A new three-dimensional fluorene pyrazolate-based metal–organic framework featuring luminescence and electrical conductivity

Metal-organic frameworks (MOFs) are crystalline porous materials that are composed of metal ions or clusters connected by organic linkers through coordination bonds to form one, two, and three-dimensional structures.Owing to their fascinating properties, these materials are employed in diverse applications such as gas storage and separation, chemical sensing, charge storage, etc.[1]Among all, the luminescent and chemically stable MOFs have gained significant attention in chemical sensors and electrochemical devices.[2-3]In particular, fluorene-based MOFs showed excellent luminescence properties and were employed as chemical sensors.[4-5]However, most of the studied MOFs using fluorene linkers are based on carboxylate linkages that may not be chemically stable in aqueous and basic conditions, which limits their utility in luminescence and conductivity properties.With this knowledge, we aim to synthesize the azolate linkers using the fluorene as an organic building unit and prepare chemically stable MOFs for studying their luminescence and conductivity properties.Herein, we report our design and synthetic route of a new fluorene-based pyrazolate linker named 2,7-bis((1H-pyrazole-4-yl)ethynyl)-9H-fluorene (PEF).Using this linker, a novel Zn-based three-dimensional pyrazolate MOF was synthesized by a solvothermal process, denoted as Zn2(PEF)4.The structural analysis displayed that this compound crystallizes in lower symmetry orthorhombic (Pccm) space group and the asymmetric unit consists of one Zn 2+ ion and one fluorene pyrazole linker.The secondary building unit (SBU) contains two different Zn 2+ metal ions with four different fluorene pyrazolate linkers to form a distorted tetrahedral geometry (Figure 1).In addition, this compound illustrated 4fold interpenetration and π -π stacking of fluorene linkers along the crystallographic c-axis with a distance of 4.117 Å .The MOF compound's phase purity, thermal stability, and porosity are characterized through various instrumental analyses.Based on the structural analysis, we anticipated that the compound shows better luminescent properties, and observed interlayer distance between the two fluorene linkers may accommodate electron transport through a through-space pathway which could display electrical conductivity in this MOF compound.Figure 1.Structural representation of new three-dimensional Zn-based fluorene pyrazolate MOF.