Structural Engineering of Two-Dimensional Covalent Organic Frameworks for Visible-Light-Driven Organic Transformations

Abstract

Covalent organic frameworks (COFs) emerging as a novel kind of visible light-responsive organic semiconductor have attracted extensive research attention in the field of photocatalytic organic transformations. However, the key parameters affecting their photocatalytic properties are still not clear. In this work, a series of [3 + 3] COFs with similar two-dimensional hexagonal structure but different compositions are synthesized and employed as model materials for investigating the key factors affecting the photocatalytic properties in the visible-light-driven reductive dehalogenation reaction and the aerobic cross-dehydrogenative coupling reaction. In comparison with −H and −CF3, the −OH substituent in the aromatic ring could narrow the band gap of the COFs. The COFs with a triazine skeleton in the framework usually boost the photocatalytic activity, possibly because of the enhanced charge separation efficiency by the formation of a donor–acceptor domain. As a combined result of the narrow band gap, efficient charge separation, and high conductivity, the COF possessing both a −OH group and triazine skeleton shows the highest activity in the photocatalytic reductive dehalogenation reaction. Notably, COFs could be easily recovered and reused several times without the loss of crystallinity. Our primary results may shed light on the design of efficient COF-based semiconductors for photocatalytic organic transformations.

Covalent Organic Frameworks in Catalytic Organic Synthesis

“…Recently, Yang and co-workers also established a series of COFs (OH-TFP-TTA, OH-TFP-TAPB et al Figure 4, Table 2) to catalyze the dehalogenation reaction and aerobic CDC reaction (Scheme 13, c, d). [53] In this work, the author found that the incorporation of hydroxyl group enhanced the photocatalytic activity by narrowing the band gap of COFs and thereby promoting charge separation efficiency and conductivity. Moreover, Zhang et al reported an asymmetric CTF (asy-CTF, Figure 4, Table 2) to catalyze the synthesis of benzophosphole oxides (Scheme 15, a).…”

Donor‐Acceptor Type Covalent Organic Frameworks

“…Similar as COF-JLU22, the photocatalytic activity of another three kinds of DÀ A COFs containing electron-donating triphenylbenzene, that is, 1,3,5-tri(4-formylphenyl)benzene (TFP), 1,3,5-tris(4-formyl-3-hydroxyphenyl)-benzene (OH-TFP), 1,3,5-tris (4-formyl-3-trifluoromethyl)benzene (CF 3 -TFP), and electron-accepting triphenyltriazine (TTA) segments toward dehalogenation of phenacyl bromide was also evaluated. [59] The reaction was carried out by green light irradiation for 18 h. Results indicated that the COF incorporating OH-functionalized triphenylbenzene moieties showed higher catalytic activity (reaction conversion:~90 %) than the ones containing CF 3 -and Hsubstituted triphenylbenzenes, which was attributed to the narrower bandgap, more efficient charge separation and high conductivity of the OH-containing COF. Besides, OH-TFP-TTA COF as catalyst demonstrated high recyclability.…”

“…In general, the formation of D−A arrays by polycondensation between donor and acceptor monomers are essential for a typical D−A COF. Thus far, only a few boronic‐ester‐, [14,34,35,46,47,49,51,63] imine‐ [40b,41b,42,48a,52–62] and C=C‐linked [44] D−A COFs have been reported (Scheme 1).…”

“…In addition to the dynamic covalent linkages, geometric matching of the monomers plays an important role in the synthesis of the targeted COFs. Based on the topological diagram, hexagonal 2D COFs could be generated with the combination of [ C 3 + C 2 ] [14,35,46,49,52,53,57,62,63] or [ C 3 + C 3 ]; [40b,48a] tetragonal 2D COFs could be formed from [ C 4 + C 2 ] [34,42,47,51] or [ C 4 + C 4 ], and [ C 2 + D 2 h ] may give rise to rhombic COFs with a single kind of pore [41b,44,54,58,59] or kagome type COFs with six small evenly‐arranged trigonal pores surrounding a large hexagonal pore. Moreover, COFs containing unique small trigonal pores could be obtained through [ C 6 + C 2 ], and asymmetric topologies could be accessed with a multi‐component (MC) approach [20] .…”

Fabrication of NanoCOF/Polyoxometallate Composites for Photocatalytic NADH Regeneration via Cascade Electron Relay

“…Apart from photoelectric properties, the porosity could also have significant influence on the catalytic activity. [7,35] The lower activity of nTp-TTA/POM-1.1 is possibly related with its low surface area. The nTp-TTA/ POM-4.1 with narrow bandgap exhibited lowest activity among nTp-TTA/POM-x samples, possibly due to its low charge separation rate.…”