Metal–organic frameworks (MOFs), also known as porous coordination polymers (PCPs), synthesized by assembling metal ions with organic ligands have recently emerged as a new class of crystalline porous materials. The amenability to design as well as fine-tunable and uniform pore structures makes them promising materials for a variety of applications. Controllable integration of MOFs and functional materials is leading to the creation of new multifunctional composites/hybrids, which exhibit new properties that are superior to those of the individual components through the collective behavior of the functional units. This is a rapidly developing interdisciplinary research area. This review provides an overview of the significant advances in the development of diverse MOF composites reported till now with special emphases on the synergistic effects and applications of the composites. The most widely used and successful strategies for composite synthesis are also presented.

Metal–organic framework composites

Gold nanorods have been receiving extensive attention owing to their extremely attractive applications in biomedical technologies, plasmon-enhanced spectroscopies, and optical and optoelectronic devices. The growth methods and plasmonic properties of Au nanorods have therefore been intensively studied. In this review, we present a comprehensive overview of the flourishing field of Au nanorods in the past five years. We will focus mainly on the approaches for the growth, shape and size tuning, functionalization, and assembly of Au nanorods, as well as the methods for the preparation of their hybrid structures. The plasmonic properties and the associated applications of Au nanorods will also be discussed in detail.

Gold nanorods and their plasmonic properties

Polyoxometalates (POMs) are discrete anionic metaloxygen clusters which can be regarded as soluble oxide fragments. They exhibit a great diversity of sizes, nuclearities, and shapes. They are built from the connection of {MOx} polyhedra, M being a d-block element in high oxidation state, usually VIV,V, MoVI, or WVI.1 While these species have been known for almost two centuries, they still attract much interest partly based on their large domains of applications. They play a great role in various areas ranging from catalysis,2 medicine,3 electrochemistry,4 photochromism,5 to magnetism.6 This palette of applications is intrinsically due to the combination of their added value properties (redox properties, large sizes, high negative charges, nucleophilicity...). Parallel to this domain, the organic-inorganic hybrids area has followed a similar expansion during the last 10 years. The concept of organic-inorganic hybrid materials * To whom correspondence should be addressed. E-mail: dolbecq@ chimie.uvsq.fr. Chem. Rev. 2010, 110, 6009–6048 6009

Hybrid Organic−Inorganic Polyoxometalate Compounds: From Structural Diversity to Applications

Higher efficiency in the end-use of energy requires substantial progress in lighting concepts. All the technologies under development are based on solid-state electroluminescent materials and belong to the general area of solid-state lighting (SSL). The two main technologies being developed in SSL are light-emitting diodes (LEDs) and organic light-emitting diodes (OLEDs), but in recent years, light-emitting electrochemical cells (LECs) have emerged as an alternative option. The luminescent materials in LECs are either luminescent polymers together with ionic salts or ionic species, such as ionic transition-metal complexes (iTMCs). Cyclometalated complexes of IrIII are by far the most utilized class of iTMCs in LECs. Herein, we show how these complexes can be prepared and discuss their unique electronic, photophysical, and photochemical properties. Finally, the progress in the performance of iTMCs based LECs, in terms of turn-on time, stability, efficiency, and color is presented.

Luminescent ionic transition-metal complexes for light-emitting electrochemical cells.

Polyoxometalate (POM)-based metal–organic framework (MOF) materials contain POM units and generally generate MOF materials with open networks. POM-based MOF materials, which utilize the advantages of both POMs and MOFs, have received increasing attention, and much effort has been devoted to their preparation and relevant applications over the past few decades. They have good prospects in catalysis owing to the electronic and physical properties of POMs that are tunable by varying constituent elements. In this review, we present recent developments in porous POM-based MOF materials, including their classification, synthesis strategies, and applications, especially in the field of catalysis.

Recent advances in porous polyoxometalate-based metal-organic framework materials.

Water-dispersible gold nanoparticles functionalized with paramagnetic gadolinium have been fully characterized, and the NMRD profiles show very high relaxivities up to 1.5 T. Characterization using TEM images and dynamic light scattering indicate a particle size distribution from 2 to 15 nm. The gold cores of the nanoparticles do not contribute significantly to the overall magnetic moment.

Gold nanoparticles functionalized with gadolinium chelates as high-relaxivity MRI contrast agents.

Hybrid Organic—Inorganic Polyoxometalate Compounds: From Structural Diversity to Applications

During the last decades, noble-metal nanoparticles have attracted a great deal of interest by their unique optical, electronic, magnetic, and catalytic properties and intense research efforts are still devoted to develop new synthetic and functionalizing strategies [1–5]. This extremely active research field was supported by the amazing chemical and physical properties displayed by the metal particles of nanometric size that are markedly different from those of the corresponding bulk materials [6]. Especially, optical and electronic properties of metal nanoparticles can be easily tuned by modifying their size and shape [7]. Regarding noble metals nanoparticles, gold nanoparticles (Au-NPs) are without contest at the forefront in this research area. Academic interest for Au-NPs, which showed fast growth over the past years, is motivated by the strong surface plasmon resonance displayed by gold nanoparticles. In addition, gold nanoparticles gained a renewal of interest by finding potential uses in medical diagnostics, imaging, and therapeutic treatments. In these last fields, preparation of Au-NPs with benign reactants is often favored to remove all potential contamination of the colloidal solutions [8–14]. To date, four different classes of biological applications for AuNPs have been identified: labeling, delivering, heating, and sensing [15]. However, the use of gold nanoparticles was not limited to biological applications and Au-NPs were also successfully employed as scaffolds for molecular recognition of elements and molecules [16], in optoelectronics and data storage [17], in nanotechnology with molecular switches [18] and motors [19], or in light-harvesting assemblies [20, 21]. Typically, gold nanoparticles are obtained by chemical reduction of tetrachloroauric acid [22, 23]. However, this conventional approach is based on the use of external chemical reductants that often produce undesired sideproducts. Therefore, a series of functionalizing agents for Au-NPs has recently been developed that display a dual role of effective reducing agents of gold salts and of stabilizers, by providing a robust coating to gold nanoparticles, within a unique reaction step. Seven different types of these reducing/ capping agents were investigated to date: microorganisms and bacteria, plants extracts and physiological molecules, inorganic reagents and metal complexes, organic molecules, organic acids and salts, liposomes, and polymers (Table 1). In this review, we propose to focus on these exciting functionalizing agents exerting the dual role of reducing and coating agents and to discuss the precise size-controlled synthesis of Au-NPs using this approach.

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Eddy Dumas

Papers

Hybrid Organic−Inorganic Polyoxometalate Compounds: From Structural Diversity to Applications

Gold nanoparticles functionalized with gadolinium chelates as high-relaxivity MRI contrast agents.

Hybrid Organic—Inorganic Polyoxometalate Compounds: From Structural Diversity to Applications

Controlled spontaneous generation of gold nanoparticles assisted by dual reducing and capping agents

Iridium(III) soft salts from dinuclear cationic and mononuclear anionic complexes for OLED devices.