Collective Dynamics of Molecular Rotors in Periodic Mesoporous Organosilica: A combined Solid-State 2H-NMR and Molecular Dynamics Simulation Study.

Molecular rotors offer a platform to realize controlled dynamics and modulate functions of solids. Motional mechanisms in arrays of rotors have not been explored in depth. Crystal-like porous organosilicas, comprising p-phenylene rotators pivoted onto the siloxane scaffold, were modelled using Molecular Dynamics (MD) simulations. Long simulations, on the 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 a ultra-fast libration motion (oscillation up to 30°) with a slow and fast 180° flip reorientation. Adopting a multiple-site model grants a more accurate simulation of the 2H-NMR spectra and a rationalization of their temperature dependence. In particular, rotators endowed with distinct rates cloud be explained by the presence of slower rings locked in T-shaped conformation.