Supramolecular Organogels Formed through Complementary Double-Helix Formation

Optically active amidine ((R)-1) and achiral carboxylic acid (2) dimers with a m-terphenyl backbone linked by a 1,4-phenylene diacetylene unit form a double helix, (R)-1⋅2, through complementary amidinium-carboxylate salt bridges in THF. Upon the addition of poor solvents, such as n-hexane, the duplex forms an organogel as a result of supramolecular polymerization of the duplex by intermolecular rearrangement of the salt bridges. In sharp contrast, an analogous racemic duplex composed of achiral amidine residues and an optically active duplex linked by a shorter diacetylene unit with a higher binding affinity than that of (R)-1⋅2 does not show any gelation. The supramolecular fluorescent gels exhibit reversible thermo- and chemoresponsive behavior. The chiroptical properties of the gels, the mechanism of gelation, and the amplification of helical chirality during the gelation of (R)-1⋅2 in the absence and presence of its enantiomeric counterpart, (S)-1⋅2, and a racemic duplex, consisting of achiral amidine and carboxylic acid dimers, were investigated by following changes in the absorption and circular dichroism spectra.