Rational Modification of Two-Dimensional Donor–Acceptor Covalent Organic Frameworks for Enhanced Visible Light Photocatalytic Activity

Abstract

Covalent organic frameworks (COFs) are promising crystalline materials for photocatalytic solar-to hydrogen-energy conversion due to the tunable chemical structures and energy band gaps. Herein, we report a chemical modification strategy for improving the photocatalytic activity of COFs. A benzene-1,3,5-tricarbaldehyde (BT)-and benzothiadiazole derivative-based twodimensional donor−acceptor (D-A) COF, denoted as BT-COF, were fabricated and further modified by using an alternative electron-donating unit, 2-hydroxybenzene-1,3,5-tricarbaldehyde (HBT), to the polycondensation reaction, yielding HBT-COF with an enhanced internal D-A effect and hydrophilicity. Interestingly, the photocatalytic H 2 production rate of HBT-COF reaches 19.00 μmol h −1 , which is 5 times higher than that of BT-COF (3.40 μmol h −1 ) under visible light irradiation. The increase in photocatalytic activity of HBT-COF is rationally attributed to finely tuned energy levels and improved wettability, which in turn leads to broadened visible light absorption, efficient photoinduced charge separation and transfer, and enhanced interactions between the COF catalyst and reaction substrates. The present results demonstrate that a subtle structural modification can significantly modulate the band structure and interfacial property, thus providing a feasible strategy for the optimization of COFbased photocatalytic systems.

Building a Pyrazole–Benzothiadiazole–Pyrazole Photosensitizer into Metal–Organic Frameworks for Photocatalytic Aerobic Oxidation

“…29 Furthermore, the donor−acceptor−donor (D-A-D) type photosensitizers have been frequently demonstrated to facilitate the charge transfer. 11,27,28,30 We thus envisioned that combining electron-donating pyrazole and electronaccepting benzothiadiazole into a D-A-D type photosensitizer and building it into a MOF framework would offer a heterogeneous platform with further enhanced photocatalytic performance. 28 Diatomic oxygen (O 2 ) is an ideal oxidant for organic transformations and has gained much attention in recent years owing to its availability and environmentally friendly characteristics.…”

“…For example, the benzothiadiazole unit is known for its photon absorption efficiency. ,− A benzothiadiazole-containing linker was introduced into the MOF framework through a mixed-linker strategy by the Farha group, and the obtained PCN-57-S showed photocatalytic activity of oxidizing alkyl sulfide to alkyl sulfoxide . Furthermore, the donor–acceptor–donor (D-A-D) type photosensitizers have been frequently demonstrated to facilitate the charge transfer. ,,, We thus envisioned that combining electron-donating pyrazole and electron-accepting benzothiadiazole into a D-A-D type photosensitizer and building it into a MOF framework would offer a heterogeneous platform with further enhanced photocatalytic performance …”

2D–2D SnS2/Covalent Organic Framework Heterojunction Photocatalysts for Highly Enhanced Solar-Driven Hydrogen Evolution without Cocatalysts

“…To overcome these bottlenecks and to boost the photocatalytic H 2 evolution efficiency, a host of strategies have been proposed, including structure engineering, − elemental doping, , and heterojunction construction. , Among them, heterojunction construction is an advanced strategy to improve the photocatalytic performance through matching another semiconductors with appropriate band positions. − Especially, 2D metal chalcogenides (MDCs, e.g., MoS 2 , WS 2 , and SnS 2 ) are regarded as notable materials for coupling with other semiconductors in terms of the narrow band gap and high conductivity. − Among these MDCs, 2D tin disulfide (SnS 2 ) possesses high carrier mobility (50 cm 2 V –1 s –1 ) and a low band gap (2.0–2.4 eV), with a typical n-type semiconductor. , Notably, the special hexagonal CdI 2 -type layered structure of SnS 2 is composed of three stable layers of S–Sn–S, in which the S atom exposed to the crystal plane with relatively high density may be conducive to the formation of a H bond with H 2 O molecules, thus potentially improving the dispersion of heterogeneous photocatalysts. − Importantly, the 2D SnS 2 and TpPa-1-COF have proper band potential positions, and when they match well, possibly to form a heterojunction, photogenerated electrons and holes can reverse migrate and enrich both sides, respectively. , Furthermore, a built-in electric field is formed potentially between the 2D–2D SnS 2 /TpPa-1-COF intimate contact interface, , which also facilitates charge transfer and the separation of photogenerated electrons and holes . Owing to the narrow band gap of 2D SnS 2 , its absorption covers the entire solar spectrum from ultraviolet to near-infrared light.…”

Nanoscale Covalent Organic Frameworks with Donor–Acceptor Structures as Highly Efficient Light-Responsive Oxidase-like Mimics for Colorimetric Detection of Glutathione

“…Covalent organic frameworks (COFs) are an emerging family of porous crystalline polymers linked via covalent bonding. , Particularly, COFs have been extensively provided a stage for photoinduced catalysis that rely on their π–π array framework, high crystallinity, tunable band gaps, and attractive optoelectronic performances. , To acquire superior photocatalytic activity, the proper combination and arrangement of complementary donor–acceptor pairs within the COF skeleton have been demonstrated to be an effective approach to achieve narrow band gaps for improving visible light absorption and boost the separation and transportation of photogenerated electron–hole pairs. , Recent reports have indicated the applicability of energy and charge transfer for donor–acceptor COFs in organic photovoltaics (OPV), electrochromic devices (ECDs), light-induced fluorescence, and photocatalysts. , Although functional donor–acceptor polymers have been reported to show light-responsive oxidase-like activity and used in biosensing, , the application of donor–acceptor COFs for light-responsive oxidase mimics has not been reported so far.…”