Chiral Amplification, Kinetic Pathways, and Morphogenesis of Helical Nanorods upon Self-assembly of Dipolar Merocyanine Dyes

In this account we report on the self-assembly of bis(merocyanine) dyes into helical nanorods by emphasizing the kinetic pathways and the mechanistic aspects leading to chiral amplification. A complex self-assembly sequence of these dipolar dyes into well-defined helical nanorod structures has been revealed spectroscopically by time-dependent circular dichroism (CD) spectroscopy and microscopically by atomic force microscopy (AFM). This self-assembly sequence proceeds over several kinetically formed supramolecular intermediates, which are distinguished by their morphology and (chir)optical properties. Moreover, the kinetics for the formation of homochiral nanorods was studied, revealing that chiral amplification mechanisms can operate at different stages of self-assembly to afford energetically favored structures. The rates of these processes decrease strongly with decreasing chiral bias in the monomers or decreasing fraction of chiral co-monomers in mixtures of chiral and achiral monomers. For the fully equilibrated nanorods, however, strong chiral amplification directed by the majority-rules or sergeants-and-soldiers effect is observed.