Effect of an Intramolecular Hydrogen Bond Belt and Complexation with the Guest on the Rotation Behavior of Phenolic Units in Pillar[5]arenes

The rotation behavior of the phenolic units in pillar[5]arenes has been studied by means of the dynamic (1)H NMR measurements. In acetone-d(6), the individual conformers of pillar[5]arene were observed under -60 degrees C, indicating that the rotation movement was slow on the NMR time scale under -60 degrees C. In permethylated pillar[5]arene in acetone-d(6), the peaks were not split even at -90 degrees C, strongly suggesting that the rotation movement was fast on the NMR time scale at -90 degrees C. This is due to the lack of the intramolecular hydrogen bond belt in permethylated pillar[5]arene. In the mixture of pillar[5]arene and viologen guest in acetone-d(6), the split peaks were found at -30 degrees C and did not change under -60 degrees C. By formation of the host-guest complex between pillar[5]arene and viologen guest, the rotation movement was slow on the NMR time scale under -30 degrees C and almost stopped under -60 degrees C.