Metal-to-metal Charge Transfer Research Articles

Metal-to-metal charge transfer in AWO4 (A = Mg, Mn, Co, Ni, Cu, or Zn) compounds with the wolframite structure.

Using a combination of UV-visible spectroscopy and electronic structure calculations, we have characterized the electronic structures and optical properties of AWO4 (A = Mn, Co, Ni, Cu, Zn, or Mg) tungstates with the wolframite structure. In MgWO4 and ZnWO4, the lowest energy optical excitation is a ligand to metal charge transfer (LMCT) excitation from oxygen 2p nonbonding orbitals to antibonding W 5d orbitals. The energy of the LMCT transition in these two compounds is 3.95 eV for ZnWO4 and 4.06 eV for MgWO4. The charge transfer energies observed for the other compounds are significantly smaller, falling in the visible region of the spectrum and ranging from 2.3 to 3.0 eV. In these compounds, the partially occupied 3d orbitals of the A(2+) ion act as the HOMO, rather than the O 2p orbitals. The lowest energy charge transfer excitation now becomes a metal-to-metal charge transfer (MMCT) excitation, where an electron is transferred from the occupied 3d orbitals of the A(2+) ion to unoccupied antibonding W 5d states. The MMCT value for CuWO4 of 2.31 eV is the lowest in this series due to distortions of the crystal structure driven by the d(9) configuration of the Cu(2+) ion that lower the crystal symmetry to triclinic. The results of this study have important implications for the application of these and related materials as photocatalysts, photoanodes, pigments, and phosphors.

Oxo-Bridged Dinuclear Chromium(III) Complexes: Correlation between the Optical and Magnetic Properties and the Basicity of the Oxo Bridge

The synthesis and X-ray structure of a new member of the series of oxo-bridged, dinuclear chromium(III) complexes, the methyl isocyanide complex [(CH3NC)5CrOCr(CNCH3)5](PF6)4·2CH3CN, is reported. This constitutes only the third oxo-bridged, dinuclear chromium(III) complex with a homoleptic auxillay ligand sphere. Experimentally, the system shows unshifted narrow nuclear magnetic resonance (NMR) spectra that are consistent with calculations using broken symmetry density functional theory (DFT), which suggests it to be the strongest coupled, dinuclear chromium(III) complex known. Furthermore, we report the crystal structure and computed magnetic properties for [(bpy)2(SCN)CrOCr(NCS)(bpy)2](ClO4)2·2H2O (bpy = 2,2'-bipyridine), which differs from other reported oxo-bridged species by featuring a bent CrOCr(4+) core. We also interpret the spectacular 10-orders-of-magnitude variation in acid dissociation constant of the bridging hydroxo ligand in mono hydroxo-bridged dinuclear chromium(III) complexes, in terms of a valence bond model parametrized by metal-to-metal charge transfer (MMCT) and ligand-to-metal charge transfer (LMCT) energies.

Effects of Co, Ni, Cu, and Zn on Photophysical and Photocatalytic Properties of Carbonate Intercalated MII/Cr LDHs for Enhanced Photodegradation of Methyl Orange

To investigate the effects of divalent metal ions on the photophysical and photocatalytic properties of MII/Cr–CO3 layered double hydroxides (LDHs), we varied the MII metal ions (Co, Ni, Cu, and Zn) at a constant MII/Cr atomic ratio of 2:1. The phase structures and textural and optical properties of these catalysts were characterized by powder X-ray diffraction (PXRD), Brunauer–Emmett–Teller (BET) surface area, Fourier transform infrared (FTIR) spectroscopy, photoluminescence (PL) spectroscopy, and diffuse-reflectance UV–vis (DRUV–vis) spectroscopy. The PXRD measurements demonstrated that all of the as-synthesized LDHs had well-defined layered structures except Ni/Cr–CO3 LDH (LDH2). Optical difference spectra revealed that absorption in the visible region can be attributed to metal-to-metal charge transfer (MMCT) excitation of oxo-bridged bimetallic linkages of MII–O–CrIII in the two neighboring MO6 octahedra. The photocatalytic performances of these catalysts were examined in the degradation of methyl or...

Activation of MCM-41 mesoporous silica by transition-metal incorporation for photocatalytic hydrogen production

A series of transition metal incorporated MCM-41 mesoporous silica is successfully synthesized by hydrothermal method, and all of them display optical absorption in ultraviolet-visible light region. Analysis results from different characterization techniques demonstrate that the transition metal ions are well dispersed in the silica framework and mainly in tetrahedral coordination. In comparison to Co and Ni incorporated MCM-41, Fe-MCM-41 shows considerable photocatalytic activity for hydrogen production, without loading any noble metal as cocatalyst. The activation for photocatalytic reaction is induced by the ligand-to-metal charge-transfer (LMCT) excitation of the monometallic centers in the framework-substituted MCM-41. Bimetal co-incorporation is found to be effective for the enhanced photocatalytic activity. For instance, Fe/Ni co-incorporated MCM-41 shows much higher photocatalytic activity for hydrogen production than either Fe or Ni incorporated MCM-41 indicating synergistic effect of co-doping with Fe and Ni. The main reason for the enhanced activity is attributed to the metal-to-metal charge–transfer (MMCT) excitation creating oxo-bridged bimetallic redox sites, which results in efficient charge separation and creation of long-lived electrons and holes to initiate redox reaction.

The impact of surface structure and band gap on the optoelectronic properties of Cu2O nanoclusters of varying size and symmetry

A systematic characterization of Cu2O nanoclusters using classical electrodynamics and time-dependent density functional theory (TDDFT) is performed to investigate their response to light with the alteration of size and symmetry. Absorption and scattering play a crucial role in tuning the surface plasmon resonance (SPR), which is the focal feature of optoelectronic properties. In larger dimensions the SPR is found to be strongly influenced by scattering and in smaller NPs it is dominated by absorption. A blue shift of the SPR peak is observed with decreasing cluster size. The optical properties of Cu2O nanoclusters are also affected by the symmetry aspect. With the variation of size and symmetry the associated surface structure and band gap are also varied. The TDDFT calculation is performed to explore the impact of these two fundamental factors on the optoelectronic nature of (Cu2O)n clusters. The TDDFT study on Cu2O nanoclusters reveals the nature of electronic excitations in photoirradiated (Cu2O)n clusters for n = 1, 2, and 3. The transitions involved in (Cu2O)n are basically categorized as ligand to metal charge transfer (LMCT) and metal to metal charge transfer (MMCT) processes. The change in absorption with varying cluster dimension and symmetry is found to be critically controlled by the relative probabilities of LMCT and MMCT processes. A competing surface reconstruction effect and occupied–virtual energy gap are also found to govern the SPR pattern of the Cu2O nanoclusters. All of these observations provide an appropriate guideline to tune SPR of Cu2O NPs for specific applications.

Design and development of a visible light harvesting Ni–Zn/Cr–CO32− LDH system for hydrogen evolution

In this work we have prepared a series of visible-light active Ni–Zn/Cr–CO32− LDHs with (Ni + Zn)/Cr ratio 2.0, while varying the Ni/Zn atomic ratio to 0 : 100, 25 : 75, 50 : 50, 75 : 25 and 100 : 0, and tested them for visible light photocatalytic hydrogen evolution. The photophysical and photocatalytic properties of the samples were evaluated thoroughly by PXRD, TEM and FESEM, UV-vis DRS, FTIR, PL and BET-Surface area. The PXRD measurement demonstrates a characteristic of hydrotalcite with long range order structure. The shifting of the d110 plane towards a lower angle clearly indicates that there is Ni2+ substitution in the brucite layer of Zn/Cr–CO32− LDH. Different diffuse reflectance spectra revealed that the absorption in the visible region is attributed to the metal-to-metal charge-transfer (MMCT) excitation of ZnII/NiII–O–CrIII to ZnI/NiI–O–CrIV. This oxo-bridged hetero-bimetallic assembly acts as a photo-induced centre for hydrogen evolution. The photocatalytic studies suggested that the Ni–Zn/Cr–CO32− LDH with Ni : Zn ratio of 75 : 25 exhibits the best photocatalytic activity under visible light radiation compared with the others. The detailed mechanism for the photocatalytic decomposition of water to hydrogen has also been discussed.

Metal-to-metal charge-transfer transitions: reliable excitation energies from ab initio calculations

A computational strategy is presented to describe excited states, involving the transfer of an electron from one metallic site to a neighboring metal center, the so-called metal-to-metal charge-transfer (MMCT) states. An accurate ab initio treatment of these states in transition metal compounds is intrinsically difficult for both time-dependent density functional and wave function-based methods. The rather large dependence of the MMCT energies on the applied functional makes difficult to extract reliable estimates from density functional theory, while the standard multiconfigurational approach (complete active space SCF + second-order perturbation theory) leads to severe intruder state problems and unrealistic, negative energies. The analysis of the failure of the multiconfigurational approach shows that the state-average orbitals are biased toward the ground state and strongly deficient to describe the MMCT state. We propose a method to improve the orbitals by gradually approaching as much as possible the state-specific description of the MMCT state in the reference wave function for the second-order perturbation treatment of the dynamic electron correlation.

Molecular charge transfer and quantum efficiency analyses of a photochemical reactor for hydrogen production

This paper examines a lab-scale, batch-type photochemical reactor design for hydrogen generation from water. The reactor comprises a transparent cylindrical reaction vessel of 100 ml volume, which is illuminated with monochromatic LED arrays from 4 sides. The reaction is kept at constant pressure by a mechanism which enlarges the volume above the liquid in correspondence to the rate of hydrogen production. The measurement of volume, pressure and temperature is used to determine the number of moles of gas generated. The vessel is filled with a non-aqueous solution containing 0.62 M water and a supramolecular device (SMD), namely {[Ru(bpy)2dpp]2Rh(OH)2}5+ (with bpy = 2,2′–bipyridine and dpp = 2,3–bis(2–pyridil)pyrazine)), which reduces water photochemically under the presence of DMA as sacrificial electron donors (DMA = n,n-dimethylaniline), according to the following reaction: 2H2O(l)+2DMA→2hνH2(g)+2OH−+2DMA+. The equations for modelling of the photo-catalytic process are identified and used for simulations and analyses to improve the apparatus design and find the best operating conditions. The molecular simulations with Discovery Studio software are used for 3D representation of the SMD to identify its fundamental properties in the relevant energetic states, i.e., ground, metal to ligand charge transfer (MLCT), metal to metal charge transfer (MMCT) and the photo-activated catalyst states. The quantum efficiency of each sub-process is determined based on specified assumptions and subsequent analysis. It shows that the quantum efficiency is 0.01, which is consistent with experimental results from past studies. Two routes of the catalysis were identified: one short (300 ns), where the catalyst is activated from the 3MLCT state, and one long (400 ns) where the catalyst is activated via a 3MMCT state. The turnover frequency is mostly influenced by the rate of incident photons. A mathematical model of five differential equations predicts the molar concentration of the relevant species in the batch reactor. The model is used to determine the variation of concentration, hydrogen production and quantum efficiency in time. It is found that the process has an initial phase, when the concentration of hydrogen dissolved in the solution increases up to saturation; thereafter it remains constant. This result is also consistent with past literature experimental results. Because of other species production (e.g., hydroxyl, DMA+), water and DMA consumption, the quantum efficiency of the process decreases in time. Also the pressure in the head space influences the process. If the reactor operates with a full head space (50 ml), the pressure increases by ∼20 kPa in 12 h. The study provides useful input to the design of the experiment and also its application to the modelling of photochemical hydrogen production.

Strong metal–metal coupling in mixed-valent intermediates [Cl(L)Ru(μ-tppz)Ru(L)Cl]+, L = β-diketonato ligands, tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine

Five diruthenium(II) complexes [Cl(L)Ru(μ-tppz)Ru(L)Cl] (1-5) containing differently substituted β-diketonato derivatives (1: L = 2,4-pentanedionato; 2: L = 3,5-heptanedionato; 3: L = 2,2,6,6-tetramethyl-3,5-heptanedionato; 4: L = 3-methyl-2,4-pentanedionato; 5: L = 3-ethyl-2,4-pentanedionato) as ancillary ligands (L) were synthesized and studied by spectroelectrochemistry (UV-Vis-NIR, electron paramagnetic resonance (EPR)). X-ray structural characterisation revealed anti (1, 2, 5) or syn (3) configuration as well as non-planarity of the bis-tridentate tppz bridge and strong dπ(Ru(II)) → π*(pyrazine, tppz) back-bonding. The widely separated one-electron oxidation steps, Ru(II)Ru(II)/Ru(II)Ru(III) and Ru(II)Ru(III)/Ru(III)Ru(III), result in large comproportionation constants (K(c)) of ≥10(10) for the mixed-valent intermediates. The syn-configurated (n) exhibits a particularly high K(c) of 10(12) for n = 1+, accompanied by density functional theory (DFT)-calculated minimum Ru-N bond lengths for this Ru(II)Ru(III) intermediate. The electrogenerated mixed-valent states 1(+)-5(+) exhibit anisotropic EPR spectra at 110 K with average values <g> of 2.304-2.234 and g anisotropies Δg = g(1)-g(3) of 0.82-0.99. Metal-to-metal charge transfer (MMCT) absorptions occur for 1(+)-5(+) in the NIR region at 1660 nm-1750 nm (ε ≈ 2700 dm(3) mol(-1) cm(-1), Δν(1/2) ≈ 1800 cm(-1)). DFT calculations of 1(+) and 3(+) yield comparable Mulliken spin densities of about 0.60 for the metal ions, corresponding to valence-delocalised situations (Ru(2.5))(2). Rather large spin densities of about -0.4 were calculated for the tppz bridges in 1(+) and 3(+). The calculated electronic interaction values (V(AB)) for 1(+)-5(+) are about 3000 cm(-1), comparable to that for the Creutz-Taube ion at 3185 cm(-1). The DFT calculations predict that the Ru(III)Ru(III) forms in 1(2+)-5(2+) prefer a triplet (S = 1) ground state with ΔE (S = 0 - S = 1) ∼5000 cm(-1). One-electron reduction takes place at the tppz bridge which results in species [Cl(L)Ru(II)(μ-tppz˙(-))Ru(II)(L)Cl](-) (1˙(-)-3˙(-), 5˙(-)) which exhibit free radical-type EPR signals and NIR transitions typical of the tppz radical anion. The system 4(n) is distinguished by lability of the Ru-Cl bonds.

Cyano‐ and Aqua‐Coordinated Diruthenium(III) Complexes with Oxo‐Bis(acetato) Bridge: Preparation and Steric and Electronic Structures

AbstractThree new μ‐oxo‐bis(μ‐acetato)diruthenium(III) complexes,trans(μ‐O,OH2)‐[Ru2(μ‐O)(μ‐CH3COO)2(bpy)2(H2O)2][PF6]2·3H2O ([1][PF6]2·3H2O; bpy = 2,2′‐bipyridine), trans(μ‐O,CN)‐[{Ru2(μ‐O)(μ‐CH3COO)2(bpy)2}2{μ‐Ag(CN)2}{μ‐Ag2(CN)3}][Ag(CN)2][Ag2(CN)3](CH3CN)4(CH2Cl2) (2), and cis(μ‐O,CN)‐[Ru2(μ‐O)(μ‐CH3COO)2(bpy)2(CN)2]·CH3OH·H2O([3]·CH3OH·H2O), have been prepared and their X‐ray crystal structures determined. Whereas [1]2+ and 3 are discrete species, 2 is a polymer in which the Ag–CN groups act as bridges through Ru–CN(or NC)–Ag linkages. The discrete complexes dissolve in common solvents, but 2 is only slightly soluble in acetonitrile and dimethyl formamide (DMF) to give fragmented species (denoted as 2′) in solution. Redox waves of 3 were observed at –0.83 (Epc) and +0.76 V (E1/2) versus Ag/AgCl in a solution of 0.1 M nBu4NPF6 in CH2Cl2, which correspond to the Ru2II,III/Ru2III,III and Ru2III,III/Ru2III,IV processes, respectively. The corresponding redox couples (E1/2) of [1]2+ were observed at –0.31 and +0.76 V in 0.1 M NaClO4 aqueous solution. Significantly more negative potentials of the Ru2II,III/Ru2III,III couple for the two cyano complexes (E1/2 = –0.77 V for 2′) are noted. Complexes [1]2+, 2′, and 3 exhibit strong visible absorption bands at λmax = 566 nm (ϵ = 12600 M–1 cm–1; in H2O), 636 nm (21000 M–1 cm–1; in acetonitrile), and 558 nm (18000 M–1 cm–1; in H2O), respectively. Density functional theory (DFT) molecular orbital calculations have been carried out for [1]2+, trans(μ‐O,CN)‐[Ru2(μ‐O)(μ‐CH3COO)2(bpy)2(CN)2] (2a, a model compound for 2′), 3, [Ru2(μ‐O)(μ‐CH3COO)2(py)6]2+ ([4]2+), and trans(μ‐O,py)‐[Ru2(μ‐O)(μ‐CH3COO)2(bpy)2(py)2]2+ ([5]2+; py = pyridine). It was revealed that the frontier orbitals of these complexes are dominated by the Ru(dπ)–μ‐O(pπ) type π systems. For the cyano complexes, energies of these π‐type MOs are lifted up by the interaction with CN–, so that the energy difference with the upper bpy π* orbitals becomes smaller. Thus, whereas the visible absorption bands of [1]2+, [4]2+, and [5]2+ are ascribed to the transitions within the π system metal‐to‐metal charge transfer (MMCT), those of 2′ and 3 are ascribed to the HOMO‐to‐ligand π* (bpy) transition metal‐to‐ligand charge transfer (MLCT) and to the mixture of MMCT and MLCT (HOMO–1 to π*), respectively.

Predicting metal-to-metal charge transfer in closed-shell transition metal oxides doped with Bi 3+ or Pb 2+

An empirical model is proposed to predict the energy position of the metal-to-metal charge transfer (MMCT) bands in closed shell d 0 transition metal complex oxides doped with Bi 3+ or Pb 2+ ions. The model is constructed on the basis of optical data compiled from the literature and from the investigation of the luminescence properties of a series of compounds (titanates, vanadates, niobates, tantalates, molybdates, and zirconates) prepared and characterized in this work.

Visible-light induced oxo-bridged ZrIV−O−CeIII redox centre in tetragonal ZrO2–CeO2 solid solution for degradation of organic pollutants

Tetragonal ZrO(2)-CeO(2) solid solutions with composition Zr(1-x)Ce(x)O(2) (x = 0.1, 0.2, and 0.3) were synthesized in a citrate complexation route and characterized by XRD, XPS, UV-vis diffuse reflectance and ESR measurements. The formation of the homogeneous solid solution Zr(1-x)Ce(x)O(2) constructed the oxo-bridged bimetallic Zr(IV)-O-Ce(III) linkage between two neighboring flattened tetrahedrons of the structural framework. As compared to their parent oxides, the ZrO(2)-CeO(2) solid solutions exhibited optical absorption extending to longer wavelengths in the visible region. The red shift in the absorption spectrum was demonstrated to be partially due to a metal-to-metal charge transfer (MMCT) transition of the oxo-bridged Zr(IV)-O-Ce(III) linkage. The visible-light induced MMCT transition of Zr(IV)-O-Ce(III)→ Zr(III)-O-Ce(IV) resulted in the generation of the additional Ce(IV) and superoxide anion radical formed by the interaction of Zr(III) with adsorbed O(2). Catalytic activity evaluation revealed that the photoexcitation of the MMCT over the solid solution can initiate the degradation of RhB and 2,4-DCP upon visible-light irradiation, whereby Zr(III) and Ce(IV) act as a site-specific reducing and oxidizing center, respectively. The structure of the solid solution Zr(1-x)Ce(x)O(2) and the oxidation states of Zr and Ce species are also discussed in detail.

Series of MI[Co(bpy)3][Mo(CN)8]·nH2O (MI= Li (1), K (2), Rb (3), Cs (4);n= 7−8) Exhibiting Reversible Diamagnetic to Paramagnetic Transition Coupled with Dehydration−Rehydration Process

In this paper we report the synthesis and the structural and magnetic properties of the series of ionic compounds with general formula: M(I)[Co(bpy)(3)][Mo(CN)(8)] x nH(2)O (M(I) = Li, n = 8 (1), M(I) = K, n = 8 (2), M(I) = Rb, n = 8 (3), M(I) = Cs, n = 7.5 (4)). Solids 1-4 are characterized by the optical outer-sphere metal-to-metal charge transfer (MMCT) transition from Mo(IV) center to Co(III) center in the visible region and the Co(III)Mo(IV) <==> Co(II)Mo(V) spin equilibrium strongly dominated by the Co(III)Mo(IV) form. We show a gentle thermal treatment of diamagnetic compounds 1-4 leading to the dehydrated forms 1a-4a, which reveal a significant increase of paramagnetic contribution (from 0.5 to 2% to 30-40%). The rehydration allows to recover the diamagnetic phases 1b-4b of compositions and properties similar to those of 1-4. The irradiation of the dehydrated form 2a within the MMCT band in the Superconducting Quantum Interference Device (SQUID) cavity at T = 10 K causes further increase of the Co(II)Mo(V) contribution giving the metastable phase annealed back to the 2a phase after heating above T = 290 K. The IR, electron paramagnetic resonance (EPR), and X-ray photoelectron spectroscopy (XPS) spectroscopic data along with the magnetic data are interpreted in terms of strong modification of the Co(III)Mo(IV) <==> Co(II)Mo(V) equilibrium occurring in these systems.

Visible Light Sensitive Metal Oxide Nanocluster Photocatalysts: Photo-Induced Charge Transfer from Ce(III) to Keggin-Type Polyoxotungstates

Anchored visible light-absorbing metal oxide nanoclusters, consisting of trivalent cerium ions and Keggin-type polyoxotungstates (PW12O403− and CuIIPW11O397−), have been synthesized on the pore surface of mesoporous silica (MCM-41) as a new class of visible light sensitive photocatalysts. The diffuse reflectance UV−vis, FT-Raman, and Ce LIII-edge X-ray absorption measurements showed that grafting of trivalent cerium ions to the preimmobilized PW12O403− cluster on the silica surface provided a broad absorption tail extending from the UV region to 530 nm assigned to a metal-to-metal charge transfer (MMCT) transition of an oxo-bridged binuclear WVI−O−CeIII unit. Photoexcitation of WVI−O−CeIII → WV−O−CeIV MMCT in the anchored clusters initiated the photooxidation reactions of gaseous isopropanol to CO2, whereby WV and CeIV work as a site-specific reducing and oxidizing centers, respectively. In addition to the PW12O403− cluster, it was also confirmed that the present methods can be extended to metal-substituted polyoxotungstates. Grafting of trivalent cerium ions to the copper-monosubstituted cluster (CuIIPW11O397−) produced an oxo-bridged trinuclear CuII/WVI/CeIII unit and exhibited photoactivity 4 times higher than that for the binuclear WVI/CeIII cluster. CuII-substituted polyoxotungstates are known to initiate a multielectron dioxygen reduction reaction and have a unique redox property in that the two-electron reduction of CuII to Cu0 occurs in a concerted manner. Therefore, the enhanced photoactivity for the trinuclear unit demonstrates that the activation of a single Cu center from CuII to Cu0 is accomplished by sequential photoinduced MMCT through the confined WVI−O−CeIII linkages, which results in acceleration of the photocatalytic two-electron O2 reduction process.

Binuclear TiOMn charge-transfer chromophore in mesoporous silica

An all-inorganic heterobinuclear chromophore consisting of Ti(IV) oxo-bridged to a Mn(II) center has been assembled on the surface of silica pores of MCM-41 material. The key step of covalent attachment on the pore surface is the reaction of a Mn(II) precursor featuring weakly held CH3CN ligands with the OH group of a previously anchored titanol site. The optical diffuse reflectance spectrum reveals a Ti(IV)OMn(II) --> Ti(III)OMn(III) metal-to-metal charge-transfer (MMCT) absorption extending from the UV throughout to visible into the red spectral region. FT-IR, FT-Raman and optical spectroscopy confirm that the material is free of Mn oxide clusters, while EPR and Mn K-edge X-ray absorption spectra indicate that the donor center is predominantly in oxidation state +2. In situ FT-IR spectroscopy allowed detection of visible light-induced redox chemistry of the MMCT unit using O2 (18O2) and methanol as acceptor and donor probe molecules, respectively. Formate and water were observed as primary products, with methyl formate emerging as a secondary condensation product. The observed photochemistry demonstrates that excitation of the Ti(IV)OMn(II) --> Ti(III)OMn(III) results in complete transfer of an electron from donor to acceptor center, with the charge separation sufficiently long lived for initiation of redox chemistry to occur. With donor and acceptor redox potentials appropriate for driving multi-electron catalysts for water oxidation, proton or CO2 reduction, the TiOMn(II) unit is an attractive candidate as a charge-transfer chromophore in a solar fuel generating system.

Enhanced Photocatalytic Degradation of RhB Driven by Visible Light-Induced MMCT of Ti(IV)−O−Fe(II) Formed in Fe-Doped SrTiO3

It is important to reveal the origin of the visible light-induced photocatalytic activity of transition metal-doped SrTiO3 in which an isolated energy level is created by the dopant in the forbidden band gap. In this particle, the photocatalytic and photophysical properties of pure SrTiO3 and Fe-doped SrTiO3 were investigated comparatively. The Fe-doped SrTiO3 has been shown to have a much higher photocatalytic activity than pure SrTiO3 for the degradation of RhB under visible light irradiation. It was showed that doping Fe into SrTiO3 exhibited an absorption extending up to the visible region in the optical absorption spectrum and established the TiIV−O−FeII heterobimetallic linkages in the host. Difference diffuse reflectance spectra revealed that the absorption in the visible region is partly attributed to the metal-to-metal charge-transfer (MMCT) excitation of TiIV−O−FeII linkage formed in the Fe-doped SrTiO3. The visible light excitation of TiIV−O−FeII linkage was demonstrated to be the cause of the enhanced degradation of RhB. Furthermore, it was noted that the TiIV−O−FeII linkages only onto and nearest the surface of the Fe-doped SrTiO3 behavior as active sites for the visible light-induced photocatalysis.

Design of All-Inorganic Molecular-Based Photocatalysts Sensitive to Visible Light: Ti(IV)−O−Ce(III) Bimetallic Assemblies on Mesoporous Silica

The first photocatalysis operated by the visible-light induced metal-to-metal charge transfer (MMCT) has been demonstrated for the Ti(IV)/Ce(III) bimetallic assemblies synthesized on the pore of mesoporous silica. The Ce LIII-edge XANES measurements combined with 18O-isotopic labeling as well as photoelectrochemical experiments proved the ability of Ti(IV)/Ce(III) assemblies to drive the site-specific photocatalytic reaction under visible-light irradiation. Photocatalytic experiments using the oxidative decomposition of 2-propanol showed that the quantum efficiency for Ti(IV)/Ce(III) bimetallic photocatalysts was remarkably higher than that for the one of the most active visible-light sensitive photocatalysts, nitrogen-doped TiO2. It was also found that the grafting of Ce(III) ions onto nanocrystalline TiO2 particles led to the appearance of intense MMCT band in the visible-light regions, thus verifying it as the generally applicable strategy to obtain visible-light photocatalysts. To date, the grafting o...

Contrasts in the 77 K Emission Spectra, Structures, and Dynamics of Metal-to-Metal and Metal-to-Ligand Charge-Transfer Excited States

The 77 K emission spectrum of trans-[(ms-Me6[14]aneN4)Cr(CNRu(NH3)5)2]5+ has components characteristic of ligand field (LF) and metal-to-metal charge transfer (MMCT) excited states (ms-Me6[14]aneN4=5,12-meso-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane). The LF component of the emission is best resolved for irradiations at appreciably higher energies than the MMCT absorption band, while only the MMCT emission is observed for irradiations on the low-energy side of the MMCT absorption band. The LF emission component from this complex has vibronic structure that is very similar to that of the trans-[(ms-Me6[14]aneN4)Cr(CN)2]+ parent, but it is red-shifted by 560 cm-1 and the bandwidths are much larger. The red shift and the larger bandwidths of the ruthenated complex are attributed to configurational mixing between the LF and MMCT excited states, and the inferred mixing parameters are shown to be consistent with the known electron-transfer properties of the Ru(NH3)5 moieties. The MMCT excited-state lifetime is about 1 micros at 77 K and am(m)ine perdeuteration of this complex leads to an isotope effect of kNH/kND approximately 15-20. However, the contribution of the N-H stretching vibration to the emission sideband is too weak for a single vibrational mode model to be consistent with the observed lifetimes or the isotope effect. These features are very similar to those reported previously (J. Phys. Chem. A 2004, 108, 5041) for the MMCT emission of trans-[([14]aneN4)Cr{CNRu(NH3)5}2]5+ ([14]aneN4=1,4,8,11-tetraazacyclotetradecane), with the exception that the higher energy LF emission was not well resolved in the earlier work. The energies of the charge transfer absorption and emission maxima of both of these Cr(CN)Ru complexes are very similar to those of [Ru(NH3)4bpy]2+, but the latter has a 50-fold shorter 77 K excited-state lifetime, a 10-fold smaller NH/ND isotope effect, and a very different structure of its vibronic sidebands. Thus, the vibronic sidebands imply that the dominant excited-state distortions are in the metal-ligand vibrational modes for the Cr(CN)Ru complexes and in the bipyridine vibrational modes for the [Ru(NH3)4bpy]2+ complex. While an "equivalent" single vibrational mode model based on the frequencies and amplitudes of the dominant distortion modes is not consistent the observed lifetimes, such models do appear to be a good basis for qualitatively distinguishing different classes of excited-state dynamic behavior. A multimode, multichannel model may be necessary to adequately describe the excited-state dynamics of these simple electron-transfer systems.

Visible light absorption of binuclear TiOCo II charge-transfer unit assembled in mesoporous silica

Grafting of Co II(NCCH 3) 2Cl 2 onto mesoporous Ti–MCM-41 silica in acetonitrile solution affords binuclear Ti–O–Co II sites on the pore surface under complete replacement of the precursor ligands by interactions with anchored Ti centers and the silica surface. The Co II ligand field spectrum signals that the Co centers are anchored on the pore surface in tetrahedral coordination. FT-infrared action spectroscopy using ammonia gas adsorption reveals Co–O–Si bond modes at 831 and 762 cm −1. No Co oxide clusters are observed in the as-synthesized material. The bimetallic moieties feature an absorption extending from the UV into the visible to about 600 nm which is attributed to the Ti IV–O–Co II → Ti III–O–Co III metal-to-metal charge-transfer (MMCT) transition. The chromophore is absent in MCM-41 containing Ti and Co centers isolated from each other; this material was synthesized by grafting Co II onto a Ti–MCM-41 sample with the Ti centers protected by a cyclopentadienyl ligand. The result indicates that the appearance of the charge-transfer absorption requires that the metal centers are linked by an oxo bridge, which is additionally supported by XANES spectroscopy. The MMCT chromophore of Ti–O–Co II units has sufficient oxidation power to serve as visible light electron pump for driving multi-electron transfer catalysts of demanding uphill reactions such as water oxidation.

Increasing transmission of electronic interaction in dinuclear unsymmetric mixed-valent ruthenium complexes

The preparation and characterization by spectroscopic and electrochemical techniques of new dinuclear complexes of the type [(tpm)(bpy)Ru II(L)Ru II/III(NH 3) 5] 4+/5+ (bpy = 2,2′-bipyridine), tpm = tris(1-pyrazolyl)methane, L = pz (pyrazine), 4,4′-bpy (4,4′-bipyridine), BPE ( trans-1,2-bis(4-pyridyl)ethylene) and PCA (= 4-pyridinecarboxaldehyde azine), are described in this work. The analysis of the visible spectral data for the metal-to-metal charge transfer (MMCT) bands in the corresponding mixed-valent species [ Ru b II , Ru a III ] (Ru b = Ru bonded to bpy; Ru a = Ru bonded to NH 3) allows the determination of the reorganization energies λ and electronic coupling elements H AB for the intramolecular electron transfers Ru b II → Ru a III mediated by L. An increased transmission of the metal-to-metal electronic interaction is disclosed for this series, when comparing the distance dependence of H AB to that of the analogous series with trpy (2,2′:6′,2″-terpyridine) acting as a spectator ligand instead of tpm.