Research News Hybrid Organic/Inorganic Langmuir±Blodgett Films. A Supramolecular Approach to Ultrathin Magnetic Films** By Eugenio Coronado* and Christophe Mingotaud* 1. Introduction The design of molecular materials with useful physical properties, in particular electrical, magnetic, and optical, is a focus of active research in materials chemistry. In this field various strategies are being used with a common aim: To control the molecular assembly as far as possible, as it very often determines the solid-state properties of the material. The old but elegant Langmuir±Blodgett (LB) technique is clearly one of those ways to arrange molecules into organized assemblies. [1] It consists of a repetitive dipping of a solid substrate through a compressed monolayer spread at the gas±water interface. During the up and down strokes, transfer of the monolayer onto the substrate may occur, leading to a material with a precise thickness and a lamellar structure. Until very recently the LB technique had only been used to organize amphiphilic organic molecules. [2] Thus, it is not surprising that most of the LB films with useful electronic properties are purely organic. For example, conducting multilayers have readily been built from molec- ular electron donors or acceptors such as tetrathiafulvalene (TTF) or tetracyanoquinodimethane (TCNQ) derivatives, [3] but also from conductive polymers based on pyrrole, thiophene or aniline. [4] Among all the studies made on LB multilayers, one should note that the possibility offered by the LB technique to build noncentrosymmetric architectures has been widely used to achieve nonlinear optical properties in these materials. [5] Deliberate incorporation of inorganic components into the LB film has been relatively limited in the past. Thus, although metal ions as cadmium(II) or aluminum(III) have already been used in earlier work, their function was mainly that of stabilizing the monolayers at the gas±water interface. Pomeranz et al., [6] in 1978, were the first to use the LB technique to organize multilayers of paramagnetic ions such as manganese(II) or iron(III). In this case, weak antiferromag- netic effects arising from the magnetic dipolar interactions within the monolayers were observed at low temperature by electron paramagnetic resonance (EPR). The use of molec- ular metal complexes as inorganic component of hybrid organic/inorganic films is more recent and emerged only in the 1980s with the preparation of LB films containing ferrocene [7] and phthalocyanine [8] molecules. The interest in these films has been related to the electrochemical behavior of these complexes as well as to the electrochromism of the films. Other relevant examples are complexes formed by the bipyridine ligand and various metallic centers such as iron(III) [9] or ruthenium(II). [10] Focusing on the magnetic properties, one should notice that, although many molecular magnets based upon metal complexes have been discovered over the past two decades, LB films with useful magnetic properties such as magnetic hysteresis have only been designed in the last two years. These well-organized supra- molecular architectures are based on oligonuclear complexes of exchange-coupled transition metal ions (magnetic clus- ters), as well as on polymeric complexes forming extended structures. In this article we discuss the possibilities offered by the LB technique for constructing these hybrid magnetic films. 2. LB Films Incorporating Polyoxometalate Clusters Molecular metal-oxide clusters (polyoxometalates) con- stitute a wide class of inorganic compounds with remarkable chemical, structural, and electronic versatility, that impact in areas as catalysis, medicine or materials science. [11] Because of their ability to act as electron acceptors and to accommodate magnetic transition metal centers in their Adv. Mater. 1999, 11, No. 10 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 1999 0935-9648/99/1007-0869 $ 17.50+.50/0 869 ± [*] Prof. E. Coronado Dept. Química Inorgµnica, Universidad de Valencia Dr. Moliner 50, E-46100 Burjasot (Spain) Dr. C. Mingotaud Centre de Recherches Paul Pascal±CNRS Av. A. Schweitzer, F-33600 Pessac (France) [**] This work has been developed in the framework of the European COST ACTION 518 (Project on Magnetic Properties of Molecular and Polymeric Materials). Financial support by the Spanish Ministerio de Educación y Cultura (Grant MAT98-0880) is gratefully acknowledged.