Memory of chirality in J-type aggregates of an achiral perylene dianhydride dye created in a chiral asymmetric catalytic synthesis

Abstract

Perylene dyes tend to form H-aggregates because of their large aromatic π-core and high hydrophobicity. We report here a chiral asymmetric catalytic synthesis of water-soluble chiral J-aggregates from highly hydrophobic and achiral perylene dianhydride (PDA). We propose to generate in situ aggregating hydrophobic species from a water soluble precursor so that it aggregates at a low concentration in solution within a predesigned microenvironment to stabilize the formed aggregates. Choosing PDA as an example, we generated it in situ from a water-soluble perylene-3,4,9,10-tetracarboxylate in acidic cetyltrimethylammonium bromide (CTAB) micelle solution, employing small molecule D- and/or L-tartaric acid as the chiral auxiliary. CD spectra of the PDA J-aggregates formed in the presence of D- and L-tartaric acid, respectively, were mirror images of each other. When the chiral auxiliary was absent, water-soluble J-aggregates of PDA were also obtained from acidic HCl-CTAB solution but they were then achiral. This confirms that the chirality of the J-aggregates is transferred from tartaric acid. Using an L-/D-tartaric acid mixture of varying ee as the chiral auxiliary, the exciton-coupled induced circular dichroism (ICD) signal of the formed PDA J-aggregates shows an “S”-shaped profile when plotted versus ee, indicating that chiral amplification occurs. Purified J-aggregates were found to contain PDA and CTAB, whereas tartaric acid was hardly detected, yet the ICD signals remained the same as those in the presence of tartaric acid before purification. This indicates memory in the J-aggregates of the chirality of tartaric acid. The ICD signal of the purified J-aggregates was also found to remain unchanged upon addition of a large excess of the other enantiomer of tartaric acid than the one used to induce the chirality, implying the imprinting of the chirality in the J-aggregates. The J-aggregates of PDA were shown to be positively charged from the polar heads of the CTAB surfactant molecules that help to disperse the aggregates and prevent their precipitation in aqueous solution. It was found that it is critical that the surfactant tail is long enough for the chiral J-aggregates to be created. This suggests that the hydrophobic tail might act as a pillar to support the J-aggregates that are driven by edge-edge hydrophobic interactions. With this in situ generation for the highly hydrophobic perylene dyes we succeeded in creating water soluble J-aggregates, chiral and achiral, in a controllable manner.