9408923 Ward This research describes strategies for controlled heterogeneous nucleation and growth of organic crystals at molecular interfaces, with emphasis on the use of substrates based on specific crystal faces of molecular crystals, and self-assembled films prepared from organosulfur and organosilane reagents. The proposed work will demonstrate that enhanced nucleation rates, specific growth orientations, and polymorph selectivity can be achieved through the use of properly designed substrates. In the case of molecular crystal substrates, these characteristics will be examined in the context of ledge-directed epitaxial nucleation, in which prenucleation aggregates are stabilized and oriented by interfacial interaction between two low energy planes of the aggregate and a geometrically similar substrate ledge. Organosulfur and organosilane films provide molecular functionality capable of specifically interacting with functional groups of molecules in the prenucleation aggregate. Atomic force microscopy of dynamic, in situ, crystal growth will also be examined. It is anticipated that these studies will lead to greater understanding of the critical factors responsible for nucleation and growth of organic crystals, and the rational design of solid substrates for controlled crystallization of technologically important reagents and materials. %%% Molecular crystals, defined as solids consisting of molecular components assembled in the crystalline state by weak, but numerous intermolecular interactions, exhibit numerous properties of fundamental and technological interest. These include electrical conductivity, superconductivity and ferromagnetism, nonlinear optical behavior, pharmaceutical activity, and photographic sensitizers. The interest in these materials also stems from the ability to rationally control bulk properties through molec ular design, a strategy commonly referred to as "crystal engineering." While there has been considerable study of the intermolecular interactions that direct solid-state packing in molecular crystals, investigations of their nucleation and growth, particularly at the molecular level, has been rather limited. This research describes strategies for controlled heterogeneous nucleation and growth of organic crystals at molecular interfaces, with emphasis on the use of substrates based on specific crystal faces of molecular crystals, and self-assembled films prepared from organosulfur and organosilane reagents. The proposed work will demonstrate that enhanced nucleation rates, specific growth orientations, and polymorph selectivity can be achieved through the use of properly designed substrates. ***
