Electron Transfer In Donor Research Articles

Multi-functional lanthanide coordination polymers for multi-modal detection of nitroaromatics and trace water in organic solvents

Multifunctional materials used for multimodal sensors are of great significance both for enriching variety of materials and overcoming the shortcomings of traditional single signal sensors. This work reports the ultrafast preparation of multifunctional coordination polymer (CP) spheres with metalloligand Ru (II)[4,4′-(COOH)2-bpy]3▪Cl2 (L) and Gd3+ ions. Gd-L CPs were used for multi-modal detection of nitroaromatic explosives and trace water in organic solvents with their red fluorescence and magnetic resonance response. The red fluorescence was derived from the ligand, which is electron-rich because of the bipyridine rings. There were different emission behaviors when Gd-L CPs reacted with nitroaromatic explosives that have various electron-deficient properties through electron transfer donor–acceptor mechanism. The changes in the emission depended closely on the nitroaromatic explosives ability to withdraw electrons. Gd-L CPs also showed magnetic resonance response with r1 of 12.1 mM−1s−1 due to the integration of the paramagnetic property of Gd3+ ions. Smart sensing of trace water in organic solvents was achieved by magnetic signals. The fluorescence of Gd-L CPs was enhanced and there was a tiny blue-shift in protonic solvents with increased polarity. The fluorescence sensor used in detecting trace water had a consistent result like the MR response. Thus, Gd-L CPs enabled the use of fluorescence and magnetic resonance sensors simultaneously for the first time to overcome the shortcoming of single modal sensor and expand the application of magnetic resonance relaxation.

Effect of Vibrational Transitions on the Nonthermal Charge Transfer Probability

In the framework of the stochastic approach, the effect of transitions between the vibrational sublevels of products on the ultrafast dynamics of “hot” (non-thermal) electron transfer in donor — acceptor complexes dissolved in a polar medium has been investigated. An analytical expression has been obtained for the probability of electron transfer, which takes into account the transition between the vibrational states of products and the mutual influence of sinks. Quantitative estimates of the scale of the mutual influence of sinks on the probability of electron transfer are made.

Stabilization of Ruthenium(II) Polypyridyl Chromophores on Mesoporous TiO2 Electrodes: Surface Reductive Electropolymerization and Silane Chemistry.

Stabilization is a critical issue in the long term operation of dye-sensitized photoelectrosynthesis cells (DSPECs) for water splitting or CO2 reduction. The cells require a stable binding of the robust molecular chromophores, catalysts, and chromophore/catalyst assemblies on metal oxide semiconductor electrodes under the corresponding (photoelectro)chemical conditions. Here, an efficient stabilization strategy is presented based on functionalization of FTO|nanoTiO2 (mesoporous, nanostructured TiO2 deposited on fluorine-doped tin oxide (FTO) glass) electrodes with a vinylsilane followed by surface reductive electropolymerization of a vinyl-derivatized Ru(II) polypyridyl chromophore. The surface electropolymerization was dominated by a grafting-through mechanism, and rapidly completed within minutes. Chromophore surface coverages were controlled up to three equivalent monolayers by the number of electropolymerization cycles. The silane immobilization and cross-linked polymer network produced highly (photo)stabilized chromophore-grafted FTO|nanoTiO2 electrodes. The electrodes showed significant improvements over structures based on atomic layer deposition and polymer dip-coating stabilization methods in a wide pH range from pH ≈ 1 to pH ≈ 12.5 under both dark and light conditions. Under illumination, with hydroquinone added as a sacrificial electron transfer donor, a photoresponse for sustained electron transfer mediation occurred for at least ∼20 h in a pH ≈ 7.5 phosphate buffer (0.1 M NaH2PO4/Na2HPO4, with 0.5 M NaClO4). The overall procedure provides an efficient way to fabricate highly stabilized molecular assemblies on electrode surfaces with potential applications for DSPECs in solar fuels.

The Impact of Driving Force and Temperature on the Electron Transfer in Donor–Acceptor Blend Systems

We discuss whether electron transfer from a photoexcited polymer donor to a fullerene acceptor in an organic solar cell is tractable in terms of Marcus theory, and whether the driving force ΔG0 is crucial in this process. Considering that Marcus rates are presumed to be thermally activated, we measured the appearance time of the polaron (i.e., the radical-cation) signal between 12 and 295 K for the representative donor polymers PTB7, PCPDTBT, and Me-LPPP in a blend with PCBM as acceptor. In all cases, the dissociation process was completed within the temporal resolution of our experimental setup (220–400 fs), suggesting that the charge transfer is independent of ΔG0. We find that for the PCPDTBT:PCBM (ΔG0 ≈ −0.2 eV) and PTB7:PCBM (ΔG0 ≈ −0.3 eV) the data is mathematically consistent with Marcus theory, yet the condition of thermal equilibrium is not satisfied. For MeLPPP:PCBM, for which electron transfer occurs in the inverted regime (ΔG0 ≈ −1.1 eV), the dissociation rate is inconsistent with Marcus theor...

Manipulating the Energetics and Rates of Electron Transfer in Rhodobacter capsulatus Reaction Centers with Asymmetric Pigment Content

Seemingly redundant parallel pathways for electron transfer (ET), composed of identical sets of cofactors, are a cornerstone feature of photosynthetic reaction centers (RCs) involved in light-energy conversion. In native bacterial RCs, both A and B branches house one bacteriochlorophyll (BChl) and one bacteriopheophytin (BPh), but the A branch is used exclusively. Described herein are the results obtained for two Rhodobacter capsulatus RCs with an unnaturally high degree of cofactor asymmetry, two BPh on the RC's B side and two BChl on the A side. These pigment changes derive, respectively, from the His(M180)Leu mutation [a BPh (ΦB) replaces the B-side BChl (BB)], and the Leu(M212)His mutation [a BChl (βA) replaces the A-side BPh (HA)]. Additionally, Tyr(M208)Phe was employed to disfavor ET to the A branch; in one mutant, Val(M131)Glu creates a hydrogen bond to HB to enhance ET to HB. In both ΦB mutants, the decay kinetics of the excited primary ET donor (P*) resolve three populations with lifetimes of ∼9 ps (50-60%), ∼40 ps (10-20%), and ∼200 ps (20-30%), with P+ΦB- formed predominantly from the 9 ps fraction. The 50-60% yield of P+ΦB- is the highest yet observed for a ΦB-containing RC. The results provide insight into factors needed for efficient multistep ET.

Increased Charge Separation Rates with Increasing Donor–Acceptor Distance in Molecular Triads: The Effect of Solvent Polarity

Distance-dependent electron transfer in donor–spacer–acceptor systems is accepted to occur via two distinct mechanisms, that is, by coherent superexchange or incoherent hopping. In general, the rate of electron transfer (kET) decreases with increasing donor–acceptor distances, irrespective of the actual mechanism being responsible for the process. However, recently Wenger and his group showed that in the frame of the superexchange mechanism electron-transfer rates can pass a maximum when increasing the transfer distance. This manuscript presents an investigation of the forward electron transfer in a series of donor (N-methylphenothiazine)–photocenter (Ru(II) bis(terpyridine) complex)–acceptor (N-methylfulleropyrrolidine) triads that reveals the control of the electron-transfer rates by solvent variation to an extent that in acetonitrile an increasing electron-transfer rate is observed with increasing donor–acceptor distance, while in dichloromethane an increase in the separation causes the electron transf...

Photoinduced electron transfer from aromatic amino acids to the excited isoalloxazine in single mutated flavin mononucleotide binding proteins: Effect of the dimer formation on the rate and the electrostatic energy inside the proteins

Conformational changes in the single mutated flavin mononucleotide binding proteins (FBP), E13K (Glu13 of wild type FBP is replaced by Lys), E13R (Glu13 replaced by Arg), E13T (Glu13 replaced by Thr) and E13Q (Glu13 replaced by Gln) upon dimer formations were studied through the phenomena of photoinduced electron transfer (ET) from tryptophan 32 (Trp32) and tryptophan 106 (Trp106) to the excited isoalloxazine (Iso∗). The ET rates were obtained with atomic coordinates of the proteins determined by a method of molecular dynamics simulation (MDS) and an ET theory using the reported experimental fluorescence decays of the FBPs. The value of the net electrostatic energy (NetES) between the photo-products and ionic groups inside the proteins increased in magnitude upon the dimer formation in every mutated FBP. Dependencies in the logarithmic ET rate (ln Rate) on the donor-acceptor distances (Rc) could not be approximated with simple linear nor parabolic functions in most ET donors, which are ascribed to the relatively greater values of the NetES in the dimers compared to the energy terms of the standard free energy gap (SFEG), solvent reorganization energy (SROE) and the electrostatic energy between the photo-products (ESDA) in total free energy gap (GT) in the ET rates. The NetES vs Rc relations displayed two distinct groups in Trp32KB, Trp32QA and Trp32QB. The ln Rate vs Rc relations in these donors displayed different behaviors between the two groups.

Impact of Temperature and Non-Gaussian Statistics on Electron Transfer in Donor-Bridge-Acceptor Molecules.

A combination of experimental data and theoretical analysis provides evidence of a bell-shaped kinetics of electron transfer in the Arrhenius coordinates ln k vs 1/T. This kinetic law is a temperature analogue of the familiar Marcus bell-shaped dependence based on ln k vs the reaction free energy. These results were obtained for reactions of intramolecular charge shift between the donor and acceptor separated by a rigid spacer studied experimentally by Miller and co-workers. The non-Arrhenius kinetic law is a direct consequence of the solvent reorganization energy and reaction driving force changing approximately as hyperbolic functions with temperature. The reorganization energy decreases and the driving force increases when temperature is increased. The point of equality between them marks the maximum of the activationless reaction rate. Reaching the consistency between the kinetic and thermodynamic experimental data requires the non-Gaussian statistics of the donor-acceptor energy gap described by the Q-model of electron transfer. The theoretical formalism combines the vibrational envelope of quantum vibronic transitions with the Q-model describing the classical component of the Franck-Condon factor and a microscopic solvation model of the solvent reorganization energy and the reaction free energy.

Influence of π-conjugation structural changes on intramolecular charge transfer and photoinduced electron transfer in donor-π-acceptor dyads.

The influence of π-conjugation structural changes on photoinduced electron transfer (PET) and intramolecular charge transfer (ICT) processes in π-conjugated donor (D)-acceptor (A) dyads (D-π-A) was investigated. Three types of D-π-A dyads were prepared through the modification of the structure of their π-conjugated linker, including D-π-A (1) and D-πtw-A (2) having a twisted π-conjugation, and D-π-Si-π-A (3) with a π-conjugation severed by a Si-atom. In these dyads, carbazole (Cz) and oxadiazole (Oz) moieties act as an electron donor and acceptor, respectively. The emission maxima of dyads 1 and 3 red-shifted with the increase in polarity, which could be attributed to the ICT process. The fluorescence lifetimes of dyads 1 and 3 were 2.64 and 4.29 ns in CH2Cl2, respectively. In contrast, dyad 2 showed dual emission at 350 and 470 nm in CH2Cl2. The emission of dyad 2 at 380 nm corresponded to the monomer fluorescence in the locally excited state. Moreover, the emission at 470 nm increased simultaneously with the diminishing of the fluorescence at 380 nm. This emission band can be assigned as the intramolecular exciplex emission, and showed a strong solvatochromic shift. The low emission quantum yield (<3%) of dyad 2 is due to the PET process. In dyad 2, the cationic and anionic radical species generated by the PET process were confirmed by femtosecond transient absorption (fs-TA) spectroscopy. Upon photoexcitation at 290 or 340 nm, the A or D moieties can be selectively excited. Upon excitation at 290 nm, the acceptor moiety can be excited to the 1A* state, thus the photoinduced hole transfer (PHT) takes place from 1A* to D through the HOMO levels within a few picoseconds. On the other hand, when the donor moiety is excited at 340 nm, the PET process occurs from 1D* to A. Based on the fs-TA studies, it was found that the dynamics and mechanisms for the electron (or charge) transfer were strongly affected by the variation of the π-conjugation of the linker. Herein, we can conclude that the PET and ICT processes are strongly influenced by the π-conjugation properties and their mechanisms are also affected by whether selective excitation of the donor or acceptor moiety occurs. Moreover, unit electron transfers (PET or PHT) were observed dominantly in the dyads having severed/twisted linkers in π-conjugation. However, dyad 1 possessing a well-conjugated linker showed a partial charge transfer character.

Phosphonate-Derivatized Porphyrins for Photoelectrochemical Applications.

A series of phosphonate-derivatized, high redox potential porphyrins with mesityl, pentafluorophenyl, and heptafluoropropyl meso-substituents were synthesized by acid-catalyzed condensation reactions. Ground and excited state redox potentials in the series were varied systematically with the electron-donating or electron-accepting nature of the meso-substitutents. The extent of excitation and injection by porphyrin singlet excited states surface-bound to SnO2/TiO2 core/shell metal oxide nanoparticle films varies with the excited state reduction potential, E°(')(P(+)/P*). With the mesityl-substituted porphyrin, high current density and sustained photocurrents are observed at pH 7 with the addition of the electron transfer donor hydroquinone.

Optical control of neuronal firing via photoinduced electron transfer in donor-acceptor conjugates.

A series of porphyrin-fullerene linked molecules has been synthesized to evaluate the effects of substituents and molecular structures on their charge-separation yield and the lifetime of a final charge-separated state in various hydrophilic environments. The selected high-performance molecule effectively achieved depolarization in a plasma cell membrane by visible light as well as two-photon excitation using a near-infrared light laser. Moreover, it was revealed that the depolarization can trigger neuronal firing in rat hippocampal neurons, demonstrating the potential and versatility for controlling cell functions using light.

Fluorescence quenching of rhodamine 6G by 1,3,5-Trinitroperhydro-1,3,5- triazine entrapped in porous sol–gel silica

Fluorescence quenching of laser dye rhodamine 6G by codoping 1,3,5-Trinitroperhydro-1,3,5-triazine (RDX) in porous sol–gel silica is reported. It was found that the fluorescence intensity of rhodamine 6G decreases with increase in the concentration of RDX. Both the samples, predoped cylindrical shaped and postdoped thick film on glass substrate with RDX show the similar fluorescence quenching behavior. Emission spectra show slight red shifting with the increase in RDX concentration. An electron-transfer donor–acceptor mechanism is proposed for the observed fluorescence quenching. On the basis of these results, candidature of dye doped porous sol–gel silica as a sensor for explosive detection seems to be promising.

Photo-initiated multi-step electron transfer in donor–acceptor systems using a novel bi-functionalized perylene chromophore

The excited state and redox properties of a new bi-functional perylene redox chromophore, 2,3-dihydro-1-azabenzo[cd]perylene (DABP), are described. Perylene has been widely used in electron donor–acceptor molecules in fields ranging from artificial photosynthesis to molecular spintronics. However, attaching multiple redox components to perylene to carry out multi-step electron transfer reactions often produces hard to separate regioisomers, which complicate data analysis. The use of DABP provides a strategy to retain the electronic properties of perylene, yet eliminate regioisomers. Ultrafast photo-initiated single- and two-step electron transfer reactions in three linear electron donor–acceptor systems incorporating DABP are described to illustrate its utility.

Structural heterogeneity among four subunits in pyranose 2-oxidase: A molecular dynamics simulation study

The homotetramer pyranose 2-oxidase (P2O) from Tetrametes multicolor contains flavin adenine dinucleotide (FAD) as a cofactor, and displays two conformers with different transient fluorescence spectra and lifetimes (ca. 0.1 ps and 360 ps). The ultrashort lifetimes of isoalloxazine (Iso) are ascribed to the photoinduced electron transfer (ET) from Trp168 to the excited Iso. Here, the structural heterogeneity among the four subunits in solution was studied by means of molecular dynamics simulation (MDS). The ET donor–acceptor distances in crystal and solution were compared. The distribution of the H-bond distances between Iso and the surrounding amino acids revealed appreciable differences among the four subunits. The structural fluctuations in two distant places were examined for the Iso-P and Iso-Q distances (where P and Q are Trp or Tyr) with the correlation coefficients between Iso-P and Iso-Q distances, revealing cooperative motions even though P and Q were more than 1 nm apart and located in different subunits. Moreover, distributions of the distances between Iso and its closest ionic amino acids markedly differed among the four subunits. Electrostatic (ES) energies between the Iso anion and the ionic amino acids in the entire protein were obtained using a static dielectric constant of 1. The ES energy in each subunit was strongly influenced by the other subunits, whilst the distributions of the ES energies greatly differed among the four subunits. This heterogeneous distribution of the ES energy between subunits may contribute to the large differences in the experimentally detected ET rates.

Role of the electrostatic energy between the photo-products and ionic groups on the photoinduced electron transfer rates from aromatic amino acids to the excited flavin in five single-point substitution isoforms of the charged amino acid residue-13 in the FMN-binding protein

Effect of the charge (negative, positive or neutral) of amino acid residue-13 on the photoinduced electron transfer (ET) from Trp32, Tyr35 and Trp106 to the excited isoalloxazine was evaluated in the flavin mononucleotide-binding protein from Desulfovibrio vulgaris isolate Miyazaki F (DvFBP). The protein structures of the wild type and the four isoforms where glutamic acid-13 is replaced with lysine (E13K), arginine (E13R), threonine (E13T) and glutamine (E13Q) in aqueous solution were obtained by molecular dynamics simulation. The distances between the amino acid residue-13 and isoalloxazine (Iso), and between the amino acid residue-13 and the ET donors were longer than 1 nm. The ET rates were evaluated with the Kakitani and Mataga model (KM theory) from their ultrafast fluorescence dynamics by means of a non-linear least squares method. Electrostatic (ES) energies between the photo-products and other ionic groups in the proteins markedly varied among ET donors and among the DvFBP isoforms, while the other physical quantities related to the ET rates, the solvent reorganisation and ES energies between the Iso anion and the donor cations did not vary much between the proteins and donors. A plot of the logarithmic ET rates versus either the total free energy gaps or the net ES energies between the photo-products and the other ionic groups both displayed a parabolic function and so the net ES energies are an important influential factor upon the ET rate, in addition to the donor–acceptor distance.

On the effect of the nature of the bridge on oxidative or reductive photoinduced electron transfer in donor–bridge–acceptor systems

Photoinduced electron transfer is a topical issue in chemistry. In multicomponent donor-bridge-acceptor systems, electron transfer is usually discussed within the frame of superexchange theory, which takes into account electronic coupling mediated by virtual states involving bridge orbitals. However, the schematization used for superexchange in thermal electron transfer processes is not suitable to immediately understand some intriguing aspects of photoinduced charge separation and recombination processes, which are only uncovered by analyzing the virtual states involved in forward and backward excited-state electron transfer. In particular, for oxidative photoinduced electron transfer, a low-energy virtual state which cannot mediate the forward charge separation can efficiently mediate charge recombination via the hole-transfer superexchange route, whereas for reductive photoinduced electron transfer, a low-energy virtual state which cannot mediate the forward process can efficiently mediate charge recombination via electron-transfer superexchange. As a consequence, to obtain long-lived charge-separated states upon oxidative photoinduced electron transfer in donor-bridge-acceptor systems it is preferable to avoid easy-to-oxidize bridges, whereas easy-to-reduce bridges should better be avoided in reductive photoinduced charge separation. These considerations, exemplified by the analysis of some literature cases, can be useful hints for the design of long-lived charge-separated states.

The mechanism of photoinduced electron transfer in the d-amino acid oxidase–benzoate complex from pig kidney: Electron transfer in the inverted region

Abstract d -Amino acid oxidase (DAAO) from pig kidney binds flavin adenine dinucleotide (FAD) as a cofactor. The DAAO–benzoate (Bz) complex (DAOB) is remarkably more stable than DAAO. It was previously reported that the fluorescence lifetime of FAD in DAAO is reduced by 66% upon Bz binding. The mechanism of photoinduced electron transfer (ET) in the DAOB monomer from Bz to the excited isoalloxazine (Iso*) was studied by molecular dynamics simulation (MDS) and by Kakitani and Mataga electron transfer theory (KM theory), and compared with that in DAAO. The ET parameters contained in KM theory were determined to obtain the fluorescence lifetime of DAOB (60 ps). The ET mechanism in DAOB is different from other flavoproteins, where neutral tryptophans and tyrosines are the ET donors, since the anionic Bz acts as the anionic ET donor in DAOB. Moreover, ET takes place in the inverted region. Charge transfer interaction (CT) was studied by semi-empirical molecular orbital (MO) methods. The dipole moment in the Iso*–Bz system was lower than that in the isoalloxazine (Iso)–Bz system, which was ascribed to CT from the negatively charged Bz to Iso*. The mean interaction energies between Iso and Bz over 100 snapshots were −16.8 and −8.1 kcal/mol in the excited and the ground states, respectively, with mean charge densities for the excited and the ground states of −0.354 and −0.013 at Iso and −0.646 and −0.987 at Bz, respectively. The ionization potential of Bz was evaluated as 7.3 eV by ET analysis, which was in excellent agreement with that obtained by MO calculations. The dielectric constant between Iso and the donors was 2.2 in DAOB, compared to 5.8 in DAAO. According to the MDS, several water molecules existed around Iso in DAAO that were all removed upon Bz binding resulting in a strongly non-polar environment near Iso in DAOB.

Exponential Distance Dependence of Photoinitiated Stepwise Electron Transfer in Donor–Bridge–Acceptor Molecules: Implications for Wirelike Behavior

Donor-bridge-acceptor (D-B-A) systems in which a 3,5-dimethyl-4-(9-anthracenyl)julolidine (DMJ-An) chromophore and a naphthalene-1,8:4,5-bis(dicarboximide) (NI) acceptor are linked by oligomeric 2,7-fluorenone (FN(n)) bridges (n = 1-3) have been synthesized. Selective photoexcitation of DMJ-An quantitatively produces DMJ(+•)-An(-•), and An(-•) acts as a high-potential electron donor. Femtosecond transient absorption spectroscopy in the visible and mid-IR regions showed that electron transfer occurs quantitatively in the sequence: DMJ(+•)-An(-•)-FN(n)-NI → DMJ(+•)-An-FN(n)(-•)-NI → DMJ(+•)-An-FN(n)-NI(-•). The charge-shift reaction from An(-•) to NI(-•) exhibits an exponential distance dependence in the nonpolar solvent toluene with an attenuation factor (β) of 0.34 Å(-1), which would normally be attributed to electron tunneling by the superexchange mechanism. However, the FN(n)(-•) radical anion was directly observed spectroscopically as an intermediate in the charge-separation mechanism, thereby demonstrating conclusively that the overall charge separation involves the incoherent hopping (stepwise) mechanism. Kinetic modeling of the data showed that the observed exponential distance dependence is largely due to electron injection onto the first FN unit followed by charge hopping between the FN units of the bridge biased by the distance-dependent electrostatic attraction of the two charges in D(+•)-B(-•)-A. This work shows that wirelike behavior does not necessarily result from building a stepwise, energetically downhill redox gradient into a D-B-A molecule.

Spin Chemistry Investigation of Peculiarities of Photoinduced Electron Transfer in Donor–Acceptor Linked System

Photoinduced intramolecular electron transfer in linked systems, (R,S)- and (S,S)-naproxen-N-methylpyrrolidine dyads, has been studied by means of spin chemistry methods [magnetic field effect and chemically induced dynamic nuclear polarization (CIDNP)]. The relative yield of the triplet state of the dyads in different magnetic field has been measured, and dependences of the high-field CIDNP of the N-methylpyrrolidine fragment on solvent polarity have been investigated. However, both (S,S)- and (R,S)-enantiomers demonstrate almost identical CIDNP effects for the entire range of polarity. It has been demonstrated that the main peculiarities of photoprocesses in this linked system are connected with the participation of singlet exciplex alongside with photoinduced intramolecular electron transfer in chromophore excited state quenching.

Determining the Conformational Change that Accompanies Donor–Acceptor Distance Fluctuations: An Umbrella Sampling Analysis

The response of a protein to variation of a specific coordinate can provide insights into the role of the overall architecture in the structural change. Given that the calculated potential of mean force governing the fluctuation of an electron transfer donor-acceptor distance in the NAD(P)H:Flavin oxidoreductase (Fre)/FAD complex was shown to agree with experiment, an analysis of the structural response of the rest of the protein to that distance change was made. Significant displacements are found throughout much of the protein, and the coupling pathway resulting in the structural changes was determined. A covariance analysis based on the quasiharmonic modes of the unperturbed protein was used to provide information concerning how the residue motions are correlated. It is found that, of the three regions identified as moving together in an NMR study, two undergo significant structural changes when the electron donor-acceptor distance is varied, and the third does not.