Structural Evolution-Driven Band-Gap Modulation in Pb(II)-Based Metal−Organic Frameworks: From Seven-fold Weaving to Two-Fold Interpenetration

Five Pb(II)-based metal−organic frameworks (crystals 1−5) have been solvothermally synthesized using H4NDISA as a bridging ligand, with systematic modulation of lead salts, additives, and solvent compositions. Single-crystal X-ray diffraction reveals a progressive structural evolution from seven-fold weaved 2D layers (1) to two-fold weaved 2D layers (2), and further to three-fold interpenetrated 3D networks (3) and two-fold interpenetrated 3D frameworks (4 and 5). The Pb(II) centers feature six-to eight-coordination geometries, which, in combination with the coordination adaptability of the deprotonated H4NDISA ligand, synergistically direct the assembly of the framework architectures. Solid-state UV−Vis spectroscopy shows tunable band gaps from 2.08 to 2.46 eV, following the order: 2 (2.08 eV) < 1 (2.10 eV) < 5 (2.28 eV) < 4 (2.46 eV). Interestingly, crystals 1 and 2 possess longer π−π stacking distances than 4 and 5, yet exhibit narrower band gaps. This counterintuitive phenomenon may primarily arise from ligand conformational distortion, which originates from the synergistic effect of variations in metal coordination numbers, ligand coordination modes, and the resultant differences in framework interpenetration or entanglement. This work demonstrates that the balance between ligand conjugation efficiency and intermolecular electronic coupling significantly modulates the band gap, offering explicit structural design references for the optoelectronic performance optimization of Pb(II)-based MOFs via topological engineering and conformational control.