Volume 104, Number 11
Introduction: Molecular Conductors This issue is about π-functional molecules and how anisotropic intermolecular interactions in solids, which represent infinite collections of the former, translate into a rich low dimensional physics of molecular semiconductors, Mott insulators, metals, and superconductors. Books and reviews of specific topics within this field have become available; however, no compendium has yet been compiled. The approach intended here is not that common, as it builds on fundamental principles of two disciplines, in the spirit of a field established in an explosive process of integrated research in chemistry and physics triggered by the discovery of a Peierls instability and its giant Kohn anomaly (Come`s et al. Phys. Rev. B 1973, 8, 571) in the Krogman salt K2[Pt(CN)4Br0.3]‚3H2O (KCP). This was soon to be followed by the observation of metallic conductivity in 1973 in a charge transfer complex, TTF-TCNQ, prepared by the Johns Hopkins group (Cowan et al. J. Am. Chem. Soc. 1973, 95, 948), and the discovery of the first organic superconductor in the Bechgaard salts (Je´rome et al. J. Phys., Lett. 1980, 41, L95). Right from the beginning, looking all the way back to McCoy and Moore’s organic amalgams and their legendary statement that “it is possible to prepare composite metallic substances from non-metallic constituent elements” (McCoy, H. N.; Moore, W. C. J. Am. Chem. Soc. 1911, 33, 273), and the observation of a finite resistivity and a Pauli-like susceptibility in a perylene-bromine complex (Akamatsu et al. Nature 1954, 173, 168), chemists have been fascinated by the immense opportunities to create molecular systems where π electrons would travel very long distances across molecular sites. A valuable, inherent feature of these systems is that local and long range embraces between π-functional molecules in the solid state are typically rather anisotropic; that is, pπ-pπ intermolecular patterns of overlap develop primarily in onedimension, eventually crossing over to two-dimensions. Thereby, in addition to the quest for superconductivity in a molecular system (Little, W. A. Phys. Rev. A 1964, 134, 1416), another reason behind this explosion is the parallel fascination exerted for more that 50 years on physicists, experimentalists, and theorists alike (Shchegolev, I. F. Phys. Status Solidi A 1972, 12, 9), by one-dimensional systems of interacting particles. In this limit, any individual excita-
tion has to become a collective one, since an electron which tries to propagate has to push its neighbors due to electron-electron interactions. Since the synthesis of the radical cation bis-1,3dithiolium, TTF•+, in 1970 (Wudl, F.; et al. Chem. Commun. 1970, 1453) and the use of the singular “electron rich” olefin tetrathiafulvalene to form charge transfer complexes, TTF has remained the most studied heterocyclic system. It is now widely seen as the gateway to organic metals, superconductors, and semiconductors. The amount of creative effort, still ongoing today, of clever organic chemists who delivered myriads of modifications of this sulfur donor is just enormous (TTF ChemistrysFundamentals and Applications of Tetrathiafulvalene; Yamada, J., Sugimoto, T., Eds.; Kodansha and Springer: Tokyo, 2004; see Fred Wudl’s Foreword to this book). The two disciplines have gone a long way to help synthetic chemists feel at home and creative on the frontier, formulating concepts and teaching each other where the molecular origins of the differences between conducting and insulating charge transfer salts lie and why mixed valence and incomplete degree of charge transfer are the expression of a balance in the molecular solid state of the redox couples of a donor and an acceptor molecule (Torrance, J. B. Acc. Chem. Res. 1979, 12, 79-86). Likewise, this crossroad of disciplines has proven to be an ideal location where fresh ideas have incubated as a response to a great need to manipulate weak intermolecular interactions competing at the organic-inorganic interface. These include hydrogen or halogen bonds and electrostatic and van der Waals dispersion forces whose complex balance interferes with the physical properties of one and the other and their interaction. Hence, a trend in materials chemistry has evolved, borrowing equally from organic synthetic chemistry and molecular inorganic, coordination, and solid state chemistries, delineating the blooming concept of organic-inorganic hybrid salts (Batail et al. Mater. Res. Bull. 1986, 21, 1223; New J. Chem. 1994, 18, 999; Chem. Rev. 2001, 101, 2064. Day, P.; et al. J. Am. Chem. Soc. 1995, 117, 12209. Coronado, E.; et al. Chem. Rev. 1998, 98, 273). Certainly, the materials chemist’s ability, today, to try and come up with a unifying picture of very many radical cation salt structures of great beauty and
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increasing complexity is a tribute to one single approach: that based on extended-Hu¨ckel tightbinding band structure calculations. Here, the field has benefitted enormously from the timely dedication of pioneers who demonstrated how to regard extendedHu¨ckel tight-binding band structure calculationssthe extension to the solid state of the molecular orbital picturesas a means of translating the anisotropy of intermolecular interactions in real space into the anisotropy of the system in the momentum space (Grant, P. J. Phys. (Paris) 1983, 44, C3-847. Whangbo, M.-H.; et al. Inorg. Chem. 1985, 24, 3500). In time, a large variety of electronic band structures of many prototypical crystal structures have been analyzed in relation to the singular features of their phase diagram (Mori, T.; et al. Bull. Chem. Soc. Jpn. 1998, 71, 2509; 1999, 72, 179; 1999, 72, 2011), and the concept of internal charge transfer within two band systems and the prediction of its applicability to the design of single component molecular metals (Canadell, E.; et al. J. Phys. (Paris) 1989, 50, 2967; New J. Chem. 1997, 21, 1147) have been formulated. Expectations run high in the search, beyond TTF, for a novel class of π-donor molecules and materials with high superconducting transition temperatures. Likewise, there is a quest for multifunctional organicinorganic hybrid molecular conductors where admixtures of several motifs would translate into magnetochiral and magneto-optical effects or allow for chemical stabilization of incommensurate fillings of the metallic band. The search for strategies and methodologies for the synthesis of crystals of thin films and for the synthesis of low dimensional π-functional monomolecular crystalline objects and the need for technological tours de force in order to connect them to patterned substrates for field effects are areas of intense activity. These are indications that a novel frontier takes shape where the concepts and vast body of 30 years of research in the field of crystalline molecular conductors meet back again with the companion approach of conducting polymers and oligomers, delineating great opportunities for creative advances and the design of nanotechnological devices. The first section of this thematic issue is a blend of chemistry, materials, and theoretical framework. It is intended to illustrate the current conceptual thinking about electron transport and charge/energy transfer phenomena in the molecular solid state, including a presentation of current dimensionality issues in interacting electronic systems. Hence, Bendikov, Wudl, and Perepichka reach out and bridge over from strategies for reducing the HOMO/ LUMO gap in TTFs to oligoacenes to Buckminsterfullerenes in a single comprehensive appreciation of the whole field offering a perspective rich with recent conceptual advances and applications in organic field effect transistors. Mori surveys the chemistry of materials with unusual band fillings and stresses the importance of the π-donor symmetry to direct anion side- or end-embraces for stoichiometry control and the need to develop a control of band filling in two dimensions. Bre´das, Beljonne, Coropceanu, and Cornil discuss recent work that addresses at the molecular level the main parameters that govern electron
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and energy transfer processes in π-conjugated oligomers, a review which also appropriately bridges to the sister field of conducting polymers and oligomers. Seo, Hotta, and Fukuyama aim for a systematic understanding of the large, prototypical family of two-donors-for-one-monoanion mixed valence phases, exploring unifying features by drawing the differences between insulating and metallic properties through an overview of the many ground states competing in one single class of compounds (Mott insulators, charge ordering, Peierls, Anderson localization and disorder effects). Giamarchi presents the main theoretical concepts pertaining to interacting electronic systems in one- and quasi-one-dimensional geometries, in a form that allows for a broad chemistry and materials science readership to grasp ideas, rather than digress into detailed derivations, and be rewarded by proposals of possible directions for research to help foster the theoretical understanding of such strongly correlated systems. The second section describes synthetic approaches to π-functional precursors. Yamada, Akutsu, Nishikawa, and Kikuchi demonstrate strategies designed to transgress the planarity paradigm by extending the σ-framework of π-functional precursors, confining the π-electronic system, increasing on-site coulomb repulsions, and delivering singular molecular superconductors. Iyoda, Hasegawa, and Miyake review the chemistry of TTF dimers and oligomers, bi-TTF and bis-TTF, which serve as building blocks in supramolecular chemistry and nanotechnological devices. Jeppesen, Brøndsted Nielsen, and Becher show the many outcomes of the versatile cyanoethyl deprotection/realkylation protocol for ready incorporation of several TTF units into cyclophane structures endowed with multiple intra- and intermolecular charge transfer interactions, en route toward nanoelectronic devices and “molecular machines”. Fabre discusses the strategies developed to deliver nonsymetrically substituted tetrachalcogenafulvalene cores and analyzes how the chemistry has evolved upon balancing the ease of access with the selectivity of dissymmetrical couplings. Gorgues, Hudhomme, and Salle´ review the chemistry of highly extended and functionalized TTF derivatives which has developed from the application of electrophilic alkynes and the introduction of the formyl group using acetylene dicarboxaldehyde or its monoacetal. Lorcy and Bellec discuss the chemistry of a class of thiazole-2-ylidene carbenes and their dimers, the dithiadiazafulvalenes, powerful electron donors whose functionalized radical cations can be stabilized within a very large potential domain, some with singular conformational flexibility and dynamics. The third section is intended as an extensive but not exhaustive coverage of the diversity and complexity of the chemistry of π-functional molecular materials. Since this represents the crossroad of the integrated effort of researchers from many walks of chemistry, materials chemistry, and physics, its scope is certain to expand. Geiser and Schlueter provide an in-depth discussion of the work carried out on the conducting salts that are formed between molecular radical cations and organic and organometallic an-
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ions, exemplifying the vast potential for tailoring crystal structures through the fine tuning of both the organic anions and electron donor molecules. A. Kobayashi, Fujiwara, and H. Kobayashi discuss the progress of the frontier orbital engineering design of single component molecular metals based on metal(dithiolene) complexes of extended-tetrathiafulvalene ligands. H. Kobayashi, Cui, and A. Kobayashi review ideas which guide the conception of molecular systems where a sizable interaction occurs between metal π electrons and localized magnetic moments and discuss their demonstration in magnetic organicinorganic hybrid superconductors based on the mixedchalcogen π-donor molecule bis(ethylenedithio)tetraselenafulvalene. Rovira discusses the chemistry of disymmetrical and inherently more rigid π-donors such as bis(ethylenethio)tetrathiafulvalene where only one heteroatom less within the outer fivemembered rings makes for salts with a wider structural diversity and recurrent polymorphism and whose development paved the way for field effects within their monomolecular, neutral solids. From the solid foundation of the two-band systems concept, Kato exposes a unifying picture of the very large effort on conducting metal dithiolene complexes where the fascinating interplay of the HOMO and LUMO orbitals within complexes where the HOMOLUMO level splitting is small has yet to reach its full potential. Concepts such as orbital degree of freedom, multisheet Fermi surface, solid crossing column structures, and spanning overlap are formulated to that effect in this review. Shibaeva and Yagubskii deliver a framework which unifies a large body of structural data on conducting and superconducting trihalides of bis(ethylenedithio)tetrathiafulvalene, formulate a generic description of their polymorphs and phase transformations, and reach out toward composite devices where nanocrystalline layers interface a polymer film surface. Fourmigue´ and Batail examine how the hydrogen and halogen bonding patterns of intermolecular interactions within the parent neutral solid constructs become activated in the structure of their radical cation children, an analysis which helps to illuminate the dynamic interdependence of redox state, hydrogen, and halogen bonding, anion recognition, and charge partition in the molecular solid state. Coronado and Day address strategies to build molecular materials that associate planar organic donors with anionic metal complexes of various nuclearities which introduce localized moments in the lattice, and they review an emerging class of molecular hybrids such as paramagnetic and antiferromagnetic superconductors, and ferromagnetic metal and ferromagnetic semiconductors. Enoki and Miyazaki discuss how a broad category of tetrathiafulvalene-based compounds where magnetic interactions dominate may be broken down into systems with π-based localized magnets, magnets in the vicinity of the Mott-Hubbard boundary, π-d composite insulating magnets, and π-d based magnetic conductors. Talham reviews the effort which has allowed both conductivity and magnetism to be achieved in Langmuir-Blodgett films and propose to further exploit this organic-inorganic
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hybrid approach toward the combination of properties in a single thin film obtained by LB nanoassembly. Inabe and Tajima discuss metal-phtalocyanide-based conductors where axially-substituted Pc complexes may be engineered to adopt various stacking modes of higher dimensionality in contrast to the preferred metal-over-metal stacking of planar M(Pc) conductors. Hu¨nig and Herberth provide an exhaustive coverage encompassing all N,N′-dicyanodiimines, powerful electron acceptors derived from quinones, spanning from the crystal chemistry of their onedimensional anion radical metal salts to the remarkable physics of their copper salts, pointing out opportunities for the development of materials for charge transport and photovoltaic cells. The final section takes us into the physics laboratory with the intent of learning how and why we should join forces, deliver crystals of high quality and purity to explore phase diagrams, and probe 1D/2D electrons in conducting molecular systems. These efforts employ combinations of pressure, temperature, and high magnetic field effects, and techniques such as X-ray diffuse scattering, NMR, ESR, optics, and Muon implantation. It is also meant to illuminate the many fascinating phenomena competing in the molecular solid state, help foster a common language, and perhaps allow, back in the chemistry laboratory, the development of fresh approaches to molecule and materials design. Thus, Je´rome reviews the progress of ideas and the very large body of experimental work which has marked the quest for organic superconductivity and the present search for new materials with the highest possible Tc, thereby exemplifying the importance of hydrostatic pressure in tuning the properties of molecular conductors. Kagoshima and Kondo discuss the use of uniaxial compression to elucidate the relationship between crystal structure and electronic properties of molecular conductors. Ravy analyzes how minute displacements of atoms (molecules) out of their average positions in a crystal lattice of otherwise perfect periodicity may be investigated through the study of “out-of-Bragg” scattering, allowing displacement and substitution disorder and their eventual coupling to be distinguished. The effort accomplished using X-ray scattering is furthermore reviewed to reveal and analyze the interplay between structural and electronic degrees of freedom in molecular conductors. Miyagawa, Kanoda, and Kawamoto discuss how NMR spectroscopy allows the effects of physical or internal chemical pressure to be evaluated, and thereby a precise control of the transfer integral relative to the Coulomb repulsive energy, within one single coherent series of two-dimensional metal, insulator, and superconductor. This article draws attention to the singularity within systems of strongly correlated electrons of triangular lattices of molecular mixed valence dimers with two kinds of interdimer transfer integrals. It also provides an overview of experimental results which allows a deep understanding of the Mott transition physics, including the occurrence of antiferromagnetic spin fluctuations, the presence of a commensurate spin structure in the low temperature phase, and the symmetry of the wave
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pairing in the superconducting state. This underscores the importance of high resolution single crystal NMR studies in conducting and magnetic molecular solids. Recalling that, because of their inherent strong anisotropy and the presence of comparatively lighter elements, electron spin resonance is detected in most low dimensional molecular metals whereas very few ordinary metals show signals even at low temperature, Coulon and Cle´rac proceed with an exhaustive overview of the many accomplishments in the field which demonstrates that ESR is a major probe for the detection and study of paramagnetic, ferromagnetic, antiferromagnetic, spin-Peierls, Peierls, and superconducting ground states. Dressel and Drichko review the optical properties of layered organic conductors in the infrared and discuss how the electromagnetic wave coupling to electronic, vibrational, and magnetic excitations allows the change of symmetry associated with charge localization and disproportionation to be identified. Blundell discusses how Muon implantation and subsequent spin precession and relaxation is extremely useful for studying magnetic and superconducting systems while providing an overview of the applications of the technique to organic donors, organic metals and superconductors, and conducting polymers. The combination of high anisotropy of the electronic system, relatively simple Fermi surface topologies, and availability of single crystals of high quality, and the discovery of magnetic quantum oscillations are singled out by Kartsovnik as being primarily responsible for
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the current intense experimental investigation of their high magnetic field properties. As a whole, the articles of this section give further substance to the idea that a high purity single crystal of a molecular material is by itself a laboratory where the chemistry and physics of anisotropic weak intermolecular interactions act in unison. Finally, I take this opportunity to acknowledge Robert Come`s, Denis Je´rome, Jerry B. Torrance, and Fred Wudl, key leaders in the modern development of this field. I have been introduced to the three others by Jerry, my postdoctoral mentor in 197880, and I, along with many, have been inspired by their exemplary qualities as scientists and individuals and wish them the best in all their future undertakings. There is no doubt in my mind that the outstanding contributions of the authors will serve to make this issue a unique resource for teaching and research in the literature of molecular materials. This was the objective in formulating this issue, and the response and dedication of all contributors have been impressive and praiseworthy; only a few articles were not able to make it on time, and these will be linked to the issue on the Journal Web site at a later date. Patrick Batail CNRS-University of Angers CR040697X
