Intervalence Charge Transfer Research Articles

Intervalence Charge Transfer in Nonbonding, Mixed-Valence, Homobimetallic Ytterbium Complexes.

There are several reports of compounds containing lanthanide ions in two different formal oxidation states; however, there are strikingly few examples of intervalence charge transfer (IVCT) transitions observed for these complexes, with those few occurrences limited to extended solids rather than molecular species. Herein, we report the synthesis, characterization, and computational analysis for a series of ytterbium complexes including a mixed-valence Yb25+ complex featuring a remarkably short Yb···Yb distance of 2.9507(8) Å. In contrast to recent reports of short Ln···Ln distances attributed to bonding through 5d orbitals, the formally Yb25+ complex presented here displays clear localization of Ln2+ and Ln3+ character and yet still displays an IVCT in the visible spectrum. These results demonstrate the ability to tune the electronic structure of formally mixed oxidation state lanthanide complexes: the high exchange stabilization of the Yb2+ 4f14 configuration disfavors the formation of a 5d1 bonding configuration, and the short metal-metal distance enforced by the ligand framework allows for the first observed lanthanide IVCT in a molecular system.

Defect compensation and intervalence charge transfer state-based Pr3+-doped niobate antithermal quenching phosphors.

Herein, a scheme of Sr2+/Ca2+ ion substitution was employed to simultaneously regulate the defect and intervalence charge transfer (IVCT) state of Sr2-xCaxNb2O7:Pr3+ phosphors, resulting in a dual-modulation strategy for enhancing phosphor thermal stability.

Enhanced thermally stable performance of Pr3+-doped vanadate phosphor by inhibiting the intervalence charge transfer quenching channel.

Praseodymium (Pr3+) ion and the transition metal vanadium (V5+) ion with d0 electronic configuration can form an intervalence charge transfer (IVCT) band, which can function both as a compensatory channel for its red emission and as a quenching channel, thus affecting the luminescence thermal stability of the phosphors. Research studies reveal that the emission of Pr3+ in the YVO4 matrix can be quenched by the IVCT mechanism, thereby limiting the application of phosphors. As such, the present contribution is based on the solid solution replacement strategy to inhibit the constitutent of the IVCT quenching channel and thus improve thermal stability. Therefore, phosphonium (P5+) with a valence state matching V5+ and a similar ion radius was selected for the V/P substitution. It lacks a d0 electron configuration, preventing the formation of an IVCT band with Pr3+ and thereby inhibiting the construction of the quenching channels to enhance thermal stability. While the empirical formula of IVCT indicates a decrease in the IVCT energy level from 3.32438 to 3.06251 eV upon the introduction of P5+, the PLE spectra demonstrate a sharp reduction in IVCT intensity, i.e., weakening of the quenching channel. The thermal stability of the phosphors at different excitation locations was enhanced with the rise of P5+ concentration. When excited at the 3P2 level, the Y0.995PO4:0.5%Pr3+ phosphor demonstrated highly stable red emission from 303 to 523 K, with a luminescence integrated intensity ranging from 95.5% to 105.3% compared to that at 303 K. This research provides a novel approach for inhibiting the IVCT quenching channel and broadens the commercial value of YVO4:Pr3+ phosphor for various applications.

Pyrene fluorescence in 2,7-di(4-phenylethynyl)pyrene-bridged bis(alkenylruthenium) complexes.

Complexes PyrDPE-RuCl and PyrDPE-Ruacac with a π-extended 2,7-di(4-phenylethynyl)pyrene linker undergo simultaneous one-electron oxidations of their {Ru}-styryl entities. The absence of an intervalence charge-transfer (IVCT) band at intermediate stages, where the mixed-valent, singly oxidized radical cation is present, and spin density confinement to the terminal styryl ruthenium site(s) are tokens of a lack of electronic coupling between the {Ru} entities across the π-conjugated linker. The close similarity of the linker-based π → π* bands in the complexes and the free ligand and their insensitivity towards oxidations at the terminal sites indicate that the central pyrenyl fluorophore is electronically decoupled from the electron-rich {Ru}-styryl termini. As a consequence, the complexes offer stable pyrene-based fluorescence emissions at 77 K, which are red-shifted from that of the linker.

Intermetallic charge transfer in MTiF6·6H2O (M = Mn, Fe, Co, and Ni): A study by X-ray photoelectron spectroscopy

The X-ray photoelectron spectra (XPS) of a series of hydrated fluoridotitanates MTiF6∙6H2O (M = Mg, Mn, Fe, Co, Ni) were measured for the first time. The compounds were grown in the form of high-quality single crystals and belong to the extensive ABF6∙6H2O family. Under the action of the XPS experimental conditions (vacuum and X-ray), the process of charge transfer from divalent transition metal to the titanium atom develops, which is described as intermetallic (intervalence) charge transfer (IMCT, IVCT): M2+ + Ti4+ → M3+ + Ti3+. Dehydration and hydrolysis of the studied complexes promote the process of charge transfer through the association of isolated octahedra by an edge or a vertex. The obtained XPS data were compared with those available in the literature, which are rather ambiguous for cobalt compounds. In our case, the binding energy of Co 2p electrons is higher for Co3+ than for Co2+ in an octahedral environment.

Shining a light on fungicide- water: enhanced photocatalytic degradation using the CoTiO3/CaTiO3 nanocomposite and experimental and theoretical viewpoints on improved intervalence charge transfer from O2− to Ti4+ and from Co2+ to Ti4+ ions and spatial charge transfer

An enhanced degradation rate is achieved for the optimized CoTiO3/CaTiO3 (CCO 75 : 25) composite for highly effective photocatalytic carbendazim degradation.

Energy-splitting from persistent luminescence nanoparticles with trivalent Cr ions for ratiometric temperature sensing.

MgGa2O4 (MGO) with the spinel structure exhibits abundance defects and could achieve the modulation of emission by ion doping as persistent luminescence nanoparticles (PLNPs). Here, we introduced Cr3+ ions into MGO to achieve near-infrared (NIR) emission, and Pr3+ ions to tune the lattice environment for enhanced NIR emission. The optimal composite, MgGa2O4: 0.005Cr3+, 0.003Pr3+ (MGCP), achieved enhanced NIR emission at 709 nm under 222 nm excitation. The concentration quenching was observed due to electric dipole-quadrupole interaction at high Cr3+ and Pr3+ content. The afterglow mechanism was revealed, while the energy-splitting occurs from trivalent Cr3+ ions at 650 and 709 nm, thanks to the complex lattice environment. We observed that the emission at 709 nm decreased, while the satellite signal at 650 nm increased first and then decreased intensity with increasing temperature, due to the intervalence charge transfer for Cr3+ ions at 303-528 K. Ratiometric temperature sensing was therefore realized with superb linearity, high absolute sensitivity at 303 K for 4.18%, and accuracy at 528 K for 2.62 K, confirming with the luminescence intensity ratio at 709 and 650 nm under excitation at 222 nm. Thus, we provide a method with energy-splitting emission of Cr3+ ions to design temperature sensing.

Modeling the polychromism of oxide minerals: The case of alexandrite and cordierite.

In this work, we investigate the spectroscopic properties of photochromic alexandrite and cordierite by TD-DFT. The objective is to assess the TD-DFT for the simulation of pleochroism (change of color depending on the crystallographic direction of the observation) and the change of color as a function of the light source. For these simulations, we compared an embedding where dangling bonds are saturated by hydrogen atoms and an electrostatic embedding. The electrostatic embedding provided numerically more stable results and allowed a good reproduction of the pleochroism of cordierite, based on a Fe2+-Fe3+ intervalence charge transfer transition. However, the pleochroism of alexandrite is not as well reproduced, suggesting that TD-DFT has some difficulties to reproduce the anisotropy of the transition dipole moment, an aspect that is not deeply documented in the literature.

Vibrational Coherences of the Photoinduced Mixed-Valent Creutz-Taube Ion Revealed by Excited State Dynamics.

A recent study of photoinduced mixed-valency in the one-electron reduced form (μ-pz)[RuII(NH3)5]24+ of the Creutz-Taube ion used transient absorption spectroscopy with vis-NIR broadband detection to uncover a mixed-valent excited state with a typical intervalence charge transfer band and a nanosecond lifetime [Pieslinger et al. Angew. Chem., Int. Ed. 2022, 61, e202211747]. Herein, we use excited state dynamics simulations with implicit solvation to elucidate the electronic and vibrational evolution in the first 10 ps after the optical excitation. A manifold of excited states with weak interaction between the metal centers is populated already at time zero due to the breakdown of the Condon approximation and dominates the population of electronic states at short time scales (<0.5 ps). A long-lived vibrational wave packet mostly confined to oscillations of the metal center-bridge distances is observed. The oscillations are traced to the electronic structure properties of states with weak metal-metal coupling. The long-lived mixed-valent excited state of the Creutz-Taube ion analogue is formed vibrationally cold and has a more compact geometry. While experimentally, intersystem crossing and vibrational relaxation were deduced to be completed within 1 ps, our analysis indicates that both processes might persist at longer times.

Modulating the antithermal-quenching of praseodymium-activated phosphor by the position of intervalence charge transfer state

So far, overcoming thermal quenching remains a huge challenge in the application of phosphor materials. The intervalence charge transfer (IVCT) state has an inestimable role in the compensation of antithermal-quenching effects and is widely used in the research of applications in optical temperature sensing, yet little study has been reported on its modulation of thermal quenching properties. In this contribution, the thermal quenching properties of praseodymium-activated solid-solution substituted GdNb1-xTaxO4 phosphors were investigated systematically for the inaugural time from the perspective of IVCT energy level positions. According to the empirical formula, the IVCT level positions in the phosphor increase sequentially from 32960.71 cm−1 to 33442.15 cm−1, the DFT calculates the extension of the substrate band gap from 3.6408 eV to 4.3066 eV, and the contributing energy level of the conduction band is determined. The variation in the position of the IVCT energy level explains the alteration of the luminescence intensity at room temperature for different composition phosphor 3P0 and 1D2 energy levels. More particularly, it was analyzed that in the variable temperature spectra, x = 0–0.75 exhibited different degrees of efficient antithermal-quenching properties at 303–523 K, while no antithermal-quenching was presented at x = 1. By analyzing the IVCT energy level position and other influencing factors such as band gap, the feasibility of the IVCT energy level position as a competing model for the compensation and quenching channels is established, and the internal mechanism of regulating thermal quenching property is revealed. Effective control of the phosphor resistance to thermal quenching performance was achieved, which directly determines whether enhanced or diminished thermal quenching can be obtained. This finding provides a new effective strategy for the tuning of thermal quenching luminescence of Pr3+-doped Nb/Ta system phosphors.

Valence Delocalization and Metal-Metal Bonding in Carbon-Bridged Mixed-Valence Iron Complexes.

The carbide ligand in the iron-molybdenum cofactor (FeMoco) in nitrogenase bridges iron atoms in different oxidation states, yet it is difficult to discern its ability to mediate magnetic exchange interactions due to the structural complexity of the cofactor. Here, we describe two mixed-valent diiron complexes with C-based ketenylidene bridging ligands, and compare the carbon bridges with the more familiar sulfur bridges. The ground state of the [Fe2 (μ-CCO)2 ]+ complex with two carbon bridges (4) is S= , and it is valence delocalized on the Mössbauer timescale with a small thermal barrier for electron hopping that stems from the low Fe-C force constant. In contrast, one-electron reduction of the [Fe2 (μ-CCO)] complex with one carbon bridge (2) affords a mixed-valence species with a high-spin ground state (S= ), and the Fe-Fe distance contracts by 1 Å. Spectroscopic, magnetic, and computational studies of the latter reveal an Fe-Fe bonding interaction that leads to complete valence delocalization. Analysis of near-IR intervalence charge transfer transitions in 5 indicates a very large double exchange constant (B) in the range of 780-965 cm-1 . These results show that carbon bridges are extremely effective at stabilizing valence delocalized ground states in mixed-valent iron dimers.

Ferrocenyl-Substituted Triazatruxenes: Synthesis, Electronic Properties, and the Impact of Ferrocenyl Residues on Directional On-Surface Switching on Ag(111).

We report on seven new ferrocenyl-(1, 3)- and ferrocenylethynyl-modified N,N',N″-triethyltriazatruxenes (EtTATs) 4-7 as well as the dodecyl counterpart 2 of compound 1 and their use as molecular switching units when deposited on a Ag(111) surface. Such functional units may constitute a new approach to molecule-based high-density information storage and processing. Besides the five compounds 1-3, 6, and 7, where the 3-fold rotational symmetry of the triazatruxene (TAT) template is preserved, we also included 2-ethynylferrocenyl-TAT 4 and 2,2'-di(ethynylferrocenyl)-TAT 5, whose mono- and disubstitution patterns break the 3-fold symmetry of the TAT core. Voltammetric studies indicate that the ferrocenyl residues of compounds 1-7 oxidize prior to the oxidation of the TAT core. We have noted strong electrostatic effects on TAT oxidation in the 2,2',2″-triferrocenyl-TAT derivatives 1 and 2 and the 3,3',3″-isomer 3. The oxidized complexes feature multiple electronic excitations in the near-infrared and the visible spectra, which are assigned to dδ/δ* transitions of the ferrocenium (Fc+) moieties, as well as TAT → Fc+ charge-transfer transitions. The latter are augmented by intervalence charge-transfer contributions Fc → Fc+ in mixed-valent states, where only a part of the available ferrocenyl residues is oxidized. EtTAT was previously identified as a directional three-level switching unit when deposited on Ag(111) and constitutes a trinary-digit unit for on-surface information storage. The symmetrically trisubstituted compound 6 retains this property, albeit at somewhat reduced switching rates due to the additional interaction between the ferrocenyl residues and the Ag surface. In particular, the high directionality at low bias and the inversion of the preferred sense of the on-surface rocking motion with either a clockwise or counterclockwise switching sense, depending on the identity of the surface enantiomer, are preserved. Unsymmetrical substitution in mono- and diferrocenylated 4 and 5 alters the underlying ratchet potential in a manner such that a two-state switching between the two degenerate surface conformations of 4 or a pronounced suppression of switching (5) is observed.

Regulating Luminescence Thermal Quenching of Praseodymium-Doped Niobo-Tantalate Phosphor through Intervalence Charge Transfer Band Displacement.

Pr3+-related intervalence charge transfer (IVCT) bands are a research hotspot owing to their amelioration in the luminescence thermal quenching of Pr3+-activated phosphors. Here, a typical IVCT band displacement strategy via a topological chemical scheme is reported to optimize the luminescence thermal quenching performance of praseodymium-doped niobo-tantalate. The substitution of Ta5+ ions for Nb5+ ions reduces the valence-weighted average cation optical electronegativity and increases the bond lengths of the activator (Pr3+) to the ligand cations (Nb5+ and Ta5+) via adjusting the crystal structure, leading to an increase in the IVCT energy level position from 3.521 to 4.139 eV. The increase in the IVCT energy level leads to an increase in the number of electrons located in the Pr3+ 3P0 energy level, which compensates for the emission of 1D2 during warming. Especially, the energy gap value of the IVCT band is positively correlated with the thermal quenching activation energy ΔE2. ΔE2 increases, the crossover point rises, and the nonradiative transition decreases, further enhancing the Pr3+ 1D2 emission. At 503 K, the 1D2 emission integral intensity increases from 14 to 224% relative to the 303 K original integral intensity. This IVCT band displacement strategy can be used as a scheme for designing antithermal quenching luminescence materials.

Visible-light-induced self-propelled nanobots against nanoplastics

The accumulation of plastic debris in aquatic organisms has raised serious concerns about the potential health implications of their incorporation into the food chain. However, conventional water remediation techniques are incapable of effectively removing nanoplastics (NPs) smaller than 200 nm, which can have harmful effect on animal and human health. Herein, we demonstrate the “on-the-fly” capture of NPs through their enlargement (approximately 4,100 times) using self-propelled nanobots composed of a metal-organic framework. Under visible-light irradiation, the iron hexacyanoferrate (FeHCF) nanobot exhibits fuel-free motion by electrostatically adsorbing NPs. This strategy can contribute to reducing plastic pollution in the environment, which is a significant environmental challenge. Light-induced intervalence charge transfer in the FeHCF nanobot lattice induces bipolarity on the nanobot surface, leading to the binding of negatively charged NPs. The local electron density in the lattice then triggers self-propulsion, thereby inducing agglomeration of FeHCF@NP complexes to stabilize their metastable state. The FeHCF nanobot exhibits a maximum removal capacity of 3,060 mg∙g−1 and rate constant of 0.69 min−1, which are higher than those recorded for materials reported in the literature.

Characterization of the Ligand Field in Pseudo‐Octahedral Ni(II) Complexes of Pincer‐Type Amido Ligands: Magnetism, Redox Behavior, Electronic Absorption and High‐Frequency and ‐Field EPR Spectroscopy

AbstractThe description of π‐donor amido moieties as ‘weak‐field’ ligands can belie the influence of metal‐ligand covalency on the overall ligand field of coordination complexes, which can in turn influence properties including the magnetic ground state and those of their excited states. In this contribution, the ligand fields of pseudo‐octahedral Ni(II) complexes supported by diarylamido pincer‐type amido ligands – three previously reported examples supported by asymmetric (2‐R‐phenanthridin‐4‐yl)(8‐quinolinyl)amido ligands (R = Cl, CF3,tBu;RL1) along with a new congener bearing a symmetricbis(8‐quinolinyl)amido ligand (BQA;L2) – were investigated in two ways. First, high‐frequency and ‐field electron paramagnetic resonance spectroscopy (HFEPR), SQUID magnetometry, and electronic absorption spectroscopy were used to determine the ligand field parameters. Second, the ability to electrochemically address ligand‐based oxidations despite metal‐centered SOMOs in the parentS=1 paramagnets was investigated, supported by time‐dependent density functional theory (TDDFT) identification of strong intervalence charge‐transfer (IVCT) transitions attributed to electronic communication between two Namidomoieties mediated by a Ni(II) bridge. These findings are discussed in the broader context of 3d transition metal coordination complexes of weak‐field π‐donor ligands.

Intervalence charge transfer of Cr3+-Cr3+ aggregation for NIR-II luminescence

The increasing demand for high-contrast biological imaging, non-destructive testing, and infrared night vision can be addressed by the development of high-performance NIR light-emitting materials. Unlike lanthanide (Ln3+) with sharp-line multiplets and isolated Cr3+ with NIR-I emission, this study reports the first-ever NIR-II broadband luminescence based on the intervalence charge transfer (IVCT) of Cr3+-Cr3+ aggregation in gallate magentoplumbite. In particular, LaMgGa11O19:0.7Cr3+ exhibits dual-emission (NIR-I, 890 nm and NIR-II, 1200 nm) with a full width at half maximum (FWHM) of 626 nm under 450 nm blue LED excitation. Moreover, this dual-emission exhibits anti-thermal quenching behavior (432% @ 290 K), attributed to the energy transfer among multiple Cr3+ centers. Cryogen absorption spectra, lifetimes decay (2.3 ms), and electron paramagnetic experiments reveal the NIR-II luminescence of the Cr3+-Cr3+ → Cr2+-Cr4+ IVCT transition. The application of LaMgGa11O19:0.7Cr3+ in NIR-II biological imaging as an optical contrast agent, non-destructive testing, and night vision is demonstrated. This work provides new insights into broadband NIR-II luminescence under UV-NIR excitation based on the IVCT of Cr3+-Cr3+ aggregation.

Kinetic Studies of WO3-Based Photochromism in Polyvinyl Alcohol Film.

Tungsten oxide (WO3) has been extensively studied for various photochromic applications. Blue coloration of WO3 is explained in terms of the intervalence charge transfer (IVCT) transition of electrons between W6+ and W5+. However, various absorption spectra with different shapes have been reported. Herein, a transparent film was prepared by drying aqueous solutions containing polyvinyl alcohol, WO3 nanoparticles and ethylene glycol (EG). For comparison, the photochromic behavior of an aqueous WO3 colloidal solution containing EG was also investigated. Under UV irradiation, a single intense peak was always observed at ca. 777 nm in the colloidal solution, but the absorption spectra of the film changed from a peak at 770 nm to two distinct peaks at 654 and 1003 nm. All absorption spectra observed with the film and the colloidal solution were deconvoluted into five peaks at 540, 640, 775, 984, and 1265 nm. Kinetic studies using the colloidal solution indicated that the coloration rates (r0) estimated at the deconvoluted peaks of 640, 775, and 984 nm followed the same rate law. On the other hand, in the case of the film, r0 evaluated at 640 or 984 nm was independent of the water amounts but increased proportionally to the EG amounts and the light intensity, although r0 at 775 nm significantly increased with the increasing amounts of water and EG. Raman and electron spin resonance spectroscopic observations of the film revealed that the photogenerated electrons migrated toward the terminal W═O moiety to accumulate and then a small anisotropic electron spin resonance signal appeared. Our study demonstrates that the absorption at 775 nm is due to IVCT between W6+ and W5+, which is stabilized with water in the bulk and the absorption peaks at 640 and 984 nm are attributable to IVCT on the WO3 surface.

Highly sensitive dual-mode thermometry over a wide temperature range based on bandgap engineering in LiNb1-xTaxO3:Tb3+

Having proved that highly sensitive dual-mode optical thermometry could be implemented based on fluorescence intensity ratio (FIR) of the same green emission driven by varying excitations and fluorescence lifetime (FL) of Tb3+, the feasibility to manage thermometric performance over a wide temperature range via bandgap engineering is validated in LiNb1-xTaxO3:Tb3+. The vital thermally activated relaxation processes via an intervalence charge transfer state are altered by formulating crystal structure via partial replacement of Nb5+ with Ta5+, allowing adjustment of the sensitive temperature range which can be measured with the highest accuracy. Specifically, maximum relative sensitivities SR up to 1.43 and 2.36 %K−1 are respectively attained in FIR and FL modes of LiNb1·0Ta0·0O3:Tb3+. The upper limit of the sensitive temperature range is proved to be greatly extended from 260 to 440 K for LiNb0·2Ta0·8O3:Tb3+. This work may provide a perspective approach to design self-calibrated thermometers targeted at different temperature ranges for varying applications.

Broad Luminescence Generated by IR Laser Excitation from CsPbBr3:Yb3+ Perovskite Ceramics.

This paper demonstrates the generation of broadband emission in the visible and infrared ranges induced by a concentrated beam of infrared radiation from CsPbBr3 ceramics doped with Yb3+ ions. The sample was obtained by the conventional solid-state reaction method, and XRD measurements confirmed the phase purity of the material crystallizing in the orthorhombic system. Spectroscopic measurements required further sample preparation in the form of ceramics using a high-pressure press. The research showed that as the excitation power increases, the emission intensity does not increase linearly from the beginning of the experiment. Irradiation of the material results in the accumulation of the delivered energy. Absorption of a sufficient number of photons triggers avalanche emission. It was found that the most intense luminescence is produced in a vacuum. Changes in conductivity were also observed, where the excitation was able to lower the resistivity of the material and it was highly dependent on the excitation power. The mechanism responsible for the generation of the observed phenomenon involving intervalence charge transfer (IVCT) transitions has been postulated.

Influence of Molecular Separation on Through‐Space Intervalence Transient Charge Transfer in Metal–Organic Frameworks with Cofacially arranged Redox Pairs

AbstractWe demonstrate direct evidence of photoinduced through‐space intervalence charge transfer (IVCT) between two cofacially arranged redox‐active pairs in metal–organic frameworks and their dynamic variation with their molecular separation. Two homologous MOFs [Co2(NDC)2(DPTTZ)2]. DPTTZ. DMF, 1 and [Co2(BDC)2(DPTTZ)2]. DMF, 2 (where NDC=naphthalene dicarboxylate, BDC=benzene dicarboxylate, DPTTZ=N, N′‐di(4‐pyridyl)thiazolo‐[5,4‐d]thiazole, DMF=N, N′‐dimethyl formamide) are considered for this, whose intra‐dimer distance of redox‐active DPTTZ ligands differs ca. 1 Å from one system to another. Spectroelectrochemical study detects the formation of IVCT band at the NIR region between cofacially oriented DPTTZ molecules in both MOFs. Transient spectroscopy shows faster charge separation as well as charge recombination when intra‐dimer distance is lesser (in MOF 2) due to stronger electronic coupling. We quantify the extent of IVCT by charge transfer integral calculation; and also by optical pump terahertz probe spectroscopy, where MOF 2 shows three times higher carrier mobility due to lesser inter‐DPTTZ distance than MOF 1. These findings reveal a more localized aspect of through‐space IVCT between cofacially organized redox‐active pair in a three‐dimensional framework.