Photoswitchable Radical State in Nanoporous Metal-Organic Framework Films with Embedded Hexaarylbiimidazole

Photoresponsive materials enable dynamic remote control of their fundamental properties. The incorporation of photochromic molecules in nanoporous metal-organic frameworks (MOFs) provides a unique opportunity to tailor the material properties, including the interaction between the MOF host and guest molecules in the pores. Here, a MOF film of type HKUST-1 with embedded hexaarylbiimidazole (HABI), undergoing reversible light-induced reactions between the stable dimer state and the metastable radical state, is presented. The switching between the dimer and radical form is shown by infrared, UV-vis, and electron paramagnetic resonance (EPR) spectroscopy. By transient uptake experiments with ethanol and methanol as probe molecules, we show that the dimer-radical switching impacts the host-guest interaction and, in particular, modifies the uptake amount and diffusion coefficient of the guest molecules. For ethanol, the diffusion slows down by 75%. This research presents the first MOF material with photoswitchable (meta)stable dimer and radical molecules, and it contributes to the advancement of photoresponsive nanoporous materials.