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Covalent organic frameworks (COFs) have gained significant attention as key photocatalysts for efficient solar light conversion into hydrogen production. Unfortunately, the harsh synthetic conditions and intricate growth process required to obtain highly crystalline COFs greatly hinder their practical application. Herein, we report a simple strategy for the efficient crystallization of 2D COFs based on the intermediate formation of hexagonal macrocycles. Mechanistic investigation suggests that the use of 2,4,6‐triformyl resorcinol (TFR) as the asymmetrical aldehyde build block allows the equilibration between irreversible enol‐to‐keto tautomerization and dynamic imine bonds to produce the hexagonal β‐ketoenamine‐linked macrocycles, the formation of which could provide COFs with high crystallinity in half hour. We show that COF‐935 with 3 wt % Pt as cocatalyst exhibit a high hydrogen evolution rate of 67.55 mmol g−1 h−1 for water splitting when exposed to visible light. More importantly, COF‐935 exhibits an average hydrogen evolution rate of 19.80 mmol g−1 h−1 even at a low loading of only 0.1 wt % Pt, which is a significant breakthrough in this field. This strategy would provide valuable insights into the design of highly crystalline COFs as efficient organic semiconductor photocatalysts. [ABSTRACT FROM AUTHOR]

The strategy of molecular engineering has been employed to build the structure–property–activity relationship for covalent organic frameworks (COFs) with efficient photocatalytic water splitting for hydrogen production. However, previous reports focus on the impact of molecular design on COFs' physicochemical properties, the crystallinity differences are usually ignored and related studies remain rare. Herein, we describe the preparation of a series of structurally related COFs with similar chemical compositions but have different numbers of nitrogen (N) atoms in the central aromatic ring via an efficient crystallization strategy using of 2,4,6-triformyl resorcinol (TFR). On one hand, the introduction of N atoms reduces π-π stacking interaction during the formation of framework, leading to a decrease in crystallinity. On the other hand, it contributes to enhancing the photocatalytic performance of the materials by varying the electronic structures. Consequently, our results revealed that the photocatalytic hydrogen evolution (PHE) rate is 61.22 mmol g−1 h−1 for COF-905-1N, significantly higher than COF-905-3N (40.60 mmol g−1 h−1) and COF-905 (26.23 mmol g−1 h−1). This study not only provide fundamental insights into formation mechanism of COFs, but also provide a practical approach to enhance COF-based photocatalytic performance. We synthesized three COFs (COF-905, COF-905-1N, and COF-905-3N) with varying number of nitrogen atoms, resulting in different crystallinity via molecular engineering. COF-905-1N shows the highest photocatalytic hydrogen production performance with a rate up to 61.22 mmol g−1 h−1. The water-drainage experiments also suggest the potential of COF-905-1N in the practical applications for photocatalytic hydrogen evolution. [Display omitted] • Three structurally related COFs were constructed via an efficient crystallization strategy based on the use of TFR. • The crystallinity of the resultant COFs decreases as the number of N atoms in the central benzene ring increases. • The π-π stacking between the intermediate macrocycles becomes less favorable, leading to incompact stacking of 2D layers. • COF-905-1N exhibits superior PHE performance under the combined influence of nitrogen content and crystallinity. [ABSTRACT FROM AUTHOR]