Selective Nucleation and Growth of an Organic Polymorph by Ledge-Directed Epitaxy on a Molecular Crystal Substrate

Selective growth of a thermodynamically less preferred polymorph of the salt (DMTC⁺)(TMO^-)•CHCl₃(DMTC = 3,3’-dimethylthiacarbocyanine; TMO = 3,3’,5,5’-tetramethyltrimethine oxonol) has been achieved from CHCl₃ solutions by nucleation on single-crystal succinic acid (SA) substrates. This process occurs by a ledgedirected epitaxy (LDE) mechanism in which the dihedral angle between two close-packed planes of a substrate ledge site matches that of two close-packed planes of a prenucleation aggregate corresponding to the observed polymorph. The observed orientation of (DMTC⁺)(TMO^-)•CHCl₃ is consistent with interaction of the close-packed (010) and (014) planes of its aggregate with succinic acid (010) and (111) planes that define ledges along the [101] direction of the substrate. These ledge sites are produced readily by cleavage of SA single crystals. The identity of the polymorph and its orientation are confirmed by visible spectroscopy, crystal morphology, and atomic force microscopy. While LDE allows for two possible orientations of (DMTC⁺)(TMO^-)•CHCl₃, the orientation observed is that in which the lattice mismatch along the interface is minimized. These observations illustrate that crystallization processes based on LDE may be valuable for selective growth of polymorphs.