Solid-state Electron Transfer Research Articles

Direct observation of electron transfer in solids through X-ray crystallography

Nanoscale electron transfer (ET) in solids is fundamental to the design of multifunctional nanomaterials, yet its process is not fully understood. Herein, through X-ray crystallography, we directly observe solid-state ET via a crystal-to-crystal process. We first demonstrate the creation of a robust and flexible electron acceptor/acceptor (A/A) double-wall nanotube crystal ([(Zn2+)4(LA)4(LA=O)4]n) with a large window (0.90 nm × 0.92 nm) through the one-dimensional porous crystallization of heteroleptic Zn4 metallocycles ((Zn2+)4(LA)4(LA=O)4) with two different acceptor ligands (2,7-bis((1-ethyl-1H-imidazol-2-yl)ethynyl)acridine (LA) and 2,7-bis((1-ethyl-1H-imidazol-2-yl)ethynyl)acridin-9(10H)-one (LA=O)) in a slow-oxidation-associated crystallization procedure. We then achieve the bottom-up construction of the electron donor incorporated-A/A nanotube crystal ([(D)2⊂(Zn2+)4(LA)4(LA=O)4]n) through the subsequent absorption of electron donor guests (D = tetrathiafulvalene (TTF) and ferrocene (Fc)). Finally, we remove electrons from the electron donor guests inside the nanotube crystal through facile ET in the solid state to accumulate holes inside the nanotube crystal ([(D•+)2⊂(Zn2+)4(LA)4(LA=O)4]n), where the solid-state ET process (D – e– → D•+) is thus observed directly by X-ray crystallography.

Improved photocatalytic decolorization of reactive black 5 dye through synthesis of graphene quantum dots-nitrogen-doped TiO2.

Graphene quantum dots (GQDs), a new solid-state electron transfer material was anchored to nitrogen-doped TiO2 via sol gel method. The introduction of GQDs effectively extended light absorption of TiO2 from UV to visible region. GQD-N-TiO2 demonstrated lower PL intensity at excitation wavelengths of 320 to 450nm confirming enhanced exciton lifespan. GQD-N-TiO2-300 revealed higher surface area (191.91m2g-1), pore diameter (1.94nm), TEM particle size distribution (4.88 ± 1.26nm) with lattice spacing of 0.45nm and bandgap (2.91eV). In addition, GQDs incorporation shifted XPS spectrum of Ti 2p to lower binding energy level (458.36eV), while substitution of oxygen sites in TiO2 lattice by carbon were confirmed through deconvolution of C 1s spectrum. Photocatalytic reaction followed the pseudo first order reaction and continuous reductions in apparent rate constant (Kapp) with incremental increase in RB5 concentration. Langmuir-Hinshelwood model showed surface reaction rate constants KC = 1.95mg L-1min-1 and KLH = 0.76 L mg-1. The active species trapping, and mechanism studies indicated the photocatalytic decolorization of RB5 through GQD-N-TiO2 was governed by type II heterojunction. Overall, the photodecolorization reactions were triggered by the formation of holes and reactive oxygen species. The presence of •OH, 1O2, and O2• during the photocatalytic process were confirmed through EPR analysis. The excellent photocatalytic decolorization of the synthesized nanocomposite against RB5 can be ascribed to the presence of GQDs in the TiO2 lattice that acted as excellent electron transporter and photosensitizer. This study provides a basis for using nonmetal, abundant, and benign materials like graphene quantum dots to enhance the TiO2 photocatalytic efficiency, opening new possibilities for environmental applications.

Effects of Different Counter Anions on Solid-State Electron Transfer in Viologen Compounds: Modulation of Color and Piezo- and Photochromic Properties.

Charge transfer (CT) and electron transfer (ET) underlie the various applications of viologen compounds. However, the CT and ET in solids are still insufficiently understood and difficult to control, which is challenging but fundamental to the design of multifunctional materials. Our strategy is to attempt a delicate control of CT and ET by subjecting model compounds to pressure. In this work, a series of viologens of the same cation with different anions show anion-dependent color and piezo-/photochromism due to ground-state CT and stimuli-induced ET; the ease of solid-state ET can be tuned in the order of Cl- > Br- > I-/BF4-/PF6-/ClO4-. With in situ high pressure UV-vis experiments, we also observed unexpectedly the pressure-induced solid-state ET in [H2bcpV]·I2 besides the pressure modulation of the CT absorption. It proves pressure is a more powerful stimulus than light in tuning solid-state CT and ET.

Impact of Lattice Water on Solid-State Electron Transfer in Viologen Pseudopolymorphs: Modulation of Photo- and Piezochromism.

Stimuli-induced solid-state electron transfer (ET) underlies the use of viologen compounds as responsive materials, but unequivocal structure-property correlations for solid-state ET are still lacking. With different pseudopolymorphic solids derived from N,N'-bis(4-carboxylphenyl)viologen ([H2bcpV]2+), here we report a systematic study on photo- and piezochromic properties associated with ET. We show that the higher the water content in the lattice, the less sensitively the compounds respond to light and pressure. It is proposed that the lattice water does not act as an electron donor but serves to change the ET energetics through its unique polarity and hydrogen bonding capability. The impedimental impact of water on solid-state ET of viologen compounds has not yet been recognized and elucidated prior to this work. The study also suggests that pressure is more powerful than light in inducing ET.

Synchronized Collective Proton-Assisted Electron Transfer in Solid State by Hydrogen-Bonding Ru(II)/Ru(III) Mixed-Valence Molecular Crystals.

A proton-coupled electron transfer (PCET) reaction was widely studied with isolated organic molecules and metal complexes in solution in view of the biological catalytic reaction, while studying this reaction in the crystalline or solid-state phase, which has a novel example, would give insight into the rather internal environment of proteins without solvation and a creation of new molecular materials. We tried to crystallize a hydrogen-bonded (H-bonded) coordination polymer with one-dimensional nanoporous channels, formed from redox-active RuIII complexes, [RuIII(Hbim)3] (Hbim- = 2,2'-biimidazolate monoanion). As a result, a synchronized collective PCET phenomenon was observed for the molecular nanoporous crystal by novel solid-state cyclic voltammetry (CV), which could be measured by only setting some crystals on the electrode surface. The nanoporous crystals, {[RuIII(Hbim)3]}n (1), are simultaneously induced to a synchronized collective RuIIRuIII mixed-valence state, {RuIIRuIII}n, with alternating arrays of RuII and RuIII complexes by PCET in a way of the reductive state of {RuIIRuII}n. Further, a new crystal with {RuIIRuIII}n, {[RuII(H2bim)(Hbim)2][RuIII(bim) (Hbim)2][K(MeOBz)6]}n (2), was also prepared, and the solid-state CV revealed the same electrochemical behavior of {RuIIRuIII}n with 1. The single crystal with {RuIIRuIII}n of 2 was unusually a semiconductor with 5.12 × 10-6 S/cm conductivity at 298 K by an impedance method (8.01 × 10-6 S/cm by a direct-current method at 277 K). Thus, an unprecedented electron-hopping conductor driven by a low-barrier proton transfer through a PCET mechanism (Ea = 0.30 eV) was realized in the H-bonding molecular crystal with {RuIIRuIII}n. Such studies on a PCET reaction in the crystalline state is not only worthwhile as a model of essential biological reactions without solvation, but also proposed to a new design of molecular materials to occur an electron transfer by using an intermolecular H-bond.

Evolution of the acceptor side of photosystem I: ferredoxin, flavodoxin, and ferredoxin-NADP+ oxidoreductase.

The development of oxygenic photosynthesis by primordial cyanobacteria ~2.7billion years ago led to major changes in the components and organization of photosynthetic electron transport to cope with the challenges of an oxygen-enriched atmosphere. We review herein, following the seminal contributions as reported by Jaganathan et al. (Functional genomics and evolution of photosynthetic systems, vol 33, advances in photosynthesis and respiration, Springer, Dordrecht, 2012), how these changes affected carriers and enzymes at the acceptor side of photosystem I (PSI): the electron shuttle ferredoxin (Fd), its isofunctional counterpart flavodoxin (Fld), their redox partner ferredoxin-NADP+ reductase (FNR), and the primary PSI acceptors F x and F A/F B. Protection of the [4Fe-4S] centers of these proteins from oxidative damage was achieved by strengthening binding between the F A/F B polypeptide and the reaction center core containing F x, therefore impairing O2 access to the clusters. Immobilization of F A/F B in the PSI complex led in turn to the recruitment of new soluble electron shuttles. This function was fulfilled by oxygen-insensitive [2Fe-2S] Fd, in which the reactive sulfide atoms of the cluster are shielded from solvent by the polypeptide backbone, and in some algae and cyanobacteria by Fld, which employs a flavin as prosthetic group and is tolerant to oxidants and iron limitation. Tight membrane binding of FNR allowed solid-state electron transfer from PSI bridged by Fd/Fld. Fine tuning of FNR catalytic mechanism led to formidable increases in turnover rates compared with FNRs acting in heterotrophic pathways, favoring Fd/Fld reduction instead of oxygen reduction.

Functionalization of TiO2 with graphene quantum dots for efficient photocatalytic hydrogen evolution

Graphene quantum dots (GQDs) serve as a novel solid-state electron transfer reagent anchored on TiO2 by in situ photo-assisted strategy and greatly enhanced photocatalytic H2 evolution activity in methanol aqueous solution without the noble mental cocatalyst. The excellent photocatalytic activities were ascribed to the GQDs which act as an excellent electron transporters and acceptors, as well as photosensitizer. GQDs not only acted as efficient electron reservoirs and a solid-state electron transfer reagent from the conduction band of TiO2 to GQDs, but also acted as an excellent photosensitizer to sensitize TiO2, in which the photoinduced electrons transfer from excited GQDs to TiO2 to produce H2. In addition, GQDs is nanoscale fragments of graphene which can provide a larger active surface and greatly increase the contact area with the TiO2, which is conducive to rapidly transfer photo-generated electrons due to the large specific area and high carrier mobility of GQDs. Thus, GQDs improved the photocatalytic activity for H2 evolution.

Direct and Sensitized Photolysis of Dispersed Photoacid Generators

This work proposes water-borne photopolymers cast and developed solely with neutral water by incorporating mechanically milled sub-micron particles of poorly water-soluble photoacid generators (PAGs) to make a poly(vinyl alcohol) film insoluble in water with aid of an acid-sensitive crosslinker. Fluorescence measurements of milled mixtures of anthracene and a tiny amount of tetracene in water revealed that the solid-state energy transfer occurs through singlet exciton migration as a result of mechanochemical crystal mixing. Solid-state electron transfer was also verified by fluorescence quenching of milled mixtures of PAG and emissive sensitizers including anthracene derivatives as a consequence of exciton diffusion in sensitizer crystals to reach at the particle surface to transfer an electron to PAG solid. Factors affecting solid-state electron transfer efficiency include the nature of PAG and sensitizers, particle sizes and exciton diffusion length, whereas bis[p-(tert-butylphenyl)]iodonium hexafluorophosphate (1a) was the best solid-state quencher. Solid-state sensitized photoacid generation was confirmed by irradiating aqueous dispersions of co-milled PAG and a sensitizer. Based on these results, three-component photopolymers comprised of an aqueous dispersion of PAG in absence or in the presence of a co-milled sensitizer, poly(vinyl alcohol) and a water-soluble diepoxy were prepared to exhibit photosensitivity of 17 mJ cm-2 by the direct photolysis of a dispersed sulfonium salt and that of ca.120 mJ cm-2 by the photolysis of 1a sensitized with co-milled 9, 10-dipropoxyanthracene.

Electron transfer in mixed-valence 1′,1‴-bis( m-bromobenzyl)biferrocenium triiodide

The solid-state physical properties of the new mixed-valence 1′,1‴-bis( m-bromobenzyl)biferrocenium triiodide are reported. Mössbauer spectra of a microcrystalline sample indicated the presence of both delocalized and localized species. In the case of a recrystallized sample, below 200 K there are two doublets in the 57Fe Mössbauer spectra which, on increasing the temperature, become a single “average-valence” doublet at ca. 250 K. The effects of the nature of the motion of charge in counterion and cation—anion interactions on solid-state electron transfer were investigated by studying intramolecular electron transfer for a series of 1′,1‴-bis(halobenzyl)biferrocenium salts in solution.

The question of solid-state electron transfer in contact charging between metal and organic materials

The question of solid-state electron transfer in contact charging between metal and organic materials