Molecular Arrangements in Two-Dimensional J-Aggregate Monolayers of Cyanine Dyes

Abstract

Atomic force microscopy was used for the study of two-dimensional J-aggregates monolayer structures of two cyanine dyes with a high-resolution. Two coexisting morphologically different types (i.e. the bilayer rectangular stripes and monolayer curve shaped leaves) were observed for 3,3’-di(γ-sulfopropyl)-4,4’,5,5’-dibenzo-9-ethylthiacarbocyanine betaine pyridinium (dye D-1). The morphological diversity, the crystallographic peculiarities and the monolayer height ~ 1.35 nm are selfconsistently explained by the difference in the molecular packing arrangements, i.e. the ladder for stripes and the staircase for leaves. A building unit of both J-aggregate types is a symmetric monolayer with the same height and slip angle ~ 15o, composed of all-trans monomers with anti-parallel up-down orientation. For the two-dimensional sheets of the second cyanine dye (5,5'-dichloro-3,3'-disulfopropyl thiacyanine sodium salt) Dye-2 the fine substructure formed by close packed parallel narrow threads was resolved. The stripes were inclined at the angle ~30o to the J-aggregates long side providing for the first time the direct microscopic support for the probable staircase molecular arrangement. It is speculated that the quazi-one-dimensional staircase-type and ladder-type units are building blocks of the two-dimensional aggregates of various cyanine dyes.