Kinetics Of Photoinduced Electron Transfer Research Articles

Charge accumulation kinetics in multi-redox molecular catalysts immobilised on TiO2.

Multi-redox catalysis requires the accumulation of more than one charge carrier and is crucial for solar energy conversion into fuels and valuable chemicals. In photo(electro)chemical systems, however, the necessary accumulation of multiple, long-lived charges is challenged by recombination with their counterparts. Herein, we investigate charge accumulation in two model multi-redox molecular catalysts for proton and CO2 reduction attached onto mesoporous TiO2 electrodes. Transient absorption spectroscopy and spectroelectrochemical techniques have been employed to study the kinetics of photoinduced electron transfer from the TiO2 to the molecular catalysts in acetonitrile, with triethanolamine as the hole scavenger. At high light intensities, we detect charge accumulation in the millisecond timescale in the form of multi-reduced species. The redox potentials of the catalysts and the capacity of TiO2 to accumulate electrons play an essential role in the charge accumulation process at the molecular catalyst. Recombination of reduced species with valence band holes in TiO2 is observed to be faster than microseconds, while electron transfer from multi-reduced species to the conduction band or the electrolyte occurs in the millisecond timescale. Finally, under light irradiation, we show how charge accumulation on the catalyst is regulated as a function of the applied bias and the excitation light intensity.

Kinetics of Photoinduced Electron Transfer in Alternately Stacked Eu3+:LaNb2O7– and W2O72– Nanosheets As Demonstrated by f–f Radiative Transition of Doped Eu3+

We successfully demonstrated that the kinetics of photoinduced electron transfer (PET) between alternately stacked Eu3+-doped LaNb2O7– (Eu3+:LaNb2O7–) nanosheets and W2O72– nanosheets can be investigated by the millisecond luminescence decay of an emissive relaxation between 4f orbitals of the doped Eu3+. The emission of the doped Eu3+ is triggered by energy transfer (EnT) from the band gap excitation energy of LaNb2O7–, and the EnT process competes with PET from LaNb2O7– to W2O72–, meaning that the kinetics of the PET can be estimated from the time-resolved emission behavior of doped Eu3+. Alternately stacked nanosheets were synthesized by a thiol-ene click reaction of organic moieties immobilized on each nanosheet where the center-to-center distance of neighboring nanosheets was precisely controlled to be 1.94, 2.04, and 2.20 nm by changing the number of carbons in the covalent bridge generated by the thiol-ene click reaction. When the center-to-center distances of neighboring nanosheets were 1.94 and 2...

Photoinduced electron transfer kinetics of linked Ru-Co photocatalyst dyads

Two new supramolecular photocatalyst dyads based on the [Ru(2,2′-bipyridine)3]2+ photosensitizer linked to a macrocyclic Co(II)tetra(pyridyl) catalyst for proton reduction are reported. The dyads differ primarily in the bridging ligand which links the molecular modules; the first being a short and flexible linker, and the second a longer and electronically conjugated linker. Ultrafast transient optical spectroscopy was used to monitor the photoinduced kinetics of the dyads following visible excitation of the photosensitizer module. Direct comparison of transient spectra and kinetics indicates that there are indeed substantial differences between the ultrafast transient optical spectroscopy of the dyads, but there is no indication of oxidative quenching of the photosensitizer module by the catalyst module. These initial design and characterization studies of the linked Ru(II)—Co(II) dyads provide an important foundation for advanced designs of systems for efficient solar energy conversion by molecular architectures.

Potent Bis-Cyclometalated Iridium Photoreductants with β-Diketiminate Ancillary Ligands.

In this work, we outline a strategy to prepare a class of improved visible-light photosensitizers. Bis-cyclometalated iridium complexes with electron-rich β-diketiminate (NacNac) ancillary ligands are demonstrated to be potent excited-state electron donors. Evaluation of the photophysical and electrochemical properties establishes the excited-state redox potentials of the complexes, and Stern-Volmer quenching experiments inform on the kinetics of photoinduced electron transfer to the model substrates methyl viologen (MV2+) and benzophenone (BP). Compared to fac-Ir(ppy)3 (ppy = 2-phenylpyridine), widely regarded as a state-of-the-art photoreductant, the complexes we describe have excited-state redox potentials that are more potent by 300-400 mV and rates for photoinduced electron transfer that are accelerated by as much as a factor of 3. These complexes emerge as promising targets for application in photocatalytic reactions and other photochemical processes.

Vliyanie energii reorganizatsii sredy i ee dinamicheskikh svoystv na kolebatelnyy spektralnyy effekt v kinetike fotoindutsirovannogo perenosa elektrona

Vliyanie energii reorganizatsii sredy i ee dinamicheskikh svoystv na kolebatelnyy spektralnyy effekt v kinetike fotoindutsirovannogo perenosa elektrona

Scanning Electrochemical Microscopy (SECM) of Photoinduced Electron Transfer Kinetics at Liquid/Liquid Interfaces

The kinetics of the photoinduced electron transfer (ET) reaction between the aqueous photosensitizer tris(2,2-bipyridyl) ruthenium(II) (Ru(bipy)32+) and the organic quencher 7,7,8,8-tetracyanoquinodimethane (TCNQ) in 1,2-dichloroethane (DCE) is investigated with a scanning electrochemical microscopy (SECM) photoelectrochemical setup. In this system, an optical fiber placed in one phase is employed to illuminate the interface between two immiscible electrolyte solutions (ITIES), while an ultramicroelectrode (UME) tip in the other phase, positioned with high precision close to the end of the fiber, is used to detect the products of the photoelectrochemical process at the interface. The possible transfer of reactants and products, Ru(bipy)32+, Ru(bipy)33+, TCNQ, and TCNQ•–, across the interface is elucidated by extraction experiments, UV–visible spectroscopy, and SECM-double potential step chronoamperometry (SECM-DPSC) and verified to have no detectable effect on the interfacial photoinduced ET processes in ...

Photoinduced Electron Transfer Dynamics of Cyclometalated Ruthenium (II)–Naphthalenediimide Dyad at NiO Photocathode

Both forward and backward electron transfer kinetics at the sensitizer/NiO interface is critical for p-type dye-sensitized photocathodic device. In this article, we report the photoinduced electron transfer kinetics of a Ru(II) chromophore–acceptor dyad sensitized NiO photocathode. The dyad (O26) is based on a cyclometalated Ru(N∧C∧N)(N∧N∧N) (Ru[II]) chromophore and a naphthalenediimide (NDI) acceptor, where N∧C∧N represents 2,2′-(4,6-dimethyl-phenylene)-bispyridine and N∧N∧N represents 2,2′,6′,6″-terpyridine ligand. When the dyad is dissolved in a CH3CN solution, electron transfer to form the Ru(III)–NDI– occurs with a rate constant kf = 1.1 × 1010 s–1 (τf = 91 ps), and electron–hole pair recombines to regenerate ground state with a rate constant kb = 4.1 × 109 s–1 (τb = 241 ps). When the dyad is adsorbed on a NiO film by covalent attachment through the carboxylic acid group, hole injection takes place first within our instrument response time (∼180 fs) followed by the subsequent electron shift onto the ...

Kinetics of Photoinduced Electron Transfer between DNA Bases and Triplet 3,3′,4,4′-Benzophenone Tetracarboxylic Acid in Aqueous Solution of Different pH's: Proton-Coupled Electron Transfer?

The kinetics of triplet state quenching of 3,3′,4,4′-benzophenone tetracarboxylic acid (BPTC) by DNA bases adenine, adenosine, thymine, and thymidine has been investigated in aqueous solution using time-resolved laser flash photolysis. The observation of the BPTC ketyl radical anion at λmax = 630 nm indicates that one electron transfer is involved in the quenching reactions. The pH-dependence of the quenching rate constants is measured in detail. As a result, the chemical reactivity of the reactants is assigned. The bimolecular rate constants of the quenching reactions between triplet BPTC and adenine, adenosine, thymine, and thymidine are kq = 2.3 × 109 (4.7 < pH < 9.9), kq = 4.0 × 109 (3.5 < pH < 4.7), kq = 1.0 × 109 (4.7 < pH < 9.9), and kq = 4.0 × 108 M–1 s–1 (4.7 < pH < 9.8), respectively. Moreover, it reveals that in strong basic medium (pH = 12.0) a keto–enol tautomerism of thymine inhibits its reaction with triplet BPTC. Such a behavior is not possible for thymidine because of its deoxyribose group. In addition, the pH-dependence of the apparent electrochemical standard potential of thymine in aqueous solution was investigated by cyclic voltammetry. The ΔE/ΔpH ≈ −59 mV/pH result is characteristic of proton-coupled electron transfer. This behavior, together with the kinetic analysis, leads to the conclusion that the quenching reactions between triplet BPTC and thymine involve one proton-coupled electron transfer.

Enhanced Photoinduced Electron-Transfer Reduction of Li+@C60in Comparison with C60

Kinetics of photoinduced electron transfer from a series of electron donors to the triplet excited state of lithium ion-encapsulated C60 (Li(+)@C60) was investigated in comparison with the corresponding kinetics of the photoinduced electron transfer to the triplet excited state of pristine C60. Femtosecond laser flash photolysis measurements of Li(+)@C60 revealed that singlet excited state of Li(+)@C60 (λmax = 960 nm) underwent intersystem crossing to the triplet excited state [(3)(Li(+)@C60)*: λmax = 750 nm] with a rate constant of 8.9 × 10(8) s(-1) in deaerated benzonitrile (PhCN). The lifetime of (3)(Li(+)@C60)* was determined by nanosecond laser flash photolysis measurements to be 48 μs, which is comparable to that of C60. Efficient photoinduced electron transfer from a series of electron donors to (3)(Li(+)@C60)* occurred to produce the radical cations and Li(+)@C60(•-). The rate constants of photoinduced electron transfer of Li(+)@C60(•-) are significantly larger than those of C60 when the rate constants are less than the diffusion-limited value in PhCN. The enhanced reactivity of (3)(Li(+)@C60)* as compared with (3)C60* results from the much higher one-electron reduction potential of Li(+)@C60 (0.14 V vs SCE) than that of C60 (-0.43 V vs SCE). The rate constants of photoinduced electron transfer reactions of Li(+)@C60 and C60 were evaluated in light of the Marcus theory of electron transfer to determine the reorganization energies of electron transfer. The reorganization energy of electron transfer of Li(+)@C60 was determined from the driving force dependence of electron transfer rate to be 1.01 eV, which is by 0.28 eV larger than that of C60 (0.73 eV), probably because of the change in electrostatic interaction of encapsulated Li(+) upon electron transfer in PhCN.

Fine-tuning alkyne cycloadditions: Insights into photochemistry responsible for the double-strand DNA cleavage via structural perturbations in diaryl alkyne conjugates

Hybrid molecules combining photoactivated aryl acetylenes and a dicationic lysine moiety cause the most efficient double-strand (ds) DNA cleavage known to date for a small molecule. In order to test the connection between the alkylating ability and the DNA-damaging properties of these compounds, we investigated the photoreactivity of three isomeric aryl–tetrafluoropyridinyl (TFP) alkynes with amide substituents in different positions (o-, m-, and p-) toward a model π-system. Reactions with 1,4-cyclohexadiene (1,4-CHD) were used to probe the alkylating properties of the triplet excited states in these three isomers whilst Stern–Volmer quenching experiments were used to investigate the kinetics of photoinduced electron transfer (PET). The three analogous isomeric lysine conjugates cleaved DNA with different efficiencies (34, 15, and 0% of ds DNA cleavage for p-, m-, and o-substituted lysine conjugates, respectively) consistent with the alkylating ability of the respective acetamides. The significant protecting effect of the hydroxyl radical and singlet oxygen scavengers to DNA cleavage was shown only with m-lysine conjugate. All three isomeric lysine conjugates inhibited human melanoma cell growth under photoactivation: The p-conjugate had the lowest CC50 (50% cell cytotoxicity) value of 1.49 × 10−7 M.

Photoinduced Electron Transfer of Oxazine 1/TiO2 Nanoparticles at Single Molecule Level by Using Confocal Fluorescence Microscopy

Kinetics of photoinduced electron transfer (ET) from oxazine 1 dye to TiO(2) nanoparticles (NPs) surface is studied at a single molecule level by using confocal fluorescence microscopy. Upon irradiation with a pulsed laser at 630 nm, the fluorescence lifetimes sampled among 100 different dye molecules are determined to yield an average lifetime of 2.9 +/- 0.3 ns, which is close to the value of 3.0 +/- 0.6 ns measured on the bare coverslip. The lifetime proximity suggests that most interfacial electron transfer (IFET) processes for the current system are inefficient, probably caused by physisorption between dye and the TiO(2) film. However, there might exist some molecules which are quenched before fluorescing and fail to be detected. With the aid of autocorrelation analysis under a three-level energy system, the IFET kinetics of single dye molecules in the conduction band of TiO(2) NPs is evaluated to be (1.0 +/- 0.1) x 10(4) s(-1) averaged over 100 single molecules and the back ET rate constant is 4.7 +/- 0.9 s(-1). When a thicker TiO(2) film is substituted, the resultant kinetic data do not make a significant difference. The trend of IFET efficacy agrees with the method of fluorescence lifetime measurements. The obtained forward ET rate constants are about ten times smaller than the photovoltage response measured in an assembled dye-sensitized solar cell. The discrepancy is discussed. The inhomogeneous and fluctuation characters for the IFET process are attributed to microenvironment variation for each single molecule. The obtained ET rates are much slower than the fluorescence relaxation. Such a small ET quantum yield is yet feasibly detectable at a single molecule level.

Photoinduced Intramolecular Electron Transfer in Conjugated Perylene Bisimide-Dithienothiophene Systems: A Comparative Study of a Small Molecule and a Polymer

The solution photophysical properties of two conjugated dithienothiophene (DTT)-perylene bisimide (PBI) systemsa polymer, poly{[N,N'-bis(2-decyl-tetradecyl)-3,4,9,10-perylene diimide-1,7-diyl]-alt-(dithieno[3,2-b:2',3'-d]thiophene-2,6-diyl)}, and a small molecule, 1,7-bis(dithieno[3,2-b:2',3'-d]thiophene-2-yl)-N,N'-bis(2-decyl-tetradecyl)-3,4,9,10-perylene diimidein solution have been investigated. Strong quenching of the fluorescence of the PBI moiety was observed in both DTT-PBI systems, suggesting the possibility of an efficient intramolecular electron-transfer process. The kinetics of photoinduced electron transfer in the DTT-PBI polymer and monomer in solutions were explored by femtosecond time-resolved transient absorption spectra. It was found that both the rates of charge separation and charge recombination in the DTT-PBI polymer were approximately double those in the small molecule. This indicates that electronic coupling plays an important role in the electron-transfer process in a polymer system.

Photopolymerization Sensitized by CdTe Nanocrystals in Ionic Liquid: Highly Efficient Photoinduced Electron Transfer

We demonstrate a photopolymerization sensitized by CdTe nanocrystals in room temperature ionic liquid. The kinetics of photoinduced electron transfer from cationic CdTe nanocrystals to polymerization initiators in several kinds of ionic liquids is also studied. From the analyses of photoluminescence quenching measurement, the electron transfer rates from CdTe nanocrystals to organic electron acceptors are anomalously large in the ionic liquid medium.

Hemicyanine sec-butyltriphenylborate salts as effective initiators of free radical polymerization initiated via photoinduced electron transfer process. Part II. Kinetic studies and application of electron transfer theory

Ability of two groups of sec-butyltriphenylborate salts of hemicyanine dyes (HCBo), namely styrylpyridinium borates (ABo group) and styrylbenzoxazole borates (BBo group), to photoinitiation of radical polymerization of 2-ethyl-2-(hydroxymethyl)-1,3-propanediol triacrylate (TMPTA) was investigated. The effects of a dye structure and free energy of activation of electron transfer between the components of photoinitiator pair (DGel) on the rate of photoinduced radical polymerization of TMPTA (Rp) were determined. The results have been analyzed on the basis of Marcus theory describing the kinetics of photoinduced electron transfer. The values of DGel were previously calculated using Rehm-Weller equation [equation (2)]. Oxidation potential of borate was determined using cyclic voltammetry method. Monomer conversion, quantum yield of the polymerization process (Fpolym) as well as photochemical bleaching yield (Fbl) dependently on chemical structure of HCBo investigated were also determined. It was found that the structure of initiator (dye) influenced Rp value. In case of use of ABo type initiators Rp values were similar - 3.24-4.88 while in case of BBo they were placed in wide range 1.00-5.05. The highest value has been reached for B3 dye. B3Bo initiator showed also the highest values of Fpolym, Fbl and monomer conversion. Linear dependence between Rp and Fbl was observed only for ABo type initiators.

Photoinduced Charge Transfer in Helical Polypeptides

AbstractFor Abstract see ChemInform Abstract in Full Text.

Oxidation-state-dependent protein docking between cytochrome c and cytochrome b(5): high-pressure laser flash photolysis study.

To characterize the protein-protein interaction during electron transfer, we used Zn-substituted cytochrome c (ZnCytc) as a model of ferrous Cytc and determined the volume change, DeltaV(d)(Zn), for the dissociation of its complex with ferric cytochrome b(5) (Cytb(5)) by the pressure dependence of its photoinduced electron-transfer kinetics. Under ambient pressure, the dissociation constant, K(d)(Zn), of the ZnCytc/Cytb(5) complex was dependent on the buffer concentration, 1.5 and 12 microM in 2 and 10 mM Tris-HCl, pH 7.4, respectively, which was consistent with formation of salt bridges in its complexation. The dissociation of one salt bridge is usually associated with large volume changes of -10 to -30 cm(3) mol(-1), while pressure dependence of K(d)(Zn) resulted in smaller value of DeltaV(d)(Zn), -8.5 cm(3) mol(-1). Therefore, the interaction between ZnCytc and Cytb(5) cannot be explained only by salt bridge interaction, and the partial cancellation by the positive volume change due to the additional hydrophobic interaction is a plausible explanation for the observed DeltaV(d)(Zn). In addition, DeltaV(d)(Zn) of -8.5 cm(3) mol(-1) was considerably smaller than the previously reported volume change, DeltaV(d)(Fe), of -122 cm(3) mol(-1) in the ferric Cytc/Cytb(5) complex dissociation [Rodgers and Sligar (1991) J. Mol. Biol. 221, 1453-1460]. ZnCytc used here has been assumed to be a reliable model of ferrous Cytc, and thus the discrepancy between our present DeltaV(d)(Zn) and the previous DeltaV(d)(Fe) is discussed on the basis of the protein docking dependent on the oxidation states of heme iron in Cytc.

Photoinduced charge transfer in helical polypeptides

Investigation of electron transfer in synthetic polypeptides provides an important probe of how charge entrainment is mediated in redox-active proteins, including photosynthetic reaction centers. Interest in this field has focused increasingly on experimental probes of photoinduced electron transfer kinetics and thermodynamics, and the influence of various features of polypeptide templates (e.g. the hydrogen bonding network, the permanent dipole moment for α-helices) that assemble redox groups for long range charge transfer. A review of the various approaches is presented here with attention to heliogenic peptides and the mediation of photoinduced charge entrainment.

Photoinduced electron transfer in multicomponent arrays of a π-stacked fullerene porphyrin dyad and diazabicyclooctane or a fulleropyrrolidine ligand

The kinetics of photoinduced electron transfer in new multicomponent arrays of a π-stacked fullerene porphyrin dyad and diazabicyclooctane or a fulleropyrrolidine ligand have been determined and depend on the molecular architecture

Photochemical accumulation and recombination of ion pairs undergoing the singlet–triplet conversion

Real time resolved kinetics of photoinduced electron transfer in solution is investigated theoretically. The results confirm that within the unified theory of bimolecular photoionization followed by geminate recombination the slow diffusional tails in long time kinetics of charged products are unavoidable, if ion pairs constitute a one component system. We demonstrate that observed quasi-exponential asymptotics can only be due to the multi-component nature of the reaction products. For the reaction under consideration we expect these components to be ion pairs in singlet and triplet spin states. The reaction pathway of spin interconversion and subsequent recombination of ions to both ground and triplet states of neutral reactants is proposed, whose prediction can be directly checked experimentally.

Kinetics of photo-induced electron transfer from high-potential iron-sulfur protein to the photosynthetic reaction center of the purple phototroph Rhodoferax fermentans.

The kinetics of photo-induced electrontransfer from high-potential iron-sulfur protein (HiPIP) to the photosynthetic reaction center (RC) of the purple phototroph Rhodoferarfermentans were studied. The rapid photooxidation of heme c-556 belonging to RC is followed, in the presence of HiPIP, by a slower reduction having a second-order rate constant of 4.8 x 10(7) M(-1) x s(-1). The limiting value of kobs at high HiPIP concentration is 95 s(-1). The amplitude of this slow process decreases with increasing HiPIP concentration. The amplitude of a faster phase, observed at 556 and 425 nm and involving heme c-556 reduction, increases proportionately. The rate constant of this fast phase, determined at 425 and 556 nm, is approximately 3 x 10(5) s(-1). This value is not dependent on HiPIP concentration, indicating that it is related to a first-order process. These observations are interpreted as evidence for the formation of a HiPIP-RC complex prior to the excitation flash, having a dissociation constant of -2.5 microM. The fast phase is absent at high ionic strength, indicating that the complex involves mainly electrostatic interactions. The ionic strength dependence of kobs for the slow phase yields a second-order rate constant at infinite ionic strength of 5.4 x 10(6) M(-1) x s(-1) and an electrostatic interaction energy of -2.1 kcal/mol (1 cal = 4.184 J). We conclude that Rhodoferar fermentans HiPIP is a very effective electron donor to the photosynthetic RC.