Improving cobalt catalysts: Tethering molecular catalysts together is a tried and trusted method for making them easier to purify and reuse. Lin et al. now show that the assembly of a covalent organic framework (COF) structure can also improve fundamental catalytic performance. They used cobalt porphyrin complexes as building blocks for a COF. The resulting material showed greatly enhanced activity for the aqueous electrochemical reduction of CO ₂ to CO. Science , this issue p. 1208

Flat perovskite crystals: Bulk crystals and thick films of inorganic-organic perovskite materials such as CH ₃ NH ₃ PbI ₃ have shown promise as active material for solar cells. Dou et al. show that thin films—a single unit cell or a few unit cells thick—of a related composition, (C ₄ H ₉ NH ₃ ) ₂ PbBr ₄ , form squares with edges several micrometers long. These materials exhibit strong and tunable blue photoluminescence. Science , this issue p. 1518