Metal-Organic Layers For Single-Site Catalysis And Light Harvesting

Two-dimensional(2D)metal-organic layers(MOLs)are the 2D version of metal-organic frameworks(MOFs)with nanometer thickness in one dimension.MOF is a kind of heterogeneous catalysts with well-defined structure which can solve the problem of homogeneous catalyst agglomeration and easy recovery.However,due to the limitation of the pore size,the diffusion rate of the substrate and the product has become one of the main factors affecting the catalytic activity.Enlarge the pore size of MOFs by extending the ligands and other methods does not completely solve this problem,and the cost is high.The ultrathin nature of 2D MOLs makes this kind of materials good candidate for catalysis because the exposure of active centers is maximized and easily modification.MOFs formed from Zr4+/Hf4+ and carboxylic acid ligands have high stability and structural diversity.Inspired by this,we have developed a highly scalable bottom-up strategy to assemble 2D MOLs directly from molecular building blocks in one-pot solvothermal reactions.The 2D MOLs are constructed from tricarboxylate ligand of C3 symmetry that connects Zr/Hf SBUs,and the 2D MOLs were functionalized with same size liagands with different properties.Different functionalized 2D MOLs obtained by ligands doping were used to study their catalytic,energy transfer and fluorescent properties.Firstly,we developed scalable solvothermal synthesis of self-supporting MOLs composed of Hf6 secondary building units(SBUs)and BTB bridging ligands.The MOLs structures were directly imaged by TEM and AFM,and doped with TPY before being with iron centers to afford highly active and reusable single-site solid catalysts for hydrosilylation of terminal olefins.MOL-based heterogeneous catalysts are free from the diffusional constraint placed on all known porous solid catalysts including metal-organic frameworks.This easily scalable synthesis of MOLs uncovers an entirely new strategy for designing single-site solid catalysts and opens the door to a new class of two-dimensional coordination materials with molecular functionalities.Secondly,the dimensionality dependency of resonance energy transfer is of great interest due to its importance in understanding energy transfer on cell membranes and in low-dimension nanostructures.We designed and synthesized 2D-MOLs and 3D-MOFs from similar hafnium SBUs and the same ligands to compare energy transfer in 2D vs.3D.We have not only quantified resonance energy transfer in 2D-MOLs and 3D-MOFs by quenching studies,but also decomposed the energy transfer efficiencies to contributions from donor-to-trap energy transfer and exciton migration through a combination of experimental measurements and simulations.Although the donor-to-trap energy transfer rate is faster in 2D-MOLs than in 3D-MOFs due to favorable transition dipole alignment in restricted dimensions,the lower inter-chromophore connectivity and slower exciton migrations in 2D materials lead to lower efficiency of overall energy transfer in the 2D-MOLs than in the 3D-MOFs.By contrast,with an external quencher in solution,the MOL exhibits much more efficient energy transfer than the MOF,highlighting better accessibility of the excitons on a 2D material than that in a 3D material.This exciton accessibility can be an important factor for efficient exciton extraction in chemical sensing and photocatalysis.These findings highlight the opportunities in using efficient exciton migration in low-dimensional materials for fluorescence sensing and light energy harvesting.Thirdly,during the preparation of 2D MOLs,monocarboxylic acid acts as modulating agent and capping molecule at the same time,making the 2D MOLs easily modified by carboxylic acid ligand exchanging.Tetraphenylethylene(TPE)is one of the typical molecules of aggregation-induced emission,and its mechanism of fluorescence emission has attracted widespread attention.In this part of the work,we used 2D MOL and MOFs as platforms to construct molecular environments with different motion constraints.We used fluorescence and ultra-fast spectroscopy to study the excited states of these samples and established a unified dynamic model of excited states.The study found that the evolutionary process of TPE-based molecular excited states is related to the molecular motions.With different motion constraints,the relative speed of the excited state evolution and the lifetime of the excited state will be affected.The increase in the radiative transition rate of the 1S*l state and the decrease in the non-radiative transition rate are both important for fluorescence enhancement.