CRYSTAL GROWTH & DESIGN 2002 VOL. 2, NO. 6 465-474
Perspective Crystal Engineering-Where Do We Go from Here? C. V. Krishnamohan Sharma* Imaging Materials & Media, R&D, Eastman Kodak Company, 1999 Lake Avenue, Rochester, New York, 14650-2002 USA Received August 7, 2002
ABSTRACT: This brief review critically assesses achievements, current trends, limitations, and prospects for crystal engineering with an emphasis on the growth opportunities for this field. 1. Introduction It has been three decades since the concepts of crystal engineering were introduced.1 The recent launch of three new journals dedicated to this subject indicates its emergence as a major discipline in the physical sciences.2 A pertinent question at this juncture would be what did we learn in the past three decades about crystal engineering, and where do we go from here? Does the launch of three new journals on this subject indicate a continuing growth of this field, or do they simply provide additional journal space to accommodate the spurt of crystal structures that are being generated by current state-of-the-art CCD X-ray diffractometers? This brief review is aimed at critically discussing achievements, current trends, limitations, and prospects of this field. The pioneering research work carried out by Desiraju and the late Etter led to the establishment of crystal engineering as an independent discipline.3 Crystal engineering principles formulated by Schmidt in the context of organic solid-state photochemistry were extended to the entire gamut of materials chemistry.1,3a Further, extensive research work on coordination polymers and supramolecular chemistry has helped develop crystal engineering as it is practiced today, i.e., a truly interdisciplinary field in which the conventional boundaries between organics and inorganics are obliterated.4-9 2. What Has Been Achieved? Ironically, the greatest achievement(s) of crystal engineering in the past three decades can be summarized in a phrase, i.e., “viewing crystal structures as networks”.3-8,10 Indeed, a plethora of research articles published recently about engineering of new materials * E-mail:
[email protected].
revolved around this simple concept. A major advantage of this concept is that we can simplify complex crystalline structural features into easily identifiable network topologies based on chemical and structural information of the constituent molecular building blocks (Figure 1). Further, the network approach is not confined to the design of divergently binding infinite crystalline networks but is also used to synthesize convergently binding discrete molecular species such as macrocycles, polygons, or nanoparticles (Figure 1d).11 However, the major limitation of this strategy is that the predictability of networks is subjective and cannot be generalized; understanding the intriguing correlation between molecular shape, symmetry, and the nature of intermolecular forces seems to be the key for successful design of crystalline materials. Although considerable progress has been made on the theoretical front of crystal engineering, structure prediction with theoretical tools has met with less success compared to network-based approaches.10b,12 Because modeling intermolecular forces and cooperative effects for calculating crystalline lattice energies is a very difficult task, it is not possible to quantify intermolecular forces and atomic charges precisely, while accounting for the directional preferences of molecules through electrostatic interactions (also, see section 5.3).12 Recent advances in structure determination from X-ray powder diffraction may give a further boost to the overall growth of crystal engineering.13 If structure determination from powder diffraction data can be simplified as has been done for single-crystal diffraction, problems associated with the growth of single crystals for structure determination and polymorphism can be avoided. 3. Engineered Solids In general, systematic evaluation of structural trends in a given series of compounds leads to identification of
10.1021/cg0200356 CCC: $22.00 © 2002 American Chemical Society Published on Web 09/06/2002
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Figure 1. A concise network approach for crystal structure prediction: Consider molecules as nodes and noncovalent interactions as node connections.6d,6e Given the 1D and 2D molecular network structures (a), (b), and (c), it is not difficult to identify molecular building blocks for the design of discrete macrocycles. For details see ref 11.
Figure 2. (a) The 1:1 molecular complex of 2,4-dinitrocinnamic acid and 2,5-dimethoxy cinnamic acid forms expected network structure (hydrogen bonded hetero dimers and stacking interactions), but is not photoreactive (designer solid). (b) The 1:1 molecular complex of 3,5-dinitrocinnamic acid and 2,5dimethoxycinnamic acid forms expected network structure and undergoes 2+2 cycloaddition (functional solid).14a,14b
materials with well-defined structural patterns (designer solids) and optimization of such structural patterns for targeted physical or chemical properties may result in engineered functional solids (Figure 2).14 Although for the precise control of functional properties one needs to control the entire three-dimensional structure of crystals, certain chemical and physical properties can be predicted simply based on gross structural information. For example, while host-guest chemistry involving two-dimensional layered materials and open
networks do not require precise molecular assembly, the design of materials with useful polar, nonlinear optical, magnetic, and conducting properties require a close-toperfect maneuvering of molecular properties, electronic interactions, and structural arrangement (Figures 3 and 4).8,15-21 However, despite the above-mentioned difficulties, significant strides were made in the rational design of these materials.18-21 Because of the limited scope of this review, the successful crystal engineering strategies behind the design of these materials are not detailed here, and readers are requested to consult the literature to grasp the breadth of this field. A wide variety of functional materials have been designed using crystal engineering concepts, but in recent years, rational design of layered structures and porous solids to mimic the functional properties of natural clays and zeolites seems to have attracted greater attention (Figure 3b,c). The reasons for such attention on these materials (apart from their usefulness) is that the design strategy is relatively simple (see Figures 1 and 3c), and the functionality of these materials is self-evident from crystal structures. For example, a three-dimensional, open networked structure filled with guest (or solvent) molecules may be presumed as a porous functional solid and that a corresponding open networked structure with self-interpenetration (and no guest molecules) may be considered as a nonporous solid. However, sometimes, simple crystal structures
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Figure 3. (a) Organic molecules with incompatible crystal packing features readily form inclusion complexes in the presence of appropriate guest (or solvent) molecules.16 (b) Layered structures with robust intralayer bonds and weak interlayer interactions readily intercalate complementary neutral or charged guest species (i-iii).17 (c) Symmetric molecules with robust directional interactions form open porous structures in the presence of guest molecular template.15
alone may not make certain key functionalities of these materials obvious, such as inclusion properties, reversible exchange of guest molecules, stability of porous framework, or application aspects. Therefore, the ultimate test for any successful example of engineered solid may hinge on the demonstration (i.e., quantification) of its attributed physical and chemical properties. 4. Criticism It is interesting that crystal engineering, especially network-based materials design, emerged as a promising field of research, despite the formidable criticism faced in the early 1990s by Maddox, then editor of Nature, who said that it remains impossible to predict crystalline architectures from a knowledge of chemical composition.22 Recently, even the network approach for materials design has come under the scrutiny of critics. The major criticism is that research articles concerning materials design overdo with graphics and jargon, and crystal structures are conveniently interpreted using carefully hand-picked structures screened out of several as rationally designed materials.23 Although any response to such a criticism is subjective (for example, one may consider combinatorial chemistry methods as unscientific) and debatable given the scientific perception of individual researchers or specific publications, such criticisms need to be welcomed and debated for healthy progression of crystal engineering into the future.24 One of the possible reasons for the above criticism could be that with the advent of CCD diffractometers, crystal structure determination has been dramatically simplified, and the number of publications on crystalline materials design is growing exponentially. In a majority
of these publications, novelty of engineered solids seems to be dependent on an author’s interpretation of crystal packing (based on network approach), lack conceptual advances, and proven functional properties. Consequently, it is becoming increasingly difficult to differentiate novel crystal structures, ordinary crystal structures, or crystal structures from crystal engineering.25 Therefore, there is a need to define and debate on the role(s) of various elements of crystal engineering that either directly or indirectly facilitate the design of materials. Otherwise, one may ask the question that one of the referees of this paper asked, why do crystal engineers (read structural chemists) always talk about physical properties without delivering something? It is important to note that crystal engineering is growing at a faster pace than one would have anticipated, and, unlike the past, scientists with diverse research backgrounds are becoming part of this field, and structure prediction is now considered to be one of the several aspects of this subject. The above discussion may explain why crystal engineering did not make significant inroads into industrial research programs for functional material design yet. In general, industry takes its cue from successful academic studies and research trends and gradually adopts promising new methods and technologies for its own research and development of new products. 5. “Growth” Opportunities for Crystal Engineering The principles/methodology of crystal engineering may be successfully applied to several exciting new areas of research. Some of these areas are briefly discussed below, considering their significance in fundamental science and industrial applications. 5.1 Nanoparticles. In recent years, a great deal of interest has been generated in the synthesis and ordered assembly of nanoparticles with controllable sizes and shapes because of their useful applications in present (e.g., paints, inks, coatings, pharmaceuticals, catalysts) and future technologies (e.g., photonics, quantum dots, photovoltaic cells, microelectronics, and digital display technology).26-32 In general, synthesis of monodispersed nanoparticles requires controlled nucleation of new particles and growth inhibition of existing particles. At present, there is a little scientific understanding of the factors that influence the size, shape, and growth of nanoparticles, and the experimental conditions for the synthesis of nanoparticles are optimized using empirical methods.26,27 Indeed, one can draw parallels between the design and synthesis of nanoparticulate materials and bulk crystalline materials as basic principles that govern the formation of either type of material are identical. However, it is not possible to get detailed structural information in most cases of nanoparticulate materials. Therefore, crystal engineering and supramolecular chemistry concepts can be readily extended to nanoparticles (which may be viewed as controlled finite self-assembly of molecules/ ions). Figure 5 schematically illustrates the intimate correlation among various self-assembled systems and highlights the importance of crystalline materials in getting precise information about molecular structures and intermolecular forces.
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Figure 4. Noncentrosymmetric hydrogen bonds between amino-nitro and carboxylic acids of donor-acceptor benzoic acids result in polar crystals, but lack of phase matchability prevents the realization of nonlinear optical properties (a).19a The two polymorphic structures of dithiadiazolyl radical are almost identical, except for the twist angles between the two rings and S‚‚‚S interactions (b, c). Consequently, the antiferromagnetic coupling in these two compounds is significantly different.20d
Figure 5. A supermolecule can be a simple convergent assembly of two molecules [solution state, (b)] or a complex divergent assembly of infinite number of molecules [solid state, (c)]. These two extreme examples of a supermolecule accentuate how the concepts of solid-state supramolecular synthesis may help in defining newly emerging areas of research such as nanoparticles (d) and (e), self-assembled monolayers, and thin films (f) and (g).
Fundamental questions that need to be answered for the design of nanoparticles are no different from those for the rational design of bulk crystalline materials. For example, why are certain organic molecules/surfactants
and experimental conditions conducive to making uniform dispersions of nanoparticles with controllable particle sizes? How do cooperative electronic effects influence the nucleation and growth of particles? How
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do the internal structure of particles and experimental conditions influence the size and shape of particles? How one can predict conditions suitable for the synthesis of nanoparticles with controllable sizes to fine-tune their physical and chemical properties? Is it possible to generate superlattice structures of nanoparticulates through controlled self-assembly for applications in microelectronics and photonics? These are some of the issues that easily fall under the realm of solid-state supramolecular synthesis. 5.2 Monocrystalline Thin Films. Conventionally, nucleation and crystal growth aspects of organic compounds have been studied in solution state, and no significant attention has been directed toward crystal growth of organic or organometallic complexes by sublimation or vapor phase deposition.19d,33 Indeed, understanding the crystallization process upon sublimation may be extremely important considering the key role organic thin films (
