Synthesis, optical and magnetic properties of hybrid α,α′-oligothiophenecarboxylates/transition metal hydroxide multilayered compounds

Abstract

New Cu(II), Ni(II) and Co(II) multilayered systems consisting of alternating transition metal hydroxide and oligothiophenedicarboxylate layers have been elaborated by hydrothermal reaction. The insertion of dicarboxylate ligands comprising 1 to 4 thiophene rings between metal hydroxides and the structures and physical properties of 10 hybrid organic/inorganic systems M–Tn (MCu, Ni, Co, n = 1–4) have been investigated. Controlled hydrothermal synthesis leads to layered compounds of transition metals in octahedral sites only. A wide palette of magnetic behaviors has been observed. The four copper derivatives with interlayer distances of 1.0–2.1 nm show either antiferromagnetic or ferromagnetic intra-layers interactions with a ferromagnetic or canted antiferromagnetic long range ordering. The nickel derivatives are antiferromagnetic and show a metamagnetic transition for interlayer distances below 1.3 nm and are ferrimagnetic above 1.7 nm, with low T_N (< 4 K) and coercivity (μ₀H_C < 30 mT). The cobalt compounds are all canted antiferromagnets at ordering temperatures as high as 40 K. Strong anisotropies induce a hard magnet behavior with coercive fields of 3–6 T. A special case concerns Co-T2 (4), for which the hard magnet state is induced by applying a field. The optical properties of the free ligands, oligothiophenedicarboxylates, were investigated. All ligands are fluorescent. In the hybrids, the main absorption bands observed in the visible region are attributed to the π–π* transition. No emission was observed, the quenching of photoluminescence can be explained by intermolecular interactions or vicinity of the paramagnetic metal ions directly coordinated via the carboxylate moieties.