Solvent-induced chirality inversion involving supramolecular helix transformation and color-tunable fluorescence of a C6-symmetric hexakis(phenylethynyl)benzene derivative

Abstract

A C₆-symmetric disk-like molecule, a hexakis(phenylethynyl)benzene derivative bearing chiral alanine parts, L-1, exhibited a solvent-induced supramolecular helix-sense inversion involving conformational changes followed by destruction of the supramolecular helical column. This phenomenon has been found by investigating the supramolecular assembly state of L-1 in mixed solvents of various chloroform (CHCl₃)/n-hexane (Hx) ratios. L-1 forms a stable helical columnar assemblyvia multiple noncovalent bonding interactions in nonpolar Hx, while the molecules in relatively polar CHCl₃ are in a molecularly dispersed state. Although one would expect disruption of the helical column with the addition of nonhelicogenic CHCl₃, an opposite-handed helical columnar structure was formed at 8–15 vol% of CHCl₃, and subsequently the inverted helical column was disassembled by a further increase of CHCl₃. In addition, this morphological transformation was accompanied by a significant change in fluorescent color, which varies over a wide visible range from orange in an original helical columnar state to light blue in a molecularly dispersed state through yellow in an inverted helical columnar state. These unprecedented behaviors are shown by the spectroscopic results, and the molecular conformations of L-1 and the driving force for the helical sense inversion are discussed.