Exploration of CO2 Capture and Gas/Vapour Adsorption in Dynamic and Photo-Responsive Porous Architectures

Porous frameworks provide a robust platform for organizing the spatial arrangement of molecular rotors and motors in the solid state, thus generating highly dynamic porous architectures. Notably, the design of these highly porous frameworks, combined with switchable dynamics and flexibility, allows for enhanced control over gas capture and selectivity. Modulation of rotor dynamics in MOFs upon guest loading. MOF architectures allow for the engineering of sophisticated machinery comprising multiple dynamic elements. In particular, a novel MOF family comprising bipyridine based pillars and bicyclo[1.1.1]pentanedicarboxylate (BCP) ligand layers organized into stable, interpenetrated architectures was developed.[1,2] Flexible 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, as demonstrated by variable temperature in-situ P XRD, 2H solid-echo, and 1H T1 relaxation NMR, supported by DFT modeling. These phenomena were assigned to the synchronous motion of co-rotating and counter rotating geared molecular rotors. Intriguingly, the manipulation of rotor dynamics by external stimuli was achieved through the diffusion of CO2 through the open pores, dramatically altering the global rotation mechanism. FTR-P2 is constructed with BCP rotors and azo-bipyridine pillars, which exhibit ring reorientation and pedal-like motion of the E-azo group. Pressurized CO2 regulates BCP dynamics upon incremental site occupation. The rotary dynamics is intrinsically coupled to the framework flexibility as demonstrated by complementary experimental evidence (multinuclear solid-state NMR down to very low temperatures, synchrotron radiation P-XRD, and gas adsorption) and computational modelling, which helps elucidate the highly sophisticated rotor-structure interplay. Surprisingly, iodine vapor adsorption drives a dramatic structural rearrangement, displacing the two distinct subnets in a concerted piston-like motion. Unconventionally, BCP mobility increases, exploring ultra-fast dynamics (107 Hz) at temperatures as low as 44 K, whereas the pyridyl rotors diverge into faster and slower dynamical regimes by symmetry lowering. Light-triggered porosity modulation in Porous Switchable Frameworks (PSFs). Highly porous yet stable architectures, referred to as Porous Switchable Frameworks (PSFs), have been designed to sustain the efficient isomerization of molecular switches and motors in the solid state. The quantitative and reversible photo isomerization of overcrowded alkene switches, followed by diffuse reflectance UV-vis and IR spectroscopies, and solid-state NMR, led to a restructuring of the framework, enabling "on-command" modulation of the adsorption properties.[3,4] Notably, installing two orthogonal types of photoswitches in an integrated solid porous framework allowed the selective switching between four independently accessible states of the porous material, providing a basis for dynamic multifunctional materials.[5] Recently, we reported the construction of a series of visible-light responsive microporous aromatic switchable framework materials grafted with o fluoroazobenzene pendants (Azo-PSFs)[6]. The materials exhibit reversible photoswitching upon irradiation with visible light, showing high cyclability between two distinct states. Remarkably, solid-state NMR revealed that the azobenzene moiety undergoes reversible bulk isomerization within the framework, which drives the substantial changes in adsorption capacity and CO2 uptake-release by the material. This work presents a remarkable example of all-visible-light-triggered bulk isomerization in an azobenzene-based porous material, providing a benchmark characterization of photoresponsive systems and paving the way for the future advancements in light-driven materials.