Microscopic Model for Photoinduced Magnetism in the Molecular Complex [Mo(IV)(CN)2(CN−CuL)6]8+[Mo(IV)(CN)_2(CN-CuL)_6]^{8+} Perchlorate

A theoretical model for understanding photomagnetism in the heptanuclear complex $[Mo(IV)(CN)_2(CN-CuL)_6]^{8+}$ perchlorate is developed. It is a many-body model involving the active orbitals on the transition metal ions. The model is exactly solved using a valence bond approach. The ground state solution of the model is highly degenerate and is spanned by five S=0 states, nine S=1 states, five S=2 states and one S=3 state. The orbital occupancies in all these states correspond to six $Cu(II)$ ions and one diamagnetic $Mo(IV)$ ion. The optically excited charge-transfer (CT) state in each spin sector occur at nearly the same excitation energy of 2.993 eV for the physically reasonable parameter values. The degeneracy of the CT states is largest in the S=3 sector and so is the transition dipole moment from the ground state to these excited states. Thus laser irradiation with light of this energy results in most intense absorption in the S=3 sector. The life-time of the S=3 excited states is also expected to be the largest as the number of states below that energy is very sparse in this spin sector when compared to other spin sectors. These twin features of our model explain the observed photomagnetism in the $[Mo(IV)(CN)_2(CN-CuL)_6]^{8+}$ complex.Comment: 8 pages, 6 figures and 1 tabl

A Kinetic Model for Photoswitching of magnetism in the High Spin Molecule [Mo(IV)(CN)2(CN-Cu(II)(tren))6](ClO4)8

The heptanuclear complex [Mo(IV)(CN)2(CN-CuL)6]8+ exhibits photomagnetism. An earlier microscopic model showed that the transition dipole moments for excitation in different spin manifolds are similar in magnitude. In this paper, we attribute photomagnetism to the long lived S=3 charge transfer excited state for which there appears to be sufficient experimental evidence. We model the photomagnetism by employing a kinetic model which includes internal conversions and intersystem crossings. The key feature of the model is assumption of the existence of two kinds of S=3 states: one which has no direct pathway for internal conversion and the other characterized by slow kinetics for internal conversion to the low-energy states. The trapped S=3 state can decay via a thermally activated barrier to the other S=3 state. The experimental temperature dependence of magnetization plot is fitted using rate constants with Arrhenius dependence. The two different experimental cMT vs. T curves obtained with different irradiation times are fitted with our model. Our studies show that the photomagnetism in these systems is governed by kinetics and not due to differences in oscillator strengths for excitation of the different spin states.Comment: 17 pages including 5 figures. Submitted to Phys. Rev.

Metal-to-metal electron transfer : a powerful tool for the design of switchable coordination compounds

Since the report of photomagnetic effects in Prussian blue Fe/Co networks 20 years ago by the Japanese group of Hashimoto, a substantial family of molecular analogs have been obtained and characterized. These compounds offer a unique opportunity to follow metal-to-metal electron transfer by investigating their structural, spectroscopic, electrochemical and magnetic properties. We propose an overview of recent results in order to highlight the common features of these coordination compounds, as well as the differences, with the well-known class of photomagnetic coordination compounds based on spin-crossover

Long-range magnetic ordering and bistability in molecular magnetism

Molecular magnetism is a new area of research dealing with the chemistry and physics. of molecular assemblies involving open-shell molecules, and polymeric structures synthesized from molecular precursors [1]. In that sense, molecular magnetism may be considered as a supramolecular function. What is important, it is to assemble the open-shell units within the lattice in such · a way that the interactions between thése units lead to cooperative phenomena. Cooperative here means that the lattice as a whole exhibits some physical properties which are much more than the sum of the physical properties of the units

New photomagnetic cyanido-bridged Cu(II)-Mo(IV) oligonuclear complexes: slight modification of the blocking ligands induces different structures

A pentanuclear complex, [Mo(CN)8{CuL1}2{CuL1(H2O)}2].6H2O 1, and a decanuclear one, [Mo(CN)8{CuL2}3{CuL2(H2O)}]2.10H2O 2, have been obtained following the building-block approach (HL1 and HL2 are tridentate Schiff bases obtained by reacting salicylaldehyde with 2-aminomethyl-pyridine and 2-aminoethyl-pyridine, respectively). Both compounds present reversible photo-induced magnetic properties interpreted as a MoIV-CuII charge transfer. Compound 2 exhibits a lower efficiency of this metal-metal charge transfer than compound 1 attributed to a less flexible structure of 2 compared to 1

Cooperative relaxation of the metastable states in the photomagnetic octacyanotungstate CsI[{CoII(3-CN-py)2}{WV(CN)8}] · H2O

The metastable states of the Cs[{Co(3-cpy)2}{W(CN)8}] .H2O (3-cpy = 3-cyanopyridine) complex,, characterized by a CTIST phenomenon with a high-temperature phase formed by CoIIHS (S = 3/2)-NC-WV (S = 1/2) units and a low-temperature phase formed by CoIIILS (S = 0)-NC-WIV (S = 0) units, have been induced at low temperature either by thermal quenching (RC state) or by irradiation (PI state). The T(TIESST) and T(LIESST) relaxation temperatures of RC and PI states, respectively, have been determined and the incompleteness of photo-excitation discussed. The presence of sigmoidal kinetics and the observation of LITH hysteresis loop under irradiation demonstrate the cooperative nature of the relaxation. The thermodynamic parameters have been determined and a simulation of the T(TIESST) curve done

Pressure response of the bimetallic chain compound MnNi(NO2)4(en)2; en = ethylenediamine

The compound MnNi(NO2)4(en)2, en = ethylendiamine, is a rare example of a ferromagnetically coupled bimetallic chain (Jshort parallel chain not, vert, similar 2–3 K). Further, for this material, a finite inter-chain coupling triggers an antiferromagnetic (AFM) transition below TN = 2.3 K. Here, we present a susceptibility study of MnNi(NO2)4(en)2 under pressure. From our data, we derive the pressure response of the antiferromagnetic transition temperature TN. Even up to highest pressure of 8.2 kbar we find a very modest increase of TN by only not, vert, similar0.1 K. Independently, from a pressure study of the lattice parameters of MnNi(NO2)4(en)2, we calculate the bulk module of the material to not, vert, similar8.8 GPa. Combining the data TN(p) with the bulk module yields an anomalously low Grüneisen parameter Γmag of only 0.5(5). We discuss possible scenarios accounting for such a low Grüneisen parameter

Synthesis, structure and magnetic properties of dinuclear cobalt-tetraoxolene complexes with bidentate terminal ligands

The design of molecule-based systems with tuneable optical and/or magnetic properties has attracted considerable attention because of their potential applications in high-performance molecule-based electronic devices, switches, sensors and displays. In this regard, a large number of valence tautomeric tetraoxolene-bridged dinuclear cobalt complexes with tetradentate ancillary ligands have been reported, but none of these complexes contained a bidentate terminal ligand. In order to increase the scope in this field, the present report describes the synthesis, structures, electrochemical and magnetic studies of two dinuclear cobalt-tetraoxolene complexes, [Co2(dhbq)(bpy)4](PF6)3 (1(PF6)3) and [Co2(dhbq)(bpa)4](PF6)3·6H2O (2(PF6)3·6H2O), where H2dhbq is 2,5-dihydroxy-1,4-benzoquinone, with the bidentate terminal ligands 2,2′-bipyridine (bpy) and 2,2′-bipyridyl amine (bpa), respectively. An X-ray diffraction study reveals that the dianionic form of the redox active tetraoxolene ligand bridges the metal centers in these complexes, and one metal center is in the high spin cobalt(II) state while the other one is in the low spin cobalt(III) state. The present report will increase knowledge on the synthesis of such systems using bidentate terminal ligands. Variable temperature magnetic susceptibility measurements show no indication for valence tautomerism (VT) in either complex. Further attempts could be made to synthesize similar complexes from different solvents and/or using different counter ions to check whether such variations can bring VT in these systems