Construction of Thiadiazole-Bridged sp 2 -Carbon-Conjugated Covalent Organic Frameworks with Diminished Excitation Binding Energy Toward Superior Photocatalysis.

Sp 2 -carbon-conjugated covalent organic frameworks (sp 2 c-COFs) have emerged as promising platforms for phototo-chemical energy conversion due to their tailorable optoelectronic properties, in-plane π-conjugations, and robust structures. However, the development of sp 2 c-COFs in photocatalysis is still highly hindered by their limited linkage chemistry. Herein, we report a novel thiadiazole-bridged sp 2 c-COF (sp 2 c-COF-ST) synthesized by thiadiazole-mediated aldol-type polycondensation. The resultant sp 2 c-COF-ST demonstrates high chemical stability under strong acids and bases (12 M HCl or 12 M NaOH). The electro-deficient thiadiazole together with fully conjugated and planar skeleton endows sp 2 c-COF-ST with superior photoelectrochemical performance and charge-carrier separation and migration ability. As a result, when employed as a photocathode, sp 2 c-COF-ST exhibits a significant photocurrent up to ∼14.5 μA cm -2 at 0.3 V vs reversible hydrogen electrode (RHE) under visible-light irradiation (>420 nm), which is much higher than those analogous COFs with partial imine linkages (mix-COF-SNT ∼ 9.5 μA cm -2 ) and full imine linkages (imi-COF-SNNT ∼ 4.9 μA cm -2 ), emphasizing the importance of the structure-property relationships. Further temperature-dependent photoluminescence spectra and density functional theory calculations demonstrate that the sp 2 c-COF-ST has smaller exciton binding energy as well as effective mass in comparison to mix-COF-SNT and imi-COF-SNNT, which suggests that the sp 2 c-conjugated skeleton enhances the exciton dissociation and carrier migration under light irradiation. This work highlights the design and preparation of thiadiazole-bridged sp 2 c-COFs with promising photocatalytic performance.