Viologen-Based\nCovalent\nOrganic Frameworks toward\nMetal-Free Highly Efficient Photocatalytic Hydrogen Evolution

Abstract

Covalent organic\nframeworks (COFs) have shown promise\nin the field\nof photocatalysts for hydrogen evolution. Many studies have been carried\nout using various electroactive and photoactive moieties such as triazine,\nimide, and porphyrin to produce COFs with different geometric structures\nand units. Electron transfer mediators like viologen and their derivatives\ncan accelerate the transfer of electrons from photosensitizers to\nactive sites. Herein, the combination of a biphenyl-bridged dicarbazole\nelectroactive donor skeleton with a viologen acceptor structure is\nreported for the photocatalytic hydrogen evolution of novel COF structures\nwith various alkyl linkers {TPCBP X-COF [X = ethyl (E), butyl (B),\nand hexyl (H)]}. The structures became more flexible and exhibited\nless crystal behavior as the length of the alkyl chain increased according\nto scanning and transmission electron microscopy images, X-ray diffraction\nanalyses, and theoretical three-dimensional geometric optimization.\nIn comparison, the H₂ evolution rate of the TPCBP B-COF\n(12.276 mmol g^–1) is 2.15 and 2.38 times higher\nthan those of the TPCBP H-COF (5.697 mmol h^–1) and\nTPCBP E-COF (5.165 mmol h^–1), respectively, under\nvisible light illumination for 8 h. The TPCBP B-COF structure is one\nof the best-performing catalysts for the corresponding photocatalytic\nhydrogen evolution in the literature, producing 1.029 mmol g^–1 h^–1 with a high apparent quantum efficiency of\n79.69% at 470 nm. Our strategy provides new aspects for the design\nof novel COFs with respect to future metal-free hydrogen evolution\nby using solar energy conversion.