Solvophobically driven self-association of a butadiyne-bridged pyridine macrocycle

A butadiyne-bridged square-shaped pyridinophane 2b having ester groups derived from triethylene glycol monomethyl ether was prepared and its self-assembling properties were investigated to assess the repulsive dipole-dipole interaction in less polar solvent and solvophobic driving force in polar solvents. Concentration-dependent 1H NMR spectra showed that whereas 2b did not self-assemble in chloroform-d, it did in polar solvents consisting of chloroform-d and methanol-d4 (4:6 to 6:4) with increasing association constants with the increasing methanol-d4 composition. Comparison of the free energies of association with those of the previously reported benzene macrocycle 1b indicates that solvophobically induced self-association of 2b is more promoted than 1b. The reasons for this difference are discussed from both enthalpy and entropy points of view.