Collective dynamics of molecular rotors in periodic mesoporous organosilica: a combined solid-state H-2-NMR and molecular dynamics simulation study

Molecular rotors offer a platform to realize controlled dynamics and modulate the functions of solids. The motional mechanisms in arrays of rotors have not been explored in depth. Crystal-like porous organosilicas, comprising p-phenylene rotators pivoted onto a siloxane scaffold, were modelled using molecular dynamics (MD) simulations. Long simulations, on a microsecond scale, allowed to follow the reorientation statistics of rotor collections and single out group configurations and frequency distributions as a function of temperature. The motions observed in the MD simulations support a multiple-site model for rotor reorientations. Computed motional frequencies revealed a complex rotatory phenomenon combining an ultra-fast libration motion (oscillation up to 30 degrees) with a slow and fast 180 degrees flip reorientation. Adopting a multiple-site model provides a more accurate simulation of the H-2-NMR spectra and a rationalization of their temperature dependence. In particular, rotators endowed with distinct rates could be explained by the presence of slower rings locked in a T-shaped conformation.