ACS Cent. Sci. http://doi.org/bqt9 (2016)
Much like their natural counterparts, artificial molecular machines are expected to operate in an environment where viscous forces dominate inertial forces (low Reynolds number). However, building machines that have no macroscopic counterpart is conceptually challenging. Now, Hai-Bao Duan, Miguel Garcia-Garibay and colleagues at the University of California, Los Angeles have reported a molecular rotor that works in a highly viscous medium and could be used to study the dynamics of fluids in nanoconfined environments.
The researchers start with an amphidynamic crystal, that is, a crystal composed of a static framework with dynamic elements. In this case, a metal–organic framework (MOF) acts as a scaffold for a triptycene group linked by an alkyne. The triptycene is free to rotate around a three-fold axis, since the alkyne linkage poses a negligible activation barrier and there is enough free volume in the pores of the MOF that the dipole interactions with the scaffold and the steric interactions between adjacent tryptycenes are minimal. The researchers then used solid-state deuterium NMR to measure this diffusion-controlled rotational frequency of 2H-labelled rotors. Moreover, temperature-dependent experiments allowed them to estimate the rotational energy barrier as 15.5 kcal mol−1, about 10-fold higher than in the bulk. This effect is due to the presence of 10 water molecules in the vicinity of the rotor, which makes the viscosity of the MOF cavities four orders of magnitude higher compared with in the bulk.