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        Crystal Engineering    
   
   
    The design of crystals with predictable packing arrangements and properties is a major challenge for supramolecular chemistry.  We entered this area by accident, through our work on anion receptors/transporters.  We transformed cholic acid into a number of bis-phenylureas, and found that some of them formed needle-like crystals (see right; X = CF3, NHPh, OBut).  When we determined the crystal structures we found that the packings were almost identical, and quite remarkable....      
       
   
   
    The molecules arrange themselves in a honeycomb pattern with broad channels running through the crystals.  The picture (right) shows the structure for X = CF3, viewed along the length of the channels.  The gaps are originally filled with solvent, but these can be replaced by other molecules.  As the channels are so wide (up to 1.4 nm) they could in principle accommodate a great variety of guests.  The group X points into the channel, so it is possible to change this unit without disrupting the structure.  This means we can tune the size and nature of the channels, and perhaps design crystals for specific purposes.  Such "nanoporous" crystals have potential for separations, catalysis and optoelectronic applications.

For more details see: A. L. Sisson et al., Angew. Chem. Int. Ed. 2005, 44, 6878.