Luminescent Cu(I)-halide(X) MOFs provide a useful testbed for relating small structural changes to band-edge electronic structure, but comparable halide series with fixed topology are still limited. Herein, three Cu(I)-halide MOFs, Tbpo-Cl, Tbpo-Br and Tbpo-I, were obtained solvothermally from a tripodal benzimidazole linker (Tbpo) and CuX (X=Cl, Br, I). Single-crystal X-ray diffraction shows that all three compounds adopt the same two-dimensional topology with similar coordination motifs. The Cu…Cu separations decrease from 0.317 nm (Tbpo-Cl) to 0.305 nm (Tbpo-Br) and 0.259 nm (Tbpo-I). Thermogravimetric analysis reveals a gradual reduction in thermal stability from Tbpo-Cl to Tbpo-I, consistent with the lengthening/softening of Cu—X bonds and weaker lattice reinforcement by intermolecular contacts. All three frameworks display solid-state photoluminescence under excitation at 340 nm, with emission maxima at 423 nm (Tbpo-Cl), 408 nm (Tbpo-Br) and 455 nm (Tbpo-I), demonstrating a non-monotonic halide dependence. Density functional theory calculations were used to clarify the origin of this behaviour. Density of states (DOS) analyses indicate a Cu—X hybridized valence-band edge, suggesting that the low-energy transition involves charge transfer between the inorganic unit and the ligand. The calculated band gaps follow the order Br>Cl>I, matching the emission-energy trend. In particular, the short Cu…Cu contact in Tbpo-I (0.259 nm) is compatible with stronger cuprophilic interactions and the stabilization of lower-energy Cu-centred states, accounting for the pronounced red shift. These results connect halide identity, metal-metal proximity and framework rigidity to the thermal and photophysical behaviour in a topology-fixed Cu(I)-halide MOFs.

Key words: Cu(I)-halide cluster, metal-organic frameworks, cuprophilic interactions, luminescence, stability