Room temperature conductance switching in a molecular iron(iii) spin crossover junction

Thermal- and light-induced valence tautomerism with a concerted spin transition in an iron tris(diimine) complex

The switching phenomena of spin crossover (SCO) and valence tautomerism (VT) are respectively dominated by iron(ii) and cobalt-dioxolene systems. To explore new possibilities for SCO or VT, the redox-active α-diimine ligand bis((phenyl)imino)acenaphthene (Ph-BIAN), which can adopt neutral (L0), monoanionic (L˙-), and dianionic (L2-) states, was paired with zinc, cobalt, manganese and iron to give [M(Ph-BIAN)3](BPh4)2 (M = Zn (1), Co (2), Mn (3), Fe (4)). Compounds 1, 2 and 3 adopt a temperature invariant MII-(L0)3 state, (2 and 3 are high spin (HS)) in the solid- and solution-states. Electrochemical measurements show the metal controls the degree of electronic communication between the Ph-BIAN ligands. In stark contrast to 1, 2 and 3, compound 4 adopts the LS-FeIII-(L˙-)(L0)2 (LS = low spin) tautomeric form as the ground state in both the solid-state and in solution. Combined variable temperature solid- and solution-state structural, Mössbauer and electronic spectroscopy, and magnetic measurements, show that 4 undergoes the thermally-induced VT process LS-FeIII-(L˙-)(L0)2 ⇌ HS-FeII-(L0)3, the only example of VT accompanied by a concerted spin transition in an iron complex with a redox-active ligand. Solid-state photomagnetic measurements suggest that the VT interconversion is also induced by light. Light-induced VT has not been previously observed for complexes other than cobalt-dioxolene, and is potentially afforded here by the unique spin-state change that results in large differences in the Fe-N bond lengths for the two valence tautomers. This study introduces a new example of VT, and suggests that optically-induced VT can be displayed by iron systems, opening alternate pathways toward molecular switches that can be controlled with multiple stimuli.

Thermal Bistability of Magnetic Susceptibility, Light Absorption, Second Harmonic Generation, and Dielectric Properties in a Polar Spin-Crossover Iron-Rhenium Chain Material

The bistability of multiple physical properties driven by external stimuli in a solid is a desired prerequisite for its application in memory devices with convenient data readout. We present a pathway for thermal bistability detectable in four physical properties: magnetic, light absorption, second-harmonic generation (SHG), and dielectric. We report a novel heterometallic (TBA){[FeII(phIN)4][ReV(CN)8]} ⋅ (phIN) (1) (TBA=tetrabutylammonium cation, phIN=phenyl isonicotinate) cyanido-bridged chain material. Owing to an appropriate {N6} coordination sphere of Fe(II) centers, 1 reveals a thermal spin crossover (SCO) effect which is complete and cooperative providing a distinct thermal hysteresis loop in magnetic measurements. Moreover, it exhibits simultaneous thermal bistability in (a) visible-light absorption due to the presence of efficient d-d electronic transitions in the low-spin (LS) state, (b) SHG activity as it crystallizes in a polar Cc space group due to the bulky substituent on phIN ligands, and (c) dielectric parameters, including dielectric constant, which can be correlated with subtle changes in polarity between LS and HS (high spin) phases. Thus, we present a remarkable thermally controlled hysteretic behavior in four physical functionalities realized by properly functionalizing an SCO-active coordination compound.

Predicting valence tautomerism in diverse cobalt-dioxolene complexes: elucidation of the role of ligands and solvent

The ability of molecular switches to reversibly interconvert between different forms promises potential applications at the scale of single molecules up to bulk materials. One type of molecular switch comprises cobalt-dioxolene compounds that exhibit thermally-induced valence tautomerism (VT) interconversions between low spin Co(iii)-catecholate (LS-CoIII-cat) and high spin Co(ii)-semiquinonate (HS-CoII-sq) forms. Two families of these compounds have been investigated for decades but have generally been considered separately: neutral [Co(diox)(sq)(N2L)] and cationic [Co(diox)(N4L)]+ complexes (diox = generic dioxolene, N2L/N4L = bidentate/tetradentate N-donor ancillary ligand). Computational identification of promising new candidate compounds prior to experimental exploration is beneficial for environmental and cost considerations but requires a thorough understanding of the underlying thermochemical parameters that influence the switching. Herein, we report a robust approach for the analysis of both cobalt-dioxolene families, which involved a quantitative density functional theory-based study benchmarked with reliable quasi-experimental references. The best-performing M06L-D4/def2-TZVPP level of theory has subsequently been verified by the synthesis and experimental investigation of three new complexes, two of which exhibit thermally-induced VT, while the third remains in the LS-CoIII-cat form across all temperatures, in agreement with prediction. Valence tautomerism in solution is markedly solvent-dependent, but the origin of this has not been definitively established. We have extended our computational approach to elucidate the correlation of VT transition temperature with solvent stabilisation energy and change in dipole moment. This new understanding may inform the development of VT compounds for applications in soft materials including films, gels, and polymers.

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.

Bistable Spin-Crossover Nanoparticles for Molecular Electronics

The field of spin-crossover complexes is rapidly evolving from the study of the spin transition phenomenon to its exploitation in molecular electronics. Such spin transition is gradual in a single-molecule, while in bulk it can be abrupt, showing sometimes thermal hysteresis and thus a memory effect. A convenient way to keep this bistability while reducing the size of the spin-crossover material is to process it as nanoparticles (NPs). Here, the most recent advances in the chemical design of these NPs and their integration into electronic devices, paying particular attention to optimizing the switching ratio are reviewed. Then, integrating spin-crossover NPs over 2D materials is focused to improve the endurance, performance, and detection of the spin state in these hybrid devices.

Iron(II) complexes of 2,6-bis(imidazo[1,2-a]pyridin-2-yl)pyridine and related ligands with annelated distal heterocyclic donors

Following a published synthesis of 2,6-bis(imidazo[1,2-a]pyridin-2-yl)pyridine (L1), treatment of α,α'-dibromo-2,6-diacetylpyridine with 2 equiv. 2-aminopyrimidine or 2-aminoquinoline in refluxing acetonitrile respectively gives 2,6-bis(imidazo[1,2-a]pyrimidin-2-yl)pyridine (L2) and 2,6-bis(imidazo[1,2-a]quinolin-2-yl)pyridine (L3). Solvated crystals of [Fe(L1)2][BF4]2 (1[BF4]2) and [Fe(L2)2][BF4]2 (2[BF4]2) are mostly high-spin, although one solvate of 1[BF4]2 undergoes thermal spin-crossover on cooling. The iron coordination geometry is consistently distorted in crystals of 2[BF4]2 which may reflect the influence of intramolecular, inter-ligand N⋯π interactions on the molecular conformation. Only 1 : 1 Fe : L3 complexes were observed in solution, or isolated in the solid state; a crystal structure of [FeBr(py)2L3]Br·0.5H2O (py = pyridine) is presented. A solvate crystal structure of high-spin [Fe(L4)2][BF4]2 (L4 = 2,6-di{quinolin-2-yl}pyridine; 4[BF4]2) is also described, which exhibits a highly distorted six-coordinate geometry with a helical ligand conformation. The iron(II) complexes are high-spin in solution at room temperature, but 1[BF4]2 and 2[BF4]2 undergo thermal spin-crossover equilibria on cooling. All the compounds exhibit a ligand-based emission in solution at room temperature. Gas phase DFT calculations mostly reproduce the spin state properties of the complexes, but show small anomalies attributed to intramolecular, inter-ligand dispersion interactions in the sterically crowded molecules.

Controlling Spin Crossover in a Family of Dinuclear Fe(III) Complexes via the Bis(catecholate) Bridging Ligand

Spin crossover (SCO) complexes can reversibly switch between low spin (LS) and high spin (HS) states, affording possible applications in sensing, displays, and molecular electronics. Dinuclear SCO complexes with access to [LS-LS], [LS-HS], and [HS-HS] states may offer increased levels of functionality. The nature of the SCO interconversion in dinuclear complexes is influenced by the local electronic environment. We report the synthesis and characterization of [{FeIII(tpa)}2spiro](PF6)2 (1), [{FeIII(tpa)}2Br4spiro](PF6)2 (2), and [{FeIII(tpa)}2thea](PF6)2 (3) (tpa = tris(2-pyridylmethyl)amine, spiroH4 = 3,3,3',3'-tetramethyl-1,1'-spirobi(indan)-5,5',6,6'-tetraol, Br4spiroH4 = 3,3,3',3'-tetramethyl-1,1'-spirobi(indan)-4,4',7,7'-tetrabromo-5,5',6,6'-tetraol, theaH4 = 2,3,6,7-tetrahydroxy-9,10-dimethyl-9,10-dihydro-9,10-ethanoanthracene), utilizing non-conjugated bis(catecholate) bridging ligands. In the solid state, magnetic and structural analysis shows that 1 remains in the [HS-HS] state, while 2 and 3 undergo a partial SCO interconversion upon cooling from room temperature involving the mixed [LS-HS] state. In solution, all complexes undergo SCO from [HS-HS] at room temperature, via [LS-HS] to mixtures including [LS-LS] at 77 K, with the extent of SCO increasing in the order 1 < 2 < 3. Gas phase density functional theory calculations suggest a [LS-LS] ground state for all complexes, with the [LS-HS] and [HS-HS] states successively destabilized. The relative energy separations indicate that ligand field strength increases following spiro4- < Br4spiro4- < thea4-, consistent with solid-state magnetic and EPR behavior. All three complexes show stabilization of the [LS-HS] state in relation to the midpoint energy between [LS-LS] and [HS-HS]. The relative stability of the [LS-HS] state increases with increasing ligand field strength of the bis(catecholate) bridging ligand in the order 1 < 2 < 3. The bromo substituents of Br4spiro4- increase the ligand field strength relative to spiro4-, while the stronger ligand field provided by thea4- arises from extension of the overlapping π-orbital system across the two catecholate units. This study highlights how SCO behavior in dinuclear complexes can be modulated by the bridging ligand, providing useful insights for the design of molecules that can be interconverted between more than two states.

Investigation of the unique magnetic behaviours of isomers in a 1,2-dithiooxalato-bridged diiron(II) complex

1,2-Dithiooxalate (dto) can be employed as a bridging ligand and it exhibits symmetric (O,S-chelation) or asymmetric (O,O- and S,S-chelation) coordination forms. In this study, we prepared a novel dto-bridged diiron(II) complex, [{Fe(TPA)}2(μ-dto)](ClO4)2 (1), where TPA is tris(2-pyridylmethyl)amine. Interestingly, the bridging dto ligand exhibited not only the asymmetric form but also a linkage isomer and a diastereomer within the same crystal. Notably, the three isomers of 1 exhibited different magnetic properties, resulting in a multi-step spin crossover behaviour.

pH-Switching of the luminescent, redox, and magnetic properties in a spin crossover cobalt(ii) molecular nanomagnet

The ability of mononuclear first-row transition metal complexes as dynamic molecular systems to perform selective functions under the control of an external stimulus that appropriately tunes their properties may greatly impact several domains of molecular nanoscience and nanotechnology. This study focuses on two mononuclear octahedral cobalt(ii) complexes of formula {[CoII(HL)2][CoII(HL)L]}(ClO4)3·9H2O (1) and [CoIIL2]·5H2O (2) [HL = 4'-(4-carboxyphenyl)-2,2':6',2''-terpyridine], isolated as a mixed protonated/hemiprotonated cationic salt or a deprotonated neutral species. This pair of pH isomers constitutes a remarkable example of a dynamic molecular system exhibiting reversible changes in luminescence, redox, and magnetic (spin crossover and spin dynamics) properties as a result of ligand deprotonation, either in solution or solid state. In this last case, the thermal-assisted spin transition coexists with the field-induced magnetisation blockage of "faster" or "slower" relaxing low-spin CoII ions in 1 or 2, respectively. In addition, pH-reversible control of the acid-base equilibrium among dicationic protonated, cationic hemiprotonated, and neutral deprotonated forms in solution enhances luminescence in the UV region. Besides, the reversibility of the one-electron oxidation of the paramagnetic low-spin CoII into the diamagnetic low-spin CoIII ion is partially lost and completely restored by pH decreasing and increasing. The fine-tuning of the optical, redox, and magnetic properties in this novel class of pH-responsive, spin crossover molecular nanomagnets offers fascinating possibilities for advanced multifunctional and multiresponsive magnetic devices for molecular spintronics and quantum computing such as pH-effect spin quantum transformers.

The Influence of the Substrate on the Functionality of Spin Crossover Molecular Materials

Spin crossover complexes are a route toward designing molecular devices with a facile readout due to the change in conductance that accompanies the change in spin state. Because substrate effects are important for any molecular device, there are increased efforts to characterize the influence of the substrate on the spin state transition. Several classes of spin crossover molecules deposited on different types of surface, including metallic and non-metallic substrates, are comprehensively reviewed here. While some non-metallic substrates like graphite seem to be promising from experimental measurements, theoretical and experimental studies indicate that 2D semiconductor surfaces will have minimum interaction with spin crossover molecules. Most metallic substrates, such as Au and Cu, tend to suppress changes in spin state and affect the spin state switching process due to the interaction at the molecule-substrate interface that lock spin crossover molecules in a particular spin state or mixed spin state. Of course, the influence of the substrate on a spin crossover thin film depends on the molecular film thickness and perhaps the method used to deposit the molecular film.

First crystal structure of an Fe(III) anionic complex based on a pyruvic acid thiosemicarbazone ligand with Li+: synthesis, features of magnetic behavior and theoretical analysis

The iron(III) anionic complex based on a pyruvic acid thiosemicarbazone ligand with the lithium cation Li[Fe^III(thpy)₂]·3H₂O (1) has been synthesized and characterized by FTIR spectroscopy, powder and single crystal X-ray diffraction, direct current magnetic susceptibility measurements, and ⁵⁷Fe Mössbauer spectroscopy. Moreover, the molecular structure of the [Fe(thpy)₂]^- anion has been determined for the first time. The [Fe(thpy)₂]^- units in the triclinic P1̄ lattice of 1 are assembled into layers parallel to the bc plane. The Li⁺ cations and water molecules are located between the layers and the structure is stabilized by hydrogen bonding. The [Fe(thpy)₂]^- anions form interconnected dimer pairs through hydrogen bonds and short contacts with Fe⋯Fe separation of 6.7861(4) Å. According to dc magnetic measurements, compound 1 demonstrates an incipient spin-crossover transition from the LS (S = 1/2) to the HS (S = 5/2) state above 250 K. The Bleaney-Bowers equation for a model of an isolated LS dimer with a mean-field correction was applied to fit the experimental data of magnetic susceptibility dependence on temperature in the temperature range of 2-250 K. The intra-dimer J₁ = -1.79(1) K and inter-dimer J₂ = -0.24(3) K antiferromagnetic coupling constants were defined. The analysis of the ⁵⁷Fe Mössbauer spectra at 80 K and 296 K confirms the presence of the shortened distances between the iron nuclei. Moreover, the influence of the lithium cation on the stabilization of the LS state was shown for the [Fe(thpy)₂]^- anion. BS-DFT calculations for the optimized structure of two isolated [Fe(thpy)₂]^- anions also correctly predict a weak exchange J₁(calc) = -0.92 K. DFT calculations revealed the OPBE (GGA-type) functional that correctly predicts the spin-crossover transition for the iron(III) thpy compounds. Besides, the effect of the N₂O₄, N₂S₂O₂, and N₂Se₂O₂ coordination environments on the energy stabilization of the LS state of iron(III) anionic thpy complexes was noted as well.

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.

Computational NMR of the iron pyrazolylborate complexes [Tp2Fe]+ and Tp2Fe including solvation and spin-crossover effects

Transition metal complexes have important roles in many biological processes as well as applications in fields such as pharmacy, chemistry and materials science. Paramagnetic nuclear magnetic resonance (pNMR) is a valuable tool in understanding such molecules, and theoretical computations are often advantageous or even necessary in the assignment of experimental pNMR signals. We have employed density functional theory (DFT) and the domain-based local pair natural orbital coupled-cluster method with single and double excitations (DLPNO-CCSD), as well as a number of model improvements, to determine the critical hyperfine part of the chemical shifts of the iron pyrazolylborate complexes [Tp₂Fe]⁺ and Tp₂Fe using a modern version of the Kurland-McGarvey theory, which is based on parameterising the hyperfine, electronic Zeeman and zero-field splitting interactions via the parameters of the electron paramagnetic resonance Hamiltonian. In the doublet [Tp₂Fe]⁺ system, the calculations suggest a re-assignment of the ¹³C signal shifts. Consideration of solvent via the conductor-like polarisable continuum model (C-PCM) versus explicit solvent molecules reveals C-PCM alone to be insufficient in capturing the most important solvation effects. Tp₂Fe exhibits a spin-crossover effect between a high-spin quintet (S = 2) and a low-spin singlet (S = 0) state, and its recorded temperature dependence can only be reproduced theoretically by accounting for the thermal Boltzmann distribution of the open-shell excited state and the closed-shell ground-state occupations. In these two cases, DLPNO-CCSD is found, in calculating the hyperfine couplings, to be a viable alternative to DFT, the demonstrated shortcomings of which have been a significant issue in the development of computational pNMR.

Smart Eutectic Gallium-Indium: From Properties to Applications

Eutectic gallium-indium (EGaIn), a liquid metal with a melting point close to or below room temperature, has attracted extensive attention in recent years due to its excellent properties such as fluidity, high conductivity, thermal conductivity, stretchability, self-healing capability, biocompatibility, and recyclability. These features of EGaIn can be adjusted by changing the experimental condition, and various composite materials with extended properties can be further obtained by mixing EGaIn with other materials. In this review, not only the are unique properties of EGaIn introduced, but also the working principles for the EGaIn-based devices are illustrated and the developments of EGaIn-related techniques are summarized. The applications of EGaIn in various fields, such as flexible electronics (sensors, antennas, electronic circuits), molecular electronics (molecular memory, opto-electronic switches, or reconfigurable junctions), energy catalysis (heat management, motors, generators, batteries), biomedical science (drug delivery, tumor therapy, bioimaging and neural interfaces) are reviewed. Finally, a critical discussion of the main challenges for the development of EGaIn-based techniques are discussed, and the potential applications in new fields are prospected.

FeIII in the high-spin state in dimethylammonium bis[3-ethoxysalicylaldehyde thiosemicarbazonato(2-)-κ3O2,N1,S]ferrate(III)

The synthesis and crystal structure (100 K) of the title compound, [(CH3)2NH2][Fe(C10H11O2N3S)2], are reported. The asymmetric unit consists of an octahedral [FeIII(L)2]- fragment, where L2- is 3-ethoxysalicylaldehyde thiosemicarbazonate(2-), and a dimethylammonium cation. Each L2- ligand binds with the thiolate S, the imine N and the phenolate O atoms as donors, resulting in an FeIIIS2N2O2 chromophore. The ligands are orientated in two perpendicular planes, with the O and S atoms in cis positions, and mutually trans N atoms. The FeIII ion is in the high-spin state at 100 K. The variable-temperature magnetic susceptibility measurements (5-320 K) are consistent with the presence of a high-spin FeIII ion with D = 0.83 (1) cm-1 and g = 2.

Anion Modified Spin Crossover in [Fe(qsal-4-F)]+ Complexes with a 4-Position Substituted Qsal Ligand

Four iron(III) complexes, [Fe(qsal-4-F)₂]Y·sol (Hqsal-4-F = 4-fluoro-N-(8-quinolyl)salicylaldimine; Y = NO₃^-, sol = 0.91MeOH·0.57H₂O (1_NO3); Y = PF₆^- (2_PF6); Y = BF₄^- (3_BF4); Y = OTf^-, sol =1.5MeOH (4_OTf)), with a new 4-position substituted qsal type ligand Hqsal-4-F have been synthesized and structurally and magnetically characterized. Complexes 1_NO3-3_BF4 consist of 1D chains formed by the [Fe(qsal-4-F)₂]⁺ cations connected by π-π and C-H···O interactions, which are further linked by more weak interactions to form 2D layers and 3D networks. On the other hand, complex 4_OTf has a structure of nearly isolated 1D column where the [Fe(qsal-4-F)₂]⁺ cations are connected by π-π, C-H···π, and C-F···π interactions. Magnetic studies revealed the occurrence of two-step symmetry-breaking SCO in 1_NO3 and two-step gradual SCO in 2_PF6. Complex 3_BF4 undergoes a gradual SCO, whereas 4_OTf remains almost high-spin. The smaller anions tend to stabilize the low-spin state, while larger anions tend to stabilize the high-spin state. In addition, the intermediate spin state of 1_NO3 could be thermally trapped by quenching from the high temperature, thereby kinetically suppressing the spin transition to the full low-spin state. This work represents a good example that the position of the substituent and the anions plays critical roles in the preparation of SCO materials with tunable properties.

FeIII in a high-spin state in bis(5-bromosalicylaldehyde 4-ethylthiosemicarbazonato-κ3O,N1,S)ferrate(III) nitrate monohydrate, the first example of such a cationic FeIII complex unit

The synthesis and crystal structure (100 K) of the title compound, [Fe(C₁₀H₁₁BrN₃OS)₂]NO₃·H₂O, is reported. The asymmetric unit consists of an octahedral [Fe^III(HL)₂]⁺ cation, where HL^- is H-5-Br-thsa-Et or 5-bromosalicylaldehyde 4-ethylthiosemicarbazonate(1-) {systematic name: 4-bromo-2-[(4-ethylthiosemicarbazidoidene)methyl]phenolate}, a nitrate anion and a noncoordinated water molecule. Each HL^- ligand binds via the thione S, the imine N and the phenolate O atom, resulting in an Fe^IIIS₂N₂O₂ chromophore. The ligands are orientated in two perpendicular planes, with the O and S atoms in cis and the N atoms in trans positions. This [Fe(HL)₂](anion)·H₂O compound contains the first known cationic Fe^III entity containing two salicylaldehyde thiosemicarbazone derivatives. The Fe^III ion is in the high-spin state at 100 K. In addition, a comparative IR spectroscopic study of the free ligand and the ferric complex is presented, demonstrating that such an analysis provides a quick identification of the degree of deprotonation and the coordination mode of the ligand in this class of metal compounds. The variable-temperature magnetic susceptibility measurements (5-320 K) are consistent with the presence of a high-spin Fe^III ion with a zero-field splitting D = 0.439 (1) cm^-1.

Amine-Anchored Aromatic Self-Assembled Monolayer Junction: Structure and Electric Transport Properties

We studied the structure and transport properties of aromatic amine self-assembled monolayers (NH₂-SAMs) on an Au surface. The oligophenylene and oligoacene amines with variable lengths can form a densely packed and uniform monolayer under proper assembly conditions. Molecular junctions incorporating an eutectic Ga-In (EGaIn) top electrode were used to characterize the charge transport properties of the amine monolayer. The current density J of the junction decreases exponentially with the molecular length (d), as J = J₀ exp(-βd), which is a sign of tunneling transport, with indistinguishable values of J₀ and β for NH₂-SAMs of oligophenylene and oligoacene, indicating a similar molecule-electrode contact and tunneling barrier for two groups of molecules. Compared with the oligophenylene and oligoacene molecules with thiol (SH) as the anchor group, a similar β value (∼0.35 Å^-1) of the aromatic NH₂-SAM suggests a similar tunneling barrier, while a lower (by 2 orders of magnitude) injection current J₀ is attributed to lower electronic coupling Γ of the amine group with the electrode. These observations are further supported by single-level tunneling model fitting. Our study here demonstrates the NH₂-SAMs can work as an effective active layer for molecular junctions, and provide key physical parameters for the charge transport, paving the road for their applications in functional devices.

Hard-Soft Chemistry Design Principles for Predictive Assembly of Single Molecule-Metal Junctions

The achievement of atomic control over the organic-inorganic interface is key to engineering electronic and spintronic properties of molecular devices. We leverage insights from inorganic chemistry to create hard-soft acid-base (HSAB) theory-derived design principles for incorporation of single molecules onto metal electrodes. A single molecule circuit is assembled via a bond between an organic backbone and an under-coordinated metal atom of the electrode surface, typically Au. Here, we study molecular composition factors affecting the junction assembly of coordination complexes containing transition metals atoms on Au electrodes. We employ hetero- and homobimetallic lantern complexes and systematically change the coordination environment to vary the character of the intramolecular bonds relative to the electrode-molecule interaction. We observe that trends in the robustness and chemical selectivity of single molecule junctions formed with a range of linkers correlate with HSAB principles, which have traditionally been used to guide atomic arrangements in the synthesis of coordination complexes. We find that this similarity between the intermolecular electrode-molecule bonding in a molecular circuit and the intramolecular bonds within a coordination complex has implications for the design of metal-containing complexes compatible with electrical measurements on metal electrodes. Our results here show that HSAB principles determine which intramolecular interactions can be compromised by inter molecule-electrode coordination; in particular on Au electrodes, soft-soft metal-ligand bonding is vulnerable to competition from soft-soft Au-linker bonding in the junction. Neutral donor-acceptor intramolecular bonds can be tuned by the Lewis acidity of the transition metal ion, suggesting future synthetic routes toward incorporation of transition metal atoms into molecular junctions for increased functionality of single molecule devices.

Abrupt Spin-State Switching in Mn(III) Complexes with BPh4 Anion: Effect of Halide Substituents on Crystal Structure and Magnetic Properties

Three tetraphenylborates of mononuclear Mn(III) cation complexes with hexadentate ligands, the products of the reaction between a N,N'-bis(3-aminopropyl)ethylenediamine and salicylaldehydes with the different haloid substitutions at the 5 or 3,5 positions, have been synthesized: [Mn(5-F-sal-N-1,5,8,12)]BPh₄ (1), [Mn(3,5-diCl-sal-N-1,5,8,12)]BPh₄ (2) and [Mn(3,5-Br,Cl-sal-N-1,5,8,12)]BPh₄ (3). Their crystal structure, dielectric constant (ϵ) and magnetic properties have been studied. Ligand substituents have a dramatic effect on the structure and magnetic properties of the complexes. With decreasing temperature, the complex (1) shows a gradual spin crossover from the high-spin state (HS) to the HS:LS intermediate phase, followed by an abrupt transition to the low-spin state (LS) without changing the crystal symmetry. The complexes 2 and 3 are isostructural, but have fundamentally different properties. Complex 2 demonstrates two structural phase transitions related to sharp spin crossovers from the HS to the HS:LS intermediate phase at 137 K and from the intermediate phase to the LS at 87 K, while complex 3 exhibits only one spin transition from the HS to the HS:LS intermediate phase at 83 K.

Room temperature optoelectronic devices operating with spin crossover nanoparticles

Molecular systems can exhibit multi-stimuli switching of their properties, with spin crossover materials having unique magnetic transition triggered by temperature and light, among others. Light-induced room temperature operation is however elusive, as optical changes between metastable spin states require cryogenic temperatures. Furthermore, electrical detection is hampered by the intrinsic low conductivity properties of these materials. We show here how a graphene underlayer reveals the light-induced heating that triggers a spin transition, paving the way for using these molecules for room temperature optoelectronic applications.

Bistable Hofmann-Type FeII Spin-Crossover Two-Dimensional Polymers of 4-Alkyldisulfanylpyridine for Prospective Grafting of Monolayers on Metallic Surfaces

Aiming at investigating the suitability of Hofmann-type two-dimensional (2D) coordination polymers {FeII(Lax)2[MII(CN)4]} to be processed as single monolayers and probed as spin crossover (SCO) junctions in spintronic devices, the synthesis and characterization of the MII derivatives (MII = Pd and Pt) with sulfur-rich axial ligands (Lax = 4-methyl- and 4-ethyl-disulfanylpyridine) have been conducted. The thermal dependence of the magnetic and calorimetric properties confirmed the occurrence of strong cooperative SCO behavior in the temperature interval of 100-225 K, featuring hysteresis loops 44 and 32.5 K/21 K wide for PtII-methyl and PtII/PdII-ethyl derivatives, while the PdII-methyl derivative undergoes a much less cooperative multistep SCO. Excluding PtII-methyl, the remaining compounds display light-induced excited spin-state trapping at 10 K with TLIESST temperatures in the range of 50-70 K. Single-crystal studies performed in the temperature interval 100-250 K confirmed the layered structure and the occurrence of complete transformation between the high- and low-spin states of the FeII center for the four compounds. Strong positional disorder seems to be the source of elastic frustration driving the multistep SCO observed for the PdII-methyl derivative. It is expected that the peripheral disulfanyl groups will favor anchoring and growing of the monolayer on gold substrates and optimal electron transport in the device.

Spin Crossover and High-Spin State in Fe(II) Anionic Polymorphs Based on Tripodal Ligands

Two new mononuclear Fe(II) polymorphs, [(C₂H₅)₄N]₂[Fe(py₃C-OEt)(NCS)₃]₂ (1) and [(C₂H₅)₄N][Fe(py₃C-OEt)(NCS)₃] (2) (py₃C-OEt = tris(pyridin-2-yl)ethoxymethane), have been synthesized and characterized by single-crystal X-ray diffraction, by magnetic and photomagnetic measurements, and by detailed variable-temperature infrared spectroscopy. The molecular structure, in both complexes, is composed of the same anionic [Fe(py₃C-OEt)(NCS)₃]^- complex (two units for 1 and one unit for 2) generated by coordination to the Fe(II) metal center of one tridentate py₃C-OEt tripodal ligand and three terminal κN-SCN coligands. Magnetic studies revealed that polymorph 2 displays a high-spin (HS) state over the entire studied temperature range (300-10 K), while complex 1 exhibits an abrupt and complete spin crossover (SCO) transition at ca. 132.3 K, the structural characterizations of which, performed at 295 and 100 K, show a strong modification, resulting from the thermal evolutions of the Fe-N bond lengths and of the distortion parameters (∑ and Θ) of the FeN₆ coordination sphere, in agreement with the presence of HS and low-spin (LS) states at 295 and 100 K, respectively. This thermal transition has been also confirmed by the thermal evolution of the maximum absorbance for ν(NCS) vibrational bands recorded in the temperature range 200-10 K. In 1 the signature of a metastable photoinduced HS state has been observed using photomagnetic and photoinfrared spectroscopy, leading to a similar T(LIESST) relaxation temperature (LIESST = light-induced excited spin-state trapping) of 70 K.