Hydrazone‐Linked Heptazine Polymeric Carbon Nitrides for Synergistic Visible‐Light‐Driven Catalysis

Abstract

Heptazine‐based conjugated polymeric carbon nitrides (PCNs) are promising metal‐free photocatalysts, yet their synthesis is challenging due to the electron‐deficiency and insolubility of heptazine units. Indeed, heptazine‐containing polymers have only been prepared through nucleophilic substitution with amines by using toxic cyameluric chloride as the starting material. Herein, we report the novel and environmentally friendly method for preparing heptazine‐based mesoporous PCNs with hydrazone links formed through a simple Schiff base condensation of melem‐NH2 and aldehydes. Unlike cyameluric chloride, melem‐NH2 is non‐toxic, stable, and can be readily obtained from melem and hydrazine in solution. We demonstrate that the hydrazone linkages and the heptazine units synergistically enhance the photocatalytic activity of PCNs in visible‐light‐driven aerobic oxidation of benzyl alcohol to benzaldehyde. In particular, the polymer constructed from melem‐NH2 and p‐phthalaldehyde shows 17 times more activity than graphitic carbon nitride (g‐C3N4).

Donor‐Acceptor Conjugated Heptazine Polymers with Highly Efficient Photocatalytic Degradations towards Tetracyclines

“…The D-A conjugated structures of three obtained polymers were assigned by the solid-state 13 C magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy (Figure S1, Supporting Information), which reveal that the carbon signal peaks of the heptazine cores appear at 157 and 164 ppm, respectively, while those of phenyl linkers are located at 120-150 ppm. [15,31,33] This is further confirmed by their FT-IR spectra (Figure 2a), which show characteristic stretching vibrations of heptazine cores at ≈824, ≈1370, and ≈1571 cm -1 . [31,34] The crystallinity of three obtained polymers has been investigated by powder X-ray diffraction (PXRD) technique.…”

“…[15,31,33] This is further confirmed by their FT-IR spectra (Figure 2a), which show characteristic stretching vibrations of heptazine cores at ≈824, ≈1370, and ≈1571 cm -1 . [31,34] The crystallinity of three obtained polymers has been investigated by powder X-ray diffraction (PXRD) technique. For comparison, that of g-C 3 N 4 has also been recorded.…”

“…[25] However, D-A conjugated polymers based on heptazine moieties have reported to show a totally different tendency, [26][27][28] which indicates the heptazine moieties are an ideal electron-acceptor unit for design of D-A conjugated photocatalysts. [29][30][31] Indeed, recently we reported a series of small-molecule D-A heptazine dyes by Friedel-Crafts reactions, which exhibited a decent efficiency for photocatalytic hydrogen evolution. [32] Herein, we extended such a facile method to the synthesis of conjugated polymers, and three novel D-A conjugated heptazine polymers, named as HPBP, HPTP, and HPF (see Figure 1), with heptazine moieties as electron acceptors, while biphenyl, terphenyl, or fluorene moieties as electron donors, respectively, have been successfully prepared in high yields via Friedel-Crafts reactions.…”

Visible Light-Induced Aerobic Oxidative Dehydrogenation of C–N/C–O to C=N/C=O Bonds Using Metal-Free Photocatalysts: Recent Developments

“…Other polymers used in this transformation were a trithiocyanuric acid-based material composed of disulfide units 89 (Figure 25) [187], an heptazine-based porous material with ethane-1,1,2,2-tetrayl tetraaniline units (POP-HE, 90) (Figure 25) [188], a flavin-based mesoporous polymeric network with ethylene glycol dimethacrylate units (RFITMA-co-EGDMA, 91) (Figure 25) [189], and the same hyper-cross-linked material was used in the aerobic oxidative coupling of amines and the dehydrogenation of dibenzylamine [111]. The recyclability of these photocatalysts was successfully carried out by filtration and washing with ethanol and acetone (for 90) and through a filter-equipped glass reactor (for 91), while moter [190], a hydrazine-linked heptazine carbon nitride 92 (Figure 26) [191] and a corallike melamine-based polymeric material [192] were employed in the mentioned transformation (92 only being employed in the oxidation of the benzyl alcohol to benzaldehyde), obtaining the corresponding aldehydes 69 in moderate to excellent conversions in all cases. The coral-like material was reused by centrifugation and washed with acetonitrile and ethanol, observing similar conversions in all the catalytic cycles.…”

Heptazine-Based π-Conjugated Materials for Light-Emitting

“…As the heptazine-based g-C 3 N 4 emerging to be a class of promising metal-free photocatalysts, it received tremendous research interests over the past decade in the fields of hydrogen evolution Wang et al (2009), Liao et al (2019), CO 2 reduction Gao et al (2016), Barrio et al (2019), photocatalytic degradation of organic pollutants Ong et al (2016), Zeng Y. et al (2018), and artificial photosynthesis (Su et al, 2010;Dai et al, 2018). Additionally, heptazine-based covalent organic frameworks (COFs) have also attracted much attention in the past several years due to the photocatalytic performance (Bojdys et al, 2010;Kailasam et al, 2016;Luo et al, 2019;Xing et al, 2020;Zhang et al, 2020). The various applications are significantly associated with the appealing heptazine-based molecular structure in which the sp 2 hybridized carbon and nitrogen induce a delocalized π-conjugated system and consequently result in a moderate band gap of around 2.7 eV, whereby a broad variety of photocatalytic reactions can be carried out (Zhang et al, 2019;Patnaik et al, 2021).…”