Hydrogen Bonding-Induced Inversion and Amplification of Circularly Polarized Luminescence (CPL) in Supramolecular Assemblies of Axially Chiral Luminogens

Abstract

Herein, we report the self-assembly and chiroptical properties of two axially chiral π-conjugated luminogens, R-NMI and S-NMI, each equipped with two pyridyl moieties for hydrogen (H)-bonding with chiral diacids. The two enantiomers display aggregation-induced emission enhancement (AIEE) and increased CD and CPL signals in the self-assembled state with a high g_lum value of 1.5 (±0.06)×10⁻² in 1:9 dioxane:methylcyclohexane. Crystallographic analysis confirmed mirror-image helical structures for R-NMI and S-NMI involving both intra- and intermolecular π-π stacking, leading to elongated hexagonal platelets. Supramolecular co-assembly of R-NMI with D- and L-tartaric acids (D-TA and L-TA) could remarkably modulate and invert the chiroptical properties of R-NMI, which is unachievable with control chiral monoacids. The co-assembled structures were driven by pyridine-carboxylic acid H-bonding as revealed from the crystal structure analysis, which was also supported by computational studies. Strikingly, R-NMI+D-TA leads to an exceptionally high fourfold amplification in the g_lum value [5.4 (±0.04)×10⁻²] with an inverted sign, which additionally demonstrates intriguing temperature-dependent switching. In contrast, R-NMI+L-TA results in a threefold reduction in the g_lum value [0.54 (±0.015)×10⁻³], also with an inverted sign compared to R-NMI alone, establishing a clear strategy for chiral discrimination between the two enantiomers of TA.