Two-Dimensional Benzobisthiazole-Vinylene-Linked Covalent Organic Frameworks Outperform One-Dimensional Counterparts in Photocatalysis

Vinylene/olefin-linked two-dimensional covalent organic frameworks (v-2D-COFs), featured with vinylene-linked in-plane conjugations, high chemical stabilities, and designable chemical structures, are promising for optoelectronic/photocatalytic applications. Developing v-2D-COFs with superior π-conjugation and optoelectronic properties is meaningful but remains challenging. In this work, we present the crystalline benzobisthiazole-bridged unsubstituted v-2D-COF (v-2D-COF-NS1 and v-2D-COF-NS2) synthesized via a benzothiazole-mediated aldol-type polycondensation. Interestingly, the resultant v-2D-COF exhibits a high chemical stability under both strong acidic (12 M HCl) and basic conditions (saturated KOH) due to the robust vinylene-linked skeletons. Moreover, the electron-deficient thiazole units and 2D π-conjugations endow v-2D-COFs (i.e., v-2D-COF-NS1) a narrow band gap of ∼1.85 eV with a conduction band of -3.65 eV vs vacuum, which are desirable for photocatalytic hydrogen evolution. As such, the v-2D-COF-NS1-based photoelectrode gives a photocurrent up to ∼47 μA cm-2 at 0.3 V vs reversible hydrogen electrode (RHE), which is much higher than the value of the corresponding linear polymer (LP-NS1) and outstanding among the reported COF photoelectrodes. Under a continuous visible light irradiation, v-2D-COF-NS1 generates hydrogen gas with an excellent rate of ∼4.4 mmol h-1 g-1 over 12 h. This work demonstrates the synthesis of unsubstituted v-2D-COFs that intrinsically contain benzobisthiazole-based building blocks and shows great potential in photocatalytic reactions.