Flavin Derivatives with Tailored Redox Properties: Synthesis, Characterization, and Electrochemical Behavior.

This study establishes structure-property relationships for four synthetic flavin molecules as bioinspired redox mediators in electro- and photocatalysis applications. The studied flavin compounds were disubstituted with polar substituents at the N1 and N3 positions (alloxazine) or at the N3 and N10 positions (isoalloxazines). The electrochemical behavior of one such synthetic flavin analogue was examined in detail in aqueous solutions of varying pH in the range from 1 to 10. Cyclic voltammetry, used in conjunction with hydrodynamic (rotating disk electrode) voltammetry, showed quasi-reversible behavior consistent with freely diffusing molecules and an overall global 2e(-) , 2H(+) proton-coupled electron transfer scheme. UV/Vis spectroelectrochemical data was also employed to study the pH-dependent electrochemical behavior of this derivative. Substituent effects on the redox behavior were compared and contrasted for all the four compounds, and visualized within a scatter plot framework to afford comparison with prior knowledge on mostly natural flavins in aqueous media. Finally, a preliminary assessment of one of the synthetic flavins was performed of its electrocatalytic activity toward dioxygen reduction as a prelude to further (quantitative) studies of both freely diffusing and tethered molecules on various electrode surfaces.

Tailoring copper oxide semiconductor nanorod arrays for photoelectrochemical reduction of carbon dioxide to methanol.

Solar photoelectrochemical reduction of carbon dioxide to methanol in aqueous media was driven on hybrid CuO/Cu2O semiconductor nanorod arrays for the first time. A two-step synthesis was designed and demonstrated for the preparation of these hybrid copper oxide one-dimensional nanostructures on copper substrates. The first step consisted in the growth of CuO nanorods by thermal oxidation of a copper foil at 400 °C. In the second step, controlled electrodeposition of p-type Cu2O crystallites on the CuO walls was performed. The resulting nanorod morphology with controllable wall thickness by adjusting the Cu2O electrodeposition time as well as their surface/bulk chemical composition were probed by scanning electron microscopy, X-ray diffraction and Raman spectroscopy. Photoelectrosynthesis of methanol from carbon dioxide was demonstrated at -0.2 V vs SHE under simulated AM1.5 solar irradiation on optimized hybrid CuO/Cu2O nanorod electrodes and without assistance of any homogeneous catalyst (such as pyridine or imidazole) in the electrolyte. The hybrid composition, ensuring double pathway for photoelectron injection to CO2, along with high surface area were found to be crucial for efficient performance in methanol generation under solar illumination. Methanol formation, tracked by gas chromatography/mass spectrometry, indicated Faradaic efficiencies of ~95%.

Long-lived, directional photoinduced charge separation in RuII complexes bearing laminate polypyridyl ligands.

RuII complexes incorporating both amide-linked bithiophene donor ancillary ligands and laminate acceptor ligands; dipyrido[3,2-a:2',3'-c]phenazine (dppz), tetrapyrido[3,2-a:2',3'-c:3'',2''-h:2''',3'''-j]phenazine (tpphz), and 9,11,20,22-tetraazatetrapyrido[3,2-a:2',3'-c:3'',2''-l:2''',3''']-pentacene (tatpp) exhibit long-lived charge separated (CS) states, which have been analyzed using time-resolved transient absorption (TA), fluorescence, and electronic absorption spectroscopy in addition to ground state electrochemical and spectroelectrochemical measurements. These complexes possess two electronically relevant ³MLCT states related to electron occupation of MOs localized predominantly on the proximal "bpy-like" portion and central (or distal) "phenazine-like" portion of the acceptor ligand as well as energetically similar ³LC and ³ILCT states. The unusually long excited state lifetimes (τ up to 7 µs) observed in these complexes reflect an equilibration of the ³MLCTprox or ³MLCTdist states with additional triplet states, including a ³LC state and a ³ILCT state that formally localizes a hole on the bithiophene moiety and an electron on the laminate acceptor ligand. Coordination of a ZnII ion to the open coordination site of the laminate acceptor ligand is observed to significantly lower the energy of the ³MLCTdist state by decreasing the magnitude of the excited state dipole and resulting in much shorter excited state lifetimes. The presence of the bithiophene donor group is reported to substantially extend the lifetime of these Zn adducts via formation of a ³ILCT state that can equilibrate with the ³MLCTdist state. In tpphz complexes, ZnII coordination can reorder the energy of the ³MLCTprox and ³MLCTdist states such that there is a distinct switch from one state to the other. The net result is a series of complexes that are capable of forming CS states with electron-hole spatial separation of up to 14 Å and possess exceptionally long lifetimes by equilibration with other triplet states.

Efficient solar photoelectrosynthesis of methanol from carbon dioxide using hybrid CuO-Cu2O semiconductor nanorod arrays.

Solar photoelectrosynthesis of methanol was driven on hybrid CuO-Cu(2)O semiconductor nanorod arrays for the first time at potentials ~800 mV below the thermodynamic threshold value and at Faradaic efficiencies up to ~95%. The CuO-Cu(2)O nanorod arrays were prepared on Cu substrates by a two-step approach consisting of the initial thermal growth of CuO nanorods followed by controlled electrodeposition of p-type Cu(2)O crystallites on their walls. No homogeneous co-catalysts (such as pyridine, imidazole or metal cyclam complexes) were used contrasting with earlier studies on this topic using p-type semiconductor photocathodes. The roles of the core-shell nanorod electrode geometry and the copper oxide composition were established by varying the time of electrodeposition of the Cu(2)O phase on the CuO nanorod core surface.

Photocatalytic generation of syngas using combustion-synthesized silver bismuth tungstate.

Silver bismuth tungstate (AgBiW(2)O(8)) nanoparticles were prepared for the first time by solution combustion synthesis by using the corresponding metal nitrates as the precursor and urea as the fuel. These nanoparticles were subsequently modified with Pt catalyst islands using a photocatalytic procedure and used for the photogeneration of syngas (CO+H(2)). Formic acid was used for this purpose for the in situ generation of CO(2) and its subsequent reduction to CO. In the absence of Pt modification, H(2) was not obtained in the gas products evolved. These results were compared with those obtained with acetic acid in place of formic acid. The combustion process was simulated by thermogravimetry and the synthesized powder was characterized using transmission electron microscopy, diffuse reflectance UV/Vis spectroscopy, X-ray diffraction, surface area measurements, and X-ray photoelectron spectroscopy. Tauc plots derived from the diffuse reflectance data yielded an optical band gap of 2.74 eV. The photocatalytic activity of these nanoparticles was superior to a sample prepared by solid-state synthesis. Mechanistic aspects are finally presented, as are structural models and electronic calculations, using density functional theory (DFT).

Ligand-triplet-fueled long-lived charge separation in ruthenium(II) complexes with bithienyl-functionalized ligands.

Ruthenium(II) polypyridyl complexes with pendant bithienyl ligands exhibiting unusually long-lived (τ ~ 3-7 µs) charge-separated excited states and a large amount of stored energy (ΔG° ~ 2.0 eV) are reported. A long-lived ligand-localized triplet acts as an energy reservoir to fuel population of an interligand charge-transfer state via an intermediate metal-to-ligand charge-transfer state in these complexes.

Photochemical two-electron reduction of a dinuclear ruthenium complex containing a bent tetraazatetrapyridopentacene bridging ligand: pushing up the LUMO for storing more energy.

The synthesis and characterization of a ditopic bridging ligand, 9,12,21,22-tetraazatetrapyrido[3,2-a:2',3'-c:3″2″-m:2''',3'''-o]pentaphene (tatppα) and its dinuclear ruthenium complex, [(phen)(2)Ru(tatppα)Ru(phen)(2)][PF(6)](4) (1(4+)), are described. The tatppα ligand is structurally very similar to 9,10,20,33-tetraazatetrapyrido[3,2-a:2',3'-c:3″,2″-l:2''',3'''-n]pentacene (tatppß), except that, instead of a linear tetraazapentacene backbone, tatppα has an ortho (or α) substitution pattern about the central benzene ring, leading to a 120° bend. Complex 1(4+) shows tatppα-based reductions at -0.73 and -1.14 V vs Ag/AgCl/saturated KCl and has an absorption spectrum showing the typical Ru(II) dπ → phen-like π* metal-to-ligand charge-transfer transition centered at ∼450 nm. In acetonitrile, visible-light irradiation of 1(4+) in the presence of triethylamine leads to two sequential changes in the absorption spectra, which are assigned to the formation of the one- and two-electron-reduced species, with the electrons stored on the tatppα ligand. These assignments were made by comparison of the spectral changes observed in 1(4+) upon stoichiometric chemical reduction with cobaltocene and by spectroelectrochemical analysis. Significantly, DFT calculations are very predictive of the optical and reductive behavior of the tatppα complex relative to the tatppß complexes and show that modeling is a useful tool for ligand design. The chemical reactivity and differential reflectance spectroelectrochemical data reveal that the reductions are accompanied by radical dimerization of the tatppα ligand to species such as σ-{1}(2)(6+), which is only slowly reversible upon exposure to air and may limit the complexe's 1(4+) utility for driving photochemical H(2) production.

Photoinduced ligand transformations in a ruthenium complex of dimethoxytetrapyridotetraazapentacene.

The dinuclear ruthenium(II) complex [(phen)(2)Ru(tatpOMe)Ru(phen)(2)](4+) (2(4+); phen is 1,10-phenanthroline and tatpOMe is 10,21-dimethoxy-9,10,20,33-tetraazatetrapyrido[3,2-a:2'3'-c:3'',2''-l:2''',3'''-n]pentacene) has been synthesized and characterized by (1)H NMR, ESI mass spectroscopy and elemental analysis. Loss of methoxy group from bridging ligand of complex 2(4+) due to irradiation is observed by (1)H NMR and photochemistry. The interrelated electronic properties UV-Vis, electrochemistry, photochemistry and molecular orbital calculation are analyzed and discussed on the bridging ligand of the complex 2(4+).

Electrochemical grafting of poly(3,4-ethylenedioxythiophene) into a titanium dioxide nanotube host network.

This study focuses on electrodeposition for infiltrating in situ a conducting polymer such as poly(3,4-ethylenedioxythiophene) (PEDOT) into a host titanium dioxide (TiO(2)) nanotube array (NTA) framework. The TiO(2) NTA was electrosynthesized on titanium foil in turn by anodization in a fluoride-containing medium. The PEDOT layer was electrografted into the TiO(2) NTA framework using a two-step potentiostatic growth protocol in acetonitrile containing supporting electrolyte. The nanoscopic features of oligomer/polymer infiltration and deposition in the NTA interstitial voids were monitored by field-emission scanning electron microscopy. Systematic changes in the nanotube inner diameter and the wall thickness afforded insights into the evolution of the TiO(2)NTA/PEDOT hybrid assembly. This assembly was subsequently characterized by UV-visible diffuse reflectance, cyclic voltammetry, and photoelectrochemical measurements. These data serve as a prelude to further use of these hybrids in heterojunction solar cells.

Synthesis of trithienylenevinylenes bearing dithiocarbonate groups and their dithiophene-tetrathiafulvalene derivatives.

A series of conjugated trithienylenevinylene compounds bearing dithiocarbonate groups were prepared by Wittig reactions. Dithiophene-tetrathiafulvalene (DT-TTF) derivatives can be readily prepared through trialkylphosphite-mediated coupling reactions of these trithienylenevinylene materials. All compounds were characterized by NMR, FT-IR, UV-vis, and mass spectroscopy.

Solution combustion synthesis of oxide semiconductors for solar energy conversion and environmental remediation.

In this tutorial review, we summarize recent research on the solution combustion synthesis of oxide semiconductors for applications related to photovoltaic solar energy conversion, photoelectrochemical hydrogen generation, and heterogeneous photocatalytic remediation of environmental pollutants. First, the advantages of combustion synthesis relative to other strategies for preparing oxide semiconductors are discussed followed by a summary of process variants in combustion synthesis. The possibility of in situ chemical modification of the oxide during its formation in the combustion environment is addressed. Morphological and crystal structure aspects of the combustion-synthesized products are discussed followed by a summary of trends in their photocatalytic activity relative to benchmark samples prepared by other methods.

Electroreduction of the ruthenium complex [(bpy)2Ru(tatpp)]Cl2 in water: insights on the mechanism of multielectron reduction and protonation of the Tatpp acceptor ligand as a function of pH.

The mononuclear ruthenium complex [(bpy) 2Ru(tatpp)] (2+) ( 1 (2+); bpy is 2,2'-bipyridine and tatpp is 9,11,20,22-tetra-aza-tetrapyrido[3,2-a:2'3'-c:3'',2''-l:2''',3''']-pentacene) undergoes up to four reversible tatpp ligand-based reductions as determined by electrochemistry in aqueous solution. Specific redox and protonation states of this complex were generated by stoichiometric chemical reduction with cobaltocene and protonation with trifluoroacetic acid in acetonitrile. These species exhibit unique UV-visible absorption spectra, which are used to determine the speciation in aqueous media as a function of the potential during the electrochemical reduction. A combination of cyclic voltammetry, differential pulse voltammetry, and spectroelectrochemistry showed that the voltammetric reduction peaks are associated with two-electron/two-proton processes in which the details of stepwise electron transfer and protonation steps vary as a function of the pH. Spectroelectrochemistry performed during potential scans with and without a small superimposed sinusoidal potential waveform was used to examine the mechanistic details of this proton-coupled multielectron reduction process. Under basic conditions, the radical [(bpy) 2Ru(tatpp (*-))] (+)( 1 (*+)) is the first electrogenerated species that converts to doubly reduced, single-protonated [(bpy) 2Ru(Htatpp-)] (+) (H 1 (+)) and doubly protonated [(bpy) 2Ru(H 2tatpp)] (2+)(H 2 1 (2+)) by subsequent electron-transfer (ET) and proton-transfer (PT) reactions. Partial dimerization of radical 1 (*+) is also observed in basic media. Neutral or acidic conditions favor an initial ET-PT reaction leading to the protonated, radical species [(bpy) 2Ru(Htatpp (*))] (2+) (H 1 (*2+)), which rapidly disproportionates to give 1 (2+) and H 2 1 (2+). This intermediate, H 1 (*2+), is only observed when potential modulation is used in the spectroelectrochemical experiment. At more negative potentials, the doubly reduced complexes (e.g., H 2 1 (2+), H 1 (+)) undergo a two-electron/two-proton reductions to give the quadruply reduced and protonated species H 4 1 (2+) and/or H 3 1 (+) throughout the pH range investigated. These species are also only detectable when potential modulation is used in the spectroelectrochemical experiment, as they rapidly comproportionate with 1 (2+) in the bulk, leading to the regeneration of intermediate double-reduced species, H 2 1 (2+).

Combustion synthesis and characterization of nanocrystalline WO3.

The energy payback time associated with the semiconductor active material is an important parameter in a photovoltaic solar cell device. Thus lowering the energy requirements for the semiconductor synthesis step or making it more energy-efficient is critical toward making the overall device economics more competitive relative to other nonpolluting energy options. In this communication, combustion synthesis is demonstrated to be a versatile and energy-efficient method for preparing inorganic oxide semiconductors such as tungsten trioxide (WO3) for photovoltaic or photocatalytic solar energy conversion. The energy efficiency of combustion synthesis accrues from the fact that high process temperatures are self-sustained by the exothermicity of the combustion process, and the only external thermal energy input needed is for dehydration of the fuel/oxidizer precursor mixture and bringing it to ignition. Importantly, we show that, in this approach, it is also possible to tune the optical characteristics of the oxide semiconductor (i.e., shift its response toward the visible range of the electromagnetic spectrum) in situ by doping the host semiconductor during the formative stage itself. As a bonus, the resultant material shows enhanced surface properties such as markedly improved organic dye uptake relative to benchmark samples obtained from commercial sources. Finally, this synthesis approach requires only very simple equipment, a feature that it shares with other "mild" inorganic semiconductor synthesis routes such as sol-gel chemistry, chemical bath deposition, and electrodeposition. The present study constitutes the first use of combustion synthesis for preparing WO3 powder comprising nanosized particles.

Formation and characterization of self-organized TiO2 nanotube arrays by pulse anodization.

This paper describes TiO2 nanotube arrays prepared by anodic oxidation of Ti substrates using pulse voltage waveforms. Voltages were pulsed between 20 and -4 V or between 20 and 0 V with varying durations from 2 to 16 s at the lower limit of the pulse waveform. Ammonium fluoride or sodium fluoride (and mixtures of both) was used as the electrolyte with or without added medium modifier (glycerol, ethylene glycol, or poly (ethylene glycol) (PEG 400)) in these experiments. The pulse waveform was optimized to electrochemically grow TiO2 nanotubes and chemically etch their walls during its cathodic current flow regime. The resultant TiO2 nanotube arrays showed a higher quality of nanotube array morphology and photoresponse than samples grown via the conventional continuous anodization method. Films grown with a 20 V/-4 V pulse sequence and pulse duration of 2 s at its negative voltage limit afforded a superior photoresponse compared to other pulse durations. Specifically, the negative voltage limit of the pulse (-4 V) and its duration promote the adsorption of NH4+ species that in turn inhibits chemical attack of the growing oxide nanoarchitecture by the electrolyte F- species. The longer the period of the pulse at the negative voltage limit, the thicker the nanotube walls and the shorter the nanotube length. At variance, with 0 V as the low voltage limit, the longer the pulse duration, the thinner the oxide nanotube wall, suggesting that chemical attack by fluoride ions is not counterbalanced by NH3/NH4+ species adsorption, unlike the interfacial situation prevailing at -4 V. Finally, the results from this study provide useful evidence in support of existing mechanistic models for anodic growth and self-assembly of oxide nanotube arrays on the parent metal surface.

Effect of vacuum annealing on the surface chemistry of electrodeposited copper(I) oxide layers as probed by positron annihilation induced auger electron spectroscopy.

Vacuum anneal induced changes in the surface layers of electrodeposited copper(I) oxide (Cu2O) were probed by time-of-flight positron annihilation induced Auger electron spectroscopy (TOF-PAES) and by electron induced Auger electron spectroscopy (EAES). Large changes in the intensity of the Cu PAES intensity resulting from isochronal in situ vacuum anneals made at increasing temperatures indicated that, before thermal treatment, the surface was completely covered by a carbonaceous overlayer and that this layer was removed, starting at a temperature between 100 and 200 degrees C, to expose an increasing amount of Cu in the top layer as the anneal temperature was increased. The thickness of this overlayer was estimated to be approximately 4 A based on analysis of the EAES data, and its variation with the thermal anneal temperature was mapped. This study demonstrated the order-of-magnitude enhancement in the sensitivity of PAES to the topmost surface layer in Cu2O relative to the EAES counterpart; factors underlying this contrast are discussed. Finally, the implications of ultrathin carbon layers on semiconductor surfaces are discussed.

Driving multi-electron reactions with photons: dinuclear ruthenium complexes capable of stepwise and concerted multi-electron reduction.

Using biological precedents, it is expected that concerted, multi-electron reduction processes will play a significant role in the development of efficient artificial photosynthetic systems. We have found that the dinuclear ruthenium complexes [(phen)(2)Ru(tatpp)Ru(phen)(2)](4+) (P) and [(phen)(2)Ru(tatpq) Ru(phen)(2)](4+) (Q) undergo photodriven 2- and 4-electron reductions, respectively, in the presence of a sacrificial reductant. Importantly, these processes are completely reversible upon exposure to air, and consequently, these complexes have the potential to be used catalytically in multi-electron transfer reactions. A localized molecular orbital description of the ligands and complexes is used to explain both the function and spectroscopy of these complexes. In both complexes, the reducing equivalents are stored in the pi* orbitals of the bridging ligands and depending on the solution pH, various protonation states of the reduced species of P and Q are obtained. Under basic conditions, the photochemical pathway favors sequential single-electron reductions, while neutral or slightly acidic conditions give rise to proton-coupled multi-electron transfer. In fact, at sufficiently acidic pH, only a coupled two-electron, 2-proton process is seen. Few molecular photocatalysts are capable of proton-coupled multi-electron transfer, which is believed to be a fundamental component of light-activated energy storage in nature.

Influence of pH on the photochemical and electrochemical reduction of the dinuclear ruthenium complex, [(phen)2Ru(tatpp)Ru(phen)2)Cl4, in water: proton-coupled sequential and concerted multi-electron reduction.

The dinuclear ruthenium complex [(phen)2Ru(tatpp)Ru(phen)2]4+ (P; in which phen is 1,10-phenanthroline and tatpp is 9,11,20,22-tetraaza tetrapyrido[3,2-a:2'3'-c:3'',2''-l:2''',3''']-pentacene) undergoes a photodriven two-electron reduction in aqueous solution, thus storing light energy as chemical potential within its structure. The mechanism of this reduction is strongly influenced by the pH, in that basic conditions favor a sequential process involving two one-electron reductions and neutral or slightly acidic conditions favor a proton-coupled, bielectronic process. In this complex, the central tatpp ligand is the site of electron storage and protonation of the central aza nitrogen atoms in the reduced products is observed as a function of the solution pH. The reduction mechanism and characterization of the rich array of products were determined by using a combination of cyclic and AC voltammetry along with UV-visible reflectance spectroelectrochemistry experiments. Both the reduction and protonation state of P could be followed as a function of pH and potential. From these data, estimates of the various reduced species' pKa values were obtained and the mechanism to form the doubly reduced, doubly protonated complex, [(phen)2Ru(H2tatpp)Ru(phen)2]4+ (H2P) at low pH (< or =7) could be shown to be a two-proton, two-electron process. Importantly, H2P is also formed in the photochemical reaction with sacrificial reducing agents, albeit at reduced yields relative to those at higher pH.

Selenium-modified titanium dioxide photochemical diode/electrolyte junctions: photocatalytic and electrochemical preparation, characterization, and model simulations.

The photoelectrochemical behavior of TiO2 thin film electrodes, photocatalytically modified with Se islands, is described. The TiO2 thin films were electrodeposited on transparent conducting oxide glass substrates. The resultant electrode forms a n-TiO2/p-Se "photochemical diode" which, in turn, contacts an electrolyte phase. Both transient photocurrent profiles (in response to excitation light that is switched on or off) and steady-state current-potential curves in response to chopped irradiation are considered. We show that the relative dominance of the contributions from the TiO2 and Se components to the overall response of the photochemical diode/electrolyte junction crucially depends on the wavelength distribution of the excitation light source. A simple equivalent circuit representation of this junction is presented, comprised of a photodiode in parallel with two photodiodes connected in series back-to-back. Simulations of the transient and steady-state photoelectrochemical response of this system are presented, and are shown to be in good agreement with the corresponding experimental profiles.

Multielectron photoreduction of a bridged ruthenium dimer, [(phen)2Ru(tatpp)Ru(phen)2][PF6]4: aqueous reactivity and chemical and spectroelectrochemical identification of the photoproducts.

The dinuclear ruthenium(II) complex [(phen)(2)Ru(tatpp)Ru(phen)(2)][PF(6)](4) (P) (where phen is 1,10-phenanthroline and tatpp is 9,11,20,22-tetraazatetrapyrido[3,2-a:2'3'-c:3' ',2' '-l:2' ",3' "]pentacene) is shown to accept up to four electrons and two protons on the central tatpp bridging ligand via a combination of stoichiometric chemical reductions and protonations and spectroelectrochemistry (SEC) in acetonitrile. The absorption spectra of seven distinct species related by reduction and/or protonation of the central tatpp ligand were obtained and the two sequential photoproducts obtained from visible irradiation of P in acetonitrile (with 0.25 M triethylamine (TEA)) thus identified as P(-) (singly reduced, nonprotonated P) and HP(-) (doubly reduced, monoprotonated P), respectively. Importantly, the photochemical activity is maintained in mixed water-acetonitrile (1:4) solutions under basic conditions, and the protonation state of the photoproducts is readily controlled by varying the solution pH between 8 and 12. Absorption spectra obtained by SEC under similar solvent conditions were virtually identical to those obtained photochemically, and thus the doubly reduced photoproducts were identified as P(2)(-) (pH 12), HP(-) (pH 10), and H(2)()P (pH 8). This last photoproduct, H(2)()P, is particularly promising with respect to solar hydrogen production in that it can be produced in the presence of water and its dehydrogenation under appropriate conditions could yield H(2) and regenerate P. A qualitative MO diagram is presented as a framework for understanding the observed optical transitions as a function of oxidation and protonation state.