Surface-Directed Adsorption in the Epitaxy Growth of Streptocyanine Dye Crystals

The simple molecular structure, relatively high stability, and color variation with molecular packing structure make the streptopolymethine dyes an ideal candidate for the study of surface effects on the nucleation and growth of organic crystals at a liquid/solid interface. In situ optical and atomic 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)heptamethinium (BDH+Cl-) in acetone yields prismatic crystals bound by the (001) and (110) faces. Optical study shows that, of all the substrates studied, mica has the highest selectivity toward the (001) face. The orientation of this face follows the 3-fold symmetry of the mica lattice. AFM measurements captured the early stages of BDH+Cldye crystallization on mica, such as the adsorption of single molecules, formation of rodlike aggregates, their organization into twodimensional brickwork aggregates/crystals, and three-dimensional crystal growth. For all stages, the dye molecules were oriented edge-on along the [100] direction of the mica, yielding the (001) BDH+Clcrystal face. It is concluded that BDH+Clcrystallizes epitaxially on mica despite the poor match in lattice symmetry and parameters. This is realized by the weak interlayer and strong intralayer interactions and the surfacedirected alignment of adsorbed BDH+ molecules.