Photoresponsive porous materials

Abstract

Molecular machines, switches, and motors enable control over nanoscale molecular motion with unprecedented precision in artificial systems. Integration of these compounds into robust material scaffolds, in particular nanostructured solids, is a fabrication strategy for smart materials with unique properties that can be controlled with external stimuli. Here, we describe a subclass of these structures, namely light-responsive porous materials metal-organic frameworks (MOFs), covalent-organic frameworks (COFs), and porous aromatic frameworks (PAFs) appended with molecular photoswitches. In this review, we provide an overview of a broad range of light-responsive porous materials focusing on potential applications.

Fig. 1. Illustration of three distinct modes of incorporation of a photoswitch in a solid material scaffold (a) as a guest in the pores, (b) pendant to the linker, (c) in the backbone of the linker.

Fig. 2. (a) Light and heat induced structural changes in an archetypical azobenzene photoswitch. (b) Schematic representation of the structural changes induced by azobenzene incorporated in a porous material as pendants (c) and backbone of the linker.

Fig. 3. (a) Light and heat induced structural changes in archetypical spiropyran photoswitch. (b) Example of dimeric spiropyran that can be incorporated in the framework. (c) Schematic representation of the structural changes induced by spiropyran depicted on panel (b) incorporated in a porous material as pendants.

Fig. 4. (a) Light and heat induced structural changes in archetypical dithienylethene photoswitch. (b) Schematic representation of the structural changes induced by dithienylethene incorporated in a porous material in a backbone of the linker. (c) Representation of a structure of a dithienylethene photoswitch that can be incorporated in a material scaffold as pendants.

Fig. 5. Schematic depiction of anthracene photoswitching.

Fig. 6. (a) Photochemical reaction pathways of an archetypical stilbene photoswitch. (b) Structure of a stiff-stilbene. (c) Structures of archetypical overcrowded-alkene based light-driven unidirectional rotary molecular motors (first, second and third generation of motors are shown from left to right).

Fig. 7. (a) Structure and switching of the incorporated azobenzene in the isoreticular MOF-5 scaffold. (b) Schematic depiction of the light and heat induced structural changes in the pore structure and gas adsorption of azobenzene functionalized isoreticular MOF-5 structural reprinted with permission from ref. 105 Copyright 2013 American Chemical Society.

Fig. 8. (a) Structure of the stilbene bispyridyl pillar and azobenzene dicarboxylic acid used as linkers. (b) Depiction of the active framework. (c) Changes in the CO₂ adsorption isotherm of the material (black line, pristine), during in situ irradiation (red line) and upon a modulated exposure to light (blue line), temperature of the sample (green line). Reprinted with permission from ref. 106. Copyright 2013 Wiley-VHC.

Fig. 9. (a) Structure and switching of the incorporated visible-light responsive ortho-fluoro azobenzene linker. Models of the solid-state structure of the isoreticular (b) UiO-66(Zr) (left panel) and MIL-53(Al) (right panel). Reprinted with permission from ref. 107. Copyright 2016 Wiley-VHC.

Fig. 10. Structures of triformylphloroglucinol (1) and diamine-functionalized azobenzenes (Azo-1, Azo-2) used to construct porous switchable organic polymer.

Fig. 11. (a) Representation of light induced structural changes in the SURMOF functionalized with azobenzene pendants. (b) Light-induced changes in selectivity of H₂/CO₂ separation on α-Al₂O₃-supported SURMOF membrane (red circles) and H₂ (black squares) and CO₂ (white squares). Adapted by permission from Springer Nature from ref. 126, Copyright 2014.

Fig. 12. (a) Packing of the tetrameric azobenzenes in the solid state along with representation of the empty channels. (b) CO₂ adsorption isotherms at 195 K of pristine material (red isotherm) and after exposure to UV light (blue isotherm). Reprinted by permission from Springer Nature from ref. 115, Copyright 2015.

Fig. 13. (a) Structure of the linkers used to construct five-fold interpenetrated pillared MOF. (b) Packing of the five independent networks in the crystal structure, viewed along the b direction. Interpenetrating networks were indicated by various colours.

Fig. 14. (a) Structure of the linkers used to construct framework and switching of the incorporated DTE pillar. (b) Representation of the porosity (grey areas) of the pristine material (top panel) and UV-treated material (bottom panel). Adapted with permission from ref. 120. Copyright 2017, Royal Society of Chemistry.

Fig. 15. (a) Structure of the linkers used to construct two-fold interpenetrated framework. (b) CO₂ adsorption isotherms of pristine (green points) and irradiated (blue points) materials. (c) Representation of the SC X-ray structure of pristine (left) material and UV-treated (right) materials viewed along b direction. Adapted by permission from Springer Nature from ref. 121, Copyright 2017.

Fig. 16. (a) Structure of the component units of anthracene-based two-dimensional COF connected by catechol boronate linkages. (b) Top and side view of structural model of pristine material (left panel) and UV-irradiated material after dimerization of anthracene building units (right panel). Reprinted with permission from ref. 122. Copyright 2015 Wiley-VHC.

Fig. 17. (a) Representation of the light-induced isomerization of the bistable overcrowded alkene switch. (b) Schematic representation of the photoswitching in the solid material. Adapted by permission from Springer Nature from ref. 125, Copyright 2020.

Fig. 18. (a) Structural the azobenzene linker. (b) Schematic representation of light-induced changes in pore aperture of MOF-74 upon photochemical isomerization of the appended azobenzenes. Adapted with permission from ref. 128. Copyright 2013, Royal Society of Chemistry.

Fig. 19. (a) Structure of the layered SURMOF consisting of non-responsive, storage compartment (yellow part) and light-responsive azobenzene appended layer (red part) with structure of the SURMOF layers viewed along c direction. (b) Comparison of the butanediol uptake by the two-layered SURMOF with Z-azobenzene (blue line), E-azobenzene (red line) pendants and lone passive layer (black line) determined by quartz microbalance. (c) Butanediol release experiment monitored by quartz microbalance. Red arrow indicates start of the irradiation at 560 nm to induce Z → E isomerization of the azobenzene pendants. Reprinted with permission from ref. 130. Copyright 2017 American Chemical Society.

Fig. 20. (a) Structure of the linkers used to construct homochiral SURMOF. (b) Uptake of (R)- and (S)-1-phenylethanol by the Z-azobenzene MOF thin films. Adapted with permission from ref. 138. Copyright 2019, Royal Society of Chemistry.

Fig. 21. (a) Structure of the imidazole derived linkers used in the synthesis of DTE functionalized ZIF-70 framework. (b) Model of DTE decorated ZIF-70 framework with ring-opened (left panel) and ring-closed (right-panel) DTE pendants. Adapted with permission from ref. 139. Copyright 2017 American Chemical Society.

Fig. 22. (a) Structure of the DTE pillars and tetrakis(carboxyphenyl) porphyrin linker. (b) Model of the 3D structure of Zn-paddlewheel porphyrin DTE-pillared MOF. (Note that the porphyrin linker coordinates Zn cation during the solvothermal synthesis).

Fig. 23. (a) Structure of ortho-fluoro azobenzene linkers and (b) switchable SURMOF viewed in c direction. (c) Changes in the proton current conduction of butanediol in SURMOF at 1 V and 1 Hz upon alternating irradiation at 530 and 400 nm. Adapted with permission from ref. 145. Copyright 2018 Wiley-VHC.

Fig. 24. (a) Single crystal X-ray structure with simulated spiropyran pillars showing isomerization to merocyanine form. (b) Light-induced changes in the conductance of the single crystal of the MOF bearing spiropyrans pillars (red line) and control non-responsive MOF (black line). Adapted with permission from ref. 146. Copyright 2019 American Chemical Society.

Fig. 25. (a) Structure of the linkers, nodes and schematic depiction of the frameworks structure used in the study. (b) Optical micrographs of the MOF bearing spiropyran pillars pristine material (left panel) and material subjected to HCl vapour at the point indicated by the black arrow (right panel). Adapted with permission from ref. 150. Copyright 2018 American Chemical Society.

Fig. 26. (a) Structure of building blocks and monotopic and ditopic spiropyran pendants used to synthesize responsive porous organic polymers. (b) Light-induced changes in colour of the porous material bearing monotopic spiropyran photoswitch. (c) Changes in the colour of the material bearing ditopic spiropyran upon alternating protonation/deprotonation cycles of merocyanine pendants. Adapted by permission from Springer Nature from ref. 71, Copyright 2014.

Fig. 27. (a) UV-light induced changes in the SC X-ray structure of the photoresponsive MOF bearing stilbene-pyridyl pendants (top panel) along with stacking of the pendants and structure of the stilbene dimer (bottom panel). (b) Optical micrographs of the photochemical crystals deformation, twisting and break-up of the crystals with different sizes: thin crystals (top and middle panels), thick crystal (bottom panel). Samples were irradiated from the left side from the perspective of the micrographs. Adapted with permission from ref. 86. Copyright 2019 Wiley-VHC.

Wojciech Danowski

Thomas van Leeuwen

Wesley R. Browne

Ben L. Feringa