Nevertheless, layers and coatings of MOF materials are highly interesting, as they can combine gas permeation and adsorption performance with optical functions. If processed in the glassy state, not only supported, but also dense, free-standing, transparent films, membranes, and functional substrates could be fabricated. Keppler et al. demonstrated recently that optical ZIF-8 coatings exhibit large shifts in their refractive index depending on the medium they have been exposed to. Similar tuning of the refractive index through the incorporation of volatile guest molecules into the porous channels of optical quality agZIF-62 can be envisioned, provided that glass processing can be achieved in the first place.