Chiral Perylene Bisimide–Melamine Assemblies: Hydrogen Bond-Directed Growth of Helically Stacked Dyes with Chiroptical Properties

Abstract

Self-aggregation and hydrogen bond-directed superstructure formation with a perylene bisimide dye were investigated for melamines bearing two or four alkyl chains. For a melamine with two n-octadecyl substituents, large micellar aggregates of several hundred nanometers were observed in methylcyclohexane solution by dynamic light scattering. By means of hydrogen bonding these micelles were loaded with various contents of perylene bisimide dyes up to an equimolar amount. Even at low dye contents, islands of strongly aggregated perylene bisimides were formed according to UV-vis absorption spectroscopy. In a series of melamines functionalized with n-octadecyl esters of chiral amino acids, no self-aggregation of the melamines was observed, and the stability of the assemblies with the perylene bisimide dyes showed strong dependence on the melamine substituents. For four melamine–perylene bisimide combinations stable assemblies formed, which exhibited exciton-coupled circular dichroism (CD) effects for the perylene bisimide absorption bands whose sign is controlled by the stereoisomerism of the respective melamine. While for (S,S)-melamines a negative Cotton effect was observed, indicating an M-helical arrangement of the chromophores within the assembly, the (R,R)-enantiomer induced P-helicity, and no CD effect was found for the achiral (R,S)-diastereomer.