Rational design of bimetallic sites in covalent organic frameworks for efficient photocatalytic oxidative coupling of amines

Donor-acceptor mixed-ligand MOF with energy transfer-mediated high-efficiency singlet oxygen generation for boosted organic photosynthesis

Integrating energy donor and acceptor chromophores as ligands within one MOF for advanced artificial photosynthesis is of great interest but appears to be a major challenge. Herein, via a simple one-pot synthetic strategy, an energy acceptor porphyrin ligand 5,15-di(p-benzoato)porphyrin (H2DPBP) was successfully integrated into an energy donor 1,4-naphthalenedicarboxylic acid (H2NDC)-based MOF (UiO-66-NDC) to construct a mixed-ligand MOF, donated as UiO-66-NDC-H2DPBP. Benefiting from the ample overlap between the emission spectrum of H2NDC and the absorption spectrum of H2DPBP, an efficient energy transfer (EnT) process from the donor H2NDC to the acceptor H2DPBP within UiO-66-NDC-H2DPBP can occur and be captured by time-resolved spectroscopy. Furthermore, the singlet oxygen (1O2) generation efficiency of UiO-66-NDC-H2DPBP mediated by this EnT process as well as the EnT process from the triplet state (T1) of the photosensitizer H2DPBP ligand to the ground state of molecular oxygen (3O2) upon light irradiation can be maximized via simply regulating the loading amount of H2DPBP, leading to boosted photocatalytic activities toward important aerobic oxidation reactions of amines and sulfides, even under sunlight and ambient air. This work explores an avenue to construct high-efficiency energy donor and acceptor-based light-harvesting systems by utilizing mixed-ligand MOFs as platforms to advanced artificial photosynthesis.

Engineering fluorene-based covalent organic framework photocatalysts toward efficient and selective aerobic oxidation of amines

Covalent organic frameworks (COFs) have attracted significant interest due to diverse applications, relying on their versatile molecular building blocks like fluorenes. However, the twisted structures of fluorenes pose substantial challenges for the construction of porous crystalline materials like COFs. Here, the couplings of 1,3,5-triformylphloroglucinol (Tp) with 9H-fluorene-2,7-diamine (DAF), 9,9-dimethyl-9H-fluorene-2,7-diamine (MFC) and 9,9-difluoro-9H-fluorene-2,7-diamine (FFC) with a pyrrolidine catalyst afford three fluorene-based COFs, TpDAF-COF, TpMFC-COF and TpFFC-COF, respectively. The resulting COFs, with distinct functional groups, exhibit high crystallinity and porosity. Optoelectronic tests reveal that TpFFC-COF demonstrates the most intense photocurrent density and the lowest interfacial charge transfer resistance. When applied to the selective aerobic oxidation of amines to imines, the efficiency follows the order of TpFFC-COF > TpMFC-COF > TpDAF-COF, consistent with the observed optoelectronic properties. Additionally, the TpFFC-COF photocatalyst showcases excellent reusability and broad applicability. This work illuminates the potential of engineering COFs with functional groups toward efficient photocatalysts.

Unveiling the influence of hydrophobicity on inhibiting hydrogen dissociation for enhanced photocatalytic hydrogen evolution of covalent organic frameworks

Covalent organic frameworks (COFs) have gained considerable interest as candidate photocatalysts for hydrogen evolution. In this work, we synthesized β-keto-enamine-based COFs (TpPa-X, TpDB, and TpDTP) to explore the relations between structures and photocatalytic hydrogen evolution. COFs were divided into two groups: (1) TpPa-X with different substituents attached to the TpPa backbone and (2) COFs featuring diamine linkers of varied lengths (TpDB and TpDTP). Experiments and density functional theory (DFT) calculations show that moderate hydrophobicity is favorable for the photocatalytic hydrogen evolution process, and acceptable contact angles are anticipated to range from 65° to 80°. Naturally, there are comprehensive factors that affect photocatalytic reactions, and the regulation of different backbones and substituents can considerably affect the performance of COFs for photocatalytic hydrogen evolution in terms of electronic structure, specific surface area, surface wettability, carrier separation efficiency, and hydrogen dissociation energy. Results show that TpPa-Cl2 (TpPa-X, X  = Cl2) demonstrates the highest photocatalytic activity, approximately 14.51 mmol g-1h-1, with an apparent quantum efficiency of 4.62 % at 420 nm. This work provides guidance for designing efficient COF-based photocatalysts.

Tailoring β-ketoenamine covalent organic framework with azo for blue light-driven selective oxidation of amines with oxygen

Covalent organic frameworks (COFs) present bright prospects in visible light photocatalysis with abundant active sites and exceptional stability. Tailoring an established COF with photoactive group is a prudent strategy to extend visible light absorption toward broad photocatalysis. Here, a β-ketoenamine COF, TpBD-COF, constructed with 1,3,5-triformylphloroglucinol (Tp) and 4,4'-biphenyldiamine (BD), is tailored with azo to validate this strategy. The insertion of azo into BD affords 4,4'-azodianiline (Azo); TpAzo-COF is successfully constructed with Tp and Azo. Intriguingly, the insertion of azo enhances π-conjugation, thereby facilitating visible light absorption and intramolecular electron transfer. Moreover, TpAzo-COF, with an appropriate electronic structure and impressive specific surface area of 1855 m2 g-1, offers substantial active sites conducive to the reduction of oxygen (O2) to superoxide. Compared with TpBD-COF, TpAzo-COF exhibits superior performance for blue light-driven oxidation of amines with O2. Superoxide controls the selective formation of product imines. This work foreshadows the remarkable capacity of tailoring COFs with photoactive group toward broad visible light photocatalysis.