Metal-to-metal Charge Transfer Research Articles

Nephelauxetic effect and metal-to-metal charge transfer in solids doped with Tl+, Sn2+, Pb2+ or Sb3+

The luminescence-structure relationships in 159 compounds doped with either Tl+, Sn2+, Pb2+ or Sb3+ are reviewed and formalized based on a model introduced for Bi3+ in 2023. This model is shown to reproduce the energy of the s2 → s1p1 transitions in these compounds within ±0.35 eV (Sb3+), ±0.45 eV (Tl+, Pb2+) and ±0.55 eV (Sn2+) based on the calculation of the nephelauxetic function he at the crystal site occupied by the s2 cation in the host lattice. In the case of Pb2+, the method allowing to calculate the energy of the s2 → d0 metal-to-metal charge transfer (MMCT) in d0 transition metal oxides is re-actualized based on a model introduced for Bi3+ in 2013. These crystal structure-based procedures constitute a set of simple tools to rationalize and predict the optical properties of s2 cations in solids in a routine manner.

Achievements in the Photomagnetic Effect of Cobalt‐Iron Prussian Blue Analogues

AbstractHarnessing light for magnetization control presents a promising approach with compelling merits such as non‐contact operation, ultrafast response, reversibility, high spatial resolution, tunability, and energy efficiency. Photoinduced metal‐to‐metal charge transfer (MMCT) is one key mechanism in achieving photo‐controlled magnetic properties, making relevant materials prime candidates for versatile switchable devices. Cobalt iron Prussian blue analogues (CoFePBA) are trailblazers in showcasing photomagnetic properties via MMCT and the numerous related studies on CoFePBA have significantly enhanced the understanding of photoinduced MMCT processes. Thus, this review embarks on a thorough examination of three decades of research on the photomagnetic of CoFePBA to deep into the art of the photocontrol of spin. In addition to an introduction to the structure and preparation of CoFePBA, the focus is on elucidating the underlying mechanisms and introducing strategies for its manipulation and control. Finally, the challenges and prospects at this moment are summarized and discussed.

Water-Mediated Charge Transfer and Electron Localization in a Co3Fe2 Cyanide-Bridged Trigonal Bipyramidal Complex.

The effects of temperature and chemical environment on a pentanuclear cyanide-bridged, trigonal bipyramidal molecular paramagnet have been investigated. Using element- and oxidation state-specific near-ambient pressure X-ray photoemission spectroscopy (NAP-XPS) to probe charge transfer and second order, nonlinear vibrational spectroscopy, which is sensitive to symmetry changes based on charge (de)localization coupled with DFT, a detailed picture of environmental effects on charge-transfer-induced spin transitions is presented. The molecular cluster, Co3Fe2(tmphen)6(μ-CN)6(t-CN)6, abbrev. Co3Fe2, shows changes in electronic behavior depending on the chemical environment. NAP-XPS shows that temperature changes induce a metal-to-metal charge transfer (MMCT) in Co3Fe2 between a Co and Fe center, while cycling between ultrahigh vacuum and 2 mbar of water at constant temperature causes oxidation state changes not fully captured by the MMCT picture. Sum frequency generation vibrational spectroscopy (SFG-VS) probes the role of the cyanide ligand, which controls the electron (de)localization via the superexchange coupling. Spectral shifts and intensity changes indicate a change from a charge delocalized, Robin-Day class II/III high spin state to a charge-localized, class I low spin state consistent with DFT. In the presence of a H-bonding solvent, the complex adopts a localized electronic structure, while removal of the solvent delocalizes the charges and drives an MMCT. This change in Robin-Day classification of the complex as a function of chemical environment results in reversible switching of the dipole moment, analogous to molecular multiferroics. These results illustrate the important role of the chemical environment and solvation on underlying charge and spin transitions in this and related complexes.

Zero-thermal-quenching broadband yellow-emitting Bi3+-activated phosphors based on metal to metal charge transfer

Bi3+-activated phosphors have been proven to have potential applications foreground in white light-emitting diodes (WLED), plant growth lamps and temperature sensing. Therefore, it is urgent to exploit high-efficiency Bi3+-activated phosphors. Herein, a novel broadband yellow-emitting phosphor Ba2GdGaO5:Bi3+ with high internal quantum efficiency (IQE = 77%) was obtained based on metal to metal charge transfer (MMCT) between Bi3+ ground state and Gd3+ excited states. The photoluminescence excitation (PLE) spectrum and photoluminescence (PL) spectrum range from 225 nm to 400 nm and 400 nm to 700 nm, respectively, which can avoid the reabsorption phenomenon efficiently. Besides, Ba2GdGaO5:Bi3+ has superior thermal stability and it shows zero-thermal-quenching at 150 °C. The K+ doping hardly changes the thermal stability and can improve the PL intensity to 133.1% when the K+ concentration is 2%. Finally, a phosphor-convert WLED (pc-WLED) was simply synthesized by Ba2GdGaO5:Bi3+ and BaMgAl10O17:Eu2+ (BAM:Eu2+) phosphors. The doping of Eu3+ can significantly enhance the color rendering index (CRI, from 88.1 to 91.5) and reduce the correlated color temperature (CCT, from 4911 K to 4014 K). The above experimental results demonstrated that the phosphor has great application prospect in WLED.

Synthesis, Optical, Dielectric, SHG, Magnetic and Visible Light Driven Catalytic Studies on Compounds Belonging to the Swedenborgite Structure.

A new compound, InBaZn3GaO7, with swedenborgite structure along with transition metal (TM) substituted variants have also been prepared. The structure contains layers of tetrahedral ions (Zn2+/Ga3+) connected by octahedrally coordinated In3+ ion forming the three-dimensional structure with voids where the Ba2+ ions occupy. The TM substituted compounds form with new colors. The origin of the color was understood based on the ligand-field transitions. The near IR reflectivity studies indicate that the Ni - substituted compounds exhibit good near - IR reflectivity behavior, making them possible candidates for 'cool pigments'. The temperature dependent dielectric studies indicate that the InBaZn3GaO7 compound undergoes a phase transition at ~360 °C. The compounds are active towards second harmonic generation (SHG). Magnetic studies show the compounds, InBaZn2CoFeO7 and InBaZn2CuFeO7 to be anti-ferromagnetic in nature. The copper containing compounds were found to be good catalysts, under visible light, for the oxidation of aromatic alkenes. The many properties observed in the swedenborgite structure-based compounds suggests that the mineral structure offers a fertile ground to investigate newer compounds and properties.

One-pot synthesis of Ni(Ⅱ)-doped UiO-66-NH2 for enhanced photocatalytic CO2 reduction to CO with efficient charge transfer

Introducing a new metal into the metal center of metal–organic framework materials (MOFs) to alter their original electron transfer pathway to improve photocatalytic performance is a promising approach at present. Here, we report a one-pot method to introduce Ni2+ into the metal-centered Zr-O clusters of UiO-66-NH2 (UN) to create a series of Ni2+-doped bimetallic materials (xNi-UN). Experimental results showed that the 0.5Ni-UN exhibited the highest photocatalytic CO2 reduction activity, with a CO yield of 9.01 µmol g-1h−1 under visible light irradiation, which was 2.46 times higher than that of UN (3.67 µmol g-1h−1). Through a number of characterization analyses, we find that the doped Ni2+ replaced the part of original metal center Zr4+ to form a bimetallic UiO-66-NH2 (Zr/Ni), which changed the energy band gap of UiO-66-NH2 and improved the charge transfer. More importantly, the introduction of Ni2+ enhanced the efficiency of interfacial charge transfer from the organic ligand (2-aminoterephthalic acid) to the Zr-O cluster, enabling a metal-to-metal charge transfer (MMCT) pathway, which exhibited higher photocatalytic CO2 to CO reduction activity.

Syntheses, crystal structures and MMCT properties of diruthenium-based cyanido-bridged RuV/VI2-NC-RuII complexes.

The goal of this study was to investigate how the electron-donating capability around the lower valent metal ion and the electron-accepting capability of the higher valent metal ion influence metal to metal charge transfer (MMCT) properties in mixed-valence complexes. A series of trinuclear ruthenium complexes represented as [Ru2(ap-4-Me)3(CH3COO)NCRuCpMex(dppe)][PF6] (CpMex = polymethylcyclopentadienyl, x = 0, 1, and 5; and dppe = 1, 2-bis(diphenylphosphino)ethane, ap-4-Me = 2-anilino-4-methylpyridine) and their one-electron oxidized products were synthesized and fully characterized. The UV-vis-NIR spectra confirmed that as the electron donor character of the CpMex(dppe)RuCN fragment enhanced or the electron-accepting capability of the higher valent diruthenium cluster increased, the RuII → RuV2 or RuVI2 Ru2 MMCT bands shifted to lower energies, which was supported by TDDFT calculations.

Energy transfer and tunable-color luminescence properties of a single-phase CaSrNb2O7:Sm3+, Bi3+

New tunable-color luminescent materials are getting a lot of attention. Double-doped activated ions are an important research method. Here, CaSrNb2O7:R3+ (R3+ = Sm3+, Bi3+, and Sm3+/Bi3+) are synthesized by solid state method in the air. Phase purity and successful synthesis of samples are confirmed by X-ray diffractometer (XRD) patterns and energy disperse spectroscopy (EDS) diagram. The luminescence properties of samples are studied. CaSrNb2O7:Sm3+ has an excitation spectrum due to the O2- - Sm3+ charge transfer band (CTB) and the 6H5/2 → 4H9/2, 4D3/2, 4D1/2, 4F7/2, and 4I11/2 transitions of Sm3+ ion. The red-orange emission of CaSrNb2O7:Sm3+ is observed because of the 4G5/2 → 6H5/2, 6H7/2, 6H9/2, and 6H11/2 transitions of Sm3+ ion. PLE spectrum of CaSrNb2O7:Bi3+ comes from the metal-to-metal charge- transfer (MMCT) Bi3+ - Nb5+ and the 1S0 → 1P1 and 3P1 transitions of Bi3+ ion. CaSrNb2O7:Bi3+ shows blue emission due to the 3P1 → 1S0 transition of Bi3+ ion. The excitation spectrum of CaSrNb2O7:Sm3+, Bi3+ contains that of CaSrNb2O7:Bi3+ and CaSrNb2O7:Sm3+. CaSrNb2O7:Sm3+, Bi3+ with excitation at 314, 363, and 406 nm glows orange and red-orange, and red emission, respectively. We observe the energy transfer from Bi3+ to Sm3+ ions and the effect of Bi3+ on luminescence properties of Sm3+ ion by their spectra. The optimal Sm3+ and Bi3+ concentrations are confirmed by the concentration dependent spectra. The energy level diagrams of Sm3+ and Bi3+ are used to analyze the luminous mechanism. The results are conducive to the development of new Sm3+ and Bi3+ co-doped luminescent materials.

Exploring LiZnNbO4 as a Host for New Colored Compounds: Synthesis, Structure, and Material Properties of NbZn1‐x Mx LiO4 (M=Mn, Co, Ni, Fe) and Nb1‐ySby Zn1‐x Mx LiO4 (M=Co, Ni)

AbstractThe non ‐ centrosymmetric tetragonal inverse spinel structure of LiZnNbO4 has been explored with a view to prepare new colored compounds. The substitution of Co2+, Ni2+, Fe2+, Mn2+, and Cu2+ ions were attempted in the place of Zn2+ ions and Sb5+ ions in place of Nb5+ ions. The studies indicated that 0.75 Zn2+ ions in LiZnNbO4 can be replaced by Co2+ ions and 0.5 Zn2+ ions in LiZnNb0.5Sb0.5O4 compound. The substitution of Co2+ ions gives rise to different shades of blue color in Li(Zn1‐xCox)NbO4 compounds and from ink blue to blue‐green color in Li(Zn1‐xCox)(Nb0.5Sb0.5)O4 compounds. The different colors observed in the present study were explained by the traditional allowed d‐d transitions as well as the metal‐to‐metal charge transfer (MMCT) transitions involving Nb5+ (4d0) ions and partially filled 3d electrons. The SHG studies indicate that the prepared compounds are SHG active. All the compounds exhibit reasonable dielectric behavior with low loss. The XPS studies confirm the oxidation states of the different substituted ions. Raman studies indicate variations in the bands due to the substitutions in the parent LiZnNbO4 phase. Magnetic studies on the Co2+ ions substituted compounds suggest antiferromagnetic behavior.

Highly Efficient Solar-driven Dry Reforming of Methane on a Rh/LaNiO3 Catalyst through a Light-induced Metal-to-metal Charge Transfer Process.

As an energy-saving and green method, solar-driven dry reforming of methane (DRM) is expected to introduce new activation processes and prevent sintering and coking of the catalysts. However, it still lacks an efficient way to the coordinate regulation of activation of reactants and lattice oxygen migration. In this work, Rh/LaNiO3 is designed as a highly efficient photothermal catalyst for solar-driven DRM, which performs production rates of 452.3mmol h-1 gRh -1 for H2 and 527.6mmol h-1 gRh -1 for CO2 under a light intensity of 1.5W cm-2 , with an excellent stability. Moreover, a remarkable light-to-chemical energy efficiency (LTCEE) of 10.72% is achieved under a light intensity of 3.5W cm-2 . The characterizations of surface electronic and chemical properties and theoretical analysis demonstrate that strong adsorption for CH4 and CO2 , light-induced metal-to-metal charge transfer (MMCT) process and high oxygen mobility together bring Rh/LaNiO3 excellent performance for solar-driven DRM. This article is protected by copyright. All rights reserved.

Electron Trap Depths in Cubic Lutetium Oxide Doped with Pr and Ti, Zr or Hf─From Ab Initio Multiconfigurational Calculations.

We propose a universal approach to model intervalence charge transfer (IVCT) and metal-to-metal charge transfer (MMCT) transitions between ions in solids. The approach relies on already well-known and reliable ab initio RASSCF/CASPT2/RASSI-SO calculations for a series of emission center coordination geometries (restricted active space self-consistent field, complete active space second-order perturbation theory, and restricted active space state interaction with spin-orbit coupling). Embedding with ab initio model potentials (AIMPs) is used to represent the crystal lattice. We propose a way to construct the geometries via interpolation of the coordinates obtained using solid-state density functional theory (DFT) calculations for the structures where the activator metal is at specific oxidation (charge) states of interest. The approach thus takes the best of two worlds: the precision of the embedded cluster calculations (including localized excited states) and the geometries from DFT, where the effects of ionic radii mismatch (and eventual nearby defects) can be modeled explicitly. The method is applied to the Pr activator and Ti, Zr, Hf codopants in cubic Lu2O3, in which the said ions are used to obtain energy storage and thermoluminescence properties. Electron trap charging and discharging mechanisms (not involving a conduction band) are discussed in the context of the IVCT and MMCT role in them. Trap depths and trap quenching pathways are analyzed.

Syntheses, crystal structures and MMCT properties of cyanidometal-bridged trinuclear Ru-Ru-Ru complexes

Two pairs of cyanidometal-bridged trinuclear complexes trans-[Cp*(modppe)Ru(μ-NC)Ru(L)4(μ-CN)Ru(modppe)Cp*][PF6]n (n = 2, 3; L = pyridine (py), 1[PF6]n; 4-methoxypyridine (mopy), 2[PF6]n; Cp* = 1,2,3,4,5-pentamethylcyclopentadiene) in two distinct states have been synthesized and fully characterized. The investigated results suggest that 1[PF6]3 and 2[PF6]3 are Class II mixed valence complexes. The IR spectroscopy and UV–vis-NIR spectroscopy analyses showed that with the increase of the electron-donating ability of the auxiliary ligand of the cyanidometal bridge, the degree of delocalization increases and the energy of the metal-to-metal charge transfer (MMCT) of the mixed valence complexes decreases, supported by TDDFT calculations.

Front Cover: Influence of Changing the Remote Substituents on Charge Transfer Properties of Cyanide‐Bridged Trinuclear Fe−Ru−Fe Complexes (Eur. J. Inorg. Chem. 12/2023)

The Front Cover shows a new series of trinuclear Fe−Ru−Fe mixed-valence compounds obtained by changing the electron-donating ability of the remote substituents (R) of the ligand on the bridging metal center. The “person” (which symbolizes the electron) in the picture has to climb up different heights to reach the next corresponding step, which denotes the metal-to-metal charge transfer (MMCT) energy barriers from Ru2+ to Fe3+ in this family of compounds. As the electron-donating ability of the remote substituents decreases, it becomes more and more difficult for the “person” to reach the corresponding step, which means higher energy barriers need to be overcome for MMCT. The cover was designed by Bing-Chang Tan (the first author). More information can be found in the Research Article by T.-L. Sheng and co-workers.

Influence of Rare-Earth Ion Radius on Metal-Metal Charge Transfer in Trinuclear Mixed-Valent Complexes.

We report the synthesis and characterization of a highly conjugated bisferrocenyl pyrrolediimine ligand, Fc2PyrDIH (1), and its trinuclear complexes with rare earth ions─(Fc2PyrDI)M(N(TMS)2)2 (2-M, M = Sc, Y, Lu, La). Crystal structures, nuclear magnetic resonance (NMR) spectra, and ultraviolet/visible/near-infrared (UV/vis-NIR) data are presented. The latter are in good agreement with DFT calculations, illuminating the impact of the rare earth ionic radius on NIR charge transfer excitations. For [2-Sc]+, the charge transfer is at 11,500 cm-1, while for [2-Y]+, only a d-d transition at 8000 cm-1 is observed. Lu has an ionic radius in between Sc and Y, and the [2-Lu]+ complex exhibits both transitions. From time-dependent density functional theory (TDDFT) analysis, we assign the 11,500 cm-1 transition as a mixture of metal-to-ligand charge transfer (MLCT) and metal-to-metal charge transfer (MMCT), rather than pure metal-to-metal CT because it has significant ligand character. Typically, the ferrocenes moieties have high rotational freedom in bis-ferrocenyl mixed valent complexes. However, in the present (Fc2PyrDI)M(N(TMS)2)2 complexes, ligand-ligand repulsions lock the rotational freedom so that rare-earth ionic radius-dependent geometric differences increasingly influence orbital overlap as the ionic radius falls. The Marcus-Hush coupling constant HAB trends as [2-Sc]+ > [2-Lu]+ > [2-Y]+.

Influence of Changing the Remote Substituents on Charge Transfer Properties of Cyanide‐Bridged Trinuclear Fe−Ru−Fe Complexes

AbstractTo investigate the effect of ligand remote (>10 Å) substituents on the bridging metal center on the metal‐to‐metal charge transfer (MMCT) properties in cyanidometa‐bridged complexes, a series of new cyanidometal‐bridged complexes and their one‐electron and two‐electron oxidation products have been synthesized and well characterized (namely, trans‐[Cp(dppe)Fe−NC−(L)Ru(PPh3)−CN−Fe(dppe)Cp][PF6]n (n=2, 3, 4) (L=dmptpy, 1[PF6]n; L=meoptpy, 2[PF6]n; L=t‐Buptpy, 3[PF6]n) (Cp=1,3‐cyclopentadiene, dppe=1,2‐bis(diphenylphosphino)ethane, PPh3=triphenylphosphine, dmptpy=4′‐(4‐dimethylaminophenyl)‐2,2′,6′,2′′‐terpyridine, meoptpy=4′‐(4‐methoxyphenyl)‐2,2′,6′,2′′‐terpyridine, t‐Buptpy=4′‐(4‐tertbutylphenyl)‐2,2′,6′,2′′‐terpyridine)). The investigations suggest that the cyanido‐stretching (νCN) vibration energy for the complexes is unsensitive to the electron‐donating ability change of the remote substituents of the cyanidometal bridging auxiliary ligand from tertbutyl, methoxy to dimethylamino group. However, the MMCT energies of the one‐ and two‐electron oxidation complexes are still sensitive to the remote substituents of the ligand on the bridging metal center, and decreases with the increase of the electron‐donating ability of the remote substituents from tertbutyl, methoxy to dimethylamino group. All one‐electron and two‐electron oxidation products belong to Class II mixed valence compounds according to the classification of Robin and Day.

Highly Sensitive Hybrid Oxide Phototransistors with Photoresponsive Zeolitic‐Imidazolate‐Frameworks for Real‐Time Light Detection

AbstractConventional oxide‐based thin‐film phototransistors (Ph‐TRs) with a hybrid structure with additional photoabsorbers show limitations in real‐time on/off operations without additional input bias. Here, hybrid InGaZnO (IGZO) Ph‐TRs utilizing zeolitic‐imidazolate‐framework‐67 (ZIF‐67), an electrically insulating material with a unique metal‐to‐metal charge transfer (MMCT) pathway enabling charge migration, are presented. A wide photodetection range and fast response/recovery speed without additional gate bias are achieved. In particular, ZIF‐67 used as a photoabsorber exhibits absorption of visible light over a wide range of wavelengths. Photoexcited electrons migrate through the MMCT pathway between Co ions and migrate to the IGZO conduction band, causing a significant change in the photocurrent (Iph) of the hybrid ZIF‐67/IGZO Ph‐TR. In the dark, ZIF‐67 immediately returns to the insulating state, resulting in a fast rise/fall time under dynamic photo‐stimulus. In addition, it exhibits repeatable photo‐sensing performance without persistent photocurrent behavior under pulsed light‐on/off cycles. A high photosensitivity of 108 for red light (592 nm, 0.1 mW cm–2) is obtained as it suppresses the dark current and amplifies photocurrent generation in the subthreshold gate voltage region. This combination of fast response time, high photosensitivity, and high stability is promising for ideal Ph‐TRs, enabling real‐time detection even for high‐frequency operations.

Construction of polyoxometalate-based metal−organic frameworks through covalent bonds for enhanced visible light-driven coupling of alcohols with amines

Broad spectral response and rapid carrier transport are essential in making efficient photocatalysts. In this work, we utilized Keggin-type polyoxometalates (POMs) [CoIIW12O40]6– as metal-to-metal charge transfer (MMCT) chromophores and assembled them into metal–organic frameworks (MOFs) to construct visible-light-responsive, noble metal-free crystalline POM-based MOFs (POMOFs) (CoW-1 and CoW-2). The precise introduction of MMCT chromophores extending the light absorption of POMOFs to the visible region and improving the solar energy utilization efficiency, meanwhile, the Cu-O-W covalent bonds constructed in CoW-1 makes the framework structure more robust, reduces the interfacial contact resistance, facilitates the electron transfer, suppresses the recombination of the electron-hole pairs, improves the charge carriers separation efficiency, and boosts the quantum efficiency, thus achieving high catalytic activity in the coupling reaction of benzyl alcohol with aniline under visible light irradiation (>400 nm). In the presence of CoW-1, the reaction conversion yield was measured as 92.6 % with turnover frequency reaching 374.4 h−1, and the apparent quantum yield at 595 nm was calculated as 17.5 %, besides, CoW-1 also exhibited high catalytic stability and reusability. To the best of our knowledge, this is the first work describing the enhanced photocatalytic performance of POMOFs based on the synergistic effect of MMCT and covalent linkage.

Impact of Solvent-Solute Hydrogen Bonding on Ultrafast Electron Transfer in a Trimetallic Iron-Ruthenium Complex

Transition metal complexes (TMCs) are widely used as light absorbers and catalysts for photochemical energy conversion processes such as photocatalytic reactions in liquid environments. Of particular interest are TMCs which incorporate multiple bridged metal centres because they represent a step towards more complex supramolecular assemblies that can ultimately enable control of the charge flow in solution-based photo-redox processes. However, the development of design guidelines for such molecular assemblies is impeded by an incomplete understanding of the coupling between their electronic and nuclear dynamics, including the interplay of TMCs with their liquid environment upon photoexcitation. Probing the involved processes requires high spatial sensitivity towards electronic motion and the ability to capture atomic motion on ultrafast timescales – a combination which is not readily available for optical transient spectroscopic techniques.X-ray free electron lasers (XFELs) have revolutionized the field of ultrafast science in recent years. XFELs generate tuneable and extremely bright X-ray pulses with a typical pulse duration of <30 fs, which are well-suited as an element-specific probe of ultrafast electronic and atomic motion in molecules and materials.1,2 Applied to TMCs, XFEL probes are sensitive to the local electronic structure in such molecular systems as well as to intra- and inter-molecular structural reorganization at an atomic level.3 In this talk, I will present work on the interaction of a linear trimetallic cyanide-bridged iron-ruthenium complex (FeRuFe) with its solvent environment, carried out using the LCLS XFEL facility at SLAC National Accelerator Laboratory. The focus of this study lies on elucidating how photoinduced intramolecular electron transfer processes in FeRuFe are affected by hydrogen bonding interactions between its cyanide ligands and the surrounding solvent molecules. In water, such solvent-solute hydrogen bonding interactions are stronger for Fe(II) than Fe(III), leading to substantial changes in solute-solvent interactions upon photoinduced electron transfer between metal centres.The experiment is performed in a pump-probe geometry on a liquid jet, where a visible/near-infrared pump induces a metal-to-metal charge transfer (MMCT) between Ru and Fe which is followed by an ultrafast back-electron transfer (BET). The excited state dynamics are monitored using hard X-ray pulses as a function of time delay between the laser pump and the X-ray probe pulses. To resolve and correlate electronic and structural dynamics upon MMCT excitation, we simultaneously perform ultrafast Fe Kβ X-ray emission spectroscopy (XES) and X-ray solution scattering (XSS) measurements, where XES is sensitive to the oxidation and spin state of the Fe centres and XSS provides information on atomic motion in solute and solvent, as demonstrated in previous work on a similar system.4 To evaluate the effect of solute-solvent hydrogen bonding interactions on the electron transfer dynamics, we perform measurements in a series of solvents with different hydrogen bonding properties, including water, methanol, and acetonitrile. We find that the BET increases as a function of decreasing hydrogen bonding ability of the solvent, and that distinct signatures of solvation dynamics are present in the scattering data. The resulting molecular level understanding of solvent reorganization coupled to electron transfer demonstrates that the strength and type of solute-solvent interactions are a central factor in determining the outcome of photoinduced charge transfer processes in TMCs.References Bergmann, U. et al. Using X-ray free-electron lasers for spectroscopy of molecular catalysts and metalloenzymes. Nat. Rev. Phys. 3, 264–282 (2021).Schoenlein, R. et al. Recent advances in ultrafast X-ray sources. Philos. Trans. R. Soc. Math. Phys. Eng. Sci. 377, 20180384 (2019).Gaffney, K. J. Capturing photochemical and photophysical transformations in iron complexes with ultrafast X-ray spectroscopy and scattering. Chem. Sci. 12, 8010–8025 (2021).Biasin, E. et al. Direct observation of coherent femtosecond solvent reorganization coupled to intramolecular electron transfer. Nat. Chem. 13, 343–349 (2021).

Thermal Quenching Mechanism of Metal-Metal Charge Transfer State Transition Luminescence Based on Double-Band-Gap Modulation.

Bi3+-related metal-to-metal charge transfer (MMCT) transition phosphors are expected to become a new class of solid-state luminescent materials due to their unique broadband long-wavelength emission; however, the main obstacle to their application is the thermal quenching effect. In this study, one novel thermal quenching mechanism of Bi3+-MMCT transition luminescence is proposed by introducing electron-transfer potential energy (ΔET). Y0.99V1-xPxO4:0.01Bi3+ (YV1-xPxO4:Bi3+) is used as the model; when the band gap of the activator Bi3+ increases from 3.44 to 3.76 eV and the band gap of the host YV1-xPxO4 widens from 2.75 to 3.16 eV, the electron-transfer potential energy (ΔET) decreases and the thermal quenching activation energy (ΔE) increases, which result in the relative emission intensity increasing from 0.06 to 0.64 at 303-523 K. Guided by density functional calculations, the thermal quenching mechanism of the Bi3+-MMCT state transition luminescence is revealed by the double-band-gap modulation model of the activator ion and the matrix. This study improves the thermal quenching theory of different types of Bi3+ transition luminescence and offers one neo-theory guidance for the contriving and researching of high-quality luminescence materials.

Ligand Modified and Light Switched On/Off Single-Chain Magnets of {Fe 2 Co} Coordination Polymers via Metal-to-Metal Charge Transfer

Ligand Modified and Light Switched On/Off Single-Chain Magnets of {Fe ₂ Co} Coordination Polymers via Metal-to-Metal Charge Transfer