Search

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]

• A thiol-ene-induced exfoliation of COFs into CONs was developed. • Vinylene groups in OPV units allows the addition of thiols under UV irradiation. • The degree of exfoliation can be quantitatively adjusted by varying the feed ratio. • The exfoliation exhibits an obvious increase in the charge/discharge capacity. Exfoliating insoluble bulk covalent organic frameworks (COFs) into covalent organic nanosheets (CONs) is an attractive way to improve their processability and the application performance. Herein, we report an efficient thiol-ene-induced exfoliation of oligo (phenylene vinylene) (OPV) based 2D COF-935 into CONs, which exhibits an obvious improvement in the reversible charge/discharge capacity as electrode materials for energy storage devices. The presence of vinylene groups in OPV units allows the addition of thiols under UV irradiation, which weaken the interlayer interactions for exfoliation by breaking the planarity of the layers in COFs. Furthermore, the degree of modification can be quantitatively controlled by varying the feeding ratio to obtain CONs with reasonable π-conjugation and short lithium-ion transfer distance. Specifically, the CON-UV-1 offers a nearly 90 % higher excellent cycle performance of 494 mA·h·g−1 at 1 A·g−1 after 500 cycles and outstanding rechargeability. This research thus provides a promising strategy for the chemical exfoliation of covalent organic frameworks into nanosheets and promotes their potential applications in energy storage. [ABSTRACT FROM AUTHOR]