Two heterometallic Cu−Ca/Ba metal organic frameworks (MOFs), {[Ca(H2O)6][Cu6Ca2(mna)6(H2O)6] ⋅ 6H2O}n (1) and {[(Cu6Ba3(mna)6(H2O)8] ⋅ 6H2O}n (2), were synthesized by using 2‐mercaptonicotinic acid (H2mna) as ligand via liquid‐liquid diffusion method. The frameworks of compounds 1 and 2 are both adopts hexanuclear copper cluster of [Cu6(mna)6]6− as building units. In 1, the two‐dimensional (2D) structure is formed by the linkage between [Cu6(mna)6]6− and monomeric Ca2+, while the three‐dimensional (3D) framework of 2 is constructed by [Cu6(mna)6]6− and trimer of Ba2+. Furthermore, compounds 1 and 2 show excellent photoluminescence with red emitting around 700 nm. Luminescent studies reveal that 1 can serve as a probe for the rapid detection of Fe3+, Cr2O72− and CrO42− ions in water with remarkable selectivity even in the presence of other analogous ions.