Zwitterionic and Hydrophilic Vinylene-Linked Covalent Organic Frameworks for Efficient Photocatalytic Hydrogen Evolution

Developing effective synthetic strategies as well asbroadeningfunctionalities for zwitterionic materials that comprise moietieswith equimolar cationic and anionic groups still remains a huge challenge.Herein, we develop two zwitterionic vinylene-linked covalent organicframeworks (Zi-VCOF-1 and Zi-VCOF-2) that are a type of novel hydrophilicmaterial. Zi-VCOF-1 and Zi-VCOF-2 are obtained directly through theconvenient Knoevenagel condensation of new sulfonic-pyridinium zwitterionicmonomers with aromatic aldehyde derivatives. This is the first reporton zwitterionic COFs being constructed by the bottom-up functionalizationapproach from predesigned zwitterionic monomers. Both Zi-VCOFs exhibita high photocatalytic hydrogen evolution rate (HER) because of theirappropriate optical property and outstanding hydrophilicity. Specifically,Zi-VCOF-1 and Zi-VCOF-2 show photocatalytic HER of 13,547 and 5057 & mu;mol h(-1) g(-1), respectively.Interestingly, the photocatalytic HER of Zi-VCOF-1 is about 2.68 timesof that of Zi-VCOF-2, although they differ by only one methyl groupin sulfonic-pyridinium zwitterionic pairs. The photocatalytic HERof Zi-VCOF-1 is not only the highest in the vinylene-linked COFs butalso outstanding among the most reported COFs. This is the first applicationof zwitterionic COFs for photocatalytic hydrogen evolution, whichwould open a new frontier in zwitterionic COFs and be helpful forthe design of other photocatalytic materials.