Blue- and white-light emitting 2D metal–organic frameworks of cis-5-norbornene-endo-2,3-dicarboxylic acid†

Abstract

Two-dimensional (2D) materials have recently gained increased attention owing to their potential applications in catalysis and as luminescent materials in optoelectronics. Herein, we report the synthesis of two distinct blue- and white-light emitting crystalline 2D-metal organic frameworks (MOFs) using cis-5-norbornene-endo-2,3-dicarboxylic acid (NDA) as a ligand with Pb(NO₃)₂ [Pb(NDA)] (1) and Zn(OAc)₂·2H₂O [Zn(NDA)] (2), respectively. Compound 1 forms hepta-coordinated Pb(II) geometry with greater distortion, resulting in a 2D MOF with a (4,4) network topology. In contrast, in 2, Zn(II) centers are linked through NDA ligands to furnish a 2D MOF. The final structural characteristics of the synthesized compounds were significantly impacted by the carboxylic acid ligand. Upon excitation at 365 nm, compound 2 displayed blue-light emission, while compound 1 demonstrated near-white-light emission of solid-state photoluminescence (PL). The differing emissions of the two 2D sheets were associated with the bridging NDA ligand and the relative structural differences between the 2D structures. The shortest non-bonded metal-to-metal distance is lower in 1 than that in 2. Compound 1 exhibits a higher packing efficiency of bridging-metal centered clusters of layers; such packing is less pronounced in compound 2 because of the differences in coordination geometries. These differences profoundly affect the luminescent characteristics of 1 and 2.