Intervalence Charge Transfer Band Research Articles

Achieved High-Sensitivity Single-Band Ratiometric Optical Thermometry by Manipulating the Intervalence Charge-Transfer Band Position.

Currently, optical thermometry has received widespread attention because of its noncontact and wide temperature range, but most of them are based on the application of dual-band optical ratiometric thermometry, so the development of a single-band ratiometric (SBR) optical thermometry, which is easier to analyze and use, is particularly important. In this work, the position of the intervalence charge-transfer (IVCT) band for Na2Gd2-xLaxTi3O10:Pr3+ (x = 0, 0.5, 1.0, 1.5, 2.0) was modulated using Gd/La substitution, enhancing the thermal response difference of Pr3+ 1D2 → 3H4 under charge-transfer band (CTB) and IVCT band excitation, thereby achieving high-sensitivity SBR optical thermometry, and the maximum relative sensitivity (Sr-max) reached 2.95% (at 298 K). In addition, this series of phosphors has high-color-purity red emission, indicating that it has potential for multifield applications.

Symmetric Electron Transfer Coordinates are Intrinsic to Bridged Systems: An ab Initio Treatment of the Creutz-Taube Ion.

A long-standing question in electron transfer research concerns the number and identity of collective nuclear motions that drive electron transfer or localisation. It is well established that these nuclear motions are commonly gathered into a so-called electron transfer coordinate. In this theoretical study, we demonstrate that both anti-symmetric and symmetric vibrational motions are intrinsic to bridged systems, and that both are required to explain the characteristic shape of their intervalence charge transfer bands. Using the properties of a two-state Marcus-Hush model, we identify and quantify these two coordinates as linear combinations of normal modes from ab initio calculations. This quantification gives access to the potential coupling, reorganization energy and curvature of the potential energy surfaces involved in electron transfer, independent of any prior assumptions about the system of interest. We showcase these claims with the Creutz-Taube ion, a prototypical Class III mixed valence complex. We find that the symmetric dimension is responsible for the asymmetric band shape, and trace this back to the offset of the ground and excited state potentials in this dimension. The significance of the symmetric dimension originates from geometry dependent coupling, which in turn is a natural consequence of the well-established superexchange mechanism. The conceptual connection between the symmetric and anti-symmetric motions and the superexchange mechanism appears as a general result for bridged systems.

Intervalence Charge Transfer in Aluminum Oxide and Aluminosilicate Minerals at Elevated Temperatures

Abstract Single-crystal optical spectra of corundum (Al2O3) and the Al2SiO5 polymorphs andalusite, kyanite, and sillimanite, containing both Fe2+ – Fe3+ and Fe2+ – Ti4+ intervalence charge transfer (IVCT) absorption bands were measured at temperatures up to 1000 °C. Upon heating, thermally equilibrated IVCT bands significantly decreased in intensity and recovered fully on cooling. These trends contrast with the behavior of crystal field bands at temperature for Fe, Cr and V in corundum, kyanite, and spinel. The effects of cation diffusion and aggregation, as well as the redistribution of band intensity at temperature, are also discussed. The loss of absorption intensity in the visible and near-infrared regions of the spectrum of these phases may point to a more general behavior of IVCT in minerals at temperatures within the Earth with implications for radiative conductivity within the Earth.

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.

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.

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.

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.

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

We 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.

Exploitation of mixed-valency chemistry for designing a monolayer with double ferroelectricity and triferroic couplings.

Mixed-valence compounds possess both intriguing chemical and physical properties such as the intervalence charge transfer band and thus have been excellent model systems for the investigation of fundamental electron- and charge-transfer phenomena. Herein, we show that valence stratification can be a source of symmetry breaking and generating ferroelectricity in two-dimensional (2D) materials. We present ab initio computation evidence of the monolayer Cu2Cl3 structure with Cu ions being stratified into two separated layers of Cu(I) and Cu(II). Chemically, this unique monolayer not only entails lower formation energy than the bulk CuCl + CuCl2, but also enables the swapping of two valences through vertical ferroelectric switching, leading to a hitherto unreported chemical valencing phenomenon. Notably, the Jahn-Teller distortion of the Cu(II) layer results in another source of symmetry breaking and thus in-plane ferroelectricity. Apart from the valence swapping and self-contained double ferroelectricity, the monolayer's ferroelasticity is also coupled with in-plane ferroelectricity, while the monolayer's ferromagnetism is coupled with vertical polarization owing to the distinct magnetization of each Cu(I) and Cu(II) layer, thereby evoking the long-sought 2D triferroicity as well as triferroic couplings.

Design of anti-thermal quenching Pr3+-doped niobate phosphors based on a charge transfer and intervalence charge transfer band excitation-driven strategy

Based on the charge transfer and intervalence charge transfer mechanism, an excitation-driven strategy was employed to achieve single compensation and dual compensation anti-thermal quenching of 1D2 red-emitting in YNbO4:Pr3+ phosphor.

Modulation of Charge Distribution in Cobalt-α-Diimine Complexes toward Valence Tautomerism.

Valence tautomerism (VT) and spin crossover (SCO) are promising avenues for developing a range of molecular materials for sensing, memory, and optoelectronic applications. However, these phenomena arise only when specific metal-ligand combinations are employed. The underexplored combination of cobalt(II/III) paired with bis((aryl)imino)acenapthene (Ar-BIAN) ligands, which can exist as neutral Ar-BIAN0 (L0), monoanionic radical Ar-BIAN•- (L•-), and dianionic Ar-BIAN2- (L2-) forms, has potential to afford both VT and SCO. Aiming to develop a new family of switchable molecules, we systematically explored a dual-tuning approach by varying the redox state and aryl substituents in a series of homoleptic [Co(Ar-BIAN)3]n+ complexes (Ar = Ph, n = 2 (12+), 1 (1+), 0 (1); Ar = 3,5-CF3-Ph, n = 0 (2); Ar = 4-MeO-Ph, n = 2 (32+), 0 (3)). As a prelude to synthetic and experimental studies, density functional theory (DFT) calculations were used to explore the structure and relative energies of the different electronic forms of each complex, comprising different cobalt oxidation and spin states and different ligand oxidation states. Except for compound 3, DFT identified a HS-CoII-L0 containing ground state for all complexes, precluding thermally induced SCO or VT. For 3, calculations suggested a possible thermally accessible LS-CoIII-(L•-)3 ⇌ HS-CoII-(L•-)2(L0) VT interconversion. Experimentally, structural and magnetic data reveal a HS-CoII-L0 containing ground state for all six compounds in the solid state, including 3, discounting thermally induced VT or SCO. In solution, electrochemical and spectroscopic analysis also indicate that all compounds exist as the HS-CoII-L0-containing electromer at 298 K. Intervalence charge transfer (IVCT) bands observed for neutral 1, 2, and 3 at room temperature suggest the mixed-valence HS-CoII-(L•-)2(L0) charge distribution. However, cooling 3 to 243 K in acetonitrile uniquely affords a substantial reduction in the intensity of this IVCT band, consistent with thermally induced VT interconversion to the LS-CoIII-(L•-)3 ground state as predicted by DFT calculations. This study emphasizes the utility of computationally guided molecular design for complicated systems with redox activity at the metal and multiple ligands, thus opening new avenues for tuning electronic structure and developing new families of switchable molecules.

Temperature stimulated bandgap tailoring in the smart chromogenic WO30.33H2O

The orthorhombic form of WO30.33H2O is prepared hydrothermally and the thermochromic property of the material is identified using spectroradiometer with in a wavelength range of 325 nm to 1075 nm. The variation of reflectance with temperature is observed and it is accompanied with a change in bandgap. The absorbance relation with temperature is analyzed and the presence of Inter Valence Charge Transfer (IVCT) band is detected in the absorbance spectra. The IVCT bands illustrate the homovalent charge transfer mechanism behind the thermochromism taking place in the compound. The change in bandgap with temperature provides further supportive evidence to the thermochromic phenomenon observed in the compound.

Photoinduced Intervalence Charge Transfers: Spectroscopic Tools to Study Fundamental Phenomena and Applications.

The exploitation of excited state chemistry for solar energy conversion or photocatalysis has been continuously increasing, and the needs of a transition to a sustainable human development indicate this trend will continue. In this scenario, the study of mixed valence systems in the excited state offers a unique opportunity to explore excited state electron transfer reactivity, and, in a broader sense, excited state chemistry. This Concept article analyzes recent contributions in the field of photoinduced mixed valence systems, i. e. those where the mixed valence core is absent in the ground state but created upon light absorption. The focus is on the utilization of photoinduced intervalence charge transfer bands, detected via transient absorption spectroscopy, as key tools to study fundamental phenomena like donor/acceptor inversion, hole delocalization, coexistence of excited states and excited state nature, together with applications in molecular electronics.

Dinuclear cobalt complexes with an asymmetric biphenyl bridging ligand, [Co2(LFtBu)(bpqa)2](PF6)2 (H4LFtBu = 5-fluoro-5′-tert-butyl-3,3′,4,4′-tetrahydroxybiphenyl, bpqa = bis(2-pyridylmethyl)(2-quinolylmethyl)amine): Spectroscopic, electrochemical and magnetic characterization

Dinuclear cobalt complexes with an asymmetric biscatechol H4LFtBu (5-fluoro-5′-tert-butyl-3,3′,4,4′-tetrahydroxybiphenyl) ligand that has different substituent groups in both phenyl rings, and some ancillary tripod ligands were synthesized. The spectroscopic, electrochemical and magnetic properties of the complexes were compared to the electronic states of dinuclear cobalt complexes with a different ancillary tripod ligand. The result was that [Co2(LFtBu)(bpqa)2](PF6)2 (bpqa: bis(2-methylpyridyl)(2-quinolylmethyl)amine) was isolated with a mixed valence electronic state of lsCoIII-[Cat-Sq]-hsCoII (Cat: catechol, Sq: semiquinone), confirmed by the appearance of the inter-valence charge transfer (IVCT) band at around 1300 nm in the solid and by X-ray Photoelectron Spectroscopy (XPS). In this report, the effect of the substituent group in the biscatechol on the electronic states of such complexes is clarified from examination of their spectroscopic, electrochemical and magnetic properties.

Intense Photoinduced Intervalence Charge Transfer in High-Valent Iron Mixed Phenolate/Carbene Complexes.

We report high‐valent iron complexes supported by N‐heterocyclic carbene (NHC)‐anchored, bis‐phenolate pincer ligands that undergo ligand‐to‐metal charge transfer (LMCT) upon photoexcitation. The resulting excited states – with a lifetime in the picosecond range – feature a ligand‐based, mixed‐valence system and intense intervalence charge transfer bands in the near‐infrared region. Upon oxidation of the complex, corresponding intervalence charge transfer absorptions are also observed in the ground state. We suggest that the spectroscopic hallmarks of such LMCT states provide useful tools to decipher excited‐state decay mechanisms in high‐valent NHC complexes. Our observations further indicate that NHC‐anchored, bis‐phenolate pincer ligands are not sufficiently strong donors to prevent the population of excited metal‐centered states in high‐valent iron complexes.

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

Thermal Properties and Solvent Polarities of Mixed-Valence Ionic Liquids Containing Cationic Biferrocenylene Derivatives.

Ionic liquids (ILs) containing cationic mixed-valence biferrocenylene derivatives were synthesized with an octanoyl or octyl substituent in each cation. Their melting points ranged between 25 and 39 °C, and the octanoyl derivatives exhibited higher melting points than the octyl derivatives. In addition, each IL exhibited a glass transition in the temperature ranging from -66 to -45 °C after melting. Their melting points were ∼10 °C higher than those of mononuclear octamethylferrocenium salts bearing the same substituents. The solvent polarity (ETN) and Kamlet-Taft parameters (π*, α, and β) of these dinuclear and mononuclear ILs were then examined. The dinuclear ILs bearing octanoyl substituents exhibited significant increases in ETN and π* and a decrease in α with the decreasing temperature, whereas the other ILs exhibited a significantly less pronounced temperature dependence. Finally, the intervalence charge-transfer (or charge-resonance) bands of the octanoyl dinuclear ILs exhibited red shifts with the decreasing temperature, which can be regarded as self-thermosolvatochromism.

Charge and Spin Delocalization in Mixed-Valent Vinylruthenium–Triarylamine-Conjugates with Planarized Triarylamines

We report on three new vinylruthenium–triarylamine (TA) conjugates with planarized TA substituents. The neutral, mono-, and dioxidized forms of the complexes were scrutinized by UV/vis/NIR, IR, and EPR spectroscopies as well as by quantum chemical calculations. By analyzing the intervalence charge-transfer (IVCT) transitions of the mixed-valent radical cations, we obtain information on the extent of electronic coupling between the chemically different redox sites. Completely delocalized Ru-MeTA+ shows a vibrationally structured, non-solvatochromic IVCT band. Decreased blueshifts of the indicative Ru(CO) stretching vibration during the first oxidation and larger A(14N) EPR hyperfine splitting constants in concert with smaller g-values indicate enhanced TA contributions to the singly occupied molecular orbital of the other two complexes Ru-TOTA•+ and Ru-DOTA•+ and hence less symmetrical charge and spin density distributions. This is also reflected by the solvatochromism and the asymmetric shape of the IVCT band with a smaller bandwidth at the low-energy side. Temperature-dependent UV/vis/NIR spectroscopy of mixed-valent Ru-TOTA•+ and Ru-DOTA•+ revealed that the band skewing is due to the vibrational coupling of the IVCT transition to symmetrical vibrations, placing these radical cations near or at the class II/III borderline according to Robin and Day.

Exciton Coupling in Redox-Active Salen based Self-Assembled Metallacycles.

The incorporation of a redox-active nickel salen complex into supramolecular structures was explored via coordination-driven self-assembly with homobimetallic ruthenium complexes (bridged by oxalato or 5,8-dihydroxy-1,4-naphthoquinato ligands). The self-assembly resulted in the formation of a discrete rectangle using the oxalato complex and either a rectangle or a catenane employing the larger naphthoquinonato complex. The formation of the interlocked self-assembly was determined to be solvent and concentration dependent. The electronic structure and stability of the oxidized metallacycles was probed using electrochemical experiments, UV-Vis-NIR absorption, EPR spectroscopy and DFT calculations, confirming ligand radical formation. Exciton coupling of the intense near-infrared (NIR) ligand radical intervalence charge transfer (IVCT) bands provided further confirmation of the geometric and electronic structures in solution.