Molecule-based materials with active physical properties, in particular electrical, magnetic, and optical, are a focus of contemporary materials chemistry research. Certainly, one reason for this interest has been the realization that these materials can exhibit cooperative properties typically associated with the inorganic network solids, as for example metallic conduction or even superconductivity,1 ferromagnetism,2 and nonlinear optical properties.3 With respect to the electrical properties, many important achievements were obtained in the 1970s with the discovery of the first molecule-based metal in 1972,4 namely the π-electron donor-acceptor complex [TTF][TCNQ] (TTF ) tetrathiafulvalene, TCNQ ) tetracyano-p-quinodimethane),5 and the report of the first molecule-based superconductors in 1979 based on the Bechgaard salts [TMTSF]2X (X ) PF6, AsF6; TMTSF ) tetramethyltetraselenafulvalene).6 The recognition of molecule-based ferromagnetic compounds is more recent and has emerged only in the past decade with the discovery of the electron-transfer salt [Fe(C5Me5)2][TCNE] (TCNE ) tetracyanoethylene) in 1985.7 Since these pioneering studies, the two areassmolecule-based metals and magnetsshave witnessed rapid development, and many new molecules have been designed which, if assembled in the appropriate manner in the solid, will enable researchers to improve the physical properties by increasing superconducting and ferromagnetic critical temperatures. A current development in the general area of molecule-based materials is to design, from a wise choice of the constituent molecules, new materials that combine properties not normally associated with a single material. Some intriguing applications of this concept would be to couple conductivity or optical phenomena with magnetic properties. In fact, this challenging goal was proposed8 in the mid-1980s but has only recently begun to be explored. Efforts in this direction nucleated with the design of hybrid materials formed by two molecular networks, such as anion/cation salts or host/guest solids, where each network furnishes distinct physical properties. In these new types of materials, each network contributes distinct physical properties to the solid. Examples include hybrid molecule-based materials, combining inorganic metal complexes that act as structural or magnetic components with an organic π-electron donor or acceptor molecule that furnishes the pathway for electronic conductivity.9 Polyoxometalates have been found to be extremely versatile inorganic building blocks for the construction of the aforementioned functionally active solids.10 The present article highlights recent results and provides a perspective of the use of polyoxometalates in the construction of molecule-based materials. We present herein the different classes of polyoxometalate-based hybrid materials that are of considerable interest due to their electrical or/and magnetic properties. These are (i) organic/inorganic hybrid salts in which the electron donors are organic molecules of the TTF type, (ii) organometallic/inorganic salts in which the electron donor is the decamethylferrocene complex, and (iii) organic/inorganic films in 273 Chem. Rev. 1998, 98, 273−296

http://www1.udel.edu/chem/polenova/PDF/Polyoxometalates/POM_materials_ChemRev1998.pdf

Polyoxometalate-Based Molecular Materials

This review attempts to present the most salient developments of research on organic conductors and superconductors during the past 10 years. A theoretical introduction treats instabilities of quasi-one-dimensional electron systems and associated precursor effects which are relevant to the experimental results on organic conductors. We then describe the characterization of quasi-one-dimensional organic conductors by their transport, optical and magnetic properties. Finally, two sections are devoted to the experimental investigation of the low temperature instabilities: lattice instability in TTF-TCNQ and related compounds, superconducting or antiferromagnetic instabilities in the (TMTSF) 2 X series. The importance of one-dimensional fluctuations is emphasized in both lattice and superconducting instabilities.

Organic conductors and superconductors

Tetrathiafulvalene (TTF) and its derivatives were originally prepared as strong electron-donor molecules for the development of electrically conducting materials. This Review emphasizes how TTF and its derivatives offer new and in some cases little-exploited possibilities at the molecular to the supramolecular levels, as well as in macromolecular aspects. TTF is a well-established molecule whose interest goes beyond the field of materials chemistry to be considered an important building block in supramolecular chemistry, crystal engineering, and in systems able to operate as machines. At the molecular level, TTF is a readily available molecule which displays a strong electron-donor ability. However, its use as a catalyst for radical-polar crossover reactions, thus mimicking samarium iodide chemistry, has only recently been addressed. Important goals have been achieved in the use of TTF at the macromolecular level where TTF-containing oligomers, polymers, and dendrimers have allowed the preparation of new materials that integrate the unique properties of TTF with the processability and stability that macromolecules display. The TTF molecule has also been successfully used in the construction of redox-active supramolecular systems. Thus, chemical sensors and redox-switchable ligands have been prepared from TTF while molecular shuttles and molecular switches have been prepared from TTF-containing rotaxanes and catenanes. A large synthetic effort has been devoted to the preparation of the so-called organic ferromagnets, many of which are derived from TTF. The main task in these systems is the introduction of ferromagnetic coupling between the conduction electrons and localized electrons. TTF has also played a prominent role in molecular electronics where TTF-containing D-sigma-A molecules have allowed the preparation of the first confirmed unimolecular rectifier. Recently, it has been confirmed that TTF can display efficient nonlinear optic (NLO) responses in the second and third harmonic generation as well as a good thermal stability. These findings can be combined with the redox ability of TTF as an external stimuli to provide a promising strategy for the molecular engineering of switchable NLO materials. Fullerenes endowed with TTF exhibit outstanding photophysical properties leading to charge-separated (CS) states that show remarkable lifetimes.

New Concepts in Tetrathiafulvalene Chemistry.

The difference between metallic and insulating salts of tetracyanoquinodimethone (TCNQ): how to design an organic metal

Coordination polymers are currently one of the hottest topics in Inorganic and Supramolecular Chemistry. This critical review summarizes the current state-of-the-art on electrical conductive coordination polymers (CPs), also named metal–organic frameworks (MOFs). The data were collected following two sort criteria of the CPs structure: dimensionality and bridging ligands (151 references).

Electrical conductive coordination polymers

Superconducting fluctuations and the Peierls instability in an organic solid

The temperature dependence of the dc electrical conductivity of tetrathiofulvalinium tetracyanoquinodimethan (TTF)(TCNQ) measured along the crystallographic $a$, $b$, and ${c}^{*}$ axes and the corresponding anisotropies $\frac{{\ensuremath{\sigma}}_{\ensuremath{\parallel}}^{b}}{{\ensuremath{\sigma}}_{\ensuremath{\perp}}^{a}}$ and $\frac{{\ensuremath{\sigma}}_{\ensuremath{\parallel}}^{b}}{{\ensuremath{\sigma}}_{\ensuremath{\perp}}^{{c}^{*}}}$ are reported. Standard four-probe measurements are supplemented by applying the technique of Montgomery to the question of inhomogeneous currents in this anisotropic conductor. We have observed that the intrinsic anisotropy contains two maxima which provide a direct internal method for unambiguously testing the validity of four-probe measurements of the conductivity along the principal conducting $b$ axis. The transverse transport properties are diffusive and yield an electrical anisotropy greater than 500 at room temperature which increases to greater than ${10}^{4}$ near 58 K. The extreme sensitivity of this one-dimensional metal to crystalline imperfections is experimentally demonstrated through temperature cycling studies. The dc results confirm (TTF)(TCNQ) is a one-dimensional metal exhibiting a strongly temperature-dependent $b$-axis conductivity that reaches maximum values exceeding ${10}^{5}$ ${(\mathrm{\ensuremath{\Omega}}\phantom{\rule{0ex}{0ex}}\mathrm{c}\mathrm{m})}^{\ensuremath{-}}$. These data are taken as confirming evidence that above 58 K the metallic state exhibits strong electron correlations associated with a many-body collective state in which the conductivity can greatly exceed the limitations of single-particle scattering.

Electrical conductivity of tetrathiofulvalinium tetracyanoquinodimethan (TTF) (TCNQ)

Electrical conductivity of (SN)x

The results of an experimental study of nonlinear electrical transport at low temperatures are described for the one-dimensional organic conductors TTF-TCNQ (tetrathiafulvalene-tetracyanoquinodimethane) and TSeF-TCNQ (tetraselenafulvalene-tetracyanoquinodimethane). The $b$-axis conductivity is nonlinear in both compounds with the onset of nonlinearity at applied electric fields of a few V/cm. Measurements along different crystallographic directions show that the nonlinear transport is directly related to the one-dimensional nature of the electronic system. Traditional single-particle mechanisms of nonlinearity in semiconductors are ruled out through direct experimental study. The experimental results are discussed in terms of nonlinear phase-kink solitons of the pinned charge-density wave condensate at low fields, and field-induced depinning at high fields.

Nonlinear transport in pinned one-dimensional charge-density wave systems

We report the first synthesis of chemically pure polymeric sulfur nitride [${(\mathrm{SN})}_{x}$]. The optical reflectance of well-characterized films of this polymeric solid has been measured from the near ultraviolet (30 000 ${\mathrm{cm}}^{\ensuremath{-}1}$) to the far infrared (500 ${\mathrm{cm}}^{\ensuremath{-}1}$). Metallike reflectance is observed in the infrared through the visible region with a well-defined plasma minimum near 22 000 ${\mathrm{cm}}^{\ensuremath{-}1}$, corresponding to light polarized parallel to the polymer chain axis. The Drude-Lorentz model is used to analyze the data, and the physical implications of the resultant parameters are discussed.

Marshall J. Cohen

Papers

Superconducting fluctuations and the Peierls instability in an organic solid

Electrical conductivity of tetrathiofulvalinium tetracyanoquinodimethan (TTF) (TCNQ)

Electrical conductivity of (SN)x

Nonlinear transport in pinned one-dimensional charge-density wave systems

Optical Reflectance of Polymeric Sulfur Nitride Films from the Ultraviolet to the Infrared