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(dimethyl amino)heptamethinium perchlorate (BDH+ClO4-). Depending on the crystallizat ion temperature, the film color and surface topography varied widely, while the crystal structure and film thickness remained the same. At low underco oling, two homogeneous regions were obtained. One showed in-plane symmetric al and the other asymmetrical growth behaviors. At high undercooling, a ban ded spherulitic structure with rainbow-like colors developed, whose formati on is attributed to the out-of-plane asymmetrical growth. The spherulitic g rowth kinetics, microstructure, and optical properties were investigated by optical microscopy, atomic force, and scanning tunneling microscopy in sta tic and real time mode. A molecular mechanism is proposed which originates from different directions of macroscopic growth and microscopic molecular a ttachment dictated by the three-dimensional crystal lattice. This mechanism is consistent with the optical spectra and surface topography observed. Th is mechanism explains the exposure of different crystal faces, asymmetrical molecular attachment, fluctuation in growth rate, film thickness, and visc osity in the amorphous phase. Understanding the molecular origins of J-aggr egation in thin dye films allows one to control and manipulate the film col or almost in the whole visible wavelength range.