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The magnetic and structural properties of three gradual spin transition monomeric compounds based on the cation [Fe(Hpt)3] 2þ (Hpt ¼ 3-(pyrid-2-yl)-1,2,4-triazole) are presented. The non-cooperative character of the spin-crossover in [Fe(Hpt)3](BF4)2·2H2 O( I) is evaluated in light of its calorimetric properties, which yielded the thermodynamic values DtrsH ¼ 5.81 kJ mol 21 and DtrsS ¼ 39.5 J mol 21 K 21 . The light-induced excited spin-state trapping effect is performed on [Fe(Hpt)3](BF4)2·2H2 O( I) and [Fe(Hpt)3](SO4)0.4(BF4)1.2·3H2 O( II), and the subsequent HS ! LS relaxations are studied. Their merely first-order kinetics are affected by disorder in the structure of both complexes, which appears in the presence of a distribution of activation energies. HS species can also be frozen-in in I by rapid cooling. Continuous irradiation is shown to induce only apparent light-induced thermal hysteresis effect in I and II, stemming from slow kinetics of relaxation with respect to the kinetics of measurement. q 2003 Elsevier Science Ltd. All rights reserved.

The synthesis and characterization of the spin transition compound hexakis(1-methyltetrazole-N4)iron(II) trifluoromethanesulfonate obtained from the reaction of 1-methyltetrazole and iron(II) triflate are described. The spin transition of [Fe(mtz)6](CF3SO3)2 (1) presents a hysteresis loop in magnetism with T↓=157.0 and T↑=179.5 K. Only one-third of the Fe(II) ions undergo this transition, the other two-thirds of the ions remain in the high-spin state. Mossbauer spectroscopy and relaxation studies have shown that the actual width of the hysteresis is only 12 K, with T↓=167.5 and T↑=179.5 K. Thermal spin-state trapping appears to be possible. The trapped high-spin state is stable up to 103 K. The relaxation is not first-order. After irradiation with red light, reverse light-induced excited spin state trapping (LIESST) is observed. The new low-spin state is stable up to 42 K. Also, its relaxation to HS is non-first-order. A reverse light-induced thermal hysteresis loop of approximately 7 K width is present too. No LIESST effect could be observed. Finally, a ferromagnetic type of coupling has been recognized, which is temperature and field dependent.

In [Fe(mtz) 6 ](BF 4 ) 2 (mtz=1-methyltetrazole), a spin transition of 50% of the Fe(II) ions corresponding to one crystallographic site occurs at 74 K. Thermal trapping in the HS state of the spin transiting site is achieved quantitatively. Relaxation back to the LS state occurs in the range 45–70 K. Non-first-order kinetics are reproduced using a phenomenological model including a self-acceleration parameter α =1.8. The Arrhenius plot shows that a thermally activated process is dominant in the range of temperatures studied, with an activation energy E a =6.02 kJ mol −1 . The speed of cooling and the applied field are shown to be of small influence on the kinetics. A new member of the propyltetrazole family, [Fe(ptz) 6 ](CF 3 SO 3 ) 2 ·2ptz (ptz=1-propyltetrazole), has been synthesized, which has two non-coordinated ptz molecules in the lattice. Its steep transition at 178 K is characterized by magnetic measurements, Mossbauer spectroscopy, reflectivity and calorimetry. The LIESST effect is shown to be present at 10 K, but only on the surface and with an extremely short lifetime.

The compounds hexakis(1-(2-chloroethyl)-tetrazole-N4)copper(II) tetrafluoroborate mono(1-(2-chloroethyl)-tetrazole) (1) and hexakis(1-(2-chloroethyl)-tetrazole-N4) copper(II) perchlorate mono(1-(2-chloroethyl)-tetrazole) (2) were obtained from the crystallization of 1-(2-chloroethyl)-tetrazole (teec) with the respective copper salt. The X-ray structures are reported. Both compounds crystallize in the monoclinic space group C2/c for 1: a = 25.754(3) A, b = 18.162(3) A, c = 10.298(2) A β = 104.294(7)° and Z = 4 and for 2: a = 25.745(3) A, b = 18.197(3) A, c = 10.29355(3) A β = 104.512(10)° and Z = 4. The CuII ions are octahedrally surrounded by six teec ligands. One free teec ligand is present in the structures. The Cu–N distances in the centrosymmetrical units are 2.01, 2.04, and 2.39 A in both compounds, consistent with a Jahn–Teller elongated octahedron. The ligand field spectra of both compounds show a signal in the visible range at about 15.200 cm−1 and by an EPR spectrum the following values are found: g| = 2.29 and g⊥ = 2.03. The compounds are both paramagnetic and the χ T value of 0.37 cm−1 K mol−1, which is associated with S = 1/2, is what would be expected for a mononuclear CuII complex.

We have synthesized, crystallized, and studied the structural and electric transport properties of organic molecular crystals based on a rubrene derivative with $t$-butyl sidegroups at the 5,11 positions. Two crystalline modifications are observed: one (A) distinct from that of rubrene with larger spacings between the naphtacene backbones, the other (B) with an in-plane structure presumably very similar compared to rubrene. The electric transport properties reflect the different structures: in the latter phase (B), the in-plane hole mobility of $12\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{2}∕\mathrm{V}\phantom{\rule{0.2em}{0ex}}\mathrm{s}$ measured on single-crystal field-effect transistors is just as high as in rubrene crystals, while in the A phase, no field effect could be measured. The high crystal quality, studied in detail for B, reflects itself in the density of gap states: The deep-level trap density of as low as ${10}^{15}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}\phantom{\rule{0.2em}{0ex}}{\mathrm{eV}}^{\ensuremath{-}1}$ has been measured, and an exponential band tail with a characteristic energy of $22\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ is observed. The bulk mobility perpendicular to the molecular planes is estimated to be of the order of ${10}^{\ensuremath{-}3}--{10}^{\ensuremath{-}1}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{2}∕\mathrm{V}\phantom{\rule{0.2em}{0ex}}\mathrm{s}$.

Spin transitions (ST) in transition-metal complexes are a well-known phenomenon in coordination chemistry. Most studied are the systems with a d 6 electron configuration, where the transition goes between essentially diamagnetic and paramagnetic states. In many cases the transition temperature is different when heating or when cooling the sample, and the resulting hysteresis can be applied for storage. So, the colour change would allow possible applications such as in storage devices. Our approach has been the development of systems with Fe(II) and heterocyclic ligands, such as the triazoles and tetrazoles. An overview of recent results is given below, with a focus upon structures and changes in the physical properties. Most structures have the general formula FeL 6 (anion) 2 , with L is a monodentate, or half of a didentate ligand. A special case deals with the compound Fe(btr) 2 (NCS) 2 (H 2 O), the first compound with a trans geometry for the NCS ligands that shows a spin transition. The role of the wa...

The title compound, C50H44, is a derivative of rubrene where tert-butyl side groups are added to two of the pendant aromatic rings. Two polymorphs of this derivative, the title compound (form A) and form B, have been identified. The mol­ecule of form A displays a strongly twisted naphthacene backbone. The in-plane arrangement differs from the classical herringbone structure, resembling a slip–stack structure type with the backbones separated by a minimum 7.0 A in the direction of possible π-stacking.

The title compound, C50H44, is a derivative of rubrene in which tert-butyl side groups are added to two of the pendant aromatic rings. The complete mol­ecule is generated by a mirror plane, and the unsubstituted and substituted pendant aromatic rings are almost perpendicular to the main backbone of the mol­ecule, which is essentially planar.

The theoretical model taking into account interactions of a spin-crossover molecule with its two nearest neighbours has been applied to the data on spin crossover equilibria in a series of Fe(II) complexes with halogenated ethyltetrazoles. Transition curves of different shapes, varying from a gradual to a two-step and incomplete spin crossover, have been successfully described by this model. The observed variation of the shape of transition curve originates from ligand-ligand interactions of neighbouring complexes achieved via electrostatic dipole-dipole interactions and hydrogen bonding.