Porphyrin-based covalent organic frameworks from design, synthesis to biological applications

Covalent organic frameworks (COFs) constitute a class of highly functional porous materials composed of lightweight elements interconnected by covalent bonds, characterized by structural order, high crystallinity, and large specific surface area. The integration of naturally occurring porphyrin molecules, renowned for their inherent rigidity and conjugate planarity, as building blocks in COFs has garnered significant attention. This strategic incorporation addresses the limitations associated with free-standing porphyrins, resulting in the creation of well-organized porous crystal structures with molecular-level directional arrangements. The unique optical, electrical, and biochemical properties inherent to porphyrin molecules endow these COFs with diversified applications, particularly in the realm of biology. This review comprehensively explores the synthesis and modulation strategies employed in the development of porphyrin-based COFs and delves into their multifaceted applications in biological contexts. A chronological depiction of the evolution from design to application is presented, accompanied by an analysis of the existing challenges. Furthermore, this review offers directional guidance for the structural design of porphyrin-based COFs and underscores their promising prospects in the field of biology.

Post-synthetic modification of covalent organic frameworks for CO2 electroreduction

To achieve high-efficiency catalysts for CO₂ reduction reaction, various catalytic metal centres and linker molecules have been assembled into covalent organic frameworks. The amine-linkages enhance the binding ability of CO₂ molecules, and the ionic frameworks enable to improve the electronic conductivity and the charge transfer along the frameworks. However, directly synthesis of covalent organic frameworks with amine-linkages and ionic frameworks is hardly achieved due to the electrostatic repulsion and predicament for the strength of the linkage. Herein, we demonstrate covalent organic frameworks for CO₂ reduction reaction by modulating the linkers and linkages of the template covalent organic framework to build the correlation between the catalytic performance and the structures of covalent organic frameworks. Through the double modifications, the CO₂ binding ability and the electronic states are well tuned, resulting in controllable activity and selectivity for CO₂ reduction reaction. Notably, the dual-functional covalent organic framework achieves high selectivity with a maximum CO Faradaic efficiency of 97.32% and the turnover frequencies value of 9922.68 h^-1, which are higher than those of the base covalent organic framework and the single-modified covalent organic frameworks. Moreover, the theoretical calculations further reveal that the higher activity is attributed to the easier formation of immediate *CO from COOH*. This study provides insights into developing covalent organic frameworks for CO₂ reduction reaction.

Construction of Atomic Metal-N2 Sites by Interlayers of Covalent Organic Frameworks for Electrochemical H2 O2 Synthesis

Electrosynthesis of H₂ O₂ is a promising alternative to the anthraquinone oxidation process because of its low energy utilization and cost-effectiveness. Heteroatom-doped carbons-based catalysts have been widely developed for H₂ O₂ synthesis. However, their doping degree, defective degree, and location of active sites are difficult to be preciously controlled at molecular level. Herein, a dioxin-linked covalent organic framework (COF) is used as the template to preciously construct different metal-N₂ sites along the porous walls for H₂ O₂ synthesis. By tuning the metal centers, the catalyst with Ca-N₂ sites enables to catalyze H₂ O₂ production with selectivity over 95% from 0.2 to 0.6 V versus RHE, while the H₂ O₂ yields for Co sites or Ni sites are 20% and 60% in the same potential range. In addition, the turnover frequency (TOF) values for Ca-N₂ sites are 11.63 e^-1 site^-1 s^-1 , which are 58 and 20 times higher than those of Co and Ni sites (0.20 and 0.57 e^-1 site^-1 s^-1 ). The theoretical calculations further reveal that the OOH* desorption on Ca sites is easier than those on Co or Ni sites, and thus catalyzes the oxygen reduction reaction in the 2e^- pathway with high efficiency.

Efficient electrocatalytic carbon dioxide reduction with tetraphenylethylene- and porphyrin-based covalent organic frameworks

TPE-CoPor-COF shows high FECO (91–95%) in the range of −0.6 to −1.0 V, and a maximum jCO of −30.4 mA cm−2 at −1.0 V, exceeding most of reported COF-based electrocatalysts.