J. Am. Chem. Soc. 1993,115, zyxwvu 11521-11535 11521 Topographically Directed Nucleation of Organic Crystals on Molecular Single-Crystal Substrates Phillip W. Carter and Michael D. Ward’ Contribution from the Department zyxwvu of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455-01 32 Received April zyxwvuts 19, 1993” Abstract: The role of specific crystal planes of single crystals of j3-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 zyxwv on these substrates occurs at low driving force at zyxwvutsrqpo [ lOi], and [OIO]v,lledge sites formed from pairs of planes identified by atomic force microscopy as (OTO), zyxw n (1 il), and (OOl)va~ n (~Ol)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 [ 1011, ledge has a 1 .O% lattice mismatch with the [110] direction of benzoic acid and a difference of only 0.6O between the ledge dihedral angle and the dihedral angle of the (001)b n (1 T2)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 [lei], and [010],,1 ledge sites, with [lOT],,,, 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],,1 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. Introduction Crystalline solids based on molecular components exhibit numerous properties of fundamental and technological interest, including electrical conductivity, superconductivity, nonlinear optical behavior, and ferromagnetism.’ The principal advantage of these materials is the ability torationally control bulk properties through molecular design, which requires a thorough under- standing of the factors responsible for the supramolecular structure of molecular crystals. Accordingly, “crystal engineering” strat- egies based on the design of thermodynamically preferred intermolecular interactions in the bulk crystal have developed.2 However, relatively little effort has been spent examining the formation of molecular crystals in the context of the nucleation and crystal growth process. This is in stark contrast to extensive investigations of the crystal growth mechanisms of ice,3inorganic, * To whom correspondence should be addressed. 0 Abstract published in Aduance ACS Abstracts, November zyxwvuts 1, 1993. (1) Miller, J. S., Ed. Extended Linear Chain Compounds; Plenum Press: New York. (2) (a) Schmidt, G. M. J. Pure Appl. Chem. 1971,27,647. (b) Desiraju, G. R. Crystal Engineering-The Design of Organic Solids; Elsevier: New York, 1989. (c) Etter, M. C. Acc. Chem. Res. 1990, 23, 120. (3) (a) Evans, L. F. Narure 1965,206,822. (b) Evans, L. F. Nature 1967, 384. (c) Edwards, G. R.; Evans, L. F. Nature 1961,192,448. (d) Vonnegut, B. J. Appl. Phys. 1947,18, 593. (e) Blair, D. N.; Davis, B. L.; Dennis, A. S. J. Appl. Meteorol. 1973, 12, 1012. (f) Davis, B. L; Johnson, L. R; Moeng, F. J. J. Appl. Meteorol. 1975, 14, 891. 0002-7863/93/1515-11521$04.00/0 and elemental systems: although these studies rarely explore these mechanisms from a molecular perspective. Crystal growth of a molecular crystal can be described as a stepwise self-assembly process in which molecules form very small prenucleation aggregates, which then form nuclei that grow into macroscopic crystals. It is commonly accepted that the su- pramolecular structure of the prenucleation aggregates resembles the crystal structure of the mature crystalline phase.5 Therefore, control of the structure of prenucleation aggregates could lead to directed self-assembly of molecular components into nuclei and crystals with preordained supramolecular structures, possibly including kinetically preferred metastable polymorphs. Since nucleation almost always occurs at heterogeneous or solid interfaces due to the lowering of the surface energy of prenu- cleation aggregates and crystal nuclei, the structure of a substrate interface can play an important role in the crystallization pathway. Limited studies of growth of molecular crystals at interfaces have appeared, including observations of oriented growth on polymeric6 and ionic substrates’ and epitaxy on semiconductor (4) (a) Brice, J. C. Crysral Growth Processes, John Wiley and Sons: New York, 1986. (b) Hartman, P., Ed. Crystal Growth: An introduction; North Holland: Amsterdam, 1973. (5) Addadi, L.; Berkovitch-Yellin, Z.; Weissbuch, I.; van Mil, J.; Shimon, L. J. W.; Lahav, M.; Leiserowitz, L. Angew. Chem.. Int Ed. Engl. 1985, 24, 466. 0 1993 American Chemical Society