Modulation of Slow Magnetic Relaxation in Gd(III)-Tetrahalosemiquinonate Complexes

Photoluminescent, dielectric, and magnetic responsivity to the humidity variation in SHG-active pyroelectric manganese(ii)-based molecular material

Multifunctional response to external stimuli which engages various properties, including optical, dielectric, magnetic, or mechanical, can be the source of new generations of highly sensitive sensors and advanced switches. Such responsivity is expected for molecular materials based on metal complexes whose properties are often sensitive to even subtle changes in a particular stimulus. We present a novel hybrid organic-inorganic salt based on earth-abundant divalent manganese ions forming two types of complexes, octahedral [MnII(Me-dppmO2)3]2+ cations with methyl-functionalized bis(diphenylphosphino)methane dioxide ligands and tetrahedral [MnIICl4]2- anions. These ions crystallize with water molecules leading to the molecular material [MnII(Me-dppmO2)3][MnIICl4]·H2O (1). We show that, due to the simple methyl substituent on the diphosphine-type ligand, 1 reveals a polar crystal structure of the Cc space group as confirmed by the single-crystal X-ray diffraction, second-harmonic generation (SHG) effect, piezoelectric response, and pyroelectricity. Besides these non-centrosymmetricity-related non-linear optical and electrical features, this material combines three other physical properties, i.e., visible room-temperature (RT) photoluminescence (PL) originating from d-d electronic transitions of octahedral Mn(ii) complexes, dielectric relaxation in ca. 170-300 K range related to Bjerrum-type orientation defects of water molecules, and slow magnetic relaxation below 3 K related to spin-phonon interactions involving paramagnetic Mn(ii) centers. We demonstrate that these three physical effects detected in 1 are sensitive to humidity variation that governs the RT-PL intensity, leads to the ON/OFF switching of dielectric relaxation around RT, and non-trivially modulates the magnetic relaxation at cryogenic temperatures. Thus, we report a unique molecular material revealing broadened multifunctionality and triple physical responsivity to the humidity change exploring luminescent, dielectric, and magnetic properties.

Redox, spectroscopic and magnetic properties of C3-symmetric rare earth complexes featuring atypical ortho-dioxolene binding

The molecular symmetry in rare earth (RE) coordination chemistry is critically important for controlling the electronic structure of the RE ion and the resulting magnetic and photophysical properties. Here, we report a family of complexes with unusual C3-point symmetry: [REIII(Br4catH)3(tpa)] (Br4catH- = tetrabromocatecholate, tpa = tris(2-pyridylmethyl)amine). The synthesis and solid-state characterisation of eleven analogues (RE = Y, Sm to Lu) were performed, enabling a systematic investigation of the effect of symmetry on various physical properties across the RE series. The crystal structures reveal a unique cooperative coordination motif, featuring a cyclic hydrogen-bonding network between the atypical monodentate monoprotonated Br4catH- ligands. Electrochemical analysis reveals a single oxidation process that suggests a concerted three-electron oxidation of all tetrabromocatecholate ligands to semiquinonate. Furthermore, single-molecule magnet (SMM) behaviour was investigated, revealing unexpected in-field slow magnetic relaxation for both Dy and Yb analogues, which can be rationalised by the effect of C3-symmetry. Finally, luminescence measurements were performed to probe the CF splitting of the Yb analogue and quantify the error in the overall CF splitting predicted by ab initio calculations. The governing effects of C3-symmetry are consistent observations in all RE3+ metals studied in this work, manifesting in the concerted three-electron oxidation, SMM behaviour, ground state composition, and luminescence properties.

Ab initio-based determination of lanthanoid-radical exchange as visualised by inelastic neutron scattering

Magnetic exchange coupling can modulate the slow magnetic relaxation in single-molecule magnets. Despite this, elucidation of exchange coupling remains a significant challenge for the lanthanoid(iii) ions, both experimentally and computationally. In this work, the crystal field splitting and 4f-π exchange coupling in the erbium-semiquinonate complex [ErTp2dbsq] (Er-dbsq; Tp- = hydro-tris(1-pyrazolyl)borate, dbsqH2 = 3,5-di-tert-butyl-1,2-semiquinone) have been determined by inelastic neutron scattering (INS), magnetometry, and CASSCF-SO ab initio calculations. A related complex with a diamagnetic ligand, [ErTp2trop] (Er-trop; tropH = tropolone), has been used as a model for the crystal field splitting in the absence of coupling. Magnetic and INS data indicate antiferromagnetic exchange for Er-dbsq with a coupling constant of Jex = -0.23 meV (-1.8 cm-1) (-2Jex formalism) and good agreement is found between theory and experiment, with the low energy magnetic and spectroscopic properties well modelled. Most notable is the ability of the ab initio modelling to reproduce the signature of interference between localised 4f states and delocalised π-radical states that is evident in the Q-dependence of the exchange excitation. This work highlights the power of combining INS with EPR and magnetometry for determination of ground state properties, as well as the enhanced capability of CASSCF-SO ab initio calculations and purposely developed ab initio-based theoretical models. We deliver an unprecedentedly detailed representation of the entangled character of 4f-π exchange states, which is obtained via an accurate image of the spin-orbital transition density between the 4f-π exchange coupled wavefunctions.

Elucidation of LMCT Excited States for Lanthanoid Complexes: A Theoretical and Solid-State Experimental Framework

Photophysical and magnetic properties arising from both ground and excited states of lanthanoid ions are relevant for numerous applications. These properties can be substantially affected, both adversely and beneficially, by ligand-to-metal charge-transfer (LMCT) states. However, probing LMCT states remains a significant challenge in f-block chemistry, particularly in the solid state. Intriguingly, the europium compounds [Eu^III(18-c-6)(X₄Cat)(NO₃)]·MeCN (18-c-6 = 18-crown-6; X = Cl (tetrachlorocatecholate, 1-Eu) or Br (tetrabromocatecholate, 2-Eu) are distinctly darkly-colored, in marked contrast to the analogues with other lanthanoid ions in the 1-Ln and 2-Ln series (Ln = La, Ce, Nd, Gd, Tb, and Dy). Herein, we report a multi-technique investigation of these compounds that has allowed elucidation of the LMCT character of the relevant absorption bands using magnetometry, absorption and emission spectroscopies, and solid-state electrochemistry. To support experimental observations, we present a semi-quantitative multireference ab initio model that (i) captures the anomalously low-lying LMCT excited state observed in the visible spectrum of 1-Eu (and its absence in the other 1-Ln analogues); (ii) elucidates the contribution of the LMCT excitation to the crystal field split ⁷F_J ground-state wave functions; and (iii) identifies the crucial role played by radial dynamical correlation of the Eu^III 4f electrons in the description of the LMCT excited state, modeled by the inclusion of 4f → 5f excitations in the optimized wave function. By providing a set of experimental and theoretical tools, this work establishes a framework for the elucidation of LMCT excited states in lanthanoid compounds in the solid state.