A light-operated pillar[6]arene-based molecular shuttle.

A molecular shuttle comprising a pillar[6]arene macrocyclic ring and an axle with two equal-energy-level stations connected by an azobenzene unit was synthesised. The E isomer of the azobenzene functioned as "open gate", allowing the pillar[6]arene ring to rapidly shuttle back-and-forth between the two stations. Ultraviolet irradiation induced photo-isomerisation of the azobenzene from E to Z form. The Z isomer of the azobenzene functioned as a "closed gate", inhibiting shuttling of the pillar[6]arene ring.

Holding of planar chirality of pillar[5]arene by kinetic trapping using host-guest interactions with achiral guest solvents.

We report a solvent-dependent switching and holding of planar chirality of pillar[5]arene with stereogenic carbons at both rims by host-guest complexation with achiral guest solvents. The planar chirality could be held for a given length of time at 25 °C in long linear guest solvents by kinetic trapping through host-guest complexation. The kinetic trapping worked at 25 °C, but not at 60 °C, thus a planar-chiral inversion using kinetic trapping based on host-guest complexation in the long linear solvents was demonstrated.

Spacer Length-Independent Shuttling of the Pillar[5]arene Ring in Neutral [2]Rotaxanes.

For a series of neutral [2]rotaxanes consisting of a pillar[5]arene ring and axles possessing two stations separated by flexible spacers of different lengths, the free energies of activation for the ring shuttling between the stations were found to be independent of the spacer length. The constitution of the spacer affects the activation energies: replacement of CH2 groups by repulsive oxygen atoms in the axle increases the barrier. The explanation for the observed length-independence lies in the presence of a barrier for re-forming the stable co-conformation, which makes the ring travel back and forth along the thread in an intermediate state.