NEW CLASSES OF MOLECULES FOR DESIGNING STRUCTURES IN THE ORGANIC SOLID STATE. John C. MacDonald, Northern Arizona University; Kathy E. Schwiebert, G. Tayhas, R. Palmore, Jon A. Zerkowski, Serge Palacin, and George M. Whitesides, Harvard University.

This talk will describe our efforts to design molecules that crystallize in well-defined structures, to rationalize the structures of crystals based on the structures of their component molecules, and to predict the structures of new molecular crystals. Our approach attempts to simplify the packing of molecules by constraining them to form tapes--that is, flat linear aggregates--that pack with their long axes parallel, an arrangement likely to minimize free volume in a crystal. We have based our design for tapes on the network of hydrogen bonds present in three separate classes of molecules: (1) 1:1 complexes between N,N'-diphenylmelamines and 5,5-diethylbarbituric acid (mel*bar complexes); (2) 4,5-disubstituted 2-benzimidazolones (cyclic ureas); (3) diketopiperazines. Series of derivatives from each of these classes of molecules have been synthesized and their structures determined in the solid state. These molecules crystallized in several different structural motifs that can be rationalized using familiar concepts such as steric repulsion and principles of closest packing. In several cases, mel*bar complexes formed tapes that crystallized in different crystalline forms; this polymorphism complicates our efforts at understanding the relationship between molecular and crystal structure. Cyclic ureas and diketopiperazines have not exhibited polymorphism under normal conditions of crystallization.