Competitive Electron Transfer Research Articles

Fine Tuning between Radical versus Nonradical States of Azoheteroarenes on Selective Osmium Platforms.

The article highlights the cooperative impact of azoheteroarenes [abbt: 2,2'-azobis(benzothiazole), L1-L3; bmpd: (E)-1,2-bis(1-methyl-1H-pyrazole-3-yl) diazene, L4] and coligands [bpy: 2,2'-bipyridine; pap: 2-phenylazopyridine] in tuning radical (N-N•-) versus nonradical (N═N0) states of L on selective OsII-platforms in structurally/spectroscopically characterized monomeric [1]ClO4-[6]ClO4 and [1](ClO4)2-[2](ClO4)2/[7](ClO4)2-[8](ClO4)2, respectively. The preferred syn-configuration of L in the complexes prevented obtaining ligand bridged dimeric species. It revealed that {Os(bpy)2} facilitated the stabilization of both nonradical ([1](ClO4)2-[2](ClO4)2) and radical ([1]ClO4-[2]ClO4) states of L1/L2, while it delivered exclusively the radical form for L3 in [3]ClO4. In contrast, {Os(pap)2} generated radical states of L1-L3 in [4]ClO4-[6]ClO4, respectively, without any alteration of the redox state of OsII and azo (N═N0) function of the pap coligand. The neutral state of L4 was, however, ascertained in [7](ClO4)2 or [8](ClO4)2 irrespective of the nature of the metal fragment {Os(bpy)2} or {Os(pap)2}, respectively. Switching between radical and nonradical forms of L in the complexes as a function L and coligand could be addressed based on their relative FMO (frontier molecular orbital) energies. Multiple close redox steps of the complexes extended a competitive electron transfer scenario between the redox active components including metal/L/bpy/pap, leading to delicate electronic forms in each case.

The three kingdoms-Photoinduced electron transfer cascades controlled by electronic couplings.

Excited states are the key species in photocatalysis, while the critical parameters that govern their applications are (i) excitation energy, (ii) accessibility, and (iii) lifetime. However, in molecular transition metal-based photosensitizers, there is a design tension between the creation of long-lived excited (triplet), e.g., metal-to-ligand charge transfer (3MLCT) states and the population of such states. Long-lived triplet states have low spin-orbit coupling (SOC) and hence their population is low. Thus, a long-lived triplet state can be populated but inefficiently. If the SOC is increased, the triplet state population efficiency is improved-coming at the cost of decreasing the lifetime. A promising strategy to isolate the triplet excited state away from the metal after intersystem crossing (ISC) involves the combination of transition metal complex and an organic donor/acceptor group. Here, we elucidate the excited state branching processes in a series of Ru(II)-terpyridyl push-pull triads by quantum chemical simulations. Scalar-relativistic time-dependent density theory simulations reveal that efficient ISC takes place along 1/3MLCT gateway states. Subsequently, competitive electron transfer (ET) pathways involving the organic chromophore, i.e., 10-methylphenothiazinyl and the terpyridyl ligands are available. The kinetics of the underlying ET processes were investigated within the semiclassical Marcus picture and along efficient internal reaction coordinates that connect the respective photoredox intermediates. The key parameter that governs the population transfer away from the metal toward the organic chromophore either by means of ligand-to-ligand (3LLCT; weakly coupled) or intra-ligand charge transfer (3ILCT; strongly coupled) states was determined to be the magnitude of the involved electronic coupling.

Dioxygen-enhanced CO2 photoreduction on TiO2 supported Cu single-atom sites

Supported Cu single-atom (Cu-SA) catalysts have exhibited unique activity and selectivity in CO2 reduction, but they generally suffer from the easy transformation of active Cuδ+ into inactive Cu0. Herein, porous TiO2 supported Cu single atoms (Cu-SAs/TiO2) photocatalyst was fabricated and dioxygen (O2) enhanced photocatalytic CO2 reduction performance of Cu-SAs/TiO2 was investigated by in-situ characterizations and theoretical calculations. The results showed that both activity and stability of Cu-SAs/TiO2 photocatalyst could be improved by feeding a small amount of O2 (117.6 ppm) in the reaction system. The competitive electron transfer to O2 on Cu-SAs/TiO2 maintained dynamic stability of Cuδ+ active sites, and O2-derived *OOH species lowered the formation energy barrier of key intermediate *COOH as well as delivered *H species for accelerating intermediates protonation process. Total yield rate of CH4 and CO reached 34.64 μmol·gcat·h−1 for CO2 photoreduction over Cu-SAs/TiO2 after introducing 117.6 ppm O2, accompanied by enhanced durability.

N-O Bond Activation by Energy Transfer Photocatalysis.

A radical shift toward energy transfer photocatalysis from electron transfer photocatalysis under visible-light photoirradiation is often due to the greener prospects of atom and process economy. Recent advances in energy transfer photocatalysis embrace unique strategies for direct small-molecule activation and sometimes extraordinary chemical bond formation in the absence of additional/sacrificial reagents. Selective energy transfer photocatalysis requires careful selection of substrates and photocatalysts for a perfect match with respect to their triplet energies while having incompatible redox potentials to prevent competitive electron transfer pathways. Substrates containing labile N-O bonds are potential targets for generating reactive key intermediates via photocatalysis to access a variety of functionalized molecules. Typically, the differential electron densities of N and O heteroatoms have been exploited for generation of either N- or O-centered radical intermediates from the functionalized substrates by the electron transfer pathway. However, the latest developments involve direct N-O bond homolysis via energy transfer to generate both N- and O-centered radicals for their subsequent utilization in diverse organic transformations, also in the absence of sacrificial redox reagents. In this Account, we highlight our key contributions in the field of N-O bond activation via energy transfer photocatalysis to generate reactive radical intermediates, with coverage of useful mechanistic insights. More specifically, well-designed N-O bond-containing substrates such as 1,2,4-oxadiazolines, oxime esters, N-indolyl carbonates, and N-enoxybenzotriazoles were successfully utilized in versatile transformations involving selective energy transfer over electron transfer from photocatalysts with high triplet state energy. Direct access to reactive N-, O-, and C-centered (if decarboxylation follows) radical intermediates was achieved for diverse cross-couplings and rearrangement processes. In particular, a variety of open-shell nitrogen reactive intermediates, including N(sp2) and N(sp3) radicals and nitrenes, have been utilized. Notably, diversified transformations of identical substrates have been achieved through careful control of the reaction conditions. 1,2,4-Oxadiazolines were converted into spiro-azolactams through iminyl intermediates in the presence of 1O2, benzimidazoles, or sulfoximines with external sulfoxide reagent through triplet nitrene intermediates under inert conditions. Besides, oxime esters underwent either intramolecular C(sp3)-N radical-radical coupling or intermolecular C(sp3)-N radical-radical coupling by a combined energy transfer-hydrogen atom transfer strategy. Furthermore, a series of electrochemical and photophysical experiments as well as computational studies were performed to substantiate the proposed selective energy-transfer-driven reaction pathways. We hope that this Account will serve as a guide for the rational design of selective energy transfer processes through the activation of further labile chemical bonds.

Optimizing the crystallization process of conjugated polymer photocatalysts to promote electron transfer and molecular oxygen activation

Photocatalytic reactive oxygen species (ROS)-induced reactions provide an appealing method to solve the environmental and energy issues, whereas the current oxidation reaction generally suffered from low efficiency and poor selectivity due to uncontrollable O2 activation process. In view of the existence of competitive electron and energy transfer pathway, we propose that highly efficient superoxide radical anion (·O2−) generation can be achieved by optimizing the order degree of the photocatalyst. Herein, by taking carbon nitride polymer as an example, we optimized the crystallization process of carbon nitride polymer by selecting precursors of different polymerization degrees with a molten salt method. Benefiting from the high crystallinity, extended π-conjugated system and strong van der-Waals interactions between interlayers, the modified carbon nitride polymer exhibited accelerated charge transport and enhancement in electron induced molecular oxygen activation reactions under visible light. Consequently, the CCN-P exhibits about 1.5 times higher conversion rate in hydroxylation of phenylboronic acid and over 6-fold faster degradation rate in Rh B organic pollutants photodegradation with respect to pristine carbon nitride. This study provides an in-depth understanding on the optimization of the O2 activation process and the design of advanced photocatalysts.

Rediverting Electron Flux with an Engineered CRISPR-ddAsCpf1 System to Enhance the Pollutant Degradation Capacity of Shewanella oneidensis.

Pursuing efficient approaches to promote the extracellular electron transfer (EET) of extracellular respiratory bacteria is essential to their application in environmental remediation and waste treatment. Here, we report a new strategy of tuning electron flux by clustered regularly interspaced short palindromic repeat (CRISPR)-ddAsCpf1-based rediverting (namely STAR) to enhance the EET capacity of Shewanella oneidensis MR-1, a model extracellular respiratory bacterium widely present in the environment. The developed CRISPR-ddAsCpf1 system enabled approximately 100% gene repression with the green fluorescent protein (GFP) as a reporter. Using a WO3 probe, 10 representative genes encoding for putative competitive electron transfer proteins were screened, among which 7 genes were identified as valid targets for EET enhancement. Repressing the valid genes not only increased the transcription level of the l-lactate metabolism genes but also affected the genes involved in direct and indirect EET. Increased riboflavin production was also observed. The feasibility of this strategy to enhance the bioreduction of methyl orange, an organic pollutant, and chromium, a typical heavy metal, was demonstrated. This work implies a great potential of the STAR strategy with the CIRPSR-ddAsCpf1 system for enhancing bacterial EET to favor more efficient environmental remediation applications.

Hai-Long Jiang.

"My biggest motivation is to make creative and significant discoveries in my research fields. I chose chemistry as a career because of a high score for chemistry in my college entrance examination …" Find out more about Hai-Long Jiang in his Author Profile.

Triplet BODIPY and AzaBODIPY Derived Donor‐acceptor Dyads: Competitive Electron Transfer versus Intersystem Crossing upon Photoexcitation

AbstractThe bis‐iodo β‐pyrrole‐substituted BF2‐chelated dipyrromethene, I2BODIPY, and its structural analogue BF2‐chelated aza dipyrromethene, I2azaBODIPY, carrying a nitrogen at the meso‐position instead of carbon, were synthesized and characterized as new set of triplet sensitizers using different techniques. These sensitizers were further functionalized with fullerene, C60, at the central boron atom to build donor‐acceptor conjugates. Using spectral, electrochemical, and computational methods, these conjugates were characterized, and the energy levels were established. Intersystem crossing to populate the triplet state was observed upon excitation of I2BODIPY and I2azaBODIPY, however, the measured rates of kISC were found to be nearly two orders of magnitude higher for I2azaBODIPY (kISC∼1011 s−1) compared to I2BODIPY (kISC∼109 s−1). The energetics, kISC, and position of HOMO and LUMO levels was found to control the ability of the dyad to undergo electron transfer, although the donor‐acceptor distances were virtually the same in both I2BODIPY‐C60 and I2azaBODIPY‐C60 conjugates. Free‐energy calculations revealed that the photoinduced electron transfer process was thermodynamically feasible from only the singlet excited states in both conjugates. Consequently, electron transfer from the 1I2BODIPY* in competition with intersystem crossing was witnessed in the case of I2BODIPY‐C60 dyad while in the case of I2azaBODIPY‐C60 dyad, excitation of azaBODIPY led to a short‐lived charge transfer state involving the catechol bridge followed by populating the low‐lying 3I2azaBODIPY* state without promoting the process of charge separation involving C60. The lifetime of the charge‐separated states was in the ns range in the I2BODIPY‐C60 conjugate both in polar and nonpolar solvents.

Cover Feature: Competitive Energy and Electron Transfer in β-Functionalized Free-Base Porphyrin-Zinc Porphyrin Dimer Axially Coordinated to C60 : Synthesis, Supramolecular Formation and Excited-State Processes (Chem. Eur. J. 52/2017)

Cover Feature: Competitive Energy and Electron Transfer in β-Functionalized Free-Base Porphyrin-Zinc Porphyrin Dimer Axially Coordinated to C₆₀ : Synthesis, Supramolecular Formation and Excited-State Processes (Chem. Eur. J. 52/2017)

Competitive electron transfer in a novel, broad-band capturing, subphthalocyanine-AzaBODIPY-C60 supramolecular triad.

A V-configured subphthalocyanine-azaBODIPY-C60 supramolecular triad has been newly synthesized, and sequential energy and electron transfer leading to the formation of charge separated states, useful properties relevant for solar energy harvesting and building optoelectronic devices, is reported.

Ternary Pt/SnOx/TiO2 photocatalysts for hydrogen production: consequence of Pt sites for synergy of dual co-catalysts

A variety of ternary nanoheterostructures composed of Pt nanoparticles (NPs), SnOx species, and anatase TiO2 are designed elaborately to explore the effect of interfacial electron transfer on photocatalytic H2 evolution from a biofuel-water solution. Among numerous factors controlling the H2 evolution, the significance of Pt sites for the H2 evolution is highlighted by tuning the loading procedure of Pt NPs and SnOx species over TiO2. A synergistic enhancement of H2 evolution can be achieved over the Pt/SnOx/TiO2 heterostructures formed by anchoring Pt NPs at atomically-isolated Sn-oxo sites, whereas the Pt/TiO2/SnOx counterparts prepared by grafting single-site Sn-oxo species on Pt/TiO2 show a marked decrease in the rate of H2 evolution. The characterization results clearly reveal that the synergy of Pt NPs and SnOx species originates from the vectorial electron transfer of TiO2 → SnOx → Pt occurring on the former, while the latter results from the competitive electron transfer from TiO2 to SnOx and to Pt NPs.

Shape‐Persistent Macrocycles as Ligands and Sensitisers of Nd3+ Ions

AbstractTwo shape‐persistent macrocycles with a hexagonal backbone incorporating two bpy units and two nonchromophoric methoxytetrahydropyran units (M), or two coumarin 2 dyes (C2–M–C2) are highly absorbing and emitting species in dichloromethane solution (ϵmax = 1.90 × 105 M–1 cm–1 at 308 nm and 1.92 × 105 M–1 cm–1 at 309 nm; φem = 0.60 and 0.73, τ = 0.76 and 3.6 ns for M and C2–M–C2, respectively). Time‐resolved fluorescence anisotropy measurements of M in dichloromethane has given information on the hydrodynamic volume of the free macrocycle in solution, which is close to the molecular volume measured by X‐ray crystallographic analysis. M and C2–M–C2 are very good ligands for lanthanide ions (Nd3+ and Gd3+): they form complexes with 1:2, 1:1 and 2:1 (clearly evident only in the case of M) metal to ligand stoichiometry. These complexes are characterised by very high association constants in dichloromethane solution, as demonstrated by significant changes to the photophysical properties of the macrocycles. In the 1:2 and 1:1 complexes, the lanthanide ion is likely sandwiched between two macrocycles and coordinated by two bpy units. The efficiency of energy transfer from the ligand to the lanthanide ion is quite high (73 % in the case of the [Nd2M]6+ complex), and it is not sensitive to the presence of oxygen. For the Nd3+ complexes with the C2–M–C2 macrocycle, sensitisation of the lanthanide NIR emission takes place with lower efficiency (ca. 50 %), due to a competitive electron transfer process from the coumarin to the complexed macrocycle. Lastly, upon addition of an excess of a competitive ligand, all the absorption and emission spectroscopic changes are completely reversible.

Competitive Electron Transfer and Enhanced Intersystem Crossing in Photoexcited Covalent TEMPO−Perylene-3,4:9,10-bis(dicarboximide) Dyads: Unusual Spin Polarization Resulting from the Radical−Triplet Interaction

A stable 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO) radical was covalently attached at its 4-position to the imide nitrogen atom of a perylene-3,4:9,10-bis(dicarboximide) (PDI) to produce TEMPO-PDI, 1, having a well-defined distance and orientation between TEMPO and PDI. Transient optical absorption experiments in toluene following selective photoexcitation of the PDI chromophore in TEMPO-PDI show that enhanced intersystem crossing occurs with tau = 45 +/- 1 ps, resulting in formation of TEMPO-(3*)PDI, while the same experiment in THF shows that the electron-transfer reaction TEMPO-(1*)PDI --> TEMPO(+*)-PDI(-*) occurs with tau = 1.2 +/- 0.2 ps and thus competes effectively with enhanced intersystem crossing. Time-resolved EPR (TREPR) spectroscopy on the photogenerated three-spin system TEMPO-(3*)PDI in toluene at 295 K initially shows a broad signal assigned to spin-polarized (3*)PDI, which thermalizes at longer times and is accompanied by formation of an emissively polarized TEMPO radical. No signals are observed in THF at 295 K. The TREPR spectrum of TEMPO-(3*)PDI at 85 K in toluene shows an emissive/absorptive signal due to TEMPO and a broad triplet signal due to (3*)PDI having a spin polarization pattern characteristic of overpopulation of its T(0) sublevel. This unusual spin polarization pattern does not result from radical pair intersystem crossing because electron transfer does not occur at 85 K. The observed spin polarization of (3*)PDI cannot be readily explained by mechanisms discussed previously, leading us to propose a new spin polarization mechanism, which requires that the radical and attached triplet are in the weak exchange regime.

Oxoporphyrinogens: From Redox and Spectroscopic Probe for Anion Sensing to a Platform for Construction of Supramolecular Donor-Acceptor Conjugates

Structures and properties of N-alkylated oxoporphyrinogens are presented with an emphasis on their potential application as optical and electrochemical anion sensors, as an electron acceptor in donor-acceptor conjugates, or as a platform to build novel supramolecular donor-acceptor assemblies. Competitive electron transfer from singlet excited zinc porphyrin to either OxP or C60, probed by various time-resolved spectroscopic techniques, in the supramolecular donor-acceptor assemblies was investigated.

Competitive Consecutive Electron Transfer in Determination of Ionization Potentials: Ketene Derivatives

Kinetics of competitive consecutive electron transfer was used to determine ionization potentials of transient species. Kinetics of two-stage electron transfer reactions in aprotic solvent was studied using 355 nm laser flash photolysis. The concentrations of transients produced by the laser flash photolysis were monitored by their light absorption. Triplet-excited tetrachloro-p-benzo-quinone (p-chloranil) generated by a 355 nm laser flash oxidized diethyl ketene, diphenyl ketene, or phenyl ethyl ketene to form radical cations. The ketene radical cations, in turn, oxidized tertiary amine, forming ground state ketene and ammonium radical cation. The kinetics of the disappearance of ketene radical cations (and/or appearance of ammonium radical cations) due to consecutive, competitive electron transfer to ketene and p-chloranil radical cations was monitored. By monitoring kinetics in the presence of tertiary amines with different oxidation potentials, it was established that in acetonitrile the oxidation potential of diethyl ketene was 5.4 eV; for phenyl ethyl ketene, it was approximately 4.8 eV; and for diphenyl ketene, it was 4.6 eV. The results were in agreement with the oxidation potentials of ketenes computed using published data.

Connecting two C60stoppers to molecular wires: ultrafast intramolecular deactivation reactions

A new series of soluble dumbbell-type architectures constituted by π-conjugated oligophenyleneethynylenes (OPEs) and two C60 stoppers have been prepared by using a convergent synthetic strategy. In particular, we base our approach on the systematic cross-coupling, palladium catalyzed Hagihara–Sonogashira reaction between aryl iodides and alkynyl derivatives. The cyclic voltammetry of the correspondingly synthesized system reveals amphoteric redox behavior and the lack of significant electronic communication between the oligo-arylalkynyl bridge and the two C60 moieties, which behave as two independent units. Excited state studies carried out by fluorescence and transition absorption spectroscopy show that, although a weak competitive electron transfer could operate in the deactivation process of the excited oligo-arylalkynyl bridges, their deactivation is mainly governed by energy transfer.

Investigations on Nanoparticle−Chromophore and Interchromophore Interactions in Pyrene-Capped Gold Nanoparticles

Three pyrene alkanethiol derivatives (P1, P2, and P3) possessing flexible alkyl groups of different lengths were attached to nanoparticles of gold (∼2−3 nm in size) along with dodecanethiol (Au−P1, Au−P2, and Au−P3). The photophysical properties of these systems were investigated as a function of (i) distance of chromophore from gold core, (ii) concentration of pyrene on gold surface, and (iii) solvent polarity. The structured absorption bands of the pyrene chromophore were significantly perturbed near the surface of gold nanoparticles (Au−P1), indicating a strong ground state interaction between the plasmon electrons of Au nanoparticles and the π-electron cloud of the chromophore. Such effects were not observed in Au−P2 and Au−P3 systems, in which the linker groups are long enough to prevent any ground state interactions. A gradual increase in the peak intensity ratio of band III/I of the normal fluorescence of pyrene chromophore was found with an increase in length of the spacer group. These results indicate that the local environment close to the surface of the Au nanoparticle is more polar compared to the bulk medium. Interchromophoric interactions are limited in the Au−P1 system due to the restriction imposed by the curvature of spherical gold nanoparticle whereas the flexible alkyl chain tethering pyrene in Au−P2/Au−P3 allows free interaction between chromophores. Steady state and time-resolved emission studies indicate that the normal fluorescence and intermolecular excimer formation are the main deactivation channels of the singlet excited state of pyrene linked to Au nanoparticles, in nonpolar solvents. In contrast, the competitive electron transfer to the gold nanocore dominates in polar solvents.

Competitive Electron Transfer from the S2 and S1 Excited States of Zinc meso-Tetraphenylporphyrin to a Covalently Bound Pyromellitimide: Dependence on Donor−Acceptor Structure and Solvent

Zinc meso-tetraphenylporphyrin (ZnTPP) is known to have a relatively slow S2 → S1 internal conversion rate, (1−3 ps)-1, so that electron transfer from S2 to a nearby acceptor is possible. We have synthesized two zinc porphyrins in which a pyromellitimide (PI) acceptor is attached to the para position of a meso-phenyl group of the porphyrin (ZnTPP−PI) or to that of a β-phenyl group of the porphyrin (ZnTPP−β-PhPI). We report here on competitive electron transfer from the S2 (excitation at 420 nm) and S1 states (excitation at 550 nm) of these zinc porphyrins to PI in 2-methyltetrahydrofuran (MTHF) and toluene. Our transient absorption studies show that efficient electron transfer occurs from S2 of ZnTPP to PI in these porphyrins despite the presence of the intervening phenyl between the zinc porphyrin macrocycle and the PI acceptor. Moreover, the results show that while charge separation from S1 is about 6 times faster for PI attached to the meso-phenyl than for PI attached to the β-phenyl, the opposite is observed for charge separation from S2. Subsequent charge recombination is 2.6−2.8 times faster for PI attached to the meso-phenyl relative to that of the β-phenyl. Comparisons of rates between MTHF and toluene show that the ordering of rates is the same for both solvents, although the rate ratios are larger in MTHF than in toluene.

Density functional theory studies of electron interaction with DNA: can zero eV electrons induce strand breaks?

The discovery of DNA strand breaks induced by low energy secondary electrons sparks a necessity to elucidate the mechanism. Through theoretical studies based on a sugar-phosphate-sugar model that mimics a backbone section of the DNA strand, it is found that bond cleavages at 3' or 5'C-O sites after addition of an electron are possible with a ca. 10 kcal/mol activation barrier. Moreover, the potential energy surfaces show that dissociation at both sites is highly favorable thermodynamically. Although the phosphate group in DNA is not a favored site for electron attachment because of competitive electron transfer to the bases, any electrons which attach to phosphates on first encounter may induce strand breaks even when the electron energy is near zero eV. These findings have profound implication as low energy secondary electrons are abundantly generated in all types of ionization radiation.

Competitive Electron Transfer and SN2 Reactions of Aromatic Radical Anions with Alkyl Halides and Methanesulfonates.

Competitive Electron Transfer and SN2 Reactions of Aromatic Radical Anions with Alkyl Halides and Methanesulfonates.