron transfer rate on the driving force. Such a dependence provides for the first time a quantitative basis for a rational design principle which has high efficiency in modulating fluorescence properties of fluorescein-based probes.

Acceleration and deceleration of photoinduced electron transfer rates by an electric field in porphyrin-fullerene dyads

Fluorescence of zinc porphyrin in zinc porphyrin–C 60 dyads is enhanced by applying an electric field ( F ) due to a field-induced deceleration of the electron transfer rate from the singlet excited state of zinc porphyrin to C 60 in the Marcus top region. Free-base porphyrin–C 60 dyads exhibit the opposite electric field effect, since the electron transfer from the singlet excited state of free-base porphyrin to C 60 is in the Marcus normal region far from the top.

Photoinduced electron-transfer along alpha-helical and coiled-coil metallopeptides.

A peptide-based electron-transfer system has been designed in which the specific positions of redox-active metal complexes appended to either an alpha-helix, or an alpha-helical coiled-coil, can be reversed to test the effect of the helix dipole in controlling photoinduced electron-transfer rates. Two 30-residue apopeptides were prepared having the following sequences: (I) Ac-K-(IEALEGK)(ICALEGK)(IEALEHK)(IEALEGK)-G-amide, and (II) Ac-K-(IEALEGK)(IHALEGK)-(IEALECK)(IEALEGK)-G-amide. Each apopeptide was reacted first with [Ru(bpy)2(phen-ClAc)]2+, where bpy = 2,2'-bipyridine and phen-ClAc = 5-chloroacetamido-1,10-phenanthroline, to attach the ruthenium polypyridyl center to the cysteine side-chain of the polypeptide. The isolated products were then reacted with [Ru(NH3)5(H2O)]2+ to yield the binuclear electron-transfer metallopeptides ET-I and ET-II. In these systems, electron-transfer occurred from the photoexcited ruthenium polypyridyl donor to the pentammine ruthenium (III) acceptor such that the electron-transfer occurred toward the negative end of the helix dipole in ET-I, and toward the positive end in ET-II. Circular dichroism spectroscopy showed that both peptides exist as dimeric alpha-helical coiled-coils in 100 mM phosphate buffer at pH 7, and as monomeric alpha-helices in the lower dielectric solvents 2,2,2-trifluoroethanol, and a 1:1 (v/v) mixture of CH2Cl2 and 2,2,2-trifluoroethanol. The peptides were predominately (i.e., 65-72%) alpha-helical in these solvents. The emission lifetime behavior of ET-I was seen to be identical to that of ET-II in each of the three solvents: no evidence for directional electron-transfer rates was observed. Possible reasons for this behavior are discussed.

Ground vs. excited state electron transfer: adsorbed monolayers and trimers in solution.

Transient emission spectroscopy has been used to probe the rate of photoinduced electron transfer between metal centres within a novel trimeric complex [[Os(bpy)2(bpe)2][Os(bpy)2Cl]2]4+, where bpy is 2,2'-bipyridyl and bpe is trans-1,2-bis-(4-pyridyl)ethylene. Transient emission experiments on the trimer, and on [Os(bpy)2 (bpe)2]2+ in which the [Os(bpy)2 Cl]+ quenching moieties are absent, reveal that the rate of photoinduced electron transfer (PET) across the bpe bridge is 1.3 +/- 0.1 x 10(8) s(-1). Investigations into the driving forces for oxidation and reduction of the electronically excited state within the trimer indicate that quenching of the [Os(bpy)2 (bpe)2]2+ centre within the trimer involves electron transfer from the [bpe Os(bpy)2 Cl]+ centres to the electronically excited state with a driving force of -0.3 eV. Monolayers of the complex, [Os(bpy)2 bpe pyridine]2+, have been formed by spontaneous adsorption onto platinum microelectrodes and used to probe the dynamics of electron transfer across the trans-1,2-bis-(4-pyridyl)ethylene bridge in the ground state. These monolayers are stable and exhibit well defined voltammetric responses for the Os2+/3+ redox reaction. Cyclic voltammograms recorded at high scan rates can be accurately modelled according to a non-adiabatic electron transfer model based on the Marcus theory using a standard heterogeneous electron transfer rate constant, k(o), of 3.1 +/- 0.2 x 10(4) s(-1) and a reorganization energy of 0.4 +/- 0.1 eV. This rate constant is a factor of approximately two orders of magnitude smaller than that found for photoinduced electron transfer across the same bpe bridge for identical driving forces. This significant difference is interpreted in terms of both the nature of the orbitals involved in electrochemically and optically driven electron transfer, as well as the strength of electronic coupling between two molecular components as opposed to a molecular component and a metal electrode.

Photoinduced electron transfer in GFP/viologen/TCNQ structured hetero-LB film

The photoinduced electron transfer rate in hetero-Langmuir–Blodgett (LB) films, composed of electron sensitizing protein and electron accepting organics, was investigated. The hetero-LB film was fabricated by sequentially depositing green fluorescence protein (GFP), viologen, and TCNQ, which functioned as an electron sensitizer, a relay, and an acceptor, respectively. To verify the hetero-LB film formation, surface morphologies of the GFP film and viologen/TCNQ LB film were observed by atomic force microscopy (AFM). The fast charge separation of the molecular interface was investigated by analyzing the time-resolved fluorescence decays of GFP homo-, GFP/viologen hetero- and GFP/viologen/TCNQ hetero-LB film using a time-correlated single photon counting (TCSPC) method. In GFP/viologen hetero-films, the fluorescence decay of excited GFP was faster than that in the GFP homo-film. Based on the decay of fluorescence the charge separation rates of excited GFP were calculated. The fluorescence lifetime and charge separation rate of the GFP/viologen/TCNQ hetero-LB film increased slightly compared to that of the GFP/viologen hetero-LB film as a result of the addition of the TCNQ layer, electron acceptor. On the basis of its molecular interface charge separation behavior, the proposed molecular array can be used as a component of the molecular photodiode.

Distance Dependence of Photoinduced Electron Transfer along α-Helical Polypeptides

α-Helical polypeptides containing a pair of L-1-pyrenylalanine and L-p-nitrophenylalanine that are separated by 0−8 amino acid units were synthesized. The rates of photoinduced electron transfer (ET) from the pyrenyl group to the nitrophenyl group were evaluated from the decay curves of pyrenyl fluorescence recorded at different temperatures from −58 to +30 °C. The rate constants showed a complex dependence on the number of spacer amino acids. In particular, recoveries of the ET rates with increasing the number of spacer amino acids from 1 to 2 and from 5 to 6 were found. The ET rate constants, however, exhibited a simple exponential dependence on the edge-to-edge distance between the two chromophores, with a distance decay factor β = 0.66 ± 0.1 (A-1). The ET data on the α-helical polypeptides were analyzed on the basis of the tunneling pathway model. The optimum ET pathways from the pyrenyl group to the nitrophenyl group were searched, and the relative values of the ET matrix elements were evaluated for ...

Control of thermal photoinduced electron transfer reactions in the activated and activationless regimes

Photoinduced electron transfer rates depend on the internal energy distribution of the locally excited donor state. This energy distribution may be hot or cold relative to the temperature of the donor in the ground electronic state and is dependent on the photoexcitation frequency. In the activated regime, the electron transfer rate depends exponentially on the temperature of the locally excited donor state. Therefore, the electron transfer rate is sensitive to the photoexcitation frequency. In the activationless regime, even if the vibrational frequencies of the locally excited donor state and the acceptor state differ, the electron transfer rate is rather insensitive to the internal energy distribution of the locally excited donor state. Therefore, changing the photoexcitation frequency does not lead to a significant change in the transfer rate. Model computations are presented to demonstrate this qualitative difference between the two regimes, as well as to confirm that the photoinduced electron transfer rate is well-approximated as a thermal electron transfer rate, but at an effective temperature of the locally excited donor state that depends on the photoexcitation frequency.

Photoinduced electron transfer in Langmuir–Blodgett films between the donor [5,10,15,20-tetra(pentyl-oxy-biphenyl-oxy- m-phenyl)-porphyrinato]zinc(II) and quinone acceptors, either separated or linked by a cyclohexylene-bridge

Complexes of zinc porphyrins with mesogenic substituents and quinones, as well as the corresponding mixtures of unlinked porphyrins and quinones, have been investigated in Langmuir–Blodgett (LB) mono- and multilayer films by means of steady-state as well as time-resolved fluorescence spectroscopy. The films were deposited onto quartz substrates. They consist of one or more layers of cadmium behenate (cadmium salt of docosanoic acid) containing either the donor (a zinc porphyrin), the acceptor (a quinone), or both. In the latter case, a covalently linked zinc porphyrin–quinone and mixtures of a zinc porphyrin and a quinone have been investigated. Steady-state S 2-fluorescence of zinc porphyrins was utilized as a reference to determine the rates of photoinduced electron transfer (PET), in order to characterize the intramolecular and intermolecular PET. This approach was found to be particularly useful for monolayer films. For PET of covalently linked zinc porphyrin–quinone compounds in monofilms, the same PET rates as in ethanol were observed (5.3×10 10 s −1). In the case of unlinked zinc-porphyrin/quinone mixtures, the PET rates depend on the concentration of quinones in the acceptor layers. Furthermore, the competition of energy transfer (between zinc porphyrins) and PET (between zinc porphyrins and quinones) in the cadmium behenate matrix is discussed.

Influence of redoxinert counterions on photoinduced intermolecular electron transfer rates in dichloromethane

Rate constants kq of the fluorescence quenching of 9,10-dicyanoanthracene by bromide and chloride ions and of 9-cyanoanthracene by bromide ions were determined in the low polar solvent dichloromethane with a series of tetra-alkylammonium counterions. Cation size dependent electron transfer rate constants ket were derived from kq, and were fitted to a nonadiabatic electron transfer model equation. Because of ion pair formation in dichloromethane electrostatic interactions of the redoxinert countercations during electron transfer were considered. Accordingly, the position of the countercation within the precursor complex had to be described.The electronic coupling matrix element derived by fitting of the experimental data is in satisfactory agreement with results of simple quantum chemical calculations.

Theoretical Study of Photoinduced Proton-Coupled Electron Transfer through Asymmetric Salt Bridges

The theoretical study in this paper is based on the experimental result that the rate of photoinduced electron transfer is 10 2 times slower through a donor-(amidinium-carboxylate)-acceptor salt bridge than through the corresponding switched interface donor-(carboxylate-amidinium)-acceptor complex (Kirby, J. P.; Roberts, J. A.; Nocera, D. G. J. Am. Chem. Soc. 1997, 119, 9230). This experimental result indicates that the proton-transfer interface plays an important role in these electron-transfer reactions. In this paper, a multistate continuum theory for proton-coupled electron transfer (PCET) is applied to analogues representing the chemical systems studied in these experiments. The solute is described with a multistate valence bond model, the solvent is represented as a dielectric continuum, and the active electrons and transferring proton(s) are treated quantum mechanically on equal footing. The application of this theory to these PCET systems provides adiabatic free energy surfaces that depend on two scalar solvent variables corresponding to the proton and electron transfer reactions. The theoretical analysis indicates that the experimentally observed difference in rates for the two chemical systems is due to differences in solute electrostatic properties, solvation energies, solvent reorganization energies, and electronic couplings. Moreover, this theoretical study provides insight into the underlying fundamental principles of PCET reactions.

Measurements of the electric-field-induced change in fluorescence decay profile of methylene-linked carbazole and terephthalic acid methyl ester in a PMMA polymer film

The electric-field-induced change in the fluorescence decay profile has been measured for methylene-linked carbazole (CZ) and terephthalic acid methyl ester (TAME) doped in a PMMA polymer film, using a picosecond time-resolved decay measurement system combined with a bipolar sample bias. The average lifetime of the fluorescence emitted from the locally excited state of CZ becomes shorter in the presence of an external electric field ( F ), confirming that the rate of photoinduced electron transfer from CZ to TAME is enhanced by F . Different decaying portions of the multi-exponential decay were also found to give different efficiencies in the electric field effect from each other, depending on the methylene chain length.

Determination of the Photoinduced Electron Transfer Rate Constant in Langmuir−Blodgett Films by Time-Resolved Fluorescence

The feasibility of a relevant analysis of the fluorescence decays of Langmuir−Blodgett−Kuhn films where electron and energy transfer occur simultaneously is demonstrated. Starting from a stretched exponential, describing Forster-type energy transfer, a model describing simultaneous photoinduced electron transfer and energy transfer is developed. In this model a binomial distribution of electron donors (pyrene) is assumed in the layer adjacent to the excited probe (Rhodamine G). When the fluorescence decays of two or three samples with different concentrations of electron donors are analyzed simultaneously with the decay of a sample containing no electron donor, reliable values of the global parameters (the fluorescence decay time of the unquenched probe, the rate constant for electron transfer, and the rate constant for energy transfer to a trap) could be obtained. The local parameters (the average number of electron donors in a site opposite the probe and the fraction of the monolayer containing an elect...

Transient fluorescence in flavin/TCNQ hetero LB film

The photoinduced electron transfer rate in hetero Langmuir–Blodgett (LB) films composed of 7,8-dimethyl-10-dodecyl isoalloxazine (flavin) and N-docosylquinolinium tetracyanoquinodimethan (TCNQ) was investigated. Flavin and TCNQ were used as electron donor and acceptor, respectively. The time-resolved fluorescence decay of three different types of flavin/TCNQ hetero LB films such as mixed layer, head (hydrophilic functional group)-to-head interfaced structure and tail (hydrophobic alkyl chain)-to-tail interfaced structure were measured by a time-correlated single photon counting (TCSPC) method. Electron transfer rate from photoexcited donor to acceptor depends on the distance between functional groups, since the fluorescence decay of head-to-head interfaced hetero LB films occurs faster than that of tail-to-tail interfaced hetero LB films. Based on the faster electron transfer of flavin/TCNQ hetero LB films than that of flavin/viologen hetero LB films, the redox potential difference between donor and acceptor affects the charge transfer rate.

Biomolecular electron transfer under high hydrostatic pressure

The dependence of the photoinduced electron transfer rate on hydrostatic pressure up to 8 kbar was studied at 295 K in a bridged Zn-porphyrin donor and pyromellitimide acceptor supermolecule dissolved in toluene. A picosecond fluorescence emission kinetics of the donor, limited by the electron transfer rate, was detected by using synchroscan streak camera. The experiment was complemented with model calculations based on modified classical and semi-classical nonadiabatic electron transfer theory. A peculiar asymmetric inverted parabola-like dependence of the electron transfer rate on pressure was observed. The dependence was successfully reproduced by nonadiabatic theory in the high-temperature limit assuming that the reorganisation free energy or both the reorganisation free energy and the reaction driving force (linearly) changed with pressure. The reaction driving force dependence on pressure alone failed to explain the asymmetry, suggesting that the electron transfer was accompanied with vibration frequency changes. It was inferred that the effective frequency in the product state should be larger than in the reactant state. The usage of the nonadiabatic theory is well justified due to the fulfilment of the inequalities V《 k B T and V《〈 τ L〉 −1 ( V is the electronic coupling matrix element, 〈 τ L〉 −1 is the solvent relaxation rate). The influence of the donor-acceptor distance reduction under compression on the electron transfer rate was found to be minor.

An Error in Determination of Photoinduced Electron Transfer Rates in Langmuir-Blodgett Films

Abstract In Langmuir-Blodgett (LB) films consisting of viologen (V2+)-pyrene (Py) dyads, the accumulation of viologen cation radical (V+·) was observed during UV irradiation under N2. The additional quenching due to energy transfer by the colored V+· resulted in an error in determination of photoinduced electron transfer rates between the V2+ and Py moieties in the LB films. This type of error can be generally expected in LB films and self-assembled monolayers.

External Electric Field Effect on Photoinduced Electron Transfer from Monomer and Dimer of Oxacarbocyanine in Langmuir-Blodgett Films

Interlayer vectorial electron transfer from the excited state of oxacarbocyanine (OCC) to viologen (VIO) occurs in a molecular assembly composed of a mixed LB monolayer of OCC and a mixed LB monolayer of VIO separated by a spacer layer of a fatty acid. Fluorescence quantum yield of OCC in this system is changed by an external electric field, indicating that the photoinduced electron transfer rate is affected by an electric field. The rate is depressed or enhanced by an electric field, depending on the direction of the applied electric field. It was also found that monomer and dimer of OCC show different field effects on the electron transfer from each other in LB films.

Use of Modern Electron Transfer Theories To Determine Electronic Coupling Matrix Elements in Intramolecular Systems

The dependence of the donor/acceptor electronic coupling on the topology of donor−bridge−acceptor (DBA) molecules is probed experimentally and theoretically. The temperature dependence of photoinduced electron-transfer rate constants is analyzed with a semiclassical electron-transfer model to extract the donor/acceptor electronic coupling matrix elements |V| and the low-frequency reorganization energy at 295 K, λo(295 K), for four rigid DBA molecules. The sensitivity of the electronic coupling |V| to the models and parameters used to fit the data are extensively investigated. The treatment of the low-frequency reorganization energy's temperature dependence has a significant impact on the analysis. The identity of the principal coupling pathways is determined for molecular linkages that propagate symmetry allowed donor/acceptor interactions and molecular linkages that propagate symmetry forbidden donor/acceptor interactions. For the symmetry forbidden case, these analyses demonstrate that solvent molecules provide the dominant coupling pathway in the nine-bond bridge, C-shaped molecule 2 but do not significantly influence |V| across the seven-bond, linear bridge in 1.

Contrasting Photoinduced Electron-Transfer Properties of Two Closely Related, Rigidly Linked Porphyrin−Quinone Dyads

Two closely related, rigidly linked porphyrin−naphthoquinone dyads have been prepared and studied using time-resolved fluorescence and absorption methods. Dyad 1, whose quinone carbonyl groups are relatively close to the porphyrin macrocycle, exhibits photoinduced electron-transfer rate constants that are virtually independent of solvent dielectric constant and temperature within the range 77−295 K. Dyad 2, which has a similar donor−acceptor linkage but whose quinone carbonyl groups are ∼2 A farther from the porphyrin, features photoinduced electron-transfer rate constants that decrease with decreasing solvent dielectric constant. Electron transfer in this molecule ceases at low temperatures. Photoinduced electron transfer in dyad 2 exhibits the usual dependence on free energy change and solvent reorganization observed in many similar porphyrin−quinone systems. The behavior of 1 may be attributed at least in part to the smaller separation of the porphyrin radical cation and the quinone radical anion, whic...