Green Synthesis of Mn-MOFs with Tunable Optical and Structural Properties Through Ligand Functionality and Framework Design

Abstract Four manganese-based metal–organic frameworks (Mn-MOFs) were synthesized via a rapid, green ultrasound-assisted aqueous method using fumarate (FA), succinate (SA), malate (MA), and tartrate (TA) as biogenic C4-dicarboxylate ligands. Their physicochemical properties were systematically characterized by PXRD, SEM-EDS, FTIR, Raman, TGA-DTA, DRS, and PL spectroscopy. PXRD confirmed that Mn-FA, Mn-SA, and Mn-TA crystallize in monoclinic systems while Mn-MA adopts a triclinic phase with reduced crystallinity, where crystallite sizes ranged from 19.9 to 28.1 nm. FTIR and Raman corroborated metal–carboxylate coordination through characteristic νₐₛ(COO⁻)/νₛ(COO⁻) shifts, along with ligand-specific bands such as C = C at 1660 cm⁻¹ in Mn-FA and O–H vibrations (3200–3400 cm⁻¹) in Mn-MA and Mn-TA. SEM images exhibited irregular polyhedra (Mn-FA), hexagonal prisms (Mn-SA), rounded grains (Mn-MA), and irregular aggregates (Mn-TA) with particle sizes between 0.49 and 7.93 μm. EDS confirmed the presence of Mn and ligand-derived elements. TGA-DTA revealed a clear trend in thermal stability: Mn-FA decomposed around 390 °C, whereas Mn-MA degraded earlier at 290 °C due to ligand rigidity. UV–Vis DRS showed direct optical band gaps ranging from 3.5 eV (Mn-FA) to 5.4 eV (Mn-MA), modulated by linker conjugation and symmetry, while PL spectra indicated UV emission maxima at 360–380 nm depending on the coordination environment and framework order. Overall, ligand identity dictates Mn-MOF structural symmetry, thermal robustness, and optical transitions. Therefore, Mn-FA with a low band gap, high thermal stability, and visible-light activity holds potential for optoelectronic and photocatalytic applications. This water-based sonochemical synthesis presents an efficient route to tunable, multifunctional Mn-MOFs with tailored properties for advanced materials design.