The controllable assembly of mixed-valence metal-organic frameworks (MOFs) remains a formidable challenge in coordination chemistry, primarily owing to the intricate balance required to stabilize disparate oxidation states within a single crystalline structure. In this study, we successfully synthesized two isoreticular copper-based mixed-valence MOFs under solvothermal conditions, employing pyrimidine-5-carboxylic acid (HL) as the bridging ligand and introducing chloride or bromide anions as coordinating modulators. The resulting MOFs, formulated as {(Cu^ⅠCl)₂[Cu^ⅠⅠ₃(L)₆(DMA)₃](H₂O)₂}_n (1) and {(Cu^ⅠBr)₂[Cu^ⅠⅠ₃(L)₆(DMA)₃](H₂O)₂}_n (2), were characterized by single-crystal X-ray diffraction, which revealed that both crystallize in the trigonal space group P3c1 and share an identical zyg-type three-dimensional network. These structures arise from the cooperative self-assembly of Cu(I)-halide secondary building units and mononuclear Cu(II) centers, which are interconnected via deprotonated pyrimidine-5-carboxylate linkers. Powder X-ray diffraction (PXRD) confirmed the high crystallinity and phase purity of the as-synthesized samples. Complementary characterization by X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), elemental analysis, verified the expected stoichiometry, the coexistence of Cu(I) and Cu(II), and thermal stability. Solvent immersion tests demonstrated that both frameworks retain their structural integrity upon exposure to common organic solvents, underscoring their good chemical stability. Ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS) revealed broad-spectrum responses with relatively narrow band gaps for both MOFs. Photoelectrochemical measurements, including transient photocurrent responses measurements and electrochemical impedance spectroscopy (EIS), indicated efficient photoinduced charge separation and interfacial charge migration. Notably, the bromide-coordinated analogue (2) exhibits a narrower band gap (1.22 eV vs 1.34 eV for 1), enhanced photocurrent density (1.06 μA•cm^-2 vs 0.42 μA•cm^-2 for 1), and reduced charge-transfer resistance relative to its chloride counterpart, clearly demonstrating that halide substitution effectively modulates the electronic band structure and promotes intramolecular electron transfer. Collectively, these mixed-valence Cu-MOFs exhibit full-spectrum absorption, robust photoelectrochemical activity, and good chemical stability, rendering them highly promising candidates for applications in photoelectrocatalysis and optical sensing.

Key words: metal-organic frameworks, isoreticular structures, mixed-valence copper, halide ions, full-spectrum absorption