Topographically Directed Nucleation of Organic Crystals on Molecular Single-Crystal Substrates

The role of specific crystal planes of single crystals of β-succinic acid (sa) and L-valine (val) as substrates for the nucleation and growth of organic crystals has been examined. Freshly cleaved crystals of these substrates provide flat terraces and ledge sites corresponding to planes whose molecular structures are well-defined by the crystallographic structure of the substrate. Nucleation of benzoic acid on these substrates occurs at low driving force at and [010]_val ledge sites formed from pairs of planes identified by atomic force microscopy as ∩ and (00l)_val ∩ respectively. Growth from these ledge sites is attributed to lowering of the prenucleation aggregate free energy via “ledge directed epitaxy” that involves a lattice match between the substrate and growing phase along the ledge direction, and equivalent dihedral angles of the substrate ledge sites and a pair of aggregate planes whose identity is assigned on the basis of the structure of the mature crystal. For example, the ledge has a 1.0% lattice mismatch with the [110] direction of benzoic acid and a difference of only 0.6° between the ledge dihedral angle and the dihedral angle of the (001)ba ∩ planes. On the basis of the crystal structures, these interfaces consist of “molecularly smooth” low-energy planes, consistent with stabilization of the prenucleation aggregates by dispersive interactions. As a consequence of these epitaxial effects and the crystallographic symmetry of the monoclinic space groups of the substrates and benzoic acid, benzoic acid growth is highly oriented. Oriented growth of 4-nitroaniline (pna) crystals is also observed on and [010]_val ledge sites, with the direction containing hydrogen-bonded 4-nitroaniline chains, aligned along the ledge directions. In each case, the lattice mismatch along the ledge direction is small and stabilization of the prenucleation aggregate by interaction with both planes of the ledge is evident from the absence of nucleation on ledge-free areas of the substrates. Experimental observations and calculations suggest that topographically directed growth orientation is observed when aggregate adsorption at the ledge is dominated by dispersive forces. Smaller contributions from dipolar and hydrogen bonding interactions may also play a role in nucleation of 4-nitroaniline on L-valine, in which 4-nitroaniline chains align with the polar [010]_val axis of the substrate ledge. These studies indicate that generally accepted epitaxy concepts involving principal lattice directions of the substrate and growing phase may be an oversimplified explanation of crystal growth on crystalline substrates. Rather, nucleation of crystalline phases on molecular crystal substrates is controlled by topographic structure, lattice parameters of ledge nucleation sites, symmetry constraints, and molecular composition of aggregate and substrate crystal planes.