Synthesis And Properties Of Phenazine-Based Metal-Organic Frameworks

The rapid development of human society is inseparable from fossil fuels,but excessive use of these fuels can lead to an increase in carbon dioxide（CO₂）levels,causing serious greenhouse effects.An ideal solution is to use photocatalysts under solar light irradiation to reduce CO₂ to high-value-added chemical materials.However,existing photocatalysts mostly only utilize the energy in the ultraviolet and visible light regions,wasting nearly half of the energy in the near-infrared（NIR）light of sunlight.To more efficiently use solar energy and achieve full-spectrum catalysis,it is essential to develop new NIR photocatalysts.The main challenge of developing NIR photocatalysts is that the energy of NIR light is insufficient to excite the electrons with sufficient oxidation-reduction ability to drive photocatalytic reactions.Photothermal catalysts do not rely on photo-generated electrons but instead increase the transfer rate by raising the temperature,reducing the catalytic energy barrier.Therefore,integrating photothermal and photocatalytic effects is expected to enhance the catalytic activity and realize full-spectrum-driven photocatalysis.Metal-organic framework materials（MOF）can integrate photothermal and photocatalytic effects through reasonable design,enhancing the MOF photocatalytic efficiency.This paper consists of the following three parts:Chapter 1:A brief introduction of common MOF synthesis methods and classic MOF series is provided.It summarizes the applications of MOF in photocatalysis,lists representative examples of MOF photocatalytic CO₂ reduction,and discusses the main reasons for the low efficiency of current MOF photocatalysts.Finally,the paper discusses the strategy of enhancing the efficiency of photocatalysts by using NIR light energy and reviews the progress of MOF in NIR photocatalysis and photothermal catalysis.Based on these foundations,this paper aims to solve the current problem of low efficiency of photocatalysts,enhance the photocatalytic performance of MOF through synergistic effects,and obtain efficient full-spectrum photocatalysts.Chapter 2:Three kinds of pyridine derivatives are designed and synthesized by Buchwald-Hartwig coupling prior to hydrolyzing reaction.Then,these three ligands are assembled with different metal ions by the solvent-thermal method,and a series of MOF materials with different dimensions are obtained.Among them,the MOF 3-Cu,formed by L₃ and Cu,has a two-dimensional layered structure and shows very strong absorption to light,spanning the ultraviolet,visible,and NIR regions.This is possibly due to the charge transfer from ligand to metal inside.Under 808 nm laser irradiation,the surface temperature of 3-Cu rapidly increases and reach a maximum of 142.2°C.The cyclic experiments verified that 3-Cu has good photothermal stability.Chapter 3:Combining the photothermal effect with photocatalytic activity,the photocatalytic activity of 3-Cu for CO₂ reduction will be enhanced.The Mott-Schottky test is used to determine the oxidation-reduction potential of the bottom of conduction band of 3-Cu,which is found to be-0.4 V vs NHE,indicating that 3-Cu is capable of reducing CO₂ to formic acid.Photocurrent testing results also shows that 3-Cu can exhibit good charge separation under illumination,which is beneficial to its photocatalytic performance.Using LED white light as the light source,triethanolamine as the hydrogen source and sacrificial reagent system,3-Cu is able to generate formate from CO₂ at a rate of 656.8μmol g^-1h^-1,which is the best result achieved for Cu-MOF photocatalytic CO₂ reduction to formic acid so far.Infrared thermal imaging shows that the temperature of the reaction system increased significantly during the photocatalytic process,confirming that the photothermal effect can play a synergistic role in the photocatalytic reaction.Thanks to the synergistic effect of photothermal effect and photocatalysis,3-Cu successfully photocatalysis the reduction of CO₂ to formic acid at a rate of 454.9μmol g^-1h^-1 under natural sunlight.The photogenerated dynamics of free radical ligand L₃^·+in DADS spectra studied by femtosecond transient absorption spectroscopy（fs-TA）shows the characteristic TA spectra with a strong Stark effect,indicating efficient charge transfer in the free radical ligand L₃^·+.3-Cu exhibits stronger signals at 580 and 620 nm,indicating enhanced charge transfer after the assembly of ligand 3 into MOF.In addition,the negative TA signal of DADS-4 in 3-Cu is inferred to be due to the long-lived electrons in the MOF framework blocking the ground state absorption,indicating the existence of trapped electrons in the band gap of 3-Cu.The weaker fluorescence intensity and shorter fluorescence lifetime of 3-Cu also confirms the occurrence of LMCT.Finally,based on the experimental results of electron paramagnetic resonance（EPR）and X-ray photoelectron spectroscopy（XPS）,we propose a photocatalytic mechanism for CO₂ reduction based on the LMCT process.This work provides a feasible strategy for the development of novel and efficient photocatalysts.