Asymmetric living supramolecular polymerization of an achiral aza-BODIPY dye by solvent-mediated chirality induction and memory

Abstract

The kinetic self-assembly properties of an achiral aza-BODIPY dye 1 bearing two hydrophobic fan-shaped tridodecyloxybenzamide pendants through 1,2,3-triazole linkages were investigated in detail in chiral solvents (S)- and (R)-limonene by UV/Vis absorption spectroscopy, CD spectroscopy, AFM, and TEM. The spectroscopic studies indicated the formation of thermodynamic H-aggregates (Agg. II_L) with a helical morphology, while metastable aggregates (Agg. I_L) could only be observed in the fast cooling process of 1 in limonene. According to further analysis of the spectroscopic data using the nucleation–elongation model, the mechanistic aspects of the self-assembly process of dye 1 were elucidated. The macroscopic chirality and helicity of supramolecular aggregates were found to be dependent on the chirality of the limonene solvents. Since the kinetic trapping of Agg. I_L could not be attained in experiments, an effective strategy for asymmetrical living supramolecular polymerization (LSP) of dye 1 was developed by employing the seeds in the chiral solvent limonene and the kinetically-trapped metastable aggregates (Agg. I_M) in methylcyclohaxane (MCH). The chiral aggregates Agg. II_L were obtained by multiple-cyclic LSP in the MCH/limonene solvent mixture even at a very low limonene ratio (less than 1%), which could be ascribed to the chiral memory effect of dye 1 aggregates upon continuous dilution of the chiral environment in the solvent mixture during the LSP process.