Strategies for Crystal Engineering of Polar Solids
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INTRODUCTION The concept of crystal engineering or rational design of solids was first invoked by G.M.J. Schmidt in the context of assembling reactant molecules in crystals in order to mimic transition states for photochemical reactions'. In more recent years crystal engineering has matured2' 3 and focussed elsewhere, including towards generation of new classes of polar solid4' 5 . Such solids have a number of potential applications in the context of materials science, including, of course, second order nonlinear optics. In this manuscript we describe how crystal engineering can be invoked to generate novel structural analogues of two prototypal second order materials, p-nitroaniline, pNA, and related compounds6 , and potassium dihydrogenphosphate, KDP7 . Our basic strategy is to mimic the structural features of pNA and KDP in order to generate structurally analogous solids. We feel that the emphasis of the work is of potential interest to materials scientists for two primary reasons: (i) the strategies invoked are generic and therefore potentially extrapolatable to a chemically diverse range of compounds. (ii) we have focussed upon multi-component systems in order to maximize the ease and feasibility of fine-tuning physical properties while retaining the crystal architecture. RESULTS AND DISCUSSION pNA analogues The key features of pNA that make it a polar solid are the nature and orientation of the nitro and amino groups, which are hydrogen bond acceptor and donor groups, respectively. Their orientation necessarily means that intermolecular hydrogen promotes self-assembly of a head-to-tail polar strand in the solid state. The relevance of head-to-tail polymers is that they are inherently polar and therefore predisposed towards crystallizing in polar space groups. Etter and Frankenbach recently illustrated this via a comprehensive analysis of crystal structures of monocarboxylic acids8 . They observed that 52% of head-to-tail 1-D strands crystallize in polar space groups (i.e. there appears to be a random chance that adjacent 107 Mat. Res. Soc. Symp. Proc. Vol. 328. ©1994 Materials Research Society
strands are aligned parallel rather than anti-parallel). Conversely, only 2% of acids that exhibit discreet dimer structures, which have an inherent center of symmetry, were found to crystallize in polar space groups. In other words polymeric motifs that are inherently polar can strongly influence space group polarity. Unfortunately, extensive functionalization of pNA can be nontrivial and can alter the crystal architecture. We have chosen to investigate another species which is capable of self-assembling in a head-to-tail manner9 , the hydrogen sulfate anion, HS0 4". In principle polarizable counter cations can be modified at will and have little effect on the formation of [HSO 4-]oo strands since the OH.. •0 hydrogen bonds are relatively strong. Marder et al. recently reported a similar strategy with polarizable organic cations however the anions were not structure determining'°. We have prepared and structurally ch
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