Modulating Helix‐Preference of an Axially‐Twisted Molecular Scaffold Through Diastereomeric Salt Formation with Tartaric Acid Stereoisomers

Abstract

AbstractHerein, we designed a chiral, axially‐twisted molecular scaffold (ATMS) using pyridine‐2,6‐dicarboxamide (PDC) unit as pivot, chiral trans‐cyclohexanediamine (CHDA) residues as linkers, and pyrene residues as fluorescent reporters. R,R‐ATMS exclusively adopted M‐helicity and produced differential response in UV‐vis, fluorescence, and NMR upon addition of tartaric acid (TA) stereoisomers allowing naked‐eye detection and enantiomeric content determination. Circular dichroism (CD) profile of R,R‐ATMS underwent unique changes during titration with TA stereoisomers ‐ while loss of CD signal at 345 nm was observed with equimolar D‐TA and meso‐TA, inversion was seen with equimolar L‐TA. Temperature increase weakened these interactions to partially recover the original CD signature of R,R‐ATMS. 2D NMR studies also indicated the significant structural changes in R,R‐ATMS in the solution state upon addition of L‐TA. Single crystal X‐ray diffraction (SCXRD) studies on the crystals of the R,R‐ATMS⊃D‐TA salt revealed the interacting partners stacked in arrays and ATMS molecules stabilized by π‐π stacking between its PDC and pyrene residues. Contrastingly, tightly‐packed supramolecular cages comprised of four molecules each of R,R‐ATMS and L‐TA were seen in R,R‐ATMS⊃L‐TA salt, and the ATMS molecules contorted to achieve CH‐π interactions between its pyrene residues. These results may have implications in modulating the helicity of topologically‐similar larger biomolecules.