Fast Charge Separation Research Articles

Exfoliated graphite nanosheets/carbon nanotubes hybrid materials for superior performance supercapacitors

Nanostructured hybrid material of exfoliated graphite nanosheets and carbon nanotubes (GNSNT) served as supercapacitor electrode materials was presented. The nanostructured hybrid was prepared by a facile chemical reduction method. The hybrid material was characterized by X-ray diffraction technique, transmission electron microscopy, scanning electron microscopy, cyclic voltammetry, galvanostatic charge/discharge cycling, and four-point probe conductivity measurement to represent a well-defined nanostructure possessing a vast number of active sites and delivering the ingredients for a fast effective charge separation network. Our results clearly demonstrated that the hybrid possess a superior performance. A specific capacitance value 266 F/g was obtained for GNSNT hybrid electrode at a current density of 0.1 A/g, while it was only 185 F/g for exfoliated graphite nanosheets (GNS). At a higher current density of 2 A/g, the GNSNT electrode still keeps a specific capacitance of 220 F/g, which is more than double that of GNS. This synergistic effect of the nanostructured hybrid material offers an effective network for charge separation and therefore renders a significantly enhanced specific capacitance and rate capability.

Hole‐Transfer Dyads and Triads Based on Perylene Monoimide, Quaterthiophene, and Extended Tetrathiafulvalene

Two families of dyad and triad systems based on perylene monoimide (PMI), quaterthiophene (QT), and 9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene (extended tetrathiafulvalene, exTTF) molecular components have been designed and synthesized. The dyads (D1 and D2) are of the PMI-QT type and the triads (T1 and T2) of the PMI-QT-exTTF type. The two families differ in the saturated or unsaturated nature of the linker groups (ethynylene in D1 and T1, ethylene in D2 and T2) that bridge the molecular components. The dyads and triads have been characterized by electrochemical, photophysical, and computational methods. Both the experimental and the computational (DFT) results indicate that in the unsaturated systems strong intercomponent interactions lead to substantial perturbation of the properties of the subunits. In particular, in T1, delocalization is particularly effective between the QT and exTTF units, which would be better viewed combined as a single electronic subsystem. For the dyad systems, the photophysics observed following excitation of the PMI unit is solvent-dependent. In moderately polar solvents (dichloromethane, diethyl ether) fast charge separation is followed by recombination to the ground state. In toluene, slow conversion to the charge-separated state is followed by intersystem crossing and recombination to yield the triplet state of the PMI unit. The behavior of the triads, on the other hand, is remarkably similar to that of the corresponding dyads, which indicates that, after primary charge separation, hole shift from the oxidized QT component to exTTF is quite inefficient. This unexpected result has been rationalized on the basis of the anomalous (simultaneous two-electron oxidation) electrochemistry of exTTF and with the help of DFT calculations. In fact, although exTTF is electrochemically easier to oxidize than QT by around 0.6 V, the one-electron redox orbitals (HOMOs) of the two units in triad T2 are almost degenerate.

Efficient Electron Transfer Processes of the Covalently Linked Perylenediimide−Ferrocene Systems: Femtosecond and Nanosecond Transient Absorption Studies

Photoinduced electron-transfer processes of the newly synthesized rodlike covalent donor−acceptor molecules consisting of electron-donating ferrocenes (Fc) with electron-accepting perylenediimides (PDIs) with core-substituted cyano and pyrrolidine groups, forming Fc-PDI(CN)2 dyad, Fc2-PDI(CN)2 triad, and Fc-PDI(Py)2 dyad, have been investigated in benzonitrile. The geometric and electronic structures of the dyads and triad were probed by ab initio B3LYP/6-311G methods. The distribution of the highest occupied molecular orbitals (HOMOs) was on the ferrocene entities, while the distribution of the lowest unoccupied molecular orbitals (LUMOs) was on the PDI entities. Free-energy calculations verify that the light-induced processes from excited states of PDIs are exothermic. The excited state photochemical events are monitored by femtosecond and nanosecond transient absorption techniques. In benzonitrile, the quenching pathway involves fast and efficient charge separation from the ferrocenes to the excited PD...

Supramolecular Tetrad of Subphthalocyanine–Triphenylamine–Zinc Porphyrin Coordinated to Fullerene as an “Antenna‐Reaction‐Center” Mimic: Formation of a Long‐Lived Charge‐Separated State in Nonpolar Solvent

We report here the formation of a long-lived charge-separated state of a self-assembled donor-acceptor tetrad, formed by axial coordination of a fulleropyrrolidine appended with an imidazole coordinating ligand (C(60)Im) to the zinc center of a subphthalocyanine-triphenylamine-zinc porphyrin (SubPc-TPA-ZnP), as a charge-stabilizing antenna reaction center mimic in toluene. The subphthalocyanine and triphenylamine entities, with their high-energy singlet states, act as an energy-transferring antenna unit to produce a singlet zinc porphyrin. The formation constant for the self-assembled tetrad was determined to be 1.0 x 10(4) M(-1), suggesting a moderately stable complex formation. The geometric and electronic structures of the covalently linked SubPc-TPA-ZnP triad and self-assembled SubPc-TPA-ZnP:C(60)Im tetrad were examined by using an ab initio B3LYP/6-31G method. The majority of the highest occupied frontier molecular orbital was found over the ZnP and TPA entities, whereas the lowest unoccupied molecular orbital was located over the fullerene entity, suggesting the formation of the radical-ion pair (SubPc-TPA-ZnP(*+):C(60)Im(*-)). The redox measurements revealed that the energy level of the radical-ion pair in toluene is located lower than that of the singlet and triplet states of the zinc porphyrin and fullerene entities. The femtosecond transient absorption measurements revealed fast charge separation from the singlet porphyrin to the coordinated C(60)Im with a lifetime of 1.1 ns. Interestingly, slow charge recombination (1.6 x 10(5) s(-1)) and the long lifetime of the charge-separated state (6.6 micros) were obtained in toluene by utilizing the nanosecond transient measurements.

Femtosecond primary charge separation in Synechocystis sp. PCC 6803 photosystem I

The ultrafast (< 100 fs) conversion of delocalized exciton into charge-separated state between the primary donor P700 (bleaching at 705 nm) and the primary acceptor A 0 (bleaching at 690 nm) in photosystem I (PS I) complexes from Synechocystis sp. PCC 6803 was observed. The data were obtained by application of pump–probe technique with 20-fs low-energy pump pulses centered at 720 nm. The earliest absorbance changes (close to zero delay) with a bleaching at 690 nm are similar to the product of the absorption spectrum of PS I complex and the laser pulse spectrum, which represents the efficiency spectrum of the light absorption by PS I upon femtosecond excitation centered at 720 nm. During the first ∼ 60 fs the energy transfer from the chlorophyll ( Chl) species bleaching at 690 nm to the Chl bleaching at 705 nm occurs, resulting in almost equal bleaching of the two forms with the formation of delocalized exciton between 690-nm and 705-nm Chls. Within the next ∼ 40 fs the formation of a new broad band centered at ∼ 660 nm (attributed to the appearance of Chl anion radical) is observed. This band decays with time constant simultaneously with an electron transfer to A 1 (phylloquinone). The subtraction of kinetic difference absorption spectra of the closed (state P700 +A 0A 1) PS I reaction center (RC) from that of the open (state P700A 0A 1) RC reveals the pure spectrum of the P700 +A 0 − ion–radical pair. The experimental data were analyzed using a simple kinetic scheme: An* → k 1 [(PA 0)*A 1 → < 100 fs P +A 0 −A 1] → k 2 P +A 0A 1 −, and a global fitting procedure based on the singular value decomposition analysis. The calculated kinetics of transitions between intermediate states and their spectra were similar to the kinetics recorded at 694 and 705 nm and the experimental spectra obtained by subtraction of the spectra of closed RCs from the spectra of open RCs. As a result, we found that the main events in RCs of PS I under our experimental conditions include very fast (< 100 fs) charge separation with the formation of the P700 +A 0 −A 1 state in approximately one half of the RCs, the ∼ 5-ps energy transfer from antenna Chl* to P700A 0A 1 in the remaining RCs, and ∼ 25-ps formation of the secondary radical pair P700 +A 0A 1 −.

Structure, Dynamics, and Power Conversion Efficiency Correlations in a New Low Bandgap Polymer: PCBM Solar Cell

Molecular packing structures and photoinduced charge separation dynamics have been investigated in a recently developed bulk heterojunction (BHJ) organic photovoltaic (OPV) material based on poly(thienothiophene-benzodithiophene) (PTB1) with a power conversion efficiency (PCE) of >5% in solar cell devices. Grazing incidence X-ray scattering (GIXS) measurements of the PTB1:PCBM ([6,6]-phenyl-C(61)-butyric acid methyl ester) films revealed pi-stacked polymer backbone planes oriented parallel to the substrate surface, in contrast to the pi-stacked polymer backbone planes oriented perpendicular to the substrate surface in regioregular P3HT [poly(3-hexylthiophene)]:PCBM films. A approximately 1.7 times higher charge mobility in the PTB1:PCBM film relative to that in P3HT:PCBM films is attributed to this difference in stacking orientation. The photoinduced charge separation (CS) rate in the pristine PTB1:PCBM film is more than twice as fast as that in the annealed P3HT:PCBM film. The combination of a small optical gap, fast CS rate, and high carrier mobility in the PTB1:PCBM film contributes to its relatively high PCE in the solar cells. Contrary to P3HT:PCBM solar cells, annealing PTB1:PCBM films reduced the device PCE from 5.24% in the pristine film to 1.92% due to reduced interfacial area between the electron donor and the acceptor. Consequently, quantum yields of exciton generation and charge separation in the annealed film are significantly reduced compared to those in the pristine film.

Internal Electrostatic Control of the Primary Charge Separation and Recombination in Reaction Centers from Rhodobacter sphaeroides Revealed by Femtosecond Transient Absorption

We report the observation of two conformational states of closed RCs from Rhodobacter sphaeroides characterized by different P(+)H(A)(-) --> PH(A) charge recombination lifetimes, one of which is of subnanosecond value (700 +/- 200 ps). These states are also characterized by different primary charge separation lifetimes. It is proposed that the distinct conformations are related to two protonation states either of reduced secondary electron acceptor, Q(A)(-), or of a titratable amino acid residue localized near Q(A). The reaction centers in the protonated state are characterized by faster charge separation and slower charge recombination when compared to those in the unprotonated state. Both effects are explained in terms of the model assuming modulation of the free energy level of the state P(+)H(A)(-) by the charges on or near Q(A) and decay of the P(+)H(A)(-) state via the thermally activated P(+)B(A)(-) state.

Helical Aromatic Oligoamide Foldamers as Organizational Scaffolds for Photoinduced Charge Transfer

Here we report the synthesis and characterization of four quinoline-derived foldamers with increasing oligomeric length; dimer O2P, tetramer O4P, pentamer O5P, and nonamer O9P functionalized with on one end an oligo(p-phenylene vinylene) (OPV) and on the other end a perylene bisimide (PB) chromophore. (1)H NMR confirms the formation of the expected folded structures in both toluene and chloroform solution. The structural predictability and rigidity of the oligomeric series enabled us to investigate the effect of a helical bridge and chromophore position on the photoinduced processes in the electron OPV-PB donor-acceptor pair in chloroform and toluene. The helical properties of the bridge ensured that the chromophore separation distance through space is different from the separation distance through the bridge. For all foldamer-solvent combinations studied, excitation of either OPV or PB results in nearly quantitative quenching of the fluorescence indicating a fast charge separation reaction between the OPV and PB. Femtosecond photoinduced absorption measurements confirmed the fast formation of a charge-separated state. The recombination reaction involves a combination of direct decay to the ground state and the formation of an intermediate triplet state, with their balance depending on the foldamer-solvent combination. Molecular orbital calculations rationalize the fast photoinduced charge separation, by revealing that the bridging foldamer mediates the charge transfer from donor to acceptor via the superexchange mechanism. Remarkably low attenuation factors (beta(CS) approximately 10(-2) A(-1)) were obtained using either through space or through bridge separation distance. However, in these calculations only three of the four foldamers show the expected linear behavior between the logarithm of the charge separation rate constant and the distance between the chromophores. The combined results show when a helical bridge is separating the charge transfer couple, hampering the usefulness of a uniform description of the charge-separation phenomena.

Zn pyro-pheophorbide a—fulleronicotine dyad; supramolecular self assembled donor–acceptor system for photoinduced charge separation

Zn pyro-pheophorbide a and fulleronicotine form in nonpolar solvents a supramolecular self-assembled electron donor-acceptor dyad, which performs fast photoinduced charge separation (0.5 ps) and slow recombination (>1 ns) as evidenced by photochemical studies.

Very Fast Single-Step Photoinduced Charge Separation in Zinc Porphyrin Bridged to a Gold Porphyrin by a Bisethynyl Quaterthiophene

A new heterometallic dyad composed of a zinc porphyrin linked by bisethynyl quaterthiophene to a gold porphyrin was synthesized according to a stepwise modular approach. The latter dyad and the parent reference compounds (porphyrin-ethynylquaterthiophene) were characterized by electrochemistry, spectroelectrochemistry, and femtosecond transient absorption spectrocopy. We showed that light excitation of the zinc or the gold porphyrin induces a very fast and quantitative charge separation over a distance of 25 A which occurs through a superexchange mechanism. The lifetime of the charge-separated state is 3.3 ns in toluene and 100 ps in dichloromethane, and it recombines to the ground state in both solvents.

Extremely long-distance electron transfer in porphyrin or phthalocyanine systems directly functionalized by an oligo(phenyleneethynylene) spacer

This review focuses on a new strategy to design covalently linked and electronically coupled porphyrin systems to achieve long-range photoinduced electron transfer. We show that the attachment of a π-conjugated bridge directly on the meso position of the porphyrin via an ethynyl or ethenyl linkage affords significant electronic coupling both on the ground- and excited-states and that oligophenyleneethynylenes represent suitable spacers to assist charge separation between porphyrins. They can indeed exhibit wire-like behaviour and promote fast interchromophore charge separation over distances up to 4.5 nm. These recent electronically coupled dyads represent attractive photochemical systems to build more complex arrays for solar energy conversion or for molecular electronic applications.

Long-lived photoinduced charge separation in new Ru(bipyridine)32+—phenothiazine dyads

Synthesis and photophysical properties of three Ru(bpy)(3)(2+)-Ptz (bpy = 2,2'-bipyridine and Ptz = phenothiazine) dyads, where the number of Ptz groups increased from one to three, are reported. The MLCT absorption bands of these compounds were slightly red shifted compared to Ru(bpy)(3)(2+). The emission, however, was highly quenched and this is attributed to electron transfer from the Ptz moiety to the excited Ru(bpy)(3)(2+) to generate the charge separated state Ru(bpy)(3)(+)-Ptz (+). Observed electron transfer rates (k(et) > 10(8) s(-1)) were much faster than those previously reported (k(et) < 10(7) s(-1)) for linked Ru(bpy)(3)(2+)-Ptz systems. Compared to the previous systems, back electron transfer rates in these systems were about 100 times slower. This has enabled us to observe the charge separated state in nanosecond flash photolysis experiments. Transient absorptions assignable to Ru(bpy)(3)(+) and Ptz (+), having lifetimes in the range of 10-30 ns were observed. In order to explain the fast charge separation and slow charge recombination rates, formation of a folded conformer where the Ptz group attached to one bpy residue comes closer to and associates with another bpy moiety was invoked. A scheme which explains the fast electron transfer and slow recombination in this pre-associated state is proposed.

Development of bioinspired artificial photosynthetic systems

The specific objective of this review is to describe recent development of bioinspired artificial photosynthetic systems and their applications. First, multi-step electron-transfer systems composed of electron donor-acceptor ensembles are presented, mimicking functions of the photosynthetic reaction center. However, a significant amount of energy is lost during the multi-step electron-transfer processes. Then, as an alternative to conventional charge-separation functional molecular models based on multi-step long-range electron transfer within redox cascades, simple electron donor-acceptor dyads linked by covalent or non-covalent bonding have been developed to attain a long-lived and high-energy charge-separated state without significant loss of excitation energy. Such simple molecular dyads, capable of fast charge separation but extremely slow charge recombination, have significant advantages with regard to synthetic feasibility, providing a variety of applications including construction of organic solar cells and development of efficient photocatalytic systems for the solar energy conversion.

Photocatalytic H 2 production from methanol aqueous solution over titania nanoparticles with mesostructures

The photocatalytic hydrogen production over TiO 2 nanoparticles with mesostructures has been studied using CH 3 OH as a sacrificial reagent. The effects of the calcination temperature, Pt-loading, amount of TiO 2 and methanol as well as irradiation time on the H 2 evolution rate were investigated in detail. The results indicated that the as-synthesized TiO 2 nanoparticles without calcination, having a large specific surface area ( 438 m 2 / g ) and small crystallite size (2.3 nm) dispersed among the amorphous mesoporous domains, exhibited the best photocatalytic activity for H 2 production compared with the samples calcined at different temperatures and the commercial photocatalyst P25 (Degussa, Germany) under the same photoreaction conditions. The presence of mesostructured TiO 2 nanoparticles with a uniform dispersion of the co-catalyst (Pt) has resulted in an effective, faster charge separation, which, in turn, leads to a higher photocatalytic activity for H 2 production.

New development of photoinduced electron-transfer catalytic systems

AbstractAs an alternative to conventional charge-separation functional molecular models based on multi-step long-range electron transfer (ET) within redox cascades, simple donor-acceptor dyads have been developed to attain a long-lived and high-energy charge-separated (CS) state without significant loss of excitation energy. In particular, a simple molecular electron donor-acceptor dyad, 9-mesityl-10-methylacridinium ion (Acr+-Mes), is capable of fast charge separation but extremely slow charge recombination. Such a simple molecular dyad has significant advantages with regard to synthetic feasibility, providing a variety of applications for photoinduced ET catalytic systems, including efficient photocatalytic systems for the solar energy conversion and construction of organic solar cells.

Photoinduced Electron Transfer and Excitation Energy Transfer in Directly Linked Zinc Porphyrin/Zinc Phthalocyanine Composite

Photoinduced electron transfer (ET) and excitation energy transfer (ENT) reactions in monomer and slipped-cofacial dimer systems of a directly linked Zn porphyrin (Por)-Zn phthalocyanine (Pc) heterodyad, ZnPc-ZnPor, were investigated by means of the picosecond and femtosecond transient absorption spectroscopies. In the dimer dyad system of two heterodyads connected through the coordination bond between two imidazolyl-substituted ZnPor bearing ZnPc, ZnPc-ZnPor(D), the rapid ENT from the ZnPor to ZnPc in the subpicosecond time region was followed by photoinduced charge separation (CS) and charge recombination (CR) with time constants of 47 and 510 ps, respectively. On the other hand in the monomer dyad system, no clear charge-separated state was observed although the CS with a time constant of 200 ps and CR with < or =70 ps were estimated. These results indicated that the dimer slipped-cofacial arrangement of pair porphyrins is advantageous for the effective production of the CS state. This advantage was discussed from the viewpoint of a decrease in the reorganization energy of the dimer relative to that of the monomer system. In addition, the electrochemical measurements indicated that the strong interaction between ZnPc and ZnPor moieties also contributed to the fast CS process despite the marginal driving force for the CS process. The dimer dyad of ZnPc-ZnPor provides full advantages in efficiencies of the light harvesting and the CS state production.

Carbon Nanotubes in Electron Donor—Acceptor Nanocomposites

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Photoinduced Electron Transfer Research to Build Model Compounds of Artificial Photosynthesis and Solar Energy Conversion

AbstractIn this article various novel synthesized dyads and triad systems containing organic based electron donor and acceptor compounds linked together by flexible and rigid spacers have been described which undergo very fast (˜ ps) charge separation reactions and relatively slow energy destructive charge recombination. Various techniques have been discussed to enhance the survival duration of the charge‐separated products, resulted from photoinduced electron transfer reactions, by retarding the energy wasting charge recombination process. It has been hinted that these systems should form the basis of future artificial photosynthetic and solar energy conversion devices.

Bioinspired Electron-Transfer Systems and Applications

Abstract Bioinspired electron-transfer systems including artificial photosynthesis and respiration are presented herein together with some of their applications. First, multi-step electron-transfer systems composed of electron donor–acceptor ensembles have been developed, mimicking functions of the photosynthetic reaction center. However, a significant amount of energy is lost during the multi-step electron-transfer processes. Then, as an alternative to conventional charge-separation functional molecular models based on multi-step long-range electron transfer within redox cascades, simple donor–acceptor dyads have been developed to attain a long-lived and high energy charge-separated state without significant loss of excitation energy, by fine control of the redox potentials and of the geometry of donor–acceptor dyads that have small reorganization energies of electron transfer. Such simple molecular dyads, capable of fast charge separation but extremely slow charge recombination, have significant advantages with regard to synthetic feasibility, providing a variety of applications including construction of organic solar cells and development of efficient photocatalytic systems for the solar energy conversion. An efficient four-electron reduction of dioxygen to water by one-electron reductants such as ferrocene derivatives as well as by an NADH analog has also been achieved as a respiration model by using a cofacial dicobalt porphyrin that can form the μ-peroxo Co(III)–O2–Co(III) complex. The catalytic mechanism of the four-electron reduction of dioxygen has been clarified based on the detailed kinetic study and the detection of the intermediate.

Charge Transfer Processes in Conjugated Triarylamine−Oligothiophene−Perylenemonoimide Dendrimers

The synthesis and charge transfer properties of triarylamine-oligothiophene-perylenemonoimide dendrimers, TPA(T2-PMI)3 and TPA(T4-PMI)3, are described. The fluorescence quantum yields indicate strong emission quenching by electron transfer [phi(THF) = 0.004 for TPA(T2-PMI)3, phi(THF) = 0.003 for TPA(T4-PMI)3, and phi(THF) = 0.8 for PMI]. Moreover, with the increase of the solvent polarity, the quantum yields decrease indicating that the A+* D-* (acceptor/donor) couple is more stabilized. The femtosecond transient absorption spectra show a very fast charge separation process (approximately 2 ps; k(cs) approximately 5 x 10(11) s(-1)) and a charge recombination of more than 1 order of magnitude slower (approximately 50 ps; k(cr) approximately 2 x 10(10) s(-1)), as observed from the rise time and decay of the radical anion and radical cation absorption bands. The analysis of the transient absorption spectroscopy and of the energetics of the process using Marcus theory indicates that in the electron transfer process the thiophene unit is the first electron donor. The triarylamine is not functioning as a second electron donor, as also substantiated by the absence of an effect of the addition of acid on the emission intensity of the dendrimers. The presence of the triarylamine and/or the proximity of the oligothiophenes does improve the donor capabilities of the oligothiophene unit slightly and enhances its conjugation as seen in the absorption spectra and the transients of the radial cations. These results can be used for a better design of molecular materials for, e.g., photovoltaic applications.