based processing methods that introduce substantial economic advantages as well as prospects for fabricating devices with Molecules in the unusual (e.g., nonplanar) geometries. History and Development of Molecular Materials Solid State The articles contained in this issue of MRS Bulletin address materials that derive their properties from molecules in the solid Bruce M. Foxman and Michael D. Ward, state. The recognition that molecules play a role in solid-state properties can be traced to Guest Editors some key historical benchmarks beginning in the latter part of the 19th century, a signif- icant period for both the history and devel- Abstract opment of molecular materials. In 1848, The design and synthesis of solid-state materials constructed from molecules has Pasteur performed the first resolution of a emerged as an important frontier of materials research. Molecular materials promise chiral compound by physically separating an unparalleled opportunity for systematic manipulation of solid-state properties and crystals of sodium ammonium tartrate into functions by using molecular design principles and capitalizing on the versatility of left- and right-handed forms (a molecule is organic synthesis. Furthermore, the use of molecular components may produce chiral if it cannot be superimposed on its considerable economic benefits, whether by reducing fabrication cost or through mirror image; the two forms are enan- increases in performance. The articles in this issue of MRS Bulletin cover recent tiomers). Pasteur brilliantly observed that discoveries and developments based on materials with properties and functions that the crystals were different: the right- and hinge on the characteristics of their molecular constituents. These materials promise left-handed forms were enantiomers and could not be superimposed, and thus were significant advances in several technologies of substantial commercial interest, chiral. More than 150 years later, the impact including organic light-emitting diodes, nonlinear optics, gas separations, chiral of this discovery is still felt as numerous separations, and molecular magnets. research efforts employ chiral reagents and catalysts for asymmetric synthesis, through which one particular enantiomer can be made with high selectivity. Ironically, Introduction molecular crystals with chiral environ- Research during the last half of the 20th In the last decade, pronounced growth ments are now being explored for asym- century led to the discovery and develop- has occurred in functional solid-state metric synthesis and selective separations ment of remarkable inorganic materials materials that rely on the properties of of enantiomers, which clearly would have which spawned numerous technologies uniquely tailored molecules embedded in delighted Pasteur. that collectively have become essential the solid state either as isolated entities in Early interest in molecular materials to modern society, as exemplified by composites or as single-phase materials was sparked further by the development silicon-based transistors, gallium arsenide containing discrete constituents—organic of alizarin and indigo dyes in Germany lasers, ceramic piezoelectrics and thermo- and organometallic molecules—held during the latter half of the 19th century, electrics, and metallic and metal oxide together by noncovalent forces. The abil- and the optical properties of molecular magnetic storage media. Without these ity of organic synthesis to produce func- solids are as important today as they were remarkable solids, progress in comput- tional molecules on demand promises then. In the 1960s, Schmidt and co- ers, electronics, optics, communication, substantial advances in materials design workers further stimulated interest in energy storage, solar cells, machine tools, for existing technologies as well as emerg- organic solid-state chemistry through sensors, and more simply would not have ing ones. As a group, molecular-based their demonstration of stereospecific syn- been possible. materials exhibit most of the properties theses and polymerizations in the solid Although inorganic materials will con- typically associated with more conven- state by ultraviolet or γ-irradiation of crys- tinue to play a central role in such applica- tional inorganic and elemental solids, talline molecular materials.2 Schmidt’s tions, materials based on molecular from superconductivity to ferromagnet- research established a connection between building blocks offer unparalleled versa- ism to piezoelectricity to second harmonic solid-state packing of different forms (i.e., tility with respect to properties and func- generation. Like most inorganic materials, polymorphs) of cinnamic acid and tion. This already is amply demonstrated solid-state materials generated by the “topochemical” reactivity in the solid by the ubiquity of polymeric materials— assembly of discrete molecules often are state. In a topochemical process, the fixed molecular building blocks linked by cova- highly crystalline (aka “molecular crys- orientation of the reactants can lead to lent bonds—that unquestionably play a tals”). Unlike their inorganic counterparts, unusual stereospecific intermolecular key role in numerous commercial arenas. however, molecular-based materials tend reactions that cannot be accomplished in More recently, the coupling of functional to be mechanically soft and typically have solution. Schmidt’s work stimulated sci- organic molecules with inorganic compo- melting or decomposition points ranging entists to think about intermolecular inter- nents has prompted considerable interest from 100–400°C, in contrast with typical actions in crystals and provided the in “hybrid” materials and devices, for ranges of 500–1500°C for inorganic mate- conceptual framework for the discipline example, semiconductor field-effect tran- rials. However, molecular-based materials of “crystal engineering.” sistors decorated with molecular and bio- usually are soluble in aqueous or organic A particularly significant burst of activ- molecular receptors.1 solvents, providing access to solution- ity in molecular crystals began in the 534 MRS BULLETIN • VOLUME 32 • JULY 2007 • www/mrs.org/bulletin Downloaded from https://www.cambridge.org/core. IP address: 170.106.33.14, on 28 Sep 2021 at 20:09:17, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/mrs2007.102 Molecules in the Solid State 1960s with the emergence of organic acene molecule, exhibit hole mobilities in bounded only by their crystal dimensions, electron-donor–electron-acceptor com- a field-effect transistor configuration that constructed from molecular “tinkertoys” plexes, also known as charge-transfer rival those measured for amorphous sili- held together by hydrogen bonding or complexes, followed by investigations of con (Figure 1).13,14 Others have built on metal-ligand coordination are emerging electronic transport in organic crystals. Schmidt’s strategy of topochemical as promising candidates for molecular This spawned a flurry of activity in solid-state reactions to make polydi- separations and gas storage.18,19 More organic metals, superconductors, and acetylenes—potentially useful as optical recently, the materials science of pharma- semiconductors,3–10 the latter reemerging limiters, waveguides, and thermometric ceutical compounds, which often consist recently in the form of new strategies for sensors—directly from crystals of discrete of small-molecule crystals, has attracted organic light-emitting diodes and field- diacetylene molecules that are aligned considerable attention, owing to the effect transistors.11,12 For example, single properly in the solid state.15–17 New mate- importance of crystal form (i.e., poly- crystals of rubrene, a well-known poly- rials based on infinite frameworks, morphs), phase stability, and mechanical properties in therapeutic formulations. The reader is directed to the article by Jones et al. in the November 2006 issue of MRS Bulletin for a comprehensive overview of this topic.20 The Emerging Importance of Solid-State Structure Collectively, these efforts and others have reinforced the notion that solid-state properties can be manipulated through careful design and selection of the molecu- lar components. But they have also demonstrated that solid-state properties strongly depend on the molecular organi- zation in the solid state and, in the case of electronic materials, the interactions between electronic states on adjacent mol- ecules. Consequently, a key ingredient in materials design from molecules is the pre- diction and control of solid-state structure, which remains one of the foremost chal- lenges in organic solid-state chemistry. Advances in materials research inar- guably rely on a thorough understanding of solid-state structure and its relationship to solid-state properties. Indeed, the evo- lution of materials, from metals to alloys to ceramics to steels to specialty electronic materials, testifies to the inextricable link- age between solid-state structure and properties. As such, the emergence of organic solid-state chemistry as a disci- pline and the associated development of functional molecular materials can be linked to advances in solid-state structure determination, particularly single-crystal x-ray diffraction. Although single-crystal x-ray diffrac- tion was used initially to determine molecular structure alone, the strong con- nection between solid-state structure and properties in molecular