Correction: Computational demonstration of isomer- and spin-state-dependent charge transport in molecular junctions composed of charge-neutral iron(II) spin-crossover complexes.

Correction for 'Computational demonstration of isomer- and spin-state-dependent charge transport in molecular junctions composed of charge-neutral iron(II) spin-crossover complexes' by Nicolás Montenegro-Pohlhammer, et al., Dalton Trans., 2023, 52, 1229-1240, https://doi.org/10.1039/D2DT02598A.

Voltage-induced modulation of the magnetic exchange in binuclear Fe(III) complex deposited on Au(111) surface.

We present a complete computational study devoted to the deposition of a magnetic binuclear complex on a metallic surface, aimed to obtain insight into the interaction of magnetically coupled complexes with their supporting substrates, as well as their response to external electrical stimuli applied through a surface-molecule-STM molecular junction-like architecture. Our results not only show that the deposition is favorable in two of the four studied orientations, but also, that the magnetic coupling is only slightly perturbed once the complex is adsorbed. We observe that the effects of the applied bias voltage on the magnetic coupling strongly depend on the molecule orientation with respect to the surface and the voltage polarity. Further analysis shows that this behavior is attributable to the stabilization/destabilization of the d-type singly occupied orbitals of the iron centers, reinforced by the strong local electric fields and induced charge densities only present in certain orientations of the deposited molecule and applied voltage polarity.

Dinitrogen Activation Mediated by the (P2PPh)Fe Complex: Electronic Structure, Dimerization Mechanism, and Magnetic Coupling.

Herein, we report the estimation of the extent of dinitrogen activation by different charged and structural forms of (P2PPh)Fe biomimetic catalysts, which, in the presence of light, exhibit significant yield in the N2-to-NH3 conversion. Complete active space self-consistent field (CASSCF) calculations have been used to determine the electronic structure of different reduced forms of the mononuclear complexes: the neutral (P2PPh)Fe(N2)2 adduct and the anionic [(P2PPh)Fe(N2)]- and [(P2PPh)Fe(N2)]2- complexes. These calculations also revealed that the extent of reduction of a dinitrogen molecule reaches up to one electron (N21-) due to the back-bonding from the Fe center, in agreement with the changes observed in the vibration frequency of the N-N bond in these complexes. In addition, the energy profile of the dimerization of the mononuclear (P2PPh)Fe(N2)2 complex to the dinuclear mono-N2-bridged [(P2PPh)Fe]2(µ-N2) complex has been determined by means of density functional theory (DFT) calculations. A three-step mechanism has been proposed for the dimerization, favored by both kinetics and thermodynamics criteria. Finally, the magnetic coupling constant in the diiron (µ-N2) complex is estimated from CASSCF/NEVPT2 calculations. Such a dinuclear complex presents a strong antiferromagnetic coupling resulting from the interaction between two S = 1 d6 Fe2+ ions, bridged by a highly activated dinitrogen molecule (N22-) with two electrons on the π* orbitals.

How complex-surface interactions modulate the spin transition of Fe(II) SCO complexes supported on metallic surfaces?

The deposition of a prototypical spin-crossover [Fe(phen)2(NCS)2] complex on Au(111), Cu(111) and Ag(111) surfaces has been investigated by means of periodic DFT+U calculations, with the aim of understanding how different metallic surfaces affect the spin state switching. Our results show that adsorption is metal- and spin-dependent, with different preferred adsorption sites for the different surfaces and spin states. For the three considered surfaces adsorption energies are larger in the LS state than in the HS one, which increases the transition enthalpy by 58.7 kJ mol-1 for Cu(111), 14.6 kJ mol-1 for Au(111) and 9.6 kJ mol-1 for Ag(111) with respect to the free molecule. There is a clear correlation between this effect and the extent of the complex-surface interaction, which can be established from adsorption energies, surface-complex distances and charge density difference plots as: Cu(111) > Au(111) > Ag(111). Therefore, a stronger interaction with the surface produces a larger energy difference between two spin states, making the spin transition less probable to occur. Finally, our calculations show that it would be possible to probe the spin-state of the deposited molecules from the STM images, in line with the recent experimental results.

Computational demonstration of isomer- and spin-state-dependent charge transport in molecular junctions composed of charge-neutral iron(II) spin-crossover complexes.

Chemistry offers a multitude of opportunities towards harnessing functional molecular materials with application propensity. One emerging area of interest is molecular spintronics, in which charge and spin degrees of freedom have been used to achieve power-efficient device architectures. Herein, we show that, with the aid of state-of-the-art quantum chemical calculations on designer molecular junctions, the conductance and spin filtering capabilities are molecular structure-dependent. As inferred from the calculations, structural control over the transport can be achieved by changing the position of the thiomethyl (SMe) anchoring groups for Au(111) electrodes in a set of isomeric 2,2'-bipyridine-based metal coordinating ligand entities L1 and L2. The computational studies on heteroleptic iron(II) coordination complexes (1 and 2) composed of L1 and L2 reveal that switching the spin-state of the iron(II) centers, from the low-spin (LS) to high-spin (HS) state, by means of an external electric field stimulus, could, in theory, be performed. Such switching, known as spin-crossover (SCO), renders charge transport through single-molecule junctions of 1 and 2 spin-state-dependent, and the HS junctions are more conductive than the LS junctions for both complexes. Additionally, the LS and HS junctions based on complex 1 are more conductive than those featuring complex 2. Moreover, it is predicted that the spin filtering efficiency (SFE) of the HS junctions strongly depends on the bridging complex geometry, with 1 showing a voltage-dependent SFE, whereas 2 exhibits an SFE of practically 100% over all the studied voltage range. To be pragmatic towards applications, the ligands L1 and L2 and complex 1 have been successfully synthesized, and the spin-state switching propensity of 1 in the bulk state has been elucidated. The results shown in this study might lead to the synthesis and characterization of isomeric SCO complexes with tuneable spin-state switching and charge transport properties.

Spin-crossover complexes in nanoscale devices: main ingredients of the molecule-substrate interactions.

Spin-crossover complexes embedded in nanodevices experience effects that are absent in the bulk that can modulate, quench and even suppress the spin-transition. In this work we explore, by means of state-of-the-art quantum chemistry calculations, different aspects of the integration of SCO molecules on active nanodevices, such as the geometry and energetics of the interaction with the substrate, extension of the charge transfer between the substrate and SCO molecule, impact of the applied external electric field on the spin-transition, and sensitivity of the transport properties on the local conditions of the substrate. We focus on the recently reported encapsulation of Fe(II) spin-crossover complexes in single-walled carbon nanotubes, with new measurements that support the theoretical findings. Even so our results could be useful to many other systems where SCO phenomena take place at the nanoscale, the spin-state switching is probed by an external electric field or current, or the substrate is responsible for the quenching of the SCO mechanism.

A photo-induced spin crossover based molecular switch and spin filter operating at room temperature.

Since Venkataramani et al. (Science, 2011, 331(6016), 445-448) reported reversible, room-temperature light-induced spin crossover in Ni-porphyrin functionalized with a phenylazopyridine ligand (NiTPP-PAPy), this complex has attracted the attention of many researchers due to its potential applications in molecular-based devices. In this work, we perform a detailed study, by means of DFT and WFT methodologies, focused on the deposition of NiTP-PAPy over an Au(111) surface, followed by DFT-NEGF calculations employing a gold surface and the tip of an STM as electrodes, in order to probe the deposited complex's transport properties. Our DFT calculations show that not only the metalled porphyrin is strongly adsorbed on the surface, in both the high (HS) and low spin (LS) configurations, but also, and more importantly, photoinduced switching is preserved upon adsorption, a fact that is also confirmed through WFT and TD-DFT calculations. Moreover, our DFT-NEGF calculations indicate that the current passing through the molecular junction-like systems is much higher in the HS configuration than in the LS one, along with the fact that the current calculated in the ferromagnetic junction is highly spin-polarized. These remarkable transport properties suggest that the complex could be used as a component in molecular switches based on the total current passing through the system, modulated by light irradiation, spin filters due to the spin polarization of the carriers in the HS configuration, or even in two-step rectifiers combining the two features mentioned above, all of these operating at room temperature, giving to this complex the potential to be an active element in all kinds of future spintronic devices.

Magnetostructural relationships in [Ni(dmit)2]- radical anions.

This work explores the relationship between the magnetic properties of salts based on [Ni(dmit)2]- radicals and different arrangements that these radicals can adopt in the crystals, induced by the packing constrains imposed by the counterions. Our analysis is based on difference dedicated configuration interaction calculations on models containing two neighbouring [Ni(dmit)2]- units with different interaction patterns. The amplitude and sign of these through-space interactions can be rationalized on the basis of a valence-only model that essentially analyzes the effective interactions between the atoms carrying the electronic density of singly occupied orbitals (SOMOs). Despite the simplicity of the model, it provides simple rules to predict the nature and the expected amplitude (strong/medium/weak) of the leading interactions in systems based on these [Ni(dmit)2]- radicals.

Spin-state-dependent electrical conductivity in single-walled carbon nanotubes encapsulating spin-crossover molecules.

Spin crossover (SCO) molecules are promising nanoscale magnetic switches due to their ability to modify their spin state under several stimuli. However, SCO systems face several bottlenecks when downscaling into nanoscale spintronic devices: their instability at the nanoscale, their insulating character and the lack of control when positioning nanocrystals in nanodevices. Here we show the encapsulation of robust Fe-based SCO molecules within the 1D cavities of single-walled carbon nanotubes (SWCNT). We find that the SCO mechanism endures encapsulation and positioning of individual heterostructures in nanoscale transistors. The SCO switch in the guest molecules triggers a large conductance bistability through the host SWCNT. Moreover, the SCO transition shifts to higher temperatures and displays hysteresis cycles, and thus memory effect, not present in crystalline samples. Our results demonstrate how encapsulation in SWCNTs provides the backbone for the readout and positioning of SCO molecules into nanodevices, and can also help to tune their magnetic properties at the nanoscale.

Theoretical inspection of the spin-crossover [Fe(tzpy)2(NCS)2] complex on Au(100) surface.

We explore the deposition of the spin-crossover [Fe(tzpy)2(NCS)2] complex on the Au(100) surface by means of density functional theory (DFT) based calculations. Two different routes have been employed: low-cost finite cluster-based calculations, where both the Fe complex and the surface are maintained fixed while the molecule approaches the surface; and periodic DFT plane-wave calculations, where the surface is represented by a four-layer slab and both the molecule and surface are relaxed. Our results show that the bridge adsorption site is preferred over the on-top and fourfold hollow ones for both spin states, although they are energetically close. The LS molecule is stabilized by the surface, and the HS-LS energy difference is enhanced by about 15%-25% once deposited. The different Fe ligand field for LS and HS molecules manifests on the composition and energy of the low-lying bands. Our simulated STM images indicate that it is possible to distinguish the spin state of the deposited molecules by tuning the bias voltage of the STM tip. Finally, it should be noted that the use of a reduced size cluster to simulate the Au(100) surface proves to be a low-cost and reliable strategy, providing results in good agreement with those resulting from state-of-the-art periodic calculations for this system.

Deposition of the Spin Crossover FeII -Pyrazolylborate Complex on Au(111) Surface at the Molecular Level.

The interaction at the molecular level of the spin-crossover (SCO) FeII ((3,5-(CH3 )2 Pz)3 BH)2 complex with the Au(111) surface is analyzed by means of rPBE periodic calculations. Our results show that the adsorption on the metallic surface enhances the transition energy, increasing the relative stability of the low spin (LS) state. The interaction indeed is spin-dependent, stronger for the low spin than the high spin (HS) state. The different strength of the Fe ligand field at low and high temperature manifests on the nature, spatial extension and relative energy of the states close to the Fermi level, with a larger metal-ligand hybridization in the LS state. This feature is of relevance for the differential adsorption of the LS and HS molecules, the spin-dependent conductance, and for the differences found in the corresponding STM images, correctly reproduced from the density of states provided by the rPBE calculations. It is expected that this spin dependence will be a general feature of the SCO molecule-substrate interaction, since it is rooted in the different ligand field of Fe site at low and high temperatures, a common hallmark of the FeII SCO complexes. Finally, the states involved in the LIESST phenomenon has been identified through NEVPT2 calculations on a model reaction path. A tentative pathway for the photoinduced LS→HS transition is proposed, that does not involve the intermediate triplet states, and nicely reproduces both the blue laser wavelength required for the activation, and the wavelength of the reverse HS → LS transition.

Theoretical study of the photoconduction and photomagnetism of the BPY[Ni(dmit)2]2 molecular crystal.

The BPY[Ni(dmit)2]2 molecular crystal synthesized by Naito and coworkers (J. Am. Chem. Soc., 2012, 134, 18656) was characterized as a photo-magnetic-conductor. This crystal is a nonmagnetic semiconductor in the dark and becomes a magnetic conductor after UV irradiation. This work analyzes the key ingredients of the observed photomagnetism and photoconduction by means of wavefunction-based calculations on selected fragments and periodic calculations on the whole crystal. Our results demonstrate that UV-Vis light induces charge transfer processes between the closest [Ni(dmit)2]- and BPY2+ units, that introduce unpaired electrons on the unoccupied orbitals of the BPY cations. Since the conduction bands present a strong mixing of BPY and Ni(dmit)2, the optically activated anion-cation charge transfer enhances the conductivity. The photoinduced (BPY2+)* radicals can indeed interact with the close Ni(dmit)2 units, with non-negligible spin-spin magnetic couplings, which are responsible for the changes induced by the irradiation on the temperature dependence of the magnetic susceptibility.

Light-Induced Control of the Spin Distribution on Cu⁻Dithiolene Complexes: A Correlated Ab Initio Study.

Metal dithiolene complexes-M(dmit)2-are key building blocks for magnetic, conducting, and optical molecular materials, with singular electronic structures resulting from the mixing of the metal and dmit ligand orbitals. Their use in the design of magnetic and conducting materials is linked to the control of the unpaired electrons and their localized/delocalized nature. It has been recently found that UV⁻Vis light can control the spin distribution of some [Cu(dmit)2]-2 salts in a direct and reversible way. In this work, we study the optical response of these salts and the origin of the differences observed in the EPR spectra under UV⁻Vis irradiation by means of wave function-based quantum chemistry methods. The low-lying states of the complex have been characterized and the electronic transitions with a non-negligible oscillator strength have been identified. The population of the corresponding excited states promoted by the UV⁻Vis absorption produces significant changes in the spin distribution, and could explain the changes observed in the system upon illumination. The interaction between neighbor [Cu(dmit)2]-2 complexes is weakly ferromagnetic, consistent with the relative orientation of the magnetic orbitals and the crystal packing, but in disagreement with previous assignments. Our results put in evidence the complex electronic structure of the [Cu(dmit)2]-2 radical and the relevance of a multideterminantal approach for an adequate analysis of their properties.

Light-Induced Spin Transitions in Copper-Nitroxide-Based Switchable Molecular Magnets: Insights from Periodic DFT+U Calculations.

The electronic structure and magnetic interactions of three members of the breathing crystal Cu(hfac)2 LR family (hfac=hexafluoroacetylacetonato, LR =pyrazole-substituted nitronyl nitroxides with R=Me, Et, Pr, iPr, Bu ), mainly Cu(hfac)2 LPr (1), Cu(hfac)2 LBu ⋅0.5 C8 H18 (2) and Cu(hfac)2 LBu ⋅0.5 C8 H10 (3), have been analyzed by means of periodic plane-wave based DFT+U calculations. These CuII -nitroxide based molecular magnets display thermally and optically induced switchable behavior and light-induced excited spin state trapping phenomena. The calculations confirm the presence of temperature-dependent exchange interaction within the spin triads formed by the nitroxide-copper(II)-nitroxide units, in line with the changes observed in the effective magnetic moment. Moreover, they quantify the interchain interaction mediated by the terminal nitroxide group of two spin triads in neighboring polymer chains. This interaction competes with the exchange interaction within the spin triads at high temperature, and introduces 1D exchange channels that do not coincide with the polymeric chains. The density of states reveal that the low-lying conduction states potentially involved in the UV/Vis transitions are located on the nitroxide radicals, the hfac groups and the Cu atoms. Then, the density of states is almost independent of the solvent and the R group. This suggests the possibility of light-induced spin switching for other members of this family. The 500 nm band of the low-temperature phase can be ascribed to a ligand-to-metal charge transfer transition between the nitroxide and Cu bands.

Electronic Structure and Magnetic Interactions in the Radical Salt [BEDT-TTF]2[CuCl4].

The magnetic behavior and electric properties of the hybrid radical salt [BEDT-TTF]2[CuCl4] have been revisited through extended experimental analyses and DDCI and periodic DFT plane waves calculations. Single crystal X-ray diffraction data have been collected at different temperatures, discovering a phase transition occurring in the 250-300 K range. The calculations indicate the presence of intradimer, interdimer, and organic-inorganic π-d interactions in the crystal, a magnetic pattern much more complex than the Bleaney-Bowers model initially assigned to this material. Although this simple model was good enough to reproduce the magnetic susceptibility data, our calculations demonstrate that the actual magnetic structure is significantly more intricate, with alternating antiferromagnetic 1D chains of the organic BEDT-TTF+ radical, connected through weak antiferromagnetic interactions with the CuCl42- ions. Combination of experiment and theory allowed us to unambiguously determine and quantify the leading magnetic interactions in the system. The density-of-states curves confirm the semiconductor nature of the system and the dominant organic contribution of the valence and conduction band edges. This general and combined approach appears to be fundamental in order to properly understand the magnetic structure of these complex materials, where experimental data can actually be fitted from a variety of models and parameters.

Evaluation of the Giant Ferromagnetic π-d Interaction in Iron-Phthalocyanine Molecule.

The interaction between itinerant π and localized d electrons in metal-phthalocyanines, namely, Jπd interaction, is considered as responsible for the giant negative magnetoresistance observed in several phthalocyanine-based conductors, among many other important physical properties. Despite the fundamental and technological importance of this on-site intramolecular interaction, its giant ferromagnetic nature has been only recently demonstrated by the experiments conducted by Murakawa et al. in the neutral radical [Fe(Pc)(CN)2]·2CHCl3 ( Phys. Rev. B 2015 , 92 , 054429 ). In this article, we present the theoretical evaluation of this interaction combining wave function-based electronic calculations on isolated Fe(Pc)(CN)2 molecules and density functional theory-based periodic calculations on the crystal. Our calculations confirm the ferromagnetic nature of the π-d interaction, with a coupling constant as large as Jπd/kB = 570 K, in excellent agreement with the experiments, and the presence of intermolecular antiferromagnetic interactions driven by the π-π overlap of neighboring phthalocyaninato molecules. The analysis of the wave function of the ground state of the Fe(Pc)(CN)2 molecule provides the clues of the origin of this giant ferromagnetic π-d interaction.

Analysis of the Magnetic Exchange Interactions in Yttrium(III) Complexes Containing Nitronyl Nitroxide Radicals.

We report a combined theoretical and experimental investigation of the exchange interactions governing the magnetic behavior of a series of nitronyl nitroxide (NIT)-based Y(III) complexes, i.e., Y(hfac)3(NIT-R)2 with R = PhOPh (1), PhOEt (2), and PhOMe (3a, 3b). Even though some of these complexes or their Dy(III) parents were previously described in the literature [ Zhao et al. Transition Met. Chem. 2006 , 31 , 593 ; Bernot et al. J. Am. Chem. Soc. 2009 , 131 , 5573 ], their synthesis procedure as well as their structural and magnetic properties were completely reconsidered. Depending on the nature of R and the crystallization conditions, Y(hfac)3(NIT-R)2 units can be organized as supramolecular dimers or linear or orthogonal chains. Such structural diversity within the series induces extremely different magnetic behaviors. The observed behaviors are rationalized by state-of-the-art wave function-based quantum-chemical approaches (CASSCF/DDCI) that demonstrate the existence of not only effective intramolecular interactions between the NIT-R radical ligands of an isolated Y(hfac)3(NIT-R)2 molecule but also intermolecular interactions between NIT-R moieties belonging to different Y(hfac)3(NIT-R)2 units. These results are supported by the use of spin Hamiltonian models going beyond the basic Bleaney-Bowers formalism to properly fit the experimental magnetic data. Finally, the microscopic mechanisms behind the evidenced intramolecular exchange interactions are elucidated through the inspection of the calculated wave functions. In particular, whereas the role of Y orbitals was already proposed, we herein demonstrate the contribution of the hfac- ancillary ligands in mediating the magnetic interactions between the NIT radicals.

Highly efficient perturbative + variational strategy based on orthogonal valence bond theory for the evaluation of magnetic coupling constants. Application to the trinuclear Cu(ii) site of multicopper oxidases.

A new strategy based on orthogonal valence-bond analysis of the wave function combined with intermediate Hamiltonian theory has been applied to the evaluation of the magnetic coupling constants in two AF systems. This approach provides both a quantitative estimate of the J value and a detailed analysis of the main physical mechanisms controlling the coupling, using a combined perturbative + variational scheme. The procedure requires a selection of the dominant excitations to be treated variationally. Two methods have been employed: a brute-force selection, using a logic similar to that of the CIPSI approach, or entanglement measures, which identify the most interacting orbitals in the system. Once a reduced set of excitations (about 300 determinants) is established, the interaction matrix is dressed at the second-order of perturbation by the remaining excitations of the CI space. The diagonalization of the dressed matrix provides J values in good agreement with experimental ones, at a very low-cost. This approach demonstrates the key role of d → d* excitations in the quantitative description of the magnetic coupling, as well as the importance of using an extended active space, including the bridging ligand orbitals, for the binuclear model of the intermediates of multicopper oxidases. The method is a promising tool for dealing with complex systems containing several active centers, as an alternative to both pure variational and DFT approaches.

Metal-Metal Interactions in Trinuclear Copper(II) Complexes [Cu3(RCOO)4(H2TEA)2] and Binuclear [Cu2(RCOO)2(H2TEA)2]. Syntheses and Combined Structural, Magnetic, High-Field Electron Paramagnetic Resonance, and Theoretical Studies.

The trinuclear [Cu3(RCOO)4(H2TEA)2] copper(II) complexes, where RCOO(-) = 2-furoate (1), 2-methoxybenzoate (2), and 3-methoxybenzoate (3, 4), as well as dimeric species [Cu2(H2TEA)2(RCOO)2]·2H2O, have been prepared by adding triethanolamine (H3TEA) at ambient conditions to hydrated Cu(RCOO)2 salts. The newly synthesized complexes have been characterized by elemental analyses, spectroscopic techniques (IR and UV-visible), magnetic susceptibility, single crystal X-ray structure determination and theoretical calculations, using a Difference Dedicated Configuration Interaction approach for the evaluation of magnetic coupling constants. In 1 and 2, the central copper atom lies on an inversion center, while in the polymorphs 3 and 4, the three metal centers are crystallographically independent. The zero-field splitting parameters of the trimeric compounds, D and E, were derived from high-field, high-frequency electron paramagnetic resonance spectra at temperatures ranging from 3 to 290 K and were used for the interpretation of the magnetic data. It was found that the dominant interaction between the terminal and central Cu sites J12 is ferromagnetic in nature in all complexes, even though differences have been found between the symmetrical or quasi-symmetrical complexes 1-3 and non-symmetrical complex 4, while the interaction between the terminal centers, J23, is negligible.

Mechanism of Magnetostructural Transitions in Copper-Nitroxide-Based Switchable Molecular Magnets: Insights from ab Initio Quantum Chemistry Calculations.

The gradual magnetostructural transition in breathing crystals based on copper(II) and pyrazolyl-substituted nitronyl nitroxides has been analyzed by means of DDCI quantum chemistry calculations. The magnetic coupling constants (J) within the spin triads of Cu(hfac)2L(Bu)·0.5C8H18 have been evaluated for the X-ray structures reported at different temperatures. The coupling is strongly antiferromagnetic at low temperature and becomes ferromagnetic when the temperature increases. The intercluster magnetic coupling (J') is antiferromagnetic and shows a marked dependence on temperature. The magnetostructural transition can be reproduced using the calculated J values for each structure in the simulation of the magnetic susceptibility. However, the µ(T) curve can be improved nicely by considering the coexistence of two phases in the transition region, whose ratio varies with temperature corresponding to both the weakly and strongly coupled spin states. These results complement a recent VT-FTIR study on the parent Cu(hfac)2L(Pr) compound with a gradual magnetostructural transition.