Unfortunately, as the tailoring of their photocatalytic performance lies on a multitude of variables (e.g., crystallinity, optoelectronic factors, etc.), it is very difficult to determine the individual contribution that is required for further modification. reported several 2D COFs with different numbers of nitrogen atoms in the central phenyl ring 35, which showed controllable photocatalytic hydrogen evolution efficiencies. However, although continuing efforts are going on developing new 2D COFs for photocatalytic HER, the rational tuning of their structures and photophysical properties for maximizing the hydrogen evolution efficiency still needs to be further clarified. Since this pioneer work, several 2D COFs bearing different photoelectric units have been successfully constructed and found interesting potential in photocatalytic hydrogen evolution 34, 35, 36, 37, 38, 39, 40, 41, 42, 43. In 2014, Lotsch and co-workers reported the first example of utilizing 2D COF to produce H 2 in the presence of metallic platinum under visible light irradiation 34. Therefore, 2D COFs have received growing interests in photocatalysis over the past few years, ranging from chemical transformation 28, 29, 30, 31, 32, 33 to solar fuel production 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48.Īmong all these tested systems, photocatalytic hydrogen evolution reaction (HER) from water is regarded as one of the most attractive ways to meet the increasing demands of clean and sustainable energy 49. In addition, the crystalline nature of 2D COFs can facilitate the establishment of structure–property–activity relationship and thus providing insights into photocatalytic processes. Accordingly, 2D COFs possess unique pre-organized transport of long-lived photoexcited states and show high charge carrier mobility, which will allow them to work as effective heterogeneous photocatalysts. From the structural viewpoint, the most important feature of 2D COFs differing from their 3D analogues and most organic systems is that they can offer a unique platform for constructing periodic columnar π arrays 27. Owing to their low density, high porosity, structural periodicity, and modular functionality, COFs have gained intensive attention and found promising applications in gas adsorption and separation 6, 7, 8, 9, 10, catalysis 11, 12, 13, 14, sensing 15, 16, 17, 18, optoelectronics 19, 20, 21, 22, and energy storage 23, 24, 25, 26. This study not only represents a simple and effective way for efficient tuning of the photocatalytic hydrogen evolution activities of covalent organic frameworks at molecular level, but also provides valuable insight on the structure design of covalent organic frameworks for better photocatalysis.Ĭovalent organic frameworks (COFs) are a novel class of porous crystalline polymer that enables the precise integration of molecular building blocks into extended two-dimensional or three-dimensional (2D or 3D) structures through covalent bonds 1, 2, 3, 4, 5. Based on the detailed experiments and calculations, this tunable performance can be mainly explained by their tailored charge-carrier dynamics via molecular engineering. Interestingly, the incorporation of different transition metals into the porphyrin rings can rationally tune the photocatalytic hydrogen evolution rate of corresponding covalent organic frameworks, with the order of CoPor-DETH-COF < H 2Por-DETH-COF < NiPor-DETH-COF < ZnPor-DETH-COF. Our results clearly show that all four covalent organic frameworks adopt AA stacking structures, with high crystallinity and large surface area. Herein, we report the designed synthesis of four isostructural porphyrinic two-dimensional covalent organic frameworks (MPor-DETH-COF, M = H 2, Co, Ni, Zn) and their photocatalytic activity in hydrogen generation. However, their structure-property-activity relationship, which should be beneficial for the structural design, is still far-away explored. Covalent organic frameworks have recently gained increasing attention in photocatalytic hydrogen generation from water.