J. Am. Chem. Soc. 134, 12458–12461 (2012)

Credit: © 2012 ACS

There is much interest in the design of functional materials from the bottom up, so that their properties can be directly related to their molecular structures. Recently, a number of materials have been reported in which the properties depend on the control of motion within individual molecules. Now, Wataru Setaka and Kentaro Yamaguchi from Tokushima Bunri University in Japan have reported a compound that they describe as a 'molecular balloon', which 'inflates' when the temperature is increased.

These researchers have previously reported the synthesis of a molecular gyrotop — a system in which a phenylene rotor could freely rotate inside a cage formed by three long alkyl loops. The molecular balloon that they now describe was discovered when studying an intermediate structure in that prior synthesis. The alkyl-chain cage was constructed using ring-closing metathesis — producing a cage in which each chain contains an alkene. In contrast to the saturated alkyl cage of their original gyrotop — which is approximately spherical — an X-ray crystal structure of this compound (at 100 K) showed a cage that was 'dented' with the alkenyl chains collapsed inwards.

Analysis of the line shapes in the 2H NMR of a deuterium-labelled version of this gyrotop enabled Setaka and Yamaguchi to estimate the rotation rate of the phenylene rotor. At low temperatures (<300 K), the phenylene ring flips slowly because its rotation is hindered by the collapsed cage structure. When the temperature was raised above 310 K, however, continuous rotation was observed.

To investigate the origin of this temperature-dependent behaviour the researchers performed a high-temperature X-ray crystallographic analysis. As expected, free rotation of the phenylene at high temperature results in a disordered structure in the centre of the cage. In addition, however, at this temperature the cage is inflated — apparently to avoid the steric clash with the rotor. The effect of this inflation is also seen in the macroscopic properties of the crystal — when the cage is inflated, the density of the unit cell is dramatically reduced.