Back Electron Transfer Reaction Research Articles

(Invited) Z-Scheme Water Splitting Using Dye-Sensitized Metal Oxide Nanosheets

Solar water splitting by a heterogeneous photocatalyst, producing H2 and O2, is a potential solution for realizing large-scale H2 production. While dye-sensitized metal oxides are good candidates as H2 evolution photocatalysts for solar-driven Z-scheme water splitting, their solar-to-hydrogen (STH) energy conversion efficiencies remains low because of uncontrolled charge recombination reactions. Here it is shown that modification of Ru dye-sensitized, Pt- intercalated HCa2Nb3O10 nanosheets (Ru/Pt/HCa2Nb3O10) with both amorphous Al2O3 and poly(styrenesulfonate) (PSS) improves the STH efficiency of Z-scheme overall water splitting by a factor of ~100, when the nanosheets are used in combination with a WO3-based O2 evolution photocatalyst and an I3 –/I– redox mediator, relative to an analogous system that uses unmodified Ru/Pt/HCa2Nb3O10. The Al2O3 and PSS modifiers, which have previously been shown to suppress back electron transfer reactions in a dye-sensitized H2 evolution photocatalyst, enabled operation of the Z-scheme system even at low intensity of simulated sunlight without a decrease in the STH values. By using the optimized photocatalyst, PSS/Ru/Al2O3/Pt/HCa2Nb3O10, a maximum STH of 0.12% and an apparent quantum yield of 4.1% at 420 nm were obtained, by far the highest among dye-sensitized water splitting systems and also comparable to conventional semiconductor-based suspended particulate photocatalyst systems.

The effects of Pt cocatalyst particle size on charge transfer kinetics in dye-sensitized SrTiO3 photocatalysts for hydrogen evolution studied by time-resolved emission/absorption spectroscopy

Dye-sensitized H2 evolution systems were constructed from Pt-loaded SrTiO3 and Ru(II) complex photosensitizers as building blocks, and the impact of the Pt cocatalyst was investigated. In the presence of a stronger electron donor (sodium ascorbate), SrTiO3 loaded with larger Pt particles showed higher activity. On the other hand, the sample having smaller Pt particles gave higher activity in the presence of a weaker electron donor (ethylenediaminetetraacetic acid disodium salt dihydrate). Time-resolved emission/absorption measurements revealed that the larger Pt particles facilitated electron injection from the excited Ru sensitizer to Pt/SrTiO3, while the smaller one improved charge carrier separation by suppressing the back electron transfer reaction from Pt/SrTiO3 to the oxidized sensitizer. These results show that the cocatalyst particle size can affect the kinetics of the electron injection and carrier separation processes. Judicious choice of the Pt particle size can therefore improve the H2 evolution activity in accordance with the reaction system.

Electrical Transport, Structural, Optical and Thermal Properties of [(1-x)Succinonitrile: xPEO]-LiTFSI-Co(bpy)3(TFSI)2-Co(bpy)3(TFSI)3 Solid Redox Mediators.

The solar cell has been considered one of the safest modes for electricity generation. In a dye-sensitized solar cell, a commonly used iodide/triiodide redox mediator inhibits back-electron transfer reactions, regenerates dyes, and reduces triiodide into iodide. The use of iodide/triiodide redox, however, imposes several problems and hence needs to be replaced by alternative redox. This paper reports the first Co2+/Co3+ solid redox mediators, prepared using [(1−x)succinonitrile: xPEO] as a matrix and LiTFSI, Co(bpy)3(TFSI)2, and Co(bpy)3(TFSI)3 as sources of ions. The electrolytes are referred to as SN_E (x = 0), Blend 1_E (x = 0.5 with the ethereal oxygen of the PEO-to-lithium ion molar ratio (EO/Li+) of 113), Blend 2_E (x = 0.5; EO/Li+ = 226), and PEO_E (x = 1; EO/Li+ = 226), which achieved electrical conductivity of 2.1 × 10−3, 4.3 × 10−4, 7.2 × 10−4, and 9.7 × 10−7 S cm−1, respectively at 25 °C. Only the blend-based polymer electrolytes exhibited the Vogel-Tamman-Fulcher-type behavior (vitreous nature) with a required low pseudo-activation energy (0.05 eV), thermal stability up to 125 °C, and transparency in UV-A, visible, and near-infrared regions. FT-IR spectroscopy demonstrated the interaction between salt and matrix in the following order: SN_E < Blend 2_E < Blend 1_E << PEO_E. The results were compared with those of acetonitrile-based liquid electrolyte, ACN_E.

Temperature dependence of nanosecond charge recombination in mutant Rhodobacter sphaeroides reaction centers: modelling of the protein dynamics

We investigated the influence of a range of factors—temperature, redox midpoint potential of an electron carrier, and protein dynamics—on nanosecond electron transfer within a protein. The model reaction was back electron transfer from a bacteriopheophytin anion, HA−, to an oxidized primary electron donor, P+, in a wild type Rhodobacter sphaeroides reaction center (RC) with a permanently reduced secondary electron acceptor (quinone, QA−). Also used were two modified RCs with single amino acid mutations near the monomeric bacteriochlorophyll, BA, located between P and HA. Both mutant RCs showed significant slowing down of this back electron transfer reaction with decreasing temperature, similar to that observed with the wild type RC, but contrasting with a number of single point mutant RCs studied previously. The observed similarities and differences are explained in the framework of a (P+BA− ↔ P+HA−) equilibrium model with an important role played by protein relaxation. The major cause of the observed temperature dependence, both in the wild type RC and in the mutant proteins, is a limitation in access to the thermally activated pathway of charge recombination via the state P+BA− at low temperatures. The data indicate that in all RCs both charge recombination pathways, the thermally activated one and a direct one without involvement of the P+BA− state, are controlled by the protein dynamics. It is concluded that the modifications of the protein environment affect the overall back electron transfer kinetics primarily by changing the redox potential of BA and not by changing the protein relaxation dynamics.

Tuning the redox potential of tyrosine-histidine bioinspired assemblies.

Photosynthesis powers our planet and is a source of inspiration for developing artificial constructs mimicking many aspects of the natural energy transducing process. In the complex machinery of photosystem II (PSII), the redox activity of the tyrosine Z (Tyrz) hydrogen-bonded to histidine 190 (His190) is essential for its functions. For example, the Tyrz-His190 pair provides a proton-coupled electron transfer (PCET) pathway that effectively competes against the back-electron transfer reaction and tunes the redox potential of the phenoxyl radical/phenol redox couple ensuring a high net quantum yield of photoinduced charge separation in PSII. Herein, artificial assemblies mimicking both the structural and redox properties of the Tyrz-His190 pair are described. The bioinspired constructs contain a phenol (Tyrz model) covalently linked to a benzimidazole (His190 model) featuring an intramolecular hydrogen bond which closely emulates the one observed in the natural counterpart. Incorporation of electron-withdrawing groups in the benzimidazole moiety systematically changes the intramolecular hydrogen bond strength and modifies the potential of the phenoxyl radical/phenol redox couple over a range of ~ 250mV. Infrared spectroelectrochemistry (IRSEC) demonstrates the associated one-electron, one-proton transfer (E1PT) process upon electrochemical oxidation of the phenol. The present contribution provides insight regarding the factors controlling the redox potential of the phenol and highlights strategies for the design of futures constructs capable of transporting protons across longer distances while maintaining a high potential of the phenoxyl radical/phenol redox couple.

Effect of TiO2 Photoanodes Morphology and Dye Structure on Dye-Regeneration Kinetics Investigated by Scanning Electrochemical Microscopy

The dye regeneration in dye-sensitized solar cells (DSSCs) is improved by optimizing the charge separation at the level of the sensitized semiconductor treatment of the mesoporous electrode by TiCl4 that passivates the surface for back electron transfer reactions. The dye-regeneration kinetics is analyzed for DN216- and D358-sensitized porous TiO2 electrodes with and without a TiCl4 treatment by means of scanning electrochemical microscopy (SECM). Different mass transport limitation of the [Co(bpy)3]3+ mediator through the porous electrode is found for the comparison of the structurally similar dyes but cannot be detected for the thin layer introduced by the TiCl4 treatment. Phototransient measurements are conducted directly in the SECM cell without any intermediated sample manipulation. The results from those measurements corroborate the findings from steady state SECM measurements.

Effect of Push-Pull Ruthenium Complex Adsorption Conformation on the Performance of Dye Sensitized Solar Cells

A new series of tris-(bipyridyl)ruthenium-like complexes based on the 4-tripheylamine-2,2’:6’,2’’-terpyridine (TPA) push-pull ligand was prepared by incorporation of 4-carboxypyridine (cpy), 4,4’-dicarboxi-2,2’-bipyridine (dcbpy) and 4-carboxyterpyridine ligands (ctpy) ligands, in order to adsorb them on TiO2 in different anchoring conformations. The electron photoinjection and electron recombination processes of the respective dye-sensitized solar cells were greatly influenced by the molecular structure, which defined the surface concentration and surface charge on TiO2, such that the photoconversion efficiency was 10 times larger for [Ru(py)(dcbpy)(TPAtpy)] than for the [Ru(cpy)(bpy)(TPAtpy)](PF6) dye. Molecules anchoring in a more upright position and by a larger number of sites were shown to enhance the electron injection into TiO2 conduction band (CB) improving the short-circuit current (JSC), open circuit voltage (Voc) and the overall photoconversion efficiency. However, a positive net charge in the dye increased the back electron-transfer reactions and induced a decrease in both Voc and conversion efficiency.

Complementary properties of silver nanoparticles on the photovoltaic performance of titania nanospheres based photoanode in dye-sensitized solar cells

In this study, the fabrication of photoanode of dye-sensitized solar cell using silver@titania nanospheres and N719 dye, and its enhanced photovoltaic performance in the dye-sensitized solar cell is described. Silver@titania with different mol. % of silver (1, 3 and 5 %) are synthesized using facile sol-gel and photochemical reduction method and are characterized by suitable analytical techniques. The synthesized nanocomposite materials showed nanospheres like morphology. Moreover, the photoluminescence studies revealed that incorporation of silver nanoparticles on the titania nanospheres surface suppressed the charge recombination process and which is more beneficial for dye-sensitized solar cells. Silver@titania nanospheres are successfully employed as a photoanode and demonstrated with an enhanced solar-to-electrical energy conversion efficiency of 5.24 %, under full sun illumination (100 mW cm−2, AM 1.5G) which is 30.67 % enhancement than that of bare titania nanospheres (4.01 %). This enhanced efficiency is attributed to the rapid interfacial charge transfer process and plasmonic effect offered by silver nanoparticles present in the silver@titania nanospheres. The improved charge transfer process led to minimize the back electron transfer reaction in the device. With complementary properties of silver nanoparticles, the high performance demonstrated by the silver@titania nanospheres could be an excellent candidate for the light energy harvesting applications.

Mechanism of the photorelease of alcohols from the 9-phenyl-9-tritylone protecting group.

The 9-phenyl-9-tritylone photoremovable protecting group was examined using both photoproduct analysis and laser flash photolysis in order to determine the details of its mechanism of alcohol release. It is shown that formation of the tritylone anion radical is required for alcohol release. Attempts to trigger release via intramolecular photoinduced electron transfer were unsuccessful due to rapid back electron transfer reactions of the triplet diradical anion.

Riboflavin-induced Type 1 photo-oxidation of tryptophan using a high intensity 365 nm light emitting diode

The mechanism of photo-oxidation of tryptophan (Trp) sensitized by riboflavin (RF) was examined employing high concentrations of Trp and RF, with a high intensity 365 nm light emitting diode (LED) source under N2, 20% and 100% O2 atmospheres. Dimerization of Trp was a major pathway under the N2 atmosphere, though this occurred with a low yield (DφTrp = 5.9 × 10−3), probably as a result of extensive back electron transfer reactions between RF•- and Trp(H)•+. The presence of O2 decreased the extent of this back electron transfer reaction, and the extent of Trp dimerization. This difference is attributed to the formation of O2•- (generated via electron transfer from RF•- to O2) which reacts rapidly with Trp• leading to extensive consumption of the parent amino acid and formation of peroxides and multiple other oxygenated products (N-formylkynurenine, alcohols, diols) of Trp, as detected by LC-MS. Thus, it appears that the first step of the Type 1 mechanism of Trp photo-oxidation, induced by this high intensity 365 nm light source, is an electron transfer reaction between the amino acid and 3RF, with the presence of O2 modulating the subsequent reactions and the products formed, as a result of O2•- formation. These data have potential biological significance as LED systems and RF-based treatments have been proposed for the treatment of pathological myopia and keratitis.

General Strategy for Improving the Quantum Efficiency of Photoredox Hydroamidation Catalysis.

The quantum efficiency in photoredox catalysis is the crucial determinant of energy intensity and, thus, is intrinsically tied to the sustainability of the overall process. Here, we track the formation of different transient species of a catalytic photoredox hydroamidation reaction initiated by the reaction of an Ir(III) photoexcited complex with 2-cyclohexen-1-yl(4-bromophenyl)carbamate. We find that the back reaction between the amidyl radical and Ir(II) photoproducts generated from the quenching reaction leads to a low quantum efficiency of the system. Using transient absorption spectroscopy, all of the rate constants for productive and nonproductive pathways of the catalytic cycle have been determined, enabling us to establish a kinetically competent equilibrium involving the crucial amidyl radical intermediate that minimizes its back reaction with the Ir(II) photoproduct. This strategy of using an off-pathway equilibrium allows us to improve the overall quantum efficiency of the reaction by a factor of 4. Our results highlight the benefits from targeting the back-electron transfer reactions of photoredox catalytic cycles to lead to improved energy efficiency and accordingly improved sustainability and cost benefits of photoredox synthetic methods.

Immobilization of Molecular Catalysts for Enhanced Redox Catalysis

AbstractIn the homogenous phase, redox catalysts are often deactivated by bimolecular reactions. For example, the charge‐separated state of photoredox catalysts decayed via bimolecular back electron transfer reactions between the charge‐separated molecules to decrease the lifetimes of the catalytically active species. When photoredox catalysts are immobilized on solid supports, the lifetime of the charge‐separated state was remarkably elongated to enhance the photocatalytic activity. Immobilization of photoredox catalysts on electrodes is required for photocurrent generation, leading to development of solar cells. Metal‐oxygen intermediates, which are active for oxidation of various substrates including water oxidation, are also deactivated via bimolecular reactions to produce inactive forms such as dinuclear metal bis‐μ‐oxo complexes. Immobilization of metal complex catalysts on solid supports prohibits the bimolecular deactivation, enhancing the catalytic activity and stability. This Review focuses on recent development of immobilization of both organic and inorganic molecular catalysts on various supports for enhancement of the catalytic activity, selectivity and stability in thermal and photoinduced redox reactions.

Ultrafast photoresponsive materials for all-optical light modulation by polymer thin films

Ultrafast and single exponential responsive materials were achieved by encapsulating donor-acceptor linked molecules into cucurbitunitl[8], CB[8], as nanocavity hosts. Various aromatic groups were linked with various types of 4,4′-bipyridinium groups through propyloxy linker. They showed characteristic charge-transfer (CT) absorption with specific colors in aqueous solutions. Upon addition of CB[8], they showed remarkably different colors due to intramolecular CT complex formation in CB[8]. Upon femtosecond laser excitation of CT band extremely fast electron transfer occurred from a donor to an acceptor unit accompanying new absorption in the visible to near-infrared region due mainly to photoreduced bipyridinium derivatives. Thermal back electron transfer reactions in CB[8] were found to follow a single exponential decay with rate constants ranging more than two orders depending on the combination of a donor and an acceptor unit. Their rate constants vs. free energy changes for oxidized donors and reduced acceptors in linked molecules were expressed by the Marcus theory.

Electron‐Transfer Reactions Triggered by Uncharged or Cationic Photosensitizer: Methodology for Generation of o‐Quinodimethane and Analysis of Back Electron‐Transfer Process

AbstractPhotoinduced electron transfer (PET) followed by back electron transfer (BET) reactions of 1,2‐bis(α‐styryl)benzenes 1, generates o‐quinodimethane derivative 2 via the corresponding radical cation 2⋅+. This process serves as a facile method to form o‐quinodimethane intermediates. The intermediacy of 2⋅+ and 2 were confirmed by using various spectroscopic methods and trapping reactions with 3O2 and dienophiles. Kinetic analysis using nanosecond time‐resolved absorption showed that 2⋅+ was transformed to 2 via a BET process, in which the decay of 2⋅+ and rise of 2 have almost the same rate constants of kDECAY=5.6×105 s−1 and kRISE=5.9×105 s−1, respectively. The net BET rate constant evaluated by using Marcus theory is much faster than these experimental values, owing to the reduction based on diffusion.

Organic Ion-Pair Charge-Transfer (IPCT) Nanoparticles: Synthesis and Photoinduced Electrochromism.

We report fabrication of photoinduced-electrochromic (or photochromic) organic nanoparticles that can be regarded as a new class of photoswitchable nanomaterials. Ion association between dicationic benzyl viologen (BzV2+) and tetraphenylborate anion (TPB-) in water produces organic ion-pair charge-transfer-type (IPCT) nanoparticles with a mean diameter of 15-20 nm. Electrostatic binding of BzV2+ with TPB- results in the prominent appearance of a new broad absorption band at 400-550 nm, which can be assign to the CT transition between these species. The IPCT nanoparticles show an interesting photoinduced electrochromic behavior caused by the electron transfer (ET) from TPB- to BzV2+, giving an intense blue-color dispersion. The feed charge stoichiometry (= [TPB-]/(2 × [BzV2+])) does not influence the kinetics of the photoinduced electrochromism, but it remarkably controls the reverse ET reaction (or retention time) in the nanoparticles; in the case of neutral stoichiometry, a very long-lived ET state can be observed, which is mainly due to decomposition of the oxidized TPB (sacrificial donor) and the resultant absence of the counterpart of benzyl viologen radical cations (BzV+•) for back ET reaction. On the other hand, the excess presence of TPB- does not cause the decomposition of oxidized TPB in the nanoparticle, making the reverse ET reaction faster, but a moderately long-lived ET state is still observable, which is probably originated from efficient electron diffusion or transport between molecules inside the nanoparticle.

EFFECTS OF TiO2 FILM THICKNESS AND ELECTROLYTE CONCENTRATION ON PHOTOVOLTAIC PERFORMANCE OF DYE-SENSITIZED SOLAR CELL

Effects of film thickness and electrolyte concentration on the photovoltaic performance of TiO2-based dye-sensitized solar cell (DSSC) were studied. Nanocrystalline anatase TiO2 thin films with varying thicknesses (3.2–18.9[Formula: see text][Formula: see text]m) have been deposited on FTO/glass substrates by screen printing method as work electrodes for DSSC. The prepared samples were characterized by UV-Vis spectroscopy, Atomic Force Microscopy/Scanning Tunneling Microscopy (AFM/STM) and X-ray diffraction (XRD). The optimal thickness of the TiO2 photoanode is 13.5[Formula: see text][Formula: see text]m. Short-circuit photocurrent density ([Formula: see text]) increases with film thickness due to enlargement of surface area whereas open-circuit voltage decreases with increase in thickness due to increase in electron diffusion length to the electrode. However, the [Formula: see text] and [Formula: see text] of DSSC with a film thickness of 18.9[Formula: see text][Formula: see text]m (7.5[Formula: see text]mA/cm2 and 0.687[Formula: see text]V) are smaller than those of DSSC with a TiO2 film thickness of 13.5[Formula: see text][Formula: see text]m (9.9[Formula: see text]mA/cm2 and 0.734[Formula: see text]V). This is because the increased thickness of TiO2 thin film resulted in the decrease in the transmittance of TiO2 thin films hence reducing the incident light intensity on the N719 dye. Photovoltaic performance also depends greatly on the redox couple concentration in iodide[Formula: see text]triiodide. [Formula: see text] decreases as the redox concentration increases as a result of increased viscosity of the solution which lowers ion mobility. Similarly, [Formula: see text] decreases as the electrolyte concentration increases due to enhanced back electron transfer reaction. An optimum power conversion efficiency of 4.3% was obtained in a DSSC with the TiO2 film thickness of 13.5[Formula: see text][Formula: see text]m and redox concentration of 0.03[Formula: see text]mol dm[Formula: see text] under AM 1.5G illumination at 100[Formula: see text]mW/cm2.

Preparation and improvement in photovoltaic performance of dye-sensitized solar cells using carbon dioxide

We report preparation and improvement in photovoltaic performance of N719-based dye-sensitized solar cells (DSSCs) using pressurized carbon dioxide (CO2) as a co-solvent for the absorption process on the TiO2 photoelectrode surface. Effective absorption of the N719 molecules on the TiO2 surface was achieved using CO2 processing, and the absorption time was shortened drastically from 24 h (in the dip process) to less than 3 h. The cells prepared under pressurized CO2 for the absorption showed greater photovoltaic performance, especially higher short-circuit current density and conversion efficiency, compared with that from typical dip method. It was revealed that the suitable CO2 pressure for the absorption was 5 MPa and the efficiency was achieved to be more than 7.5 %. Prevention of back electron transfer reactions from TiO2 to oxidized dyes or iodides was caused currently, because the homogeneous coverage of N719 molecules on the TiO2 surface was obtained by the use of pressurized CO2.

Fluorescence quenching of 9-cyanoanthracene by metallo-octaethylporphyrins in cyanobenzene

Investigations are carried out on the nature of photoinduced electron transfer reactions within some metallo-octaethylporphyrin (zinc octaethylporphyrin and magnesium octaethylporphyrin) donors and acceptor 9-cyanoanthracene in cyanobenzene at 300 K by steady state and time-resolved spectroscopic methods. Acceptor is selectively excited at 366 nm and 374 nm for steady state and time-resolved measurements, respectively, where donor metalloporphyrins possess negligible absorbance. Evidence is found for simultaneous occurrence of Förster's type singlet–singlet energy transfer process along with electron transfer. High exergonic values of Gibbs free energy change for both forward and back electron transfer reactions indicate the possibility of occurrences of these two consecutive processes in the Marcus inverted region.

Small Reorganization Energies of Photoinduced Electron Transfer between Spherical Fullerenes

Rate constants of photoinduced electron transfer between spherical fullerenes were determined using triscandium nitride encapsulated C80 fullerene (Sc3N@C80) as an electron donor and the triplet excited state of lithium ion-encapsulated C60 fullerene (Li(+)@C60) as an electron acceptor in polar and less polar solvents by laser flash photolysis measurements. Upon nanosecond laser excitation at 355 nm of a benzonitrile (PhCN) solution of Li(+)@C60 and Sc3N@C80, electron transfer from Sc3N@C80 to the triplet excited state [(3)(Li(+)@C60)*] occurred to produce Sc3N@C80(•+) and Li(+)@C60(•-) (λ(max) = 1035 nm). The rates of the photoinduced electron transfer were monitored by the decay of absorption at λ(max) = 750 nm due to (3)(Li(+)@C60)*. The second-order rate constant of electron transfer from Sc3N@C80 to (3)(Li(+)@C60)* was determined to be k(et) = 1.5 × 10(9) M(-1) s(-1) from dependence of decay rate constant of (3)(Li(+)@C60)* on the Sc3N@C80 concentration. The rate constant of back electron transfer from Li(+)@C60(•-) to Sc3N@C80(•+) was also determined to be k(bet) = 1.9 × 10(9) M(-1) s(-1), which is close to be the diffusion limited value in PhCN. Similarly, the rate constants of photoinduced electron transfer from C60 to (3)(Li(+)@C60)* and from Sc3N@C80 to (3)C60* were determined together with the back electron-transfer reactions. The driving force dependence of log k(et) and log k(bet) was well fitted by using the Marcus theory of outer-sphere electron transfer, in which the internal (bond) reorganization energy (λi) was estimated by DFT calculations and the solvent reorganization energy (λs) was calculated by the Marcus equation. When PhCN was replaced by o-dichlorobenzene (o-DCB), the λ value was decreased because of the smaller solvation changes of highly spherical fullerenes upon electron transfer in a less polar solvent.

Interfacial charge recombination of Os(ii)–polypyridyl–resorcinol complex on oleic acid capped TiO2 surface: what determines the dynamics?

Femtosecond transient absorption spectroscopic studies of an osmium(II)–polypyridyl–resorcinol complex, 1, on oleic acid capped TiO2 have been carried out and the results have been compared with our previous studies on poly(vinyl)alcohol capped TiO2. While on both these surfaces the complex exhibits a single exponential <120 fs injection, the back electron transfer reaction is slower on the oleic acid capped TiO2 surface early on while it is faster on the same surface at longer times than on poly(vinyl)alcohol capped TiO2. The probable origin of these disparate observations has been analyzed.