Alkene-Linked Covalent Organic Frameworks Boosting Photocatalytic Hydrogen Evolution by Efficient Charge Separation and Transfer in the Presence of Sacrificial Electron Donors

Abstract

Covalent organic frameworks (COFs) are potential photocatalysts for artificial photosynthesis but they are much less explored for photocatalytic hydrogen evolution (PHE). COFs, while intriguing due to crystallinity, tunability, and porosity, tend to have low apparent quantum efficiency (AQE) and little is explored on atomistic structure-performance correlation. Here, adopting triphenylbenzene knots and phenyl linkers as a proof of concept, three structurally related COFs with different linkages are constructed to achieve a tunable COF platform and probe the effect of the linkage chemistry on PHE. Cyano-substituted alkene-linked COF (COF-alkene) yields a stable 2330 µmol h^-1 g^-1 PHE rate, much superior to imine- and imide-linked counterparts (<40 µmol h^-1 g^-1) under visible light irradiation. Impressively, COF-alkene achieves an AQE of 6.7% at 420 nm. Combined femtosecond transient absorption spectroscopy and theoretical calculation disclose the critical role of cyano-substituted alkene linkages toward high efficiency of charge separation and transfer in the presence of sacrificial electron donors-the decisive key to the superior PHE performance. Such alkene linkages can also be extended to design a series of high-performance polymeric photocatalysts, highlighting a general design idea for efficient PHE.

Keywords: alkene linkages; charge separation; charge transfer; covalent organic frameworks; photocatalysis.

Figure 1

a) Chemical structure of three COFs with different linkages. b–d) Simulated crystal structure of many layers of COF–alkene, COF–imide, and COF–imine, respectively. PXRD patterns of e) COF–alkene, f) COF–imide, and g) COF–imine, SEM images of h) COF–alkene, i) COF–imide, and j) COF–imine, respectively.

Figure 2

a) UV–vis diffuse reflectance spectra (DRS) of COF–alkene, COF–imide, and COF–imine. b) Energy diagram of three COFs. c) PHE activities of three COF catalysts under visible light irradiation (λ > 420 nm) using TEOA as sacrificial agent and Pt as cocatalyst. The inset is the comparison of PHE rate. d) Isotope labeling experiment using D₂O replace H₂O for PHE exhibiting the evolution of D₂ gas, demonstrating the nonoxidative nature of COF material. e) Typical time course of hydrogen production under visible light irradiation using COF–alkene for 30 h. f) Wavelength‐dependent AQE of PHE by COF–alkene photocatalysts. The UV–vis DRS of the photocatalyst is superimposed for comparison.

Figure 3

The fs‐TA spectra obtained from suspensions of a) COF–alkene, b) COF–imide, and c) COF–imine in 0.1 mg mL⁻¹ aqueous solution containing 30% TEOA, and the corresponding kinetics of characteristic fs‐TA absorption bands observed at 700 nm for the spectra observed of b) COF–alkene, d) COF–imide, and f) COF–imine, respectively.

Figure 4

a) Molecular junctions of COF–alkene (1), COF–imide (2), and COF–imine (3) and b) their transmission functions. [P₁₁, P₁₂], [P₂₁, P₂₂], and [P₃₁, P₃₂] present the [HOMO, LUMO] of the transmission functions of COF–alkene, COF–imide, and COF–imine, respectively. c) Chemical structure and d) PHE activities of TPAL–polymer and BPDA–polymer.