Negative nonlinear CD-<italic>ee</italic> dependence in dibenzoyltartaric acid induced aggregates of perylenebisimide

Supramolecular chirality of aggregates can be described in terms of the CD signals of the aggregates as a function of the enantiomeric excess of the chiral building block or chiral inducer. While many examples have been reported that show a positive nonlinear CD- ee relationship, indicating chiral amplification, the negative nonlinear dependence has rarely been observed. We recently proposed to build such supramolecular aggregates by using the mesomer of the enantiomeric inducers. Indeed we found that in the presence of enantiomeric dibenzoyltartaric acids (DboTar), achiral perylenebisimide (PBI) dye equipped with two terminal guanidinium groups in alkaline aqueous buffer solution is induced to form chiral supramolecular aggregates, via the known guanidinium-carboxylate salt bridge, that show a positive nonlinear CD- ee relationship, and is turned into negatively nonlinear when 40% or more molar fraction of meso -DboTar was introduced. Absorption and CD spectral titrations suggest that the aggregates led by enantiomers and by mesomer are similar in terms of the supramolecular structure and stability, and when the two kinds of aggregates are mixed no molecular exchange occurs between them. Fluorescence spectral traces of the aggregates show that the mesomer is more powerful in inducing the aggregation of the PBI dyes, and thus dominates the aggregation. Our findings that the mesomer of the enantiomeric inducer could lead to aggregates to exhibit a negative nonlinear CD- ee relationship provide a promising approach to such supramolecular aggregates, which could be of significance for better insight into the mechanism leading to the negative nonlinear dependence and eventually a comprehensive understanding of the supramolecular chirality.