Distance dependence of photoinduced electron transfer through non-conjugated bridges

Picosecond time-resolved emission studies of a series of molecules containing an electron donor-acceptor pair interconnected by a series of rigid non-conjugated bridges reveal the occurrence of very fast photoinduced intramolecular electron transfer. The length of the bridge was varied to provide donor-acceptor centre-to-centre separations ranging from 8.1 to 13.3 Å (edge-to-edge 5 to 10.2 Å). At centre-to-centre separations up to 10.7 Å the rate of photoinduced electron transfer exceeded 5×10¹⁰ s⁻¹ (τ<20 ps); at the largest separation, 13.3 Å, the rate was 1.47×10¹⁰ s⁻¹ (τ = 68 ps).     

Combining Zinc Phthalocyanines,Oligo(p-Phenylenevinylenes), and Fullerenes to Impact Reorganization Energies and Attenuation Factors

A study on electron transfer in three electron donor-acceptor complexes is reported. These architectures consist of a zinc phthalocyanine (ZnPc) as the excited-state electron donor and a fullerene (C₆₀) as the ground-state electron acceptor. These complexes are brought together by axial coordination at ZnPc. The key variable in our design is the length of the molecular spacer, namely, oligo-p-phenylenevinylenes. The lack of appreciable ground-state interactions is in accordance with strong excited-state interactions, as inferred from the quenching of ZnPc centered fluorescence and the presence of a short-lived fluorescence component. Full-fledged femtosecond and nanosecond transient absorption spectroscopy assays corroborated that the ZnPc ⋅ ⁺-C₆₀ ⋅ ⁻ charge-separated state formation comes at the expense of excited-state interactions following ZnPc photoexcitation. At a first glance, the ZnPc ⋅ ⁺-C₆₀ ⋅ ⁻ charge-separated state lifetime increased from 0.4 to 86.6 ns as the electron donor-acceptor separation increased from 8.8 to 29.1 Å. A closer look at the kinetics revealed that the changes in charge-separated state lifetime are tied to a decrease in the electronic coupling element from 132 to 1.2 cm⁻¹, an increase in the reorganization energy of charge transfer from 0.43 to 0.63 eV, and a large attenuation factor of 0.27 Å⁻¹.     

Photoinduced electron transfer in a triarylamine-organoboron-Ru(2,2′-bipyridine)32+ compound

Long-range electron transfer reactions play a key role in biological photosynthesis, and they are likely to play an important role for future artificial photosynthetic endeavors as well. The possibility to control the rates for long-range electron transfer with external stimuli is of particular interest in this context. In the work presented herein, we explored a donor–bridge–acceptor compound in which intramolecular electron transfer from a triarylamine donor to a photoexcited Ru(bpy)₃²⁺ (bpy = 2,2′-bipyridine) acceptor occurs across an organoboron bridge over a distance of approximately 22 Å. Fluoride has a high binding affinity to the organoboron bridge in apolar solutions, and the resulting organofluoroborate has a significantly different electronic structure. We explored to what extent the change from an electron-deficient organoboron wire to an electron-rich organofluoroborate bridge affects long-range electron transfer between the distant triarylamine donor and the Ru(bpy)₃²⁺ acceptor.     

Importance of covalence,conformational effects and tunneling-barrier heights for long-range electron transfer: Insights from dyads with oligo-p-phenylene,oligo-p-xylene and oligo-p-dimethoxybenzene bridges

This review reports on our recent studies of phototriggered charge transfer in rigid rod-like donor-bridge-acceptor molecules in liquid solution as well as between randomly dispersed electron donors and acceptors in frozen organic glasses. Investigation of the distance dependence of the rates of these reactions provides detailed insight into the various factors that govern long-range charge transfer efficiencies. The importance of covalence can be probed by a comparison of charge tunneling through a frozen toluene matrix to tunneling across an oligo-p-xylene bridge. The distance decay constants for these two processes are β = 1.26 Å^?1 and β = 0.52 Å^?1, respectively, indicating that charge tunneling across a covalent xylene–xylene contact is ～2 orders of magnitude more efficient than that across a noncovalent toluene–toluene contact. Conformational effects were investigated by comparing hole tunneling across oligo-p-xylene and oligo-p-phenylene bridges. The latter are significantly more π-conjugated and mediate long-range hole tunneling with β = 0.21 Å^?1 between a ruthenium–phenothiazine donor–acceptor couple. Quantitative analysis indicates that in this particular instance, tunneling across a phenylene–phenylene contact is roughly 50 times more efficient than tunneling across a xylene–xylene contact. The use of oligo-p-dimethoxybenzene wires instead of the structurally very similar oligo-p-xylene bridges was found to lead to a strong acceleration of long-range hole transfer rates: The 23.5-Å charge transfer step across four xylene units occurs within 20 μs, but the charge transfer over the same distance across four dimethoxybenzene units takes only 17 ns. This is attributed to a tunneling-barrier effect that is caused by a large difference in oxidation potentials between the two types of bridges.     

DNA-mediated electron transfer from a modified base to ethidium: π-stacking as a modulator of reactivity

Background: The DNA double helix is composed of an array of aromatic heterocyclic base pairs and, as a molecular π-stack, represents a novel system for studying long-range electron transfer. Because many base damage and repair processes result from electron-transfer reactions, the ability of DNA to mediate charge transport holds important biological implications. Seemingly contradictory conclusions have been drawn about electron transfer in DNA from the many different studies that have been carried out. These studies must be reconciled so that this phenomenon can be understood both at a fundamental level and in the context of biological systems.Results: The photoinduced oxidation of a modified base, 7-deazaguanine, has been examined as a function of distance, sequence, and base stacking in DNA duplexes covalently modified with ethidium. Over ethidium/deazaguanine separations of 6–27 Å, the photooxidation reaction proceeded on a subnanosecond time scale, and the quenching yield exhibited a shallow distance dependence. The efficiency of the reaction was highly sensitive to small changes in base composition. Moreover, the overall distance-dependence of the reaction is sensitive to sequence, despite the constancy of photoexcited ethidium as acceptor.Conclusions: The remarkable efficiency of deazaguanine photooxidation by intercalated ethidium over long distances provides new evidence for fast electron-transfer pathways through DNA. By varying sequence as well as reactant separation, this work provides the first experimental demonstration of the importance of reactant stacking in the modulation of long-range DNA-mediated electron transfer.     

Increasing Electron‐Transfer Rates with Increasing Donor–Acceptor Distance

Electron transfer can readily occur over long (≥15 Å) distances. Usually reaction rates decrease with increasing distance between donors and acceptors, but theory predicts a regime in which electron‐transfer rates increase with increasing donor–acceptor separation. This counter‐intuitive behavior can result from the interplay of reorganization energy and electronic coupling, but until now experimental studies have failed to provide unambiguous evidence for this effect. We report here on a homologous series of rigid rodlike donor‐bridge‐acceptor compounds in which the electron‐transfer rate increases by a factor of 8 when the donor–acceptor distance is extended from 22.0 to 30.6 Å, and then it decreases by a factor of 188 when the distance is increased further to 39.2 Å. This effect has important implications for solar energy conversion.     

The electron spin resonance spectrum of the triphenylsilyl radical formed by the X-irradiation of monocrystalline triphenylsilane

The electron spin resonance spectrum of X-irradiated monocrystalline triphenylsilane has been studied at room temperature. The main radical species present is the triphenylsilyl radical, formed both as isolated radicals and pair-wise trapped radicals. For the radical pairs the analysis of the dipole coupling tensor indicates an inter-radical distance of 7.36 Å. The ²⁹Si hyperfine tensor has been determined, and the values discussed in terms of the electronic structure of the triphenylsilyl radical.     

Distance-Dependent Electron Transfer Kinetics in Axially Connected Silicon Phthalocyanine-Fullerene Conjugates

The effect of donor-acceptor distance in controlling the rate of electron transfer in axially linked silicon phthalocyanine-C₆₀ dyads has been investigated. For this, two C₆₀-SiPc-C₆₀ dyads, 1 and 2 , varying in their donor-acceptor distance, have been newly synthesized and characterized. In the case of C₆₀-SiPc-C₆₀ 1 where the SiPc and C₆₀ are separated by a phenyl spacer, faster electron transfer was observed with k_cs equal to 2.7×10⁹ s⁻¹ in benzonitrile. However, in the case of C₆₀-SiPc-C₆₀ 2 , where SiPc and C₆₀ are separated by a biphenyl spacer, a slower electron transfer rate constant, k_cs=9.1×10⁸ s⁻¹, was recorded. The addition of an extra phenyl spacer in 2 increased the donor-acceptor distance by ∼4.3 Å, and consequently, slowed down the electron transfer rate constant by a factor of ∼3.7. The charge separated state lasted over 3 ns, monitoring time window of our femtosecond transient spectrometer. Complimentary nanosecond transient absorption studies revealed formation of ³SiPc* as the end product and suggested the final lifetime of the charge separated state to be in the 3–20 ns range. Energy level diagrams established to comprehend these mechanistic details indicated that the comparatively high-energy SiPc^.+-C₆₀^.− charge separated states (1.57 eV) populated the low-lying ³SiPc* (1.26 eV) prior returning to the ground state.     

Mixed Ligand Palladium(Ⅱ) Complex with NS-Bidentate S-Allyldithiocarbazate Schiff Base: Synthesis, Spectral Char-acterization, Crystal Structure and Decoding Intermolecular Interactions with Hirshfeld Surface Analysis

A new mixed ligand palladium(II) complex with bidentate NS‐donor chelate, [PdCl(PPh₃)L] (L: S‐allyl β‐N‐(benzylidene)dithiocarbazate), has been prepared and characterized using single crystal X‐ray diffraction and spectroscopic (electronic, IR, ¹H NMR and ¹³C NMR) techniques. The shorter Pd? P bond distance, 2.255(7) Å, than the sum of the single bond radii for palladium and phosphorus (2.41 Å), showed partial double bond character. Visualizing and exploring the crystal structure using Hirshfeld surface analysis showed the presence of π··· π, N··· π, C? H··· π, Cl···H and weak C? H···S interactions as most important intermolecular interactions in the crystal lattice, which are responsible to extension of the supramolecular network of the compound and stabilization of the crystal structure.     

A π-Conjugated Anion-Exchange Membrane with an Ordered Ion-Conducting Channel via the McMurray Coupling Reaction

The McMurry coupling is a facile, gentle and low-cost chemical reaction for synthesizing. Here, for the first time, we employed the McMurry coupling reaction to prepare π-conjugated anion exchange membranes (AEMs). The inter-chain π-π stacking between adjacent benzene rings induces directional self-assembly aggregation and enables highly ordered ion-conductive channels. The resulting structure was characterized through UV/VIS spectrum, X-ray diffraction (XRD) pattern, small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM) and density functional theory (DFT) calculations, leading to high OH⁻ conductivity of 135.5 mS cm⁻¹ at 80 °C. Furthermore, the double bonds in the π-conjugated system also trigger in situ self-crosslinking of the AEMs to enhance dimensional and alkaline stability. Benefiting from this advantage, the as-obtained Cr-QPPV-2.51 AEM exhibits superior alkaline stability (95 % conductivity retention after 3000 hrs in 1 M KOH at 80 °C) and high mechanical strength of 34.8 MPa. Moreover, the fuel cell using Cr-QPPV-2.51 shows a maximum peak power density of 1.27 W cm⁻² at 80 °C.     

Stabilization of Pancake Bonding in (TCNQ)2.− Dimers in the Radical-Anionic Salt (N−CH3−2-NH2−5Cl−Py)(TCNQ)(CH3CN) Solvate and Antiferromagnetism Induction

We report a new antiferromagnetic radical-anion salt (RAS) formed from 7,7,8,8-tetracyanquinonedimethane (TCNQ) anion and 2-amino-5-chloro-pyridine cation with the composition of (N−CH₃−2-NH₂−5Cl−Py)(TCNQ)(CH₃CN). The crystallographic data indicates the formation of (TCNQ)₂^.− radical-anion π-dimers in the synthesized RAS. Unrestricted density functional theory calculations show that the formed π-dimers characterize with strong π-stacking “pancake” interactions, resulting in high electronic coupling, enabling efficient charge transfer properties, but π-dimers cannot be stable in the isolated conditions as a result of strong Coulomb repulsions. In a crystal, where (TCNQ)₂^.− π-dimers bound in the endless chainlets via supramolecular bonds with (N−CH₃−2-NH₂−5-Cl−Py)⁺ cations, the repulsion forces are screened, allowing for specific parallel π-stacking interactions and stable radical-anion dimers formation. Measurements of magnetic susceptibility and magnetization confirm antiferromagnetic properties of RAS, what is in line with the higher stability of ground singlet state of the radical-anion pair, calculated by means of the DFT. Therefore, the reported radical-anion (N−CH₃−2-NH₂−5Cl−Py)(TCNQ)(CH₃CN) solvate has promising applications in novel magnetics with supramolecular structures.     

Long-distance electron transfer from a triplet excited state

Electron transfer from the triplet excited state of N,N,N',N'-tetramethylphenylene diamine to phthalic anhydride has been monitored by phosphorescence emission decay. The kinetics of the transfer process were observed directly and the rate constant depends exponentially on the reacting distance, k(r) = 1 × 10⁴ exp(−0.58r) s⁻¹. The electron transfer rate has been found to be invariant over the temperature interval 77–143 K.     

Anion⋅⋅⋅Anion Attraction in Complexes of MCl3− (M=Zn,Cd, Hg) with CN−

High-level ab initio calculations show that the MCl₃⁻ anions comprising Group 2B M atoms Zn, Cd, and Hg form a stable complex with the CN⁻ anion, despite the like charge of the two ions. The complexation occurs despite a negative π-hole region above the M atom of MCl₃⁻. The dimerization distorts the planar geometry of MCl₃⁻ into a pyramidal shape which reduces the negative potential above the M atom, facilitating a close approach of the two anions, with R(M⋅⋅⋅C)∼2 Å, and an overall attractive electrostatic attraction within the dimer. In the gas phase, this dimer is less stable than the pair of separated ions by some 30 kcal/mol. However, the dissociation must surmount an energy barrier of roughly 25 kcal/mol which occurs at an intermolecular distance of 4 Å. In aqueous solution, the dimerization process is exothermic and barrier-free, with a binding energy in the 11–18 kcal/mol range.     

Isomer Effect on Energy Storage of π-Extended S-Shaped Double[6]Heterohelicene

Recently, chiral and nonplanar cutouts of graphene have been the favorites due to their unique optical, electronic, and redox properties and high solubility compared with their planar counterparts. Despite the remarkable progress in helicenes, π-extended heterohelicenes have not been widely explored. As an anode in a lithium-ion battery, the racemic mixture of π-extended double heterohelical nanographene containing thienothiophene core exhibited a high lithium storage capability, attaining a specific capacity of 424 mAh g⁻¹ at 0.1 A g⁻¹ with excellent rate capability and superior long-term cycling performance over 6000 cycles with negligible fade. As a first report, the π-extended helicene isomer (PP and MM), with the more interlayer distance that helps faster diffusion of ions, has exhibited a high capacity of 300 mAh g⁻¹ at 2 A g⁻¹ with long-term cycling performance over 1500 cycles compared to the less performing MP and PM isomer and racemic mixture (150 mAh g⁻¹ at 2 A g⁻¹). As supported by single-crystal X-ray analysis, a unique molecular design of nanographenes with a fixed (helical) molecular geometry, avoiding restacking of the layers, renders better performance as an anode in lithium-ion batteries. Interestingly, the recycled nanographene anode material displayed comparable performance.     

Long-range oxidation of guanine by Ru(III) in duplex DNA

Background: Theoretical and experimental studies have demonstrated that 5′-GG-3′ sequences in DNA are ‘hot spots’ for oxidative damage, but few studies have definitively addressed whether oxidative damage to DNA may arise from a distance via long-range charge migration. Towards this end, we have prepared tethered ruthenium (Ru)-oligonucleotide duplexes and used a flash—quench strategy to demonstrate long-range charge transport through the DNA double helix.Results: DNA assemblies containing a tethered Ru(II) intercalator have been synthesized. Ru(III), generated in situ in the presence of externally bound electron-transfer quenchers, promotes base damage selectively at the 5′-G of a 5′-GG-3′ doublet located ∼ 37 Å from the binding site of the oxidant. In the absence of a guanine doublet, oxidative damage occurs equally at all guanine bases in the strand. Oxidative damage is also observed at long range for guanine in a G·A mismatch but not in a G·T mismatch.Conclusions: The present study expands the scope of long-range electron-transfer chemistry in terms of experiments, applications, and possible reactions within the cell. Here we demonstrate oxidative damage to DNA occurring with a high quantum yield over a distance of ∼37 Å using a ground-state oxidant. These results point to the equilibration of the radical across the DNA duplex to the sites of lowest energy. In addition, this charge migration is sensitive to the intervening π-stack formed by DNA base pairs and hence may be useful for the detection of mismatches.     

Femtosecond electron-transfer reactions in mono- and polynucleotides and in DNA

Quenching of redox active, intercalating dyes by guanine bases in DNA can occur on a femtosecond time scale both in DNA and in nucleotide complexes. Notwithstanding the ultrafast rate coefficients, we find that a classical, nonadiabatic Marcus model for electron transfer explains the experimental observations, which allows us to estimate the electronic coupling (330 cm(-1)) and reorganization (8070 cm(-1)) energies involved for thionine-[poly(dG-dC)](2) complexes. Making the simplifying assumption that other charged, pi-stacked DNA intercalators also have approximately these same values, the electron-transfer rate coefficients as a function of the driving force, DeltaG, are derived for similar molecules. The rate of electron transfer is found to be independent of the speed of molecular reorientation. Electron transfer to the thionine singlet excited state from DNA obtained from calf thymus, salmon testes, and the bacterium, micrococcus luteus (lysodeikticus) containing different fractions of G-C pairs, has also been studied. Using a Monte Carlo model for electron transfer in DNA and allowing for reaction of the dye with the nearest 10 bases in the chain, the distance dependence scaling parameter, beta, is found to be 0.8 +/- 0.1 A(-1). The model also predicts the redox potential for guanine dimers, and we find this to be close to the value for isolated guanine bases. Additionally, we find that the pyrimidine bases are barriers to efficient electron transfer within the superexchange limit, and we also infer from this model that the electrons do not cross between strands on the picosecond time scale; that is, the electronic coupling occurs predominantly through the pi-stack and is not increased substantially by the presence of hydrogen bonding within the duplex. We conclude that long-range electron transfer in DNA is not exceptionally fast as would be expected if DNA behaved as a "molecular wire" but nor is it as slow as is seen in proteins, which do not benefit from pi-stacking.     

Precise Control of Intramolecular Charge‐Transport: The Interplay of Distance and Conformational Effects

A new series of donor–bridge–acceptor (D–B–A) compounds consisting of π‐conjugated oligofluorene (oFL) bridges between a ferrocene (Fc) electron‐donor and a fullerene (C₆₀) electron‐acceptor have been synthesized. In addition to varying the length of the bridge (i.e., mono‐ and bi‐fluorene derivatives), four different ways of linking ferrocene to the bridge have been examined. The Fc moiety is linked to oFL: 1) directly without any spacer, 2) by an ethynyl linkage, 3) by a vinylene linkage, and 4) by a p‐phenylene unit. The electronic interactions between the electroactive species have been characterized by cyclic voltammetry, absorption, fluorescence, and transient absorption spectroscopy in combination with quantum chemical calculations. The calculations reveal exceptionally close energy‐matching between the Fc and the oFL units, which results in strong electronic‐coupling. Hence, intramolecular charge‐transfer may easily occur upon exciting either the oFLs or Fcs. Photoexcitation of Fc–oFL–C₆₀ conjugates results in transient radical‐ion‐pair states. The mode of linkage of the Fc and FL bridge has a profound effect on the photophysical properties. Whereas intramolecular charge‐separation is found to occur rather independently of the distance, the linker between Fc and oFL acts (at least in oFL) as a bottleneck and significantly impacts the intramolecular charge‐separation rates, resulting in beta values between β_CS 0.08 and 0.19 Å^?1. In contrast, charge recombination depends strongly on the electron‐donor–acceptor distance, but not at all on the linker. A value of β_CR (0.35±0.01 Å^?1) was found for all the systems studied. Oligofluorenes prove, therefore, to be excellent bridges for probing how small structural variations affect charge transport in D–B–A systems.     

Ir-spectroscopic evidence for NaSiO and KSiO in solid argon

Condensation of SiO with sodium or potassium, respectively, leads to the formation of Na⁺(SiO)⁻ and K⁺(SiO)⁻. The SiO force constant is reduced to 6.15 mdyn/Å and 6.30 mdyn/Å with respect to uncoordinated SiO (f = 9.0 mdyn/Å). From these results a similar type of bonding can be concluded for the charge transfer complexes Ag⁺(SiO)⁻ and Ag⁺(SiS)⁻ published recently.     

Theoretical study of 3<Emphasis Type="Italic">d</Emphasis>-metal porphyrin π-complexes acetylene

The structural parameters, energies, and spectroscopic characteristics of acetylene π-complexes of metalloporphyrins M(P)(π-C₂H₂), where M is a 3d-metal atom in different multiplicity states have been calculated by the density functional theory B3LYP method. It has been shown that the activation of the coordinated acetylene molecule is manifested in (i) a sharp weakening of its C-C bond, (ii) a 20°–40° decrease of the bond angles φ(HCC) and an elongation by 0.05–0.10 Å of the R(CC) bond, (iii) a long-wavelength shift of the ν_str(CC) stretching mode by 300–500 cm^?1, (iv) considerable electron density transfer from the porphyrin ring (P ring) to the π-ligand, and (iv) a strong displacement (0.5–0.6 Å) of the M atom from the P ring plane toward the π-ligand and the dome distortion of the P ring. There is a trend in the behavior of the activation effects along the 3d series and with a change in the electronic state multiplicity of the complexes.     

Long-Range LMCT Coupling in EuIII Coordination Polymers for an Effective Molecular Luminescent Thermometer**

A design for an effective molecular luminescent thermometer based on long-range electronic coupling in lanthanide coordination polymers is proposed. The coordination polymers are composed of lanthanide ions Eu^III and Gd^III, three anionic ligands (hexafluoroacetylacetonate), and a chrysene-based phosphine oxide bridges (6,12-bis(diphenylphosphoryl)chrysene). The zig-zag orientation of the single polymer chains induces the formation of packed coordination structures containing multiple sites for CH-F intermolecular interactions, resulting in thermal stability above 350 °C. The electronic coupling is controlled by changing the concentration of the Gd^III ion in the Eu^III-Gd^III polymer. The emission quantum yield and the maximum relative temperature sensitivity (S_m) of emission lifetimes for the Eu^III-Gd^III polymer (Eu:Gd=1:1, Φ_tot=52 %, S_m=3.73 % K⁻¹) were higher than those for the pure Eu^III coordination polymer (Φ_tot=36 %, S_m=2.70 % K⁻¹), respectively. Enhanced temperature sensing properties are caused by control of long-range electronic coupling based on phosphine oxide with chrysene framework.