Magnetochiral dichroism (MChD), a fascinating manifestation of the light-matter interaction characteristic for chiral systems under magnetic fields, has become a well-established optical phenomenon reported for many different materials. However, its interpretation remains essentially phenomenological and qualitative, because the existing microscopic theory has not been quantitatively confirmed by confronting calculations based on this theory with experimental data. Here, we report the experimental low-temperature MChD spectra of two archetypal chiral paramagnetic crystals taken as model systems, tris(1,2-diaminoethane)nickel(II) and cobalt(II) nitrate, for light propagating parallel or perpendicular to the c axis of the crystals, and the calculation of the MChD spectra for the Ni(II) derivative by state-of-the-art quantum chemical calculations. By incorporating vibronic coupling, we find good agreement between experiment and theory, which opens the way for MChD to develop into a powerful chiral spectroscopic tool and provide fundamental insights for the chemical design of new magnetochiral materials for technological applications.

The ultimate limit of miniaturization is that of molecules, which are the smallest composite entities with definite size, shape and controllable properties. Much efforts has been devoted lately to the design at the molecular level of spin crossover systems to process and store information in binary form, due to the possibility of thermal and photoinduced switching between low-spin and high-spin state. Understanding the properties of such systems is of interest for the design of improved systems. To do so it is most informative to cross data gathered from structural determinations under varied conditions (temperature, light irradiation or pressure) and properties determination under similar conditions (magnetism, spectroscopy,...). We will present two studies of spin crossover complexes. SCO compounds show a change of spin state under application of an external stimuli (pressure, temperature, photoexcitation) and therefore have been proposed for a number of potential applications, including piezochromism.[1-5] This change goes together with modification of the population of molecular orbitals, with anti-bonding orbitals corresponding to longer metalligand bond lengths being populated in the High Spin (HS) state. Pressure is one of the relevant thermodynamic constraints to consider: in SCO materials based on Fe (d) ions, it usually favors the diamagnetic low-spin (LS) state, which has a lower volume than the paramagnetic HS state.

We present a theoretical model of spin transitions in stacks of molecular layers. Our model captures the already established physics of these systems (thermal hysteretic transitions and crossovers) and suggests a way towards in situ control of this physics by means of an external electric field. Our results pave the way toward both temperature and voltage controllable organic memory.

Linear polynuclear paddlewheel complexes-extended atom chains or metal strings-have provided attractive models for the study of metal-metal bonding, magnetism and conductivity since their discovery in the 1990s [1]. Their helicoidal chirality, arising from mutual steric hindrance of the 3-pyridyl protons, resulting in the twisting of the equatorial ligand around the axis (see figure), has been less studied. Nonetheless, in one of the few examples of chiral resolution, the obtained enantiomers of a trinickel complex showed a remarkably high specific rotation of 5000 deg•mL•g −1 •dm −1 [2], motivating us to seek a general technique for the chiral resolution of such racemates. We have developed a procedure based on anion exchange for the chiral resolution of [M3(dpa)4] 2+ salts (M = Co(II) or Ni(II), Hdpa = 2,2'-dipyridylamine). Homochiral arsenyl tartrate (AsT) salts promoted the selective crystallization of [-M3(dpa)4(MeCN)2](NBu4)2[-AsT]2, or [-M3(dpa)4(MeCN)2](NBu4)2[-AsT]2 in the P4212 space group. The enantiopure compounds demonstrated surprisingly large optical activities using UV-vis, Raman and infrared spectroscopy in solution and, for the cobalt derivatives, in the X-ray range at the Co K-edge in single crystals. An intense X-ray linear dichroism was observed in the orthoaxial crystal orientation, whereas it vanished in the axial confirmation, while the angular dependence of the circular dichroism spectra followed the expected (3cos 2 − 1) function, thus spectroscopically confirming the D4 crystal symmetry. X-ray magnetic circular dichroism and X-ray magnetochiral dichroism signals at the Co K-edge were not detected, likely due to a strongly delocalized spin density on the metal-metal bonded tricobalt core. Nevertheless, these results establish that chiral polynuclear paddlewheel complexes can be cleanly resolved using selective crystallization and demonstrate considerable optical activity in the infrared, UV-vis and X-ray energy ranges, thus potentially offering future perspectives in non-linear optics and asymmetric synthesis [3].

Iron borate FeBO 3 is an excellent example of the materials called “transparent magnets” [1], associating room temperature magnetic ordering (weak ferromagnetism) with transmission windows in visible spectral range [2]. The persisting research interest in iron borate is stimulated by its outstanding magnetic, magneto-acoustical, optical, magneto-optical, resonance, etc. characteristics [3–7]. Recently, new iron borate-based materials - Fe x Ga 1-x BO 3 crystals - have been synthesized by the solution in the melt technique [8] and studied by Electron Paramagnetic Resonance (EPR) [9], Nuclear Magnetic Resonance [10, 11] as well as by optical and magnetooptical techniques [12]. These crystals: (i) per se possess extraordinary physical characteristics suitable for practical applications, and these characteristics can be monitored in the synthesis process; (ii) allow comprehensive studies of diamagnetic dilution - isomorphous substitution of iron by gallium - effect on the properties of magnetic materials, viz., gradual transition from magnetically ordered to paramagnetic state; (iii) allow understanding the nature of various mechanisms responsible for magnetic properties of iron borate, e.g., magnetocrystalline anisotropy, the former having different concentration and temperature dependences. Different types of Electron Magnetic Resonance (EMR) have been observed in Fe x Ga 1-x BO 3 crystals depending on iron contents and the temperature. Figure 1 (a) shows the spectra transformation with x. At x = 1 (pure iron borate), only a low-field resonance is observed, earlier identified as Antiferromagnetic Resonance (AFMR) [7]. At a lower iron content, x = 0.75, besides the low-field line a new broad resonance emerges at higher magnetic fields, with the effective g-factor g≈2. Since iron substitution for gallium occurs more or less randomly, such crystals are expected to contain regions with different local iron concentrations, implying different magnetic ordering. The low-field line observed in the mixed crystals, by analogy with iron borate [7], can be identified as AFMR line arising from magnetically ordered regions, whereas the high-field line can be ascribed to Cluster Magnetic Resonance (CMR), i.e., EMR arising from only partially magnetically ordered regions. Both the low- and highfield EMR lines are present in all crystals with 0.35 ≤ x −1 Curie law, confirming its attribution to magnetic clusters. At still lower iron contents, x = 0.2, the AFMR line disappears and the high-field line increases in intensity. The EMR spectra for x = 0.2 crystal consist of a single line at g≈2, quite similar to the high-field line observed for higher x, consequently, in this case the antiferromagnetic regions are absent in the whole temperature range. The temperature dependence of the intensity of this line shown in Figure 1 (c), confirms that the Curie law for this resonance is not respected. For x = 0.04, the latter line also disappears and the EPR spectrum of diluted Fe 3+ ions, broadened by dipole-dipole interactions, comes into view. At a still lower iron content, x = 0.003, this spectrum is spectacularly narrowed. A detailed account of the EPR studies of these crystals has been recently carried out using laboratory-developed codes [9]. In order to confirm or infirm the existence of magnetic clusters in the mixed iron-gallium borates, we have carried out SQUID measurements. The temperature dependence of the magnetic susceptibility in crystals with intermediate x-values, e.g., see Figure 2, reveals the presence of a strong out-of-phase component, thereby confirming the existence of magnetic clusters at intermediate x values. Acknowledgments This work was partially supported by the V.I. Vernadsky Crimean Federal University Development Program for 2015 – 2024.

We measure linear absorption, circular dichroism, second harmonic, and sum frequency generation in the [Δ-Fe(phen)3](Δ-As2(tartarate)2), [Λ-Fe(phen)3](Λ-As2(tartarate)2) enantiomers of an Fe(II) complex. In the solid state, the chirality of this compound results from the introduction of the (As2(tartarate)2) chiral anions. Linear absorption and X-ray diffraction indicate that Fe(II) is in the low-spin state. Circular dichroïsm reveals that in the solid state, these compounds are chiral, whereas the complexes racemize in solution. A large second harmonic generation signal is recorded using thin films from these two enantiomers. The second-order susceptibility χ111(2) of these compounds is evaluated across the visible spectral range. It displays a resonance at 520 nm, which is associated with the metal-to-ligand charge transfer occurring within the complex. At its maximum, χ111(2) = 6.4 pm V−1 is more than 1.4 times larger than the well-known beta-BaB2O4 nonlinear crystal (χ122(2) ∼ 4.4 pm V−1). Finally, we demonstrate a useful application for a thin film from this compound, which characterizes the cross-correlation of two femtosecond laser pulses.

The thermally induced Spin-CrossOver (SCO) undergone by the mononuclear iron(II) complex [Fe(PM-AzA)(2) (NCS)(2)] (PM = N-2'-pyridylmethylene, AzA = 4-(phenylazo)aniline) is fully pictured by a quasi-continuous structural determination all along the spin-state modification within the sample. This large scale multi-temperature Single-Crystal X-Ray Diffraction (SCXRD) investigation leads to making structural movies. The latter reveal or confirm some features of the SCO that are subsequently validated by the same systematic investigation performed on a zinc isostructural analogue complex. Notably, the continuous views of the temperature dependencies of the unit-cell parameters, the dilatation tensors, the metal coordination sphere geometry and the intermolecular distances confirm a few of the structure-property relationships already known for SCO materials. In parallel, the examination of the temperature dependencies of the atomic coordinates and the atomic displacement parameters reveals unexpected behaviours in this gradual SCO material such as antagonistic atomic movements due to the single SCO and the pure thermal effects.

Temperature-and coverage-dependent studies of the Au(1 1 1)-supported spin crossover Fe(II) complex (SCO) of the type [Fe(H2B(pz)(2))(2)(bipy)] with a suite of surface-sensitive spectroscopy and microscopy tools show that the substrate inhibits thermally induced transitions of the molecular spin state, so that both high-spin and low-spin states are preserved far beyond the spin transition temperature of free molecules. Scanning tunneling microscopy confirms that [Fe(H2B(pz) 2) 2(bipy)] grows as ordered, molecular bilayer islands at sub-monolayer coverage and as disordered film at higher coverage. The temperature dependence of the electronic structure suggest that the SCO films exhibit a mixture of spin states at room temperature, but upon cooling below the spin crossover transition the film spin state is best described as a mix of high-spin and low-spin state molecules of a ratio that is constant. This locking of the spin state is most likely the result of a substrate-induced conformational change of the interfacial molecules, but it is estimated that also the intra-atomic electron-electron Coulomb correlation energy, or Hubbard correlation energy U, could be an additional contributing factor.

We present a new continuously-loaded high-pressure cell for neutron diffraction made from TiZr 'null-matrix' alloy that combines high mechanical resistance below 100 degrees C and negligible coherent neutron scattering. This cell operates at a maximum pressure of 700 MPa down to a temperature of 1.5 K. A sapphire optical window allows simultaneous illumination of the sample over the broad wavelength range similar to 0.4 to almost 5 mu m. The pressure is applied with Fluorinert or helium gas to ensure the best possible hydrostatic conditions at cryogenic temperatures.

The intricate phase diagram of the binuclear iron(II) spin-crossover complex [{Fe(3-bpp)(NCS)(2)}(2)(4,4'-bypiridine)]center dot 2CH(3)OH where 3-bpp is 2,6-bis(pyrazol-3-yl) pyridine has been investigated by variable temperature single crystal X-ray diffraction including a study into the effect of photo-irradiation. This sample is known to exhibit an incomplete spin transition at low temperature. At room temperature, in phase I, iron ions are all crystallographically equivalent, adopting the high spin state (HS). X-Ray structural investigation has revealed two phase transitions in the range (300-30 K). The first transition (T similar to 161 K) leading to phase II is of a purely structural nature and corresponds to a break in symmetry as a result of a twist of the two rings of 4,4'-bipyridine; the two iron sites of the binuclear unit becoming crystallographically independent but remaining all HS. The second structural transition corresponds to the spin crossover, one of the two Fe(II) ions of the binuclear complex being in the low spin state (LS) in phase III. The crystal structure shows an ordered HS-LS crystal packing where HS and LS sites are clearly identified and not randomly distributed in the metal ion sites as often observed. Moreover, light irradiation of single crystals in phase III at 30 K, leading to phase III*, induces a light-induced spin-state trapping (LIESST) effect corresponding to the full conversion of all the iron sites to HS. The crystal packing in phase III* is closer to that of phase III than to those observed in the other HS phases, I and II. This reveals an unusual differentiation between the thermal and light-induced HS states. A deeper analysis of the structural properties first demonstrates the key role of the bipyridine bridge in the peculiar preliminary pure structural transition shown by the title compound. Elsewhere, it also shows that the molecular packing is strongly dependent on the nature of the external perturbation contrary to the iron coordination sphere geometry that appears to be only dependent on the spin state. Moreover, in the HS phase II, the distortion of the iron sites that will subsequently undergo a spin crossover demonstrates some differences with the distortion of the iron sites that remain HS. The predominant role of the iron environment distortion in the spin crossover phenomenon is thus clearly evidenced.

Much of our recent experimental work has focused on the use of X-ray and neutron diffraction to study the structural evolution of hydrogen bonded molecular systems, including polymorphic materials, molecular complexes, tautomeric molecular materials, systems exhibiting hydrogen atom transfer and disorder, and magnetic systems coupled through hydrogen bond motifs.In particular we have focused on multi-temperature and pressure approaches to these studies, revealing often subtle behaviour of the hydrogen bonding, the structural evolution and on some cases of evolution of physical properties.Some of these effects are sufficiently subtle as to challenge the limits of current experimental diffraction, and also to challenge our theories of hydrogen bond formation.As a complementary approach to understanding these systems, we have for some years been applying developing plane-wave (periodic) density functional theory calculations for studying hydrogen bonds in the solid state.These are shown to have real potential in the study of a variety of hydrogen bonding systems.In addition MD approaches have been developed for these calculations, which allow us to examine the temperature evolution of molecular structures in the solid state and to quantify proton transfer effects.This leads to a fuller understanding of hydrogen bond formation and offers an improved description of the structural evolution observed in experiments.These approaches will be illustrated by results from a range of studies including: proton transfer systems, including examples in which the rational design and control of the degree of proton transfer is achieved, with effect on optical and nonlinear optical properties; materials with potentially cooperative hydrogen bonding; tautomeric hydrogen bonded systems in which very small experimental energy differences can be reproduced and understood; prediction of energy scales for polymorphism in hydrogen-bonded molecular complexes, and optimisation of magnetic coupling in inorganic materials by design of simple hydrogen-bonded linkages.

The spin-crossover system [Fe(ptz)(6)](BF4)(2) has been studied for more than 25 years and is used as a model system to the understanding of the eponymous phenomenon in solid-state materials. However, the structural properties of the low-spin phase formed at low temperature after a slow cooling have never been elucidated due to a splitting of the Bragg peaks. We report here a reinvestigation of this low-spin phase by single-crystal x-ray diffraction. This study demonstrates the perfect matching between the structural and magnetic transitions temperatures and hysteresis width through careful unit-cell temperature dependence. The Bragg splitting is also unambiguously associated to the spin transition. Above all, this work reveals a reversible doubling of the unit-cell parameters a and b corresponding to the spin transition. A preliminary solution for the crystal structure of the low-spin phase notably shows the potential major role of the deformation of the n-propyl groups in the physical behavior of this material.

A new iron(II) spin-crossover compound containing a phosphane ligand has been synthesized and characterized by magnetic susceptibility, Mossbauer spectroscopy, and heat capacity and photomagnetic experiments. The [Fe(tppb)Br-2] complex [tppb = 1,2,4,5-tetrakis(diphenylphosphanyl) benzene] exhibits a gradual spin conversion at T-1/2 = 184 K, and displays at low temperature a partial photoconversion of the low-spin (LS) state into the metastable high-spin (HS) state by irradiating the sample at 530 nm. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004).