Triazine–Porphyrin-Based Hyperconjugated Covalent Organic Framework for High-Performance Photocatalysis

Abstract

Covalent organic frameworks (COFs) with porphyrins as structural units are a new kind of porous organic polymers, which have a regular and ordered structure, abundant porosity, and good stability. In the past, the construction of porphyrin COFs was generally synthesized by routes such as a Schiff base reaction. Here, we report a new COF structure by linking the porphyrin with the triazine ring. Using a cyano group-terminated porphyrin as a structural unit precursor, a new triazine-porphyrin hyperconjugated COF (TA-Por-sp2-COF) was constructed through the cyano group’s self-polymerization. The extension of porphyrin units in two directions that stemmed from the cyano group at para-positions accounts for the establishment of a highly ordered two-dimensional topological structure. Attributing to the collaboration of electron-donating and withdrawing blocks for photo-induced carrier separation and adequate porosity for mass diffusion, this hyperconjugated system showed high photocatalytic performance in organic reactions such as the aerobic coupling reaction of benzylamine and thioanisole selective oxidation.

Blending Aryl Ketone in Covalent Organic Frameworks to Promote Photoinduced Electron Transfer

“…The increasing trend in photocurrent density, when switching substituted groups from −F and −OH to −OMe (Figure d), strongly validated that the charge carrier mobility in TRO-COFs could be effectively tuned by the type of groups on linkers. These results were also supported by the electrochemical impedance spectroscopy (EIS) study (Figure e), revealing that the radius of the semicircle in the Nyquist plot for TRO-COFs varied with the functional groups (the wider the radius, the larger the charge transfer resistance). − The lowest charge transfer resistance of TRO-OMe implies the highest charge separation rate of the electron–hole pairs, indicating the highest photocatalytic efficiency.…”

Porphyrin-Based Covalent Organic Frameworks: Design, Synthesis, Photoelectric Conversion Mechanism, and Applications

“…The strong aromatic group in the triazine unit and ring fusion of C=N bonds endow them with high chemical stability. However, these porphyrin-based COFs are usually amorphous and lack long-range molecular orderings due to the nonplanar trimerization route [ 98 ]. In addition, only limited members of triazine porphyrin-based COFs were reported due to the lack of suitable monomers (P5, P6) [ 83 , 99 , 100 , 101 , 102 , 103 ].…”

Direct C-H bond activation using visible light and a donor-acceptor polymer with tunable valence band

“…Driven by the push-pull effect between electron-rich donors and electron-de cient acceptors 22 , metal-free D-A-based polymers have been demonstrated to be e cient photocatalysts for water splitting 23 , hydrogen peroxide production 24, 25 , CO 2 reduction 26 , and so on 27, 28 . However, given the high bond dissociation energy and weak polarity of C-H bonds, it is highly challenging to explore effective methods to trigger the oxidation power of D-A polymers; thus, a method is urgently needed to directly activate C-H bond…”

“…To clarify the knowledge on p-D x A y samples, density functional theory calculations were performed to visualize the photoinduced carrier separation promoted by the formed D-A pairs. 27,32 The estranged density distributions at the valence band maximum and conduction band minimum were mainly localized on the donor and acceptor units, respectively, which ensured the separation of charge carriers at a molecular level under light radiation (Fig. 2a).…”

“…[19][20][21] The selective oxidation of C-H bonds over a semiconductor usually requires a very low valence band to activate the C-H bonds; thus, the band gap of the semiconductor is largely broadened to scarify the visible light responsiveness and the charge mobility for long-distance migration of charge carriers to sur cial active centers. As a result, it is still challenging to develop metal-free photocatalysts that exhibit properties of heterogeneous catalysts, i.e., tunable electronic structure and recyclability, and homogeneous catalysts, i.e., fast charge separation at the molecular level, with the goal of sustainably and e ciently converting hydrocarbons.Driven by the push-pull effect between electron-rich donors and electron-de cient acceptors 22 , metal-free D-A-based polymers have been demonstrated to be e cient photocatalysts for water splitting 23 , hydrogen peroxide production 24, 25 , CO 2 reduction 26 , and so on 27, 28 . However, given the high bond dissociation energy and weak polarity of C-H bonds, it is highly challenging to explore effective methods to trigger the oxidation power of D-A polymers; thus, a method is urgently needed to directly activate C-H bond…”