Preparation Of Near-Infrared Photoinduced Spin-Crossover Materials

Owing to the performance of reversible switching between high and low spin states,spin crossover materials have attracted wide interest with numerous applications in sensing,storage,switching and so on.MOFs which exhibiting spin crossover properties,as promising versatile materials,show prominent features such as porosity,flexible and controllable morphologies,and exposed active sites.In this dissertation,two types of MOFs with transition metal ion Co（II）as the metal center were selected,and their 3D bulk precursors,2D nanosheets,doped composite gels were studied respectively.The spin behaviors under near-infrared light（NIR）,the changes in optical and magnetic properties under dimensional evolution and the influence of gel doping on the SCO behavior of MOFs were explored.The details are as follows:（1）For two MOFs with completely different structures,namely 3D Hofmann-type Co（py）₂Ni（CN）₄and 3D[Co（NCS）₂（pyz）₂]_n,the materials were creatively triggered by low-energy NIR.The SCO behaviors triggered by NIR were judged from the changes in optical properties.More importantly,by in-situ processing of 3D Co（py）₂Ni（CN）₄and 3D[Co（NCS）₂（pyz）₂]_nwith NIR,it is demonstrated that both show good cycling performance for NIR triggering,which are extremely suitable for application in molecular switches and sensing devices irradiated by NIR.（2）By using the top-down physics-assisted method,3D[Co（NCS）₂（pyz）₂]_nmaterials were stripped to obtain 2D nanosheets,and the effects of dimensional evolution on the optical properties and magnetism of the materials were investigated.Due to the significantly enhancedπ-π^*conjugated system,the exfoliated 2D nanosheets exhibit a sensitive response to temperature and NIR,together with the spin transition of the 2D nanosheets from LS to HS,providing a short and efficient magnetic exchange path.In addition,when the 3D MOF precursor was transformed into 2D nanosheet,the magnetic properties of the material also changed from paramagnetic to spin canting of antiferromagnetic,demonstrating the feasibility of controlling the magnetic behavior of MOFs by dimensional evolution design.（3）Using PVDF gel as a carrier,3D and 2D MOFs are interspersed into the gel network to prepare composite gels.The PVDF@3D-Co（py）₂Ni（CN）₄and PVDF@3D-[Co（NCS）₂（pyz）₂]_nexhibited significant d-d transition enhancement under NIR light triggering.However,the PVDF@2D-[Co（NCS）₂（pyz）₂]_nexhibited spin switching from HS to LS.Therefore,this design method of loading SCO-type MOFs materials in PVDF networks is expected to achieve synergistic SCO behavior in composite gels and broaden the practical application of SCO-type MOF materials.The above work expands the multifunctional properties of SMOFs by using NIR to trigger and control SMOFs,design multifunctional materials with magnetic conversion,and construct composite gels with good synergy,providing new ideas for future research on SMOFs.