Are U-U Bonds Inside Fullerenes Really Unwilling Bonds?

Previous characterizations of diactinide endohedral metallofullerenes (EMFs) Th2@C80 and U2@C80 have shown that although the two Th3+ ions form a strong covalent bond within the carbon cage, the interaction between the U3+ ions is weaker and described as an "unwilling" bond. To evaluate the feasibility of covalent U-U bonds, which are neglected in classical actinide chemistry, we have first investigated the formation of smaller diuranium EMFs by laser ablation using mass spectrometric detection of dimetallic U2@C2n species with 2n ≥ 50. DFT, CASPT2 calculations, and MD simulations for several fullerenes of different sizes and symmetries showed that thanks to the formation of strong U(5f3)-U(5f3) triple bonds, two U3+ ions can be incarcerated inside the fullerene. The formation of U-U bonds competes with U-cage interactions that tend to separate the U ions, hindering the observation of short U-U distances in the crystalline structures of diuranium endofullerenes as in U2@C80. Smaller cages like C60 exhibit the two interactions, and a strong triple U-U bond with an effective bond order higher than 2 is observed. Although 5f-5f interactions are responsible for the covalent interactions at distances close to 2.5 Å, overlap between 7s6d orbitals is still detected above 4 Å. In general, metal ions within fullerenes should be regarded as templates in cage formation, not as statistically confined units that have little chance of being observed.

Comparison of Computational Strategies for the Calculation of the Electronic Coupling in Intermolecular Energy and Electron Transport Processes.

Electronic couplings in intermolecular electron and energy transfer processes calculated by six different existing computational techniques are compared to nonorthogonal configuration interaction for fragments (NOCI-F) results. The paper addresses the calculation of the electronic coupling in diketopyrrolopyrol, tetracene, 5,5'-difluoroindigo, and benzene-Cl for hole and electron transport, as well as the local exciton and singlet fission coupling. NOCI-F provides a rigorous computational scheme to calculate these couplings, but its computational cost is rather elevated. The here-considered ab initio Frenkel-Davydov (AIFD), Dimer projection (DIPRO), transition dipole moment coupling, Michl-Smith, effective Hamiltonian, and Mulliken-Hush approaches are computationally less demanding, and the comparison with the NOCI-F results shows that the NOCI-F results in the couplings for hole and electron transport are rather accurately predicted by the more approximate schemes but that the NOCI-F exciton transfer and singlet fission couplings are more difficult to reproduce.

Insights from Adsorption and Electron Modification Studies of Polyoxometalates on Surfaces for Molecular Memory Applications.

This Account highlights recent experimental and theoretical work focusing on the development of polyoxometalates (POMs) as possible active switching units in what may be called "molecule-based memory cells". Herein, we critically discuss how multiply charged vanadium-containing POMs, which exhibit stable metal-oxo bonds and are characterized by the excellent ability to change their redox states without significant structural distortions of the central polyoxoanion core, can be immobilized best and how they may work optimally at appropriate surfaces. Furthermore, we critically discuss important issues and challenges on the long way toward POM-based nanoelectronics. This Account is divided into four sections shedding light on POM interplay in solution and on surfaces, ion soft-landing of mass-selected POMs on surfaces, electronic modification of POMs on surfaces, and computational modeling of POMs on surfaces. The sections showcase the complex nature of far-reaching POM interactions with the chemical surroundings in solution and the properties of POMs in the macroscopic environment of electrode surfaces. Section 2 describes complex relationships of POMs with their counter-cations, solvent molecules, and water impurities, which have been shown to exhibit a direct impact on the resulting surface morphology, where a concentration-dependent formation of micellar structures can be potentially observed. Section 3 gives insights into the ion soft-landing deposition of mass-selected POMs on electrode surfaces, which emerges as an appealing method because the simultaneous deposition of agglomeration-stimulating counter-cations can be avoided. Section 4 provides details of electronic properties of POMs and their modification by external electronic stimuli toward the development of multiple-state resistive (memristive) switches. Section 5 sheds light on issues of the determination of the electronic structure properties of POMs across their interfaces, which is difficult to address by experiment. The studies summarized in these four sections have employed various X-ray-scattering, microscopy, spectroscopy, and computational techniques for imaging of POM interfaces in solution and on surfaces to determine the adsorption type, agglomeration tendency, distribution, and oxidation state of deposited molecules. The presented research findings and conceptual ideas may assist experimentalists and theoreticians to advance the exploration of POM electrical conductivity as a function of metal redox and spin states and to pave the way for a realization of ("brain-inspired") POM-based memory devices, memristive POM-surface device engineering, and energy efficient nonvolatile data storage and processing technologies.

Quantum dynamics simulations of the thermal and light-induced high-spin to low-spin relaxation in Fe(bpy)3 and Fe(mtz)6.

First row transition metal complexes with d4 to d7 electronic configurations exhibit spin-crossover (SCO), which can be induced by external stimuli, such as temperature, pressure and light. The low-spin to high-spin transition has been widely studied, but very little is known about the reverse process. Here, we present a theoretical study of thermal and light-induced high-to-low spin crossover in prototypical Fe(II) complexes. The lifetime of the high-spin state in the thermal process is determined using Fermi's golden rule. With this methodology, we have accurately computed the transfer rate of the HS state thermal relaxation at several time scales (from sub-nanosecond to a few seconds) in two different iron complexes. The use of quasi-degenerate perturbation theory (QDPT2) in the analysis of the LS-HS spin-orbit coupling has allowed us to identify 3T1 as the main intermediate state coupling the LS and HS states. The light-induced process has been studied using wavepacket quantum dynamics along the main vibrational coordinates (one symmetric and two asymmetric Fe-N stretchings). The study suggests that after the initial excitation from the 5T2g to the 5Eg state, the population is transferred back to a vibrationally hot 5T2g state, from which a small amount of the population is transferred to the 1A1g state via the intermediate 3T1g. Most of the population remains trapped in the HS state at the time scale of the simulation.

Molecular Transition Metal Oxide Electrocatalysts for the Reversible Carbon Dioxide-Carbon Monoxide Transformation.

Carbon monoxide dehydrogenase (CODH) enzymes are active for the reversible CO oxidation-CO2 reduction reaction and are of interest in the context of CO2 abatement and carbon-neutral solar fuels. Bioinspired by the active-site composition of the CODHs, polyoxometalates triply substituted with first-row transition metals were modularly synthesized. The polyanions, in short, {SiM3 W9 } and {SiM'2 M''W9 }, M, M', M''=CuII , NiII , FeIII are shown to be electrocatalysts for reversible CO oxidation-CO2 reduction. A catalytic Tafel plot showed that {SiCu3 W9 } was the most reactive for CO2 reduction, and electrolysis reactions yielded significant amounts of CO with 98 % faradaic efficiency. In contrast, Fe-Ni compounds such as {SiFeNi2 W9 } preferably catalyzed the oxidation of CO to CO2 similar to what is observed for the [NiFe]-CODH enzyme. Compositional control of the heterometal complexes, now and in the future, leads to control of reactivity and selectivity for CO2 electrocatalytic reduction.

Ultrafast Intersystem Crossing in Xanthone from Wavepacket Dynamics.

Most aromatic ketones containing first-row elements undergo unexpectedly fast intersystem crossing in a few tens of picoseconds and a quantum yield close to unity. Among them, xanthone (9H-xanthen-9-one) possesses one of the fastest singlet-triplet rates of only ∼1.5 ps. The exact mechanism of this unusually fast transition is still under debate. Here, we perform wavepacket dynamics of the photochemistry of xanthone in the gas phase and in polar solvents. We show that xanthone follows El-Sayed's rule for intersystem crossing. From the second singlet excited state, the mechanism is sequential: (i) an internal conversion between singlets 1ππ* → 1nπ* (85 fs), (ii) an intersystem crossing 1nπ* → 3ππ* (2.0 ps), and (iii) an internal conversion between triplets 3ππ* → 3nπ* (602 fs). Each transfer finds its origin in a barrierless access to electronic state intersections. These intersections are close to minimum energy structures, allowing for efficient transitions from the initial singlet state to the triplets.

Influence of the crystal packing in singlet fission: one step beyond the gas phase approximation.

Singlet fission (SF), a multiexciton generation process, has been proposed as an alternative to enhance the performance of solar cells. The gas phase dimer model has shown its utility to study this process, but it does not always cover all the physics and the effect of the surrounding atoms has to be included in such cases. In this contribution, we explore the influence of crystal packing on the electronic couplings, and on the so-called exciton descriptors and electron-hole correlation plots. We have studied three tetracene dimers extracted from the crystal structure, as well as several dimers and trimers of the α and ß polymorphs of 1,3-diphenylisobenzofuran (DPBF). These polymorphs show different SF yields. Our results highlight that the character of the excited states of tetracene depends on both the mutual disposition of molecules and inclusion of the environment. The latter does however not change significantly the interpretation of the SF mechanism in the studied systems. For DPBF, we establish how the excited state analysis is able to pinpoint differences between the polymorphs. We observe strongly bound correlated excitons in the ß polymorph which might hinder the formation of the 1TT state and, consequently, explain its low SF yield.

Gating the conductance of extended metal atom chains: a computational analysis of Ru3(dpa)4(NCS)2 and [Ru3(npa)4(NCS)2].

The effects of a gate potential on the conductance of two members of the EMAC family, Ru3(dpa)4(NCS)2 and its asymmetric analogue, [Ru3(npa)4(NCS)2]+, are explored with a density functional approach combined with non-equilibrium Green's functions. From a computational perspective, the inclusion of an electrochemical gate potential represents a significant challenge because the periodic treatment of the electrode surface resists the formation of charged species. However, it is possible to mimic the effects of the electrochemical gate by including a very electropositive or electronegative atom in the unit cell that will effectively reduce or oxidize the molecule under study. In this contribution we compare this approach to the more conventional application of a solid-state gate potential, and show that both generate broadly comparable results. For two extended metal atom chain (EMAC) compounds, Ru3(dpa)4(NCS)2 and [Ru3(npa)4(NCS)2], we show that the presence of a gate potential shifts the molecular energy levels in a predictable way relative to the Fermi level, with distinct peaks in the conductance trace emerging as these levels enter the bias window.

Photoinduced Mo-CN Bond Breakage in Octacyanomolybdate Leading to Spin Triplet Trapping.

The photoinduced properties of the octacoordinated complex K4 MoIV (CN)8 ⋅2 H2 O were studied by theoretical calculations, crystallography, and optical and magnetic measurements. The crystal structure recorded at 10 K after blue light irradiation reveals an heptacoordinated Mo(CN)7 species originating from the light-induced cleavage of one Mo-CN bond, concomitant with the photoinduced formation of a paramagnetic signal. When this complex is heated to 70 K, it returns to its original diamagnetic ground state, demonstrating full reversibility. The photomagnetic properties show a partial conversion into a triplet state possessing significant magnetic anisotropy, which is in agreement with theoretical studies. Inspired by these results, we isolated the new compound [K(crypt-222)]3 [MoIV (CN)7 ]⋅3 CH3 CN using a photochemical pathway, confirming that photodissociation leads to a stable heptacyanomolybdate(IV) species in solution.

Deactivation of Excited States in Transition-Metal Complexes: Insight from Computational Chemistry.

Investigation of the excited-state decay dynamics of transition-metal systems is a crucial step for the development of photoswitchable molecular based materials with applications in growing fields as energy conversion, data storage, or molecular devices. The photophysics of these systems is an entangled problem arising from the interplay of electronic and geometrical rearrangements that take place on a short time scale. Several factors play a role in the process: various electronic states of different spin and chemical character are involved, the system undergoes important structural variations and several nonradiative processes can occur. Computational chemistry is a useful tool to get insight into the microscopic description of the photophysics of these materials, since it provides unique information about the character of the electronic spin states involved, the energetics and time evolution of the system. In this review article, we present an overview of the state of the art methodologies available to address the several aspects that have to be incorporated to properly describe the deactivation of excited states in transition-metal complexes. The most recent developments in theoretical methods are discussed and illustrated with examples.

How Does the Redox State of Polyoxovanadates Influence the Collective Behavior in Solution? A Case Study with [I@V18O42] q- ( q = 3, 5, 7, 11, and 13).

A series of stable reduction-oxidation states of the cagelike [I@VIV xVV18- xO42]5- x polyoxovanadate (POV) with x = 8, 10, 12, 16, and 18 were studied with density functional theory and molecular dynamics to gain insight into the structural and electron distribution characteristics of these metal-oxo clusters and to analyze the charge/redox-dependent assemblage processes in water and acetonitrile (MeCN) solutions. The calculations show that the interplay between the POV redox state (molecular charge) and the solvent polarity, countercation size, and hydrophilicity (or hydrophobicity) controls the POV agglomeration phenomena, which substantially differ between aqueous and MeCN media. In MeCN, agglomeration is more pronounced for intermediate-charged POVs, whereas in water, the lowest-charged POVs and organic countercations tend to agglomerate into a microphase. Tests made on wet MeCN show diminished agglomeration with respect to pure MeCN. Simulations with alkali countercations in water show that only the highest-charged POV can form agglomerates. The herein presented theoretical investigation aims to support experimental studies of POVs in the field of functional nanomaterials and surfaces, where controlled molecular deposition from the liquid phase onto solid substrates requires knowledge about the features of these metal-oxo clusters in discrete solutions.

Theoretical studies on the energy structures and optical properties of copper cysteamine - a novel sensitizer.

Copper cysteamine (Cu-Cy) is a new type of photosensitizer, which can be activated not only by ultraviolet light, but also by X-rays, microwaves and ultrasound to generate reactive oxygen species for treating cancer and infection diseases. Moreover, copper cysteamine has a strong luminescence, which can be used for both therapeutics and imaging. In addition, it can also be used for solid state lighting, radiation detection and sensing. However, its electronic structures, and particularly its excited states, are not yet clear. Here, we present a computational study aiming to determine the nature of the excited states involved in the photophysical processes that lead to the luminescence of this compound. This study has been conducted using density functional theory (DFT), using both hybrid functionals and time-dependent DFT. It is found that both absorption and emission involve the replacement of an electron among the 3d and 4s orbitals of one or the other of the two types of Cu atoms found in the system. Our computed results compared well with the experimental absorption and emission results. These results are very helpful for the understanding of the experimental observations.

Trends in the Bond Multiplicity of Cr2, Cr3, and Cr2M (M = Zn, Ni, Fe, Mn) Complexes Extracted from Multiconfigurational Wave Functions.

Extended metal atom chains constitute an interesting class of molecules from both theoretical and applied points of view. In the chromium-based series Cr2M(dpa)4X2 (with M = Zn, Ni, Fe, Mn, Cr), the direct metal-metal interactions span a wide range of possibilities and so do their associated properties. The multiplicity and symmetry components of the metal-metal bond are herein analyzed via the effective bond order (EBO) concept using complete active space self-consistent field wave functions and compared with similar bimetallic Cr2L4X2 systems. The bond multiplicity follows a trend dominated by the Cr-Cr distance which, in turn, depends on the nature of the axial ligand (X). Cr2M compounds present asymmetric structures with virtually no interaction between the Cr2 unit and M, whereas fully symmetric structures with delocalized bonding among the three metals are also possible in Cr3 complexes. In such cases, a strategy that involves localization of the molecular orbitals into each Cr-Cr pair is applied to quantify the contribution of each pair to the overall metal-metal bond multiplicity.

Addressing Multiple Resistive States of Polyoxovanadates: Conductivity as a Function of Individual Molecular Redox States.

The sustainable development of IT-systems requires a quest for novel concepts to address further miniaturization, performance improvement, and energy efficiency of devices. The realization of these goals cannot be achieved without an appropriate functional material. Herein, we target the technologically important electron modification using single polyoxometalate (POM) molecules envisaged as smart successors of materials that are implemented in today's complementary metal-oxide-semiconductor (CMOS) technology. Lindqvist-type POMs were physisorbed on the Au(111) surface, preserving their structural and electronic characteristics. By applying an external voltage at room temperature, the valence state of the single POM molecule could be changed multiple times through the injection of up to 4 electrons. The molecular electrical conductivity is dependent on the number of vanadium 3d electrons, resulting in several discrete conduction states with increasing conductivity. This fundamentally important finding illustrates the far-reaching opportunities for POM molecules in the area of multiple-state resistive (memristive) switching.

U2@ I h(7)-C80: Crystallographic Characterization of a Long-Sought Dimetallic Actinide Endohedral Fullerene.

The nature of actinide-actinide bonds has attracted considerable attention for a long time, especially since recent theoretical studies suggest that triple and up to quintuple bonds should be possible, but little is known experimentally. Actinide-actinide bonds inside fullerene cages have also been proposed, but their existence has been debated intensively by theoreticians. Despite all the theoretical arguments, critical experimental data for a dimetallic actinide endohedral fullerene have never been obtained. Herein, we report the synthesis and isolation of a dimetallic actinide endohedral metallofullerene (EMF), U2@C80. This compound was fully characterized by mass spectrometry, single crystal X-ray crystallography, UV-vis-NIR spectroscopy, Raman spectroscopy, cyclic voltammetry, and X-ray absorption spectroscopy (XAS). The single crystal X-ray crystallographic analysis unambiguously assigned the molecular structure to U2@ I h(7)-C80. In particular, the crystallographic data revealed that the U-U distance is within the range of 3.46-3.79 Å, which is shorter than the 3.9 Å previously predicted for an elongated weak U-U bond inside the C80 cage. The XAS results reveal that the formal charge of the U atoms trapped inside the fullerene cage is +3, which agrees with the computational and crystallographic studies that assign a hexaanionic carbon cage, ( I h-C80)6-. Theoretical studies confirm the presence of a U-U bonding interaction and suggest that the weak U-U bond in U2@ I h(7)-C80 is strengthened upon reduction and weakened upon oxidation. The comprehensive characterization of U2@ I h(7)-C80 and the overall agreement between the experimental data and theoretical investigations provide experimental proof and deeper understanding for actinide metal-metal bonding interactions inside a fullerene cage.

Effect of Second-Order Spin-Orbit Coupling on the Interaction between Spin States in Spin-Crossover Systems.

The second-order spin-orbit coupling is evaluated in two transition-metal complexes to establish the effect on the deactivation mechanism of the excited low-spin state in systems that undergo spin transitions under the influence of light. We compare the standard perturbational approach to calculate the second-order interaction with a variational strategy based on the effective Hamiltonian theory and show that the former one can only be applied in some special cases and even then gives results that largely overestimate the interaction. The combined effect of geometry distortions and second-order spin-orbit coupling leads to sizeable interactions for states that are nearly uncoupled in the symmetric (average) structure of the complex. This opens the possibility of a direct deactivation from the singlet and triplet states of the metal-to-ligand charge-transfer manifold to the final high-spin state as suggested from the interpretation of experimental data but so far not supported by theoretical descriptions of the light-induced spin crossover.

Theoretical evidence for the direct 3MLCT-HS deactivation in the light-induced spin crossover of Fe(ii)-polypyridyl complexes.

Spin-orbit couplings have been calculated in twenty snapshots of a molecular dynamics trajectory of [Fe(bpy)3]2+ to address the importance of geometrical distortions and second-order spin-orbit coupling on the intersystem crossing rate constants in the light-induced spin crossover process. It was found that the effective spin-orbit coupling between the 3MLCT and 5T2 state is much larger than the direct coupling in the symmetric structure, which opens the possibility of a direct 3MLCT-5T2 deactivation without the intervention of triplet metal-centered states. Based on the calculated deactivation times, we conclude that both the direct pathway and the one involving intermediate triplet states are active in the ultrafast population of the metastable HS state, bringing in agreement two experimental observations that advocate for either deactivation mechanism. This resolves a long-standing dispute about the deactivation mechanism of Fe(ii)-polypyridyl complexes in particular, and about light-induced magnetism in transition metal complexes in general.

The magnetic fingerprint of dithiazolyl-based molecule magnets.

Magnetic bistability in organic-radical based materials has attracted significant interest due to its potential application in electronic devices. The first-principles bottom-up study herein presented aims at elucidating the key factors behind the different magnetic response of the low and high temperature phases of four different switchable dithiazolyl (DTA)-based compounds. The drastic change in the magnetic response upon spin transition is always due to the changes in the JAB magnetic interactions between adjacent radicals along the π-stacks of the crystal, which in turn are driven mostly by the changes in the interplanar distance and degree of lateral slippage, according to the interpretation of a series of magneto-structural correlation maps. Furthermore, specific geometrical dispositions have been recognized as a ferromagnetic fingerprint in such correlations. Our results thus show that an appropriate substitution of the chemical skeleton attached to the DTA ring could give rise to new organic materials with dominant ferromagnetic interactions.

Decoding the role of encapsulated ions in the electronic and magnetic properties of mixed-valence polyoxovanadate capsules {X@V22O54} (X = ClO4-, SCN-, VO2F2-): a combined theoretical approach.

The electronic structure and magnetism of mixed-valence, host-guest polyoxovanadates {X@VO54} with diamagnetic (X =) ClO4- (Td, 1) and SCN- (C∞v, 2) template anions are assessed by means of two theoretical methods: density functional theory and effective Hamiltonian calculations. The results are compared to those obtained for another member of this family with X = VO2F2- (C2v, 3) (see P. Kozlowski et al., Phys. Chem. Chem. Phys., 2017, 19, 29767-29771), for which complementary data are also acquired. It is demonstrated that the X guest anions strongly influence the electronic and magnetic properties of the system, leading to various valence states of vanadium and modifying V-O-V exchange interactions. Our findings are concordant with and elucidate the available experimental data (see K. Y. Monakhov et al., Chem. - Eur. J., 2015, 21, 2387-2397).

Quantum Chemical Study of the Interligand Electron Transfer in Ru Polypyridyl Complexes.

Quantum chemical calculations have been performed to study the photocycle of [Ru(bpy)3]2+, a complex that is extensively used as an electron donor in photocatalytic reactions. After the initial spin-allowed excitation from the nonmagnetic ground state to a singlet state of metal-to-ligand charge transfer character, the system undergoes a rapid intersystem crossing to a triplet state of equal character. The calculations indicate a lifetime of 10 fs, in good agreement with experimental estimates. Important factors for this extremely fast intersystem crossing are the large spin-orbit coupling and the large vibrational overlap of the states involved. Both MLCT states are delocalized over the three bipyridine ligands, but the delocalized electron can easily increase its degree of localization. The hopping parameters have been calculated and found to be large for the localization on two ligands and subsequently on one. The combination of localization and geometry relaxation creates a rather long-lived trapped triplet MLCT state with a calculated lifetime of 9 µs. The addition of methyl groups on the bipyridine ligands decreases the ligand field and consequently lowers the metal-centered triplet states. This could eventually lead to opening of a fast deactivation channel of the 3MLCT states to the initial nonmagnetic states via the triplet ligand field states as occurs in the corresponding Fe(II) complex.