Asymmetrical Molecular Aggregation in Spherulitic Dye Films

A recently developed method of spherulitic crystallization was used for the preparation of highly ordered 80 nm thin films of the dye 1.7-bis(dimethylamino)heptamethinium perchlorate (BDHClO4). Depending on the crystallization temperature, the film color and surface topography varied widely, while the crystal structure and film thickness remained the same. At low undercooling, two homogeneous regions were obtained. One showed in-plane symmetrical and the other asymmetrical growth behaviors. At high undercooling, a banded spherulitic structure with rainbow-like colors developed, whose formation is attributed to the outof-plane asymmetrical growth. The spherulitic growth kinetics, microstructure, and optical properties were investigated by optical microscopy, atomic force, and scanning tunneling microscopy in static and real time mode. A molecular mechanism is proposed which originates from different directions of macroscopic growth and microscopic molecular attachment dictated by the three-dimensional crystal lattice. This mechanism is consistent with the optical spectra and surface topography observed. This mechanism explains the exposure of different crystal faces, asymmetrical molecular attachment, fluctuation in growth rate, film thickness, and viscosity in the amorphous phase. Understanding the molecular origins of J-aggregation in thin dye films allows one to control and manipulate the film color almost in the whole visible wavelength range.