Studies of a Large Odd-Numbered Odd-Electron Metal Ring: Inelastic Neutron Scattering and Muon Spin Relaxation Spectroscopy of Cr8 Mn.

The spin dynamics of Cr8 Mn, a nine-membered antiferromagnetic (AF) molecular nanomagnet, are investigated. Cr8 Mn is a rare example of a large odd-membered AF ring, and has an odd-number of 3d-electrons present. Odd-membered AF rings are unusual and of interest due to the presence of competing exchange interactions that result in frustrated-spin ground states. The chemical synthesis and structures of two Cr8 Mn variants that differ only in their crystal packing are reported. Evidence of spin frustration is investigated by inelastic neutron scattering (INS) and muon spin relaxation spectroscopy (µSR). From INS studies we accurately determine an appropriate microscopic spin Hamiltonian and we show that µSR is sensitive to the ground-spin-state crossing from S=1/2 to S=3/2 in Cr8 Mn. The estimated width of the muon asymmetry resonance is consistent with the presence of an avoided crossing. The investigation of the internal spin structure of the ground state, through the analysis of spin-pair correlations and scalar-spin chirality, shows a non-collinear spin structure that fluctuates between non-planar states of opposite chiralities.

Light-emission and excited-state dynamics in Tm2+ doped CsCaCl3, CsCaBr3, and CsCaI3.

The light-emission and photophysical properties of CsCaCl3:Tm2+ (1.04%), CsCaBr3:Tm2+ (0.48%), and CsCaI3:Tm2+ (0.76%) are presented. We find that Tm2+ is a multiple emitter under 21,834 cm-1 laser excitation at low temperatures in all three compounds. Several distinct types of emission are observed and characterized: sharp and long-lived 4f-4f emission in the infrared (IR) and up to four broad and fast decaying emission bands in the near-IR and visible, originating from the 4f-5d states of Tm2+. The optical spectroscopic properties of the samples are compared, and we find that the measured differences in the relative intensities and the shifts in the position of the emissions can be related to the chemical influence on the absorption and emission properties of Tm2+. Thus, it nicely illustrates the principle of chemical variation on the optical spectroscopic properties. An investigation of the temperature dependence of the luminescence yields important information about the dynamics of the excited states. The interplay and competition between radiative and nonradiative pathways is explained and modeled using a single configurational coordinate approach.

A family of heterometallic wheels containing potentially fourteen hundred siblings.

The synthesis and structure of new heterometallic wheels are reported, with preliminary studies of selected compounds.

Increasing the crystallisation temperature to access new spin clusters: conversion of [Ni8(cit)6(OH)2(H2O)2]10- to [Ni8(cit)6(OH)2]10-.

The role of temperature in the formation of high nuclearity nickel(II) citrate spin clusters is explored, revealing how changes in structure and hence magnetic properties can be triggered through desolvation and ligand reorganisation.

Absorption and Luminescence Spectroscopy of MnO(4)(2)(-)-Doped Crystals of BaSO(4).

The first polarized low-temperature absorption and luminescence spectra of manganese-doped crystals of BaSO(4) containing essentially MnO(4)(2)(-) are reported. By using a flux composed of NaCl, KCl, and CsCl we were able to grow BaSO(4):Mn(6+) crystals below 620 degrees C. This prevents the simultaneous presence of MnO(4)(3)(-) besides MnO(4)(2)(-), which was mainly responsible for the erroneous assignments of the absorption spectrum in the literature. In the BaSO(4) host the MnO(4)(2)(-) ion occupies a site of C(s)() symmetry, and the orbital degeneracies of the E and T states are thus lifted. Above 16 000 cm(-)(1) the absorption spectra consist of a series of intense ligand-to-metal charge transfer (LMCT) excitations. Their marked polarization dependence allows an unambiguous band assignment in the parent T(d)() symmetry. The three origins of the (2)E --> (2)T(2) ligand-field (LF) transition peak at 11 074, 11 570, and 11 790 cm(-)(1). The lowest-energy component of (2)T(2) serves as the initial state for broadband luminescence in the near-infrared (near-IR) region with a maximum at 9300 cm(-)(1). Below 100 K the quantum yield is unity and the radiative lifetime is 2.75 &mgr;s, and at 300 K the quantum yield is still 20%. In both the (2)E <--> (2)T(2) (d --> d) absorption and luminescence spectra the vibrational structure is dominated by progressions in O-Mn-O bending modes whereas coupling to the totally symmetric Mn-O stretching mode is less pronounced. The luminescence band shapes for the transitions to the two orbital components of (2)E are strikingly different; the Huang-Rhys parameters for the bending-mode progressions obtained from fits of simulated band shapes to the experimental spectra are 1.3 and 3.7, respectively. This is due to weak E multiply sign in circlee and stronger T(2) multiply sign in circlee Jahn-Teller (JT) effects in the ground and excited LF states, respectively. The linear vibronic coupling constants are f(E)() approximately 180 cm(-)(1) and f(T)() approximately -730 cm(-)(1) and the corresponding JT stabilization energies E(JT)((2)E) approximately 50 cm(-)(1) and E(JT)((2)T(2)) approximately 780 cm(-)(1), respectively.

Ruthenium(VI) Doped into Single Crystals with the BaSO(4) and beta-K(2)SO(4) Structures: Optical Absorption Spectra of RuO(4)(2)(-).

By establishing suitable high-temperature preparation conditions we were able to grow the first RuO(4)(2)(-)-doped crystals of K(2)CrO(4), K(2)SeO(4), Cs(2)MoO(4), BaSO(4), BaCrO(4), and BaSeO(4). Their polarized absorption spectra at low temperatures are reported and discussed. These are very different from the ruthenate(VI) spectra in the literature, and the lack of resemblance between calculated and measured spectra of ruthenate(VI)-assumed to be RuO(4)(2)(-) in the past-is easily understood. The (3)A(2) --> (1)A(1) spin-flip transition of RuO(4)(2)(-) peaks at approximately 7100 cm(-)(1). Stronger bands due to (3)A(2) --> (3)T(2) and (3)T(1) are observed in the vis around 13 500 and 16 500 cm(-)(1), respectively, giving rise to the green-blue color of most samples. In the BaSO(4) host the (1)A(1) absorption line is split into three differently polarized components exhibiting a strong temperature dependence. This can be analyzed in terms of a Boltzmann population with three levels at 0, 13, and 18 cm(-)(1), corresponding to the three spinor components of the (3)A(2) ground state. In order to better understand the major changes which occur on going from the 3d(2) to the 4d(2) electron configuration the spectroscopic data of RuO(4)(2)(-) are compared with those of CrO(4)(4)(-), MnO(4)(3)(-), and FeO(4)(2)(-).

Quantitative Interpretation of the Goodenough-Kanamori Rules: A Critical Analysis.

Expressions in the early literature for kinetic exchange in magnetically coupled systems are critically analyzed by second-order perturbation theory and substituted by corrected versions. The widely held belief that kinetic exchange is always antiferromagnetic is found to be incorrect. Ferromagnetic kinetic exchange terms are found to be more important than hitherto assumed. The quantity I /U, where I stands for intraatomic exchange interactions and U is the energy difference between the ground electron configuration and a charge-transfer configuration, plays a crucial role in the competition between ferromagnetic and antiferromagnetic contributions. I /U readily exceeds the value (1)/(5) considered as an upper limit by Anderson. I is found to be proportional to the number of unpaired electrons on a given magnetic center.

Spectroscopy and Dynamics of Re(4+) Near-IR-to-Visible Luminescence Upconversion.

Intense visible upconversion luminescence is observed in a Re(4+)-doped low-phonon host material, Cs(2)ZrCl(6), with excitation in the near-infrared. The upconversion excitation wavelength, temperature, power, and time dependence is studied using variable-temperature absorption and luminescence spectroscopies, and modeled using a three-level set of coupled nonlinear rate equations. On the basis of analysis of these data, energy-transfer upconversion is shown to be the dominant mechanism responsible for this phenomenon. The main properties of this upconversion mechanism are correlated to properties of the Re(4+) electronic structure, in particular the large energy gaps, the small excited-state distortions, and the lack of cross-relaxation pathways for radiationless decay. These properties make this ion attractive for upconversion phosphor and laser materials applications, and two potential upconversion laser schemes are proposed. The contributions of these same properties to the photoredox chemistry of Re(4+) and related d(3) ions are discussed.

Exchange Interactions Derived from Electron-Transfer Processes in [Cr(2)(OH)(3)(tmtame)(2)](NO(3))(3).

From polarized optical absorption and emission spectra of [Cr(2)(OH)(3)(tmtame)(2)](NO(3))(3) (tmtame = N,N',N' '-trimethyl-1,1,1-tris(aminomethyl)ethane) in the visible and near UV, the exchange splittings of the (4)A(2)(4)A(2) ground as well as the (2)E(4)A(2) and (2)T(1)(4)A(2) singly and (2)E(2)E, (2)E(2)T(1), (2)T(1)(2)T(1), (2)E(2)T(2), and (2)T(1)(2)T(2) doubly excited states of the ground electron configuration are determined, the latter corresponding to simultaneous pair excitations by a single photon. The bulk of intensity in the region of these doubly excited states is found to be vibronically induced by an electric-dipole exchange mechanism. From single-crystal and glass absorption spectra a ferromagnetic energy ordering of the lowest energy ligand-to-metal charge transfer (LMCT) states is derived, whereas the ground state is antiferromagnetically split. The observed splittings are rationalized using a model based on a valence bond approach (VBCI), where the exchange interactions are derived from configuration interaction of LMCT and metal-to-metal charge transfer (MMCT) electron configurations with the ground configuration. The splittings are well reproduced by this model over a range of about 40 000 cm(-)(1). Trigonal orbital exchange parameters J(a) and J(e) are derived, revealing that the direct pathway along the Cr-Cr axis is the dominant one. This gives rise to a double exchange situation in the LMCT configuration, leading to the observed ferromagnetic energy ordering of LMCT levels. Magnetostructural correlations are established from a comparison of the title compound with the similar complex [Cr(2)(OH)(3)(tmtacn)(2)](ClO(4))(3) (tmtacn = 1,4,7-trimethyl-1,4,7-triazacyclononane).

Modification of the magnetic properties of a heterometallic wheel by inclusion of a Jahn-Teller distorted Cu(II) ion.

An investigation into the physical consequences of including a Jahn-Teller distorted Cu(II) ion within an antiferromagnetically coupled ring, [R(2)NH(2)][Cr(7)CuF(8)((O(2)C(t)Bu)(16))] is reported. Inelastic neutron scattering (INS) and electron paramagnetic resonance (EPR) spectroscopic data are simulated using a microscopic spin Hamiltonian, and show that the two Cr-Cu exchange interactions must be inequivalent. One Cr-Cu exchange is found to be antiferromagnetic and the other ferromagnetic. The geometry of the Jahn-Teller elongation is deduced from these results, and shows that a Jahn-Teller elongation axis must lie in the plane of the Cr(7)Cu wheel; the elongation is not observed by X-ray crystallography, due to positional disorder of the Cu site within the wheel. An electronic structure calculation confirms the structural distortion of the Cu site.

Varying spin state composition by the choice of capping ligand in a family of molecular chains: detailed analysis of magnetic properties of chromium(III) horseshoes.

We report a detailed physical analysis on a family of isolated, antiferro-magnetically (AF) coupled, chromium(III) finite chains, of general formula (Cr(RCO(2))(2)F)(n) where the chain length n = 6 or 7. Additionally, the chains are capped with a selection of possible terminating ligands, including hfac (= l,l,l,5,5,5-hexafluoropentane-2,4-dionate(l-)), acac (= pentane-2,4-dionate(l-)) or (F)(3). Measurements by inelastic neutron scattering (INS), magnetometery and electron paramagnetic resonance (EPR) spectroscopy have been used to study how the electronic properties are affected by n and capping ligand type. These comparisons allowed the subtle electronic effects the choice of capping ligand makes for odd member spin 3/2 ground state and even membered spin 0 ground state chains to be investigated. For this investigation full characterisation of physical properties have been performed with spin Hamiltonian parameterisation, including the determination of Heisenberg exchange coupling constants and single ion axial and rhombic anisotropy. We reveal how the quantum spin energy levels of odd or even membered chains can be modified by the type of capping ligand terminating the chain. Choice of capping ligands enables Cr-Cr exchange coupling to be adjusted by 0, 4 or 24%, relative to Cr-Cr exchange coupling within the body of the chain, by the substitution of hfac, acac or (F)(3) capping ligands to the ends of the chain, respectively. The manipulation of quantum spin levels via ligands which play no role in super-exchange, is of general interest to the practise of spin Hamilton modelling, where such second order effects are generally not considered of relevance to magnetic properties.

Assessment of the anisotropy in the molecule Mn19 with a high-spin ground state S = 83/2 by 35 GHz electron paramagnetic resonance.

35 GHz electron paramagnetic resonance experiments on a powder sample of the magnetic molecule Mn 19 with a high-spin ground state S = 83/2 are presented. At low temperatures, the data are well described by the simulated spectra for an isolated spin with a zero-field-splitting parameter D = 0.004 cm (-1), which is, in particular, positive. Hence, Mn 19 is not a single-molecule magnet; the previously observed magnetic hysteresis at ultralow temperatures is likely due to intermolecular dipolar interactions.

Studies of finite molecular chains: synthesis, structural, magnetic and inelastic neutron scattering studies of hexa- and heptanuclear chromium horseshoes.

We report the synthesis and structural characterisation of a family of finite molecular chains, specifically [{[R(2)NH(2)](3)[Cr(6)F(11)(O(2)CCMe(3))(10)]}(2)] (in which R=nPr 1, Et 2, nBu 3), [{Et(2)NH}(2){[Et(2)NH(2)](3)[Cr(7)F(12)(O(2)CCMe(3))(12)][HO(2)CCMe(3)](2)}(2)] (4), [{[Me(2)NH(2)](3)[Cr(6)F(11)(O(2)CCMe(3))(10)]2.5 H(2)O}(4)] (5) and [{[iPr(2)NH(2)](3)[Cr(7)F(12)(O(2)CCMe(3))(12)]}(2)] (6). The structures all contain horseshoes of chromium centres, with each Cr...Cr contact within the horseshoe bridged by a fluoride and two pivalates. The horseshoes are linked through hydrogen bonds to the secondary ammonium cations in the structure, leading to di- and tetra-horseshoe structures. Through magnetic measurements and inelastic neutron scattering studies we have determined the exchange coupling constants in 1 and 6. In 1 it is possible to distinguish two exchange interactions, J(A)=-1.1 meV and J(B)=-1.4 meV; J(A) is the exchange interactions at the tips of the horseshoe and J(B) is the exchange within the body of the horseshoe (1 meV=8.066 cm(-1)). For 6 only one interaction was needed to model the data: J=-1.18 meV. The single-ion anisotropy parameters for Cr(III) were also derived for the two compounds as: for 1, D(Cr)=-0.028 meV and |E(Cr)|=0.005 meV; for 6, D(Cr)=-0.031 meV. Magnetic-field-dependent inelastic neutron scattering experiments on 1 allowed the Zeeman splitting of the first two excited states and level crossings to be observed. For the tetramer of horseshoes (5), quantum Monte Carlo calculations were used to fit the magnetic susceptibility behaviour, giving two exchange interactions within the horseshoe (-1.32 and -1.65 meV) and a weak inter-horseshoe coupling of +0.12 meV. Multi-frequency variable-temperature EPR studies on 1, 2 and 6 have also been performed, allowing further characterisation of the spin Hamiltonian parameters of these chains.

Odd-numbered Fe(III) complexes: synthesis, molecular structure, reactivity, and magnetic properties.

Three isostructural disklike heptanuclear FeIII compounds of the general formula [FeIII7(mu3-O)3(L)3(mu-O2CCMe3)6(eta1-O2CCMe3)3(H2O)3], where L represents a di- or triethanolamine moiety, display a three-blade propeller topology, with the central Fe atom representing the axle or axis of the propeller. This motif corresponds to the theoretical model of a frustrated Heisenberg star, which is one of the very few solvable models in the area of frustrated quantum-spin systems and can, furthermore, be converted to an octanuclear cage for the case where L is triethanolamine to give [FeIII8(mu4O)3(mu4-tea)(teaH)3(O2CCMe3)6(N3)3].1/2MeCN.1/2H2O or [FeIII8(mu4O)3(mu4-tea)(teaH)3(O2CCMe3)6(SCN)3].2MeCN when treated with excess NaN3 or NH4SCN, respectively. The core structure is formally derived from that of the heptanuclear compounds by the replacement of the three aqua ligands by an {Fe(tea)} moiety, so that the 3-fold axis of the propeller is now defined by two central FeIII atoms. Magnetic studies on two of the heptanulcear compounds established unequivocally S = 5/2 spin ground state for these complexes, consistent with overall antiferromagnetic interactions between the constituent FeIII ions.

A wheel-shaped single-molecule magnet of [MnII 3MnIII 4]: quantum tunneling of magnetization under static and pulse magnetic fields.

The reaction of N-(2-hydroxy-5-nitrobenzyl)iminodiethanol (=H3(5-NO2-hbide)) with Mn(OAc)2* 4 H2O in methanol, followed by recrystallization from 1,2-dichloroethane, yielded a wheel-shaped single-molecule magnet (SMM) of [MnII 3MnIII 4(5-NO2-hbide)6].5 C2H4Cl2 (1). In 1, seven manganese ions are linked by six tri-anionic ligands and form the wheel in which the two manganese ions on the rim and the one in the center are MnII and the other four manganese ions are MnIII ions. Powder magnetic susceptibility measurements showed a gradual increase with chimT values as the temperature was lowered, reaching a maximum value of 53.9 emu mol(-1) K. Analyses of magnetic susceptibility data suggested a spin ground state of S=19/2. The zero-field splitting parameters of D and B 0 4 were estimated to be -0.283(1) K and -1.64(1)x10(-5) K, respectively, by high-field EPR measurements (HF-EPR). The anisotropic parameters agreed with those estimated from magnetization and inelastic neutron scattering experiments. AC magnetic susceptibility measurements showed frequency-dependent in- and out-of-phase signals, characteristic data for an SMM, and an Arrhenius plot of the relaxation time gave a re-orientation energy barrier (DeltaE) of 18.1 K and a pre-exponential factor of 1.63x10(-7) s. Magnetization experiments on aligned single crystals below 0.7 K showed a stepped hysteresis loop, confirming the occurrence of quantum tunneling of the on magnetization (QTM). QTM was, on the other hand, suppressed by rapid sweeps of the magnetic field even at 0.5 K. The sweep-rate dependence of the spin flips can be understood by considering the Landau-Zener-Stückelberg (LZS) model.

Crystal absorption spectra in the region of 4f-4f and 4f-5d excitations in Tm2+-doped CsCaCl3, CsCaBr3, and CsCaI3.

Low-temperature absorption spectra of single crystals of Tm2+-doped CsCaCl3, CsCaBr3, and CsCaI3 in the spectral range from 8700 to 47000 cm-1 are presented. Weak sharp-line 4f-4f absorptions around 8800 cm-1 are essentially independent of the nature of the halide. More-intense broad absorptions cover the region between 12000 and 47000 cm-1. They are assigned to 4f-5d excitations and interpreted in terms of a simple qualitative picture taking into account the most important interactions. As a result of two counterbalancing effects, the onsets of the 4f-5d spectra are almost coincident in the three materials: The blue-shift of about 3000 cm-1 between chloride and iodide resulting from the decreasing crystal field splitting of 5d is roughly balanced by the red-shift resulting from the reduced energy gap between the average energy of the 4f13 and the 4f125d1 electron configurations. The absorption helps the understanding of the most unusual light emission properties of these materials.

New type of near-infrared to visible photon upconversion in Tm2+-doped CsCaI3.

Tm2+ doped in CsCaI3 displays unusual optical properties that are characterized by the existence of two metastable 4f-5d excited states in the near-infrared and visible spectral region, respectively. For the first time, a photon upconversion process based on sequential absorption of light by 4f-5d states is reported. The large absorption cross-section of the involved transitions allows highly efficient pumping in the NIR. An efficiency of 11% for the green upconversion luminescence is reached at 10 K, and the upconversion luminescence remains visible by eye up to room temperature. The energy positions of the relevant 4f-5d states and thus the photophysical and light emission properties can be tuned by chemical variation, such as placing the Tm2+ ion into the isostructural CsCaBr3 and CsCaCl3 lattices.