We herein present the synthesis, crystal structure, and magnetic properties of a new heterometallic chain of MnIII and NiII ions, [Mn2(saltmen)2Ni(pao)2(py)2](ClO4)2 (1) (saltmen2- = N,N'-(1,1,2,2-tetramethylethylene) bis(salicylideneiminate) and pao- = pyridine-2-aldoximate). The crystal structure of 1 was investigated by X-ray crystallographic analysis: compound 1 crystallized in monoclinic, space group C2/c (No. 15) with a = 21.140(3) A, b = 15.975(1) A, c = 18.6212(4) A, beta = 98.0586(4) degrees , V = 6226.5(7) A3, and Z = 4. This compound consists of two fragments, the out-of-plane dimer [Mn2(saltmen)2]2+ as a coordination acceptor building block and the neutral mononuclear unit [Ni(pao)2(py)2] as a coordination donor building block, forming an alternating chain having the repeating unit [-Mn-(O)2-Mn-ON-Ni-NO-]n. In the crystal structure, each chain is well separated with a minimum intermetallic distance between Mn and Ni ions of 10.39 A and with the absence of interchain pi overlaps between organic ligands. These features ensure a good magnetic isolation of the chains. The dc and ac magnetic measurements were performed on both the polycrystalline sample and the aligned single crystals of 1. Above 30 K, the magnetic susceptibility of this one-dimensional compound was successfully described in a mean field approximation as an assembly of trimers (Mn...Ni...Mn) with a NiII...MnIII antiferromagnetic interaction (J = -21 K) connected through a ferromagnetic MnIII...MnIII interaction (J'). However, the mean field theory fails to describe the magnetic behavior below 30 K emphasizing the one-dimensional magnetic character of the title compound. Between 5 and 15 K, the susceptibility in the chain direction was fitted to a one-dimensional Ising model leading to the same value of J'. Hysteresis loops are observed below 3.5 K, indicating a magnet-type behavior. In the same range of temperature, combined ac and dc measurements show a slow relaxation of the magnetization. This result indicates the presence of a metastable state without magnetic long-range order. This material is the first experimental design of a heterometallic chain with ST = 3 magnetic units showing a "single-chain magnet" behavior predicted in 1963 by R. J. Glauber for an Ising one-dimensional system. This work opens new perspectives for one-dimensional systems to obtain high temperature metastable magnets by combining high spin magnetic units, strong interunit interactions, and uniaxial anisotropy.

Dedicated to Professor Hansgeorg Schnöckel on the occasion of his 65th birthday.The first evidence of single molecule magnet (SMM) behavior was discovered in the mixed-valence compounds [Mn [1] In the quest to synthesize SMMs that show hysteresis at higher temperatures, it has been recognized that large ground-state spins and a uniaxial anisotropy (large negative D and small E considering the following Hamiltonian anisotropy term:It is thus of interest to discover how to obtain the largest-spin ground state possible for a given size of aggregate. As well as having four unpaired electrons in its high-spin state, the Mn III ion is particularly useful for introducing large anisotropies through the presence of Jahn-Teller distortions in this configuration and has been the most thoroughly studied candidate for synthesizing new SMMs. Amongst the large number of aggregates containing manganese(III) in the literature, a Mn 25 cluster has been reported as having a ground spin state of 51/2.[4] Herein we report on the realization of the maximum-spin ground state of 83/2 for the aggregate [Mn

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A soluble molecular analogue of photoresponsive Co/Fe Prussian blues is described within this report. As judged via a variety of spectroscopic, magnetic, and crystallographic methods, electron transfer within the octanuclear complex (below 250 K) converts paramagnetic red crystals into green diamagnetic ones. The color and magnetic changes are associated with the transformation of FeIIILS-CN-CoIIHS units into FeIILS-CN-CoIIILS fragments in manner that is identical to that found for the An[Co(OH2)(6-6m)][Fe(CN)6]m.xH2O (An = alkali metal cation) family of three-dimensional Prussian blues. Moreover, this intramolecular electron transfer can be quantitatively circumvented via rapid thermal quenching and reversed via simple white light irradiation at low temperatures. Remarkably the data suggests that thermally or photoinduced paramagnetic metastable phases are identical and exhibit long relaxation times that approach 10 years at 120 K.

With the long term objective to build the next generation of devices from the molecular scale, scientists have explored extensively in the past two decades the Prussian blue derivatives and their remarkable physico-chemical properties. In particular, the exquisite Fe/Co system displays tuneable optical and magnetic behaviours associated with thermally and photo-induced metal-to-metal electron transfer processes. Recently, numerous research groups have been involved in the transfer of these electronic properties to new Fe/Co coordination networks of lower dimensionality as well as soluble molecular analogues in order to facilitate their manipulation and integration into devices. In this review, the most representative examples of tridimensional Fe/Co Prussian blue compounds are described, focusing on the techniques used to understand their photomagnetic properties. Subsequently, the different strategies employed toward the design of new low dimensional Prussian blue analogues based on a rational molecular building block approach are discussed emphasizing the advantages of these functional molecular systems.

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Slow relaxation of the magnetization (i.e., "magnet-like" behavior) in materials composed of magnetically isolated chains was observed for the first time in 2001. This type of behavior was predicted in the 1960s by Glauber in a chain of ferromagnetically coupled Ising spins (the so-called Glauber dynamics). In 2002, this new class of nanomagnets was named single-chain magnets (SCMs) by analogy to single-molecule magnets that are isolated molecules displaying related superparamagnetic properties. A long-range order occurs only at T = 0 K in any pure one-dimensional (1D) system, and thus such systems remain in their paramagnetic state at any finite temperature. Nevertheless, the combined action of large uniaxial anisotropy and intrachain magnetic interactions between high-spin magnetic units of the 1D arrangement promotes long relaxation times for the magnetization reversal with decreasing temperature, and finally at significantly low temperatures, the material can behave as a magnet. In this Forum Article, we summarize simple theoretical approaches used for understanding typical SCM behavior and some rational synthetic strategies to obtain SCM materials together with representative examples of SCMs previously reported.

The Glauber dynamics is studied in a single-chain magnet (SCM). As predicted a single relaxation mode of the magnetization is found. Above 2.7 K, the thermally activated relaxation time is mainly governed by the effect of magnetic correlations and the energy barrier experienced by each magnetic unit. This result is in perfect agreement with independent thermodynamical measurements. Below 2.7 K, a crossover towards a relaxation regime is observed that is interpreted as the manifestation of finite-size effects. The temperature dependences of the relaxation time and of the magnetic susceptibility reveal the importance of the boundary conditions. PACS numbers: 75.10. Pq, 75.40.Gb, 76.90.+d The design of new slow-relaxing magnetic nanosystems is a very challenging goal for both applications (as information storage) and fundamental research. A wellknown example of such systems is the single-molecule magnet (SMM) that shows slow reversal of the magnetization due to the combined effect of a high spin ground state and uniaxial anisotropy producing an energy barrier between spin-up and spin-down states [1]. When a magnetic field is initially applied to magnetize this system and then removed, the magnetization decays with a material-inherent relaxation time depending on the temperature. The corresponding relaxation time, τ , follows an Arrhenius law at high temperatures and the activation energy is equal to the barrier height, being roughly |D|S 2 , where D is the negative uniaxial anisotropy constant and S is the spin ground state of the molecule. At lower temperatures, τ may saturate when quantum tunneling through the barrier becomes relevant [2].Another research route of metastable magnetism has recently been explored with the synthesis of single-chain magnets (SCMs) [3,4,5]. In these materials, the slow relaxation of magnetization is not solely the consequence of the uniaxial anisotropy seen by each spin on the chain but depends also on magnetic correlations. The effect of the short-range order becomes more and more important when the temperature is reduced until a critical point is reached at T = 0 K for 1D systems. In fact, the relaxation time is found to be exponentially enhanced at low temperatures in agreement with the pioneer work of R. J. Glauber devoted to the dynamics of the 1D Ising model [6]. Although it seems that there is a reasonable agreement between the experimental data and the Glauber's theory [3,4,5], we show in this communication that several other arguments should be considered to fill the gap between the theory and the experimental results. Firstly, it should be mentioned that the experimental sys- tems are not strictly Ising-like. In the simplest case, they are rather described by an anisotropic Heisenberg model:where J is the ferromagnetic exchange constant between the spin units and D is the single-ion anisotropy. Secondly, the relaxation time of each magnetic unit, introduced phenomenologically in Glauber's study, is a priori temperature dependent [7] and this argument should also be consid...