ENGINEERING POROSITY MODULATION, MULTI-STIMULI RESPONSIVITY AND ORTHOGONAL ISOMERIZATION IN POROUS SWITCHABLE FRAMEWORKS

Porous aromatic frameworks (PAFs) possess a low-density, yet robust scaffold with a well-defined topology, which facilitates the organization of dynamic building blocks such as molecular switches and motors in the solid state. Highly porous and stable architectures, denominated Porous Switchable Frameworks (PSFs), were engineered to support the effective isomerization of molecular switches. Light-responsive PSFs based on a bistable chiroptical overcrowded alkene, endowed with an exceptional surface area up to 3950 m2 g-1, exhibited quantitative bulk photoisomerization in the solid state. Remarkably, the porosity of the framework and the consequent gas sorption capabilities can be reversibly modulated in response to light and heat.[1] An improved hexadentate switching monomer yielded swellable, hierarchical micro/mesoporous architectures with densely integrated photoswitches.[2] The interplay between hierarchical porosity, a flexible backbone, and the reversible photoisomerization between the two isomers resulted in three unique and distinct porosity states that can be accessed in sequence upon application of external stimuli, creating a sponge-like behavior (Figure 1A). Multi-stimuli responsive, yet structurally robust, spiropyran-based materials were prepared using an innovative in-situ solid-state grafting approach, preserving their high porosity. This effective strategy produced dynamic materials that undergo reversible transformation of spiropyran to zwitterionic merocyanine by chemical and physical stimulation, showing potential for pH-active control, modulated gas uptake and release, ion capture and immobilization, and water harvesting (Figure 1B). Furthermore, orthogonal photoswitching PSFs were fabricated by integrating distinct classes of molecular switches, e.g. o-fluoroazobenzene and nitro-spiropyran, into porous architectures. Different wavelengths of light generate four distinct states that can be selectively populated. The juxtaposition of two or more dynamic elements in flexible frameworks points to the development of complex machinery endowed with collective and emergent properties. References [1] F. Castiglioni et al. Nature Chemistry 2020, 12, 595-602. [2] J. Sheng, J. Perego et al. Chem 2023, 9, 2701-2716. [3] J. Sheng, J. Perego et al. Adv. Mater. 2024, 36, 2305783. [4] J. Sheng, J. Perego et al. Angew. Chem. Int. Ed. 2024, 63, e202404878.