Precision engineering of hyper-fast molecular rotors and “on-command” photoswitches in rigid and flexible porous architectures

Nanoporous materials with large free volumes and fine-tuned architectures provide an ideal framework for organizing molecular rotors, motors, and switches in the condensed phase, facilitating the development of stimuli-responsive and dynamic materials. Frustrated bicyclo[1.1.1]pentane dicarboxylate (BCP) molecular rotors arranged in a Zn-based Metal-Organic Framework (Zn-FTR) sustain fast molecular reorientation in the GHz regime at temperature as low as 2 K with activation energy for rotational motion as low as 6 cal/mol, a significant breakthrough in the design of dynamic frameworks. MOF architectures allow for the engineering of sophisticated machinery comprising multiple dynamic elements. Pillared-layer MOFs, FTR-P1 and FTR-P2, comprising 2-D layers of BCP molecular rotors, were co-assembled with bipyridine and azo-bipyridine ligands, respectively. FTR-P1 features an off-centered interpenetrated structure, producing two arrays of interacting neighbor rotors, which display multiple motional processes with decreasing activation energies as low as 24 cal/mol. These phenomena were assigned to the synchronous motion of co-rotating and counter-rotating geared molecular rotors. FTR-P2 is constructed with hyper-fast BCP rotors and azo-bipyridine pillars, which exhibit ring reorientation of the two distinct pyridyl rings and pedal-like motion of the E-azo group. Surprisingly, iodine vapor sorption drives a dramatic structural rearrangement, increasing the dynamics of both BCP rotors and the channel-exposed pyridyl ring. The inclusion of molecular rotors with geminal fluorine atoms (BCP-F2) in a stable Al-based MOF (Al-FTR-F2) allowed for generating a highly dynamical MOF with exceptional mobility at 2 K. The interacting dipolar rotors, arranged in 2D layers, can reorient with an ultra-low activation barrier (Ea ≈ 17 cal/mol) following correlated mechanisms active even at liquid He temperature. These fast responsive dipolar materials can find applications in sensing, ferroelectric switching and the control of solid-state dynamics with external electric fields minimizing energy dissipation. Highly porous, yet stable architectures, denominated porous switchable frameworks (PSFs), were engineered to sustain the effective isomerization of molecular switches in the solid state. The quantitative and reversible photo-isomerization induces a restructuring of the extended framework, producing "on-command” modulation of the adsorption properties.