Surface-directed adsorption in the epitaxy growth of streptocyanine dye crystals

The simple molecular structure, relatively high stability, and color variat ion with molecular packing structure make the streptopolymethine dyes an id eal candidate for the study of surface effects on the nucleation and growth of organic crystals at a liquid/solid interface. In situ optical and atomi c force microscopy;(AFM) experiments were conducted on amorphous glass and crystalline muscovite mica in order to correlate the crystal morphology and orientation with early molecular events during adsorption and nucleation. Bulk crystallization of the chloride salt of 1,7-bis(dimethylamino)heptamet hinium (BDH+Cl-) in acetone yields prismatic crystals bound by the (001) an d (110) faces. Optical study shows that, of all the substrates studied, mic a has the highest selectivity toward the (001) face. The orientation of thi s face follows the 3-fold symmetry of the mica lattice. AFM measurements ca ptured the early stages of BDH+Cl- dye crystallization on mica, such as the adsorption of single molecules, formation of rodlike aggregates, their org anization into two-dimensional brickwork aggregates/crystals, and three-dim ensional crystal growth. For all stages, the dye molecules were oriented ed ge-on along the [100] direction of the mica, yielding the (001) BDH+Cl- cry stal face. It is concluded that BDH+Cl- crystallizes epitaxially on mica de spite the poor match in lattice symmetry and parameters. This is realized b y the weak interlayer and strong intralayer interactions and the surface-di rected alignment of adsorbed BDH+ molecules.