Directing the Assembly of Molecular Crystals
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Directing the
Assembly of Molecular Crystals Michael D. Ward
Abstract Crystalline materials made from molecular components can possess useful properties that can be tailored through judicious selection of their molecular building blocks. The utility of these materials, however, depends on molecular packing in the crystal lattice as well as the properties of the individual molecules themselves. Consequently, further advances hinge on our ability to manipulate solid-state structure in a rational and systematic manner. Although computational prediction of crystal structure remains elusive, empirical guidelines for assembling molecules into preordained crystal architectures are emerging rapidly. This article briefly describes the current state of the field, emphasizing the design of crystalline materials with structures reinforced by a twodimensional hydrogen-bonded network, which serves as a platform for the synthesis of a diverse collection of compounds. These include host frameworks with cavities supported by organic “pillars” that can be interchanged to manipulate the size, shape, and character of the inclusion cavities as well as the overall lattice architecture and metrics. This research has revealed some principles for crystal design that may prove useful in general while enabling exploration of the utility of these compounds. Keywords: crystal engineering, framework materials, liquid crystals, nonlinear optics, self-assembly, separations, structure.
Introduction A broad range of scientific endeavors has established a growing interest in the “bottom-up” design and synthesis of materials based on well-defined building blocks. In this regard, considerable attention has been focused on molecular crystals, which are appealing because their solid-state properties (e.g., conductivity, superconductivity, magnetism, second harmonic generation, light-emission) can be regulated, in principle, by tapping the vast arsenal of synthetic methodologies developed by organic chemists to create molecules with desirable attributes. Most solid-state properties, however, hinge on collective interactions, which depend on molecular packing in the crystal lattice as well as the properties of the individual molecules themselves. Consequently, realizing the full potential of these materials depends on reliable manipulation of solidstate structure across several length scales. Molecular crystals are usually prepared by conventional crystallization, which can
MRS BULLETIN • VOLUME 30 • OCTOBER 2005
be viewed as a type of self-assembly that relies on specific molecular recognition events between molecular building blocks to generate a single crystal with periodic order. Unfortunately, the delicate, noncovalent forces that govern molecular organization in the solid state usually frustrate attempts at de novo design of molecular crystals, and the prediction of crystal structure remains one of the foremost challenges in organic solid-state chemistry.1,2 The lattice energy of different calculated forms of even the simplest compounds may dif
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