Photoinduced reversible switching of porosity in molecular crystals based on star-shaped azobenzene tetramers

Abstract

The development of solid materials that can be reversibly interconverted by light between forms with different physico-chemical properties is of great interest for separation, catalysis, optoelectronics, holography, mechanical actuation and solar energy conversion. Here, we describe a series of shape-persistent azobenzene tetramers that form porous molecular crystals in their E-configuration, the porosity of which can be tuned by changing the peripheral substituents on the molecule. Efficient E→Z photoisomerization of the azobenzene units takes place in the solid state and converts the crystals into a non-porous amorphous melt phase. Crystallinity and porosity are restored upon Z→E isomerization promoted by visible light irradiation or heating. We demonstrate that the photoisomerization enables reversible on/off switching of optical properties such as birefringence as well as the capture of CO2 from the gas phase. The linear design, structural versatility and synthetic accessibility make this new family of materials potentially interesting for technological applications.

Photoinduced Phase Transitions in Rod-shaped Azobenzene with Different Alkyl Chain Length

“…Similar phenomena have been reported by several groups in azobenzene derivatives [14][15][16][17] and a spiropyran [18]. In an application point of view, this phase transition is of importance because this kind of phase transition material can be potentially be used in reusable light adhesives (dismantlable adhesives), photoresists, and other functional materials.…”

Efficient Light-Induced Phase Transitions in Halogen-Bonded Liquid Crystals

“…49−53 Photoinduced crystal-to-isotropic transitions have been studied in azobenzene-based tetragonal compounds, ionic crystals, and cyclophanes. 54−56 However, no such reversible isotropization has been previously reported in supramolecular complexes. Figure 7a shows a POM image of the complex 1 -10· 2 -8 at 85 °C, i.e., 10 °C below the melting point and 30 °C below the clearing point of the complex.…”

Tunable molecular separation by nanoporous membranes

“…Photoswitchable azobenzene-containing polymers and liquid crystals have been used as actuators1718 and light-driven motors19. Furthermore, photoswitchable molecular scaffolds20 and gel assemblies21 were realized utilising azobenzene photoisomerization. Also, for MOFs in the form of powders or thin films, photoswitchable moieties222324 were used to demonstrate the remote-control of the MOF properties like colour25, release262728 and adsorption2930313233 of the guest molecules, in particular of CO 2 (refs 32, 33).…”