Molecular Geometry Modulation of Cd(II) Metal-Organic Frameworks for Enhanced Two-Photon Excited Fluorescence in...

Two-photon excited fluorescence (TPEF) materials have found applications in laser and imaging fields; however, directly enhancing the TPEF emission properties, particularly for metal-organic framework (MOF) materials, remains a challenge. In this study, three MOFs, CdCl₂-TPPE-2D, Cd-TPPE-cationic, and Cd₃-TPPE, were constructed by tetrakis(4-pyridylphenyl)ethylene (TPPE) and Cd²⁺ ions via solvothermal reactions. By regulation of the solution environment and temperature, the assembled MOFs exhibit different structural characteristics with distinct molecular geometries and stacking modes. These structural characteristics further influence the fluorescence properties and TPEF emissions. The TPEF results show that CdCl₂-TPPE-2D, Cd-TPPE-cationic, and Cd₃-TPPE exhibited strong emissions at 485, 504, and 534 nm, respectively, when excited with a 700 or 740 nm laser. Notably, Cd₃-TPPE and Cd-TPPE-cationic show similar two-photon absorption cross sections, while CdCl₂-TPPE-2D exhibited a much higher value, approximately 6.76 times that of the other two MOFs. Structure analysis and density functional theory calculations revealed that the characteristic TPEF properties are closely related to the molecular geometry and molecular interactions. This work expands the utilization of TPEF secondary building blocks and can bring some new insights into the design of high-efficiency TPEF-MOF materials.