One of the key factors determining the photocatalytic capacity of a compound is its optical band gap, which, within an optimal range, maximizes the absorption and utilization of solar light. Recent research shows that modifying organic ligands and metal centers in metal–organic frameworks (MOFs) plays a crucial role in tailoring specific band gap characteristics. Many research articles use different structural MOF motives for photocatalysis, but few delve into band gap engineering using quantum chemistry calculations. Density functional theory (DFT) has been widely used to understand the possible mechanisms of action of some photocatalysts, but few studies directly relate the band gap to the ligands present in the compounds. In our study, DFT calculations are used to get a deeper understanding of the relationship between crystal and band structures aimed at predicting and enhancing the photocatalytic properties of new isostructural coordination polymers (CPs) with the general formula [ZnX₂(L)₂]_n or [Cu₂X₂(L)]_n from X = Cl and Br to X = I, and from 1,2-bis(4-pyridyl) ethane (BPE) to 1,2-bis(4-pyridyl)ethylene (BPEE) offering significant time and cost savings by enabling predictions. Despite the good band gap value of [ZnI₂(BPEE)₂]_n, all members of the Zn/BPEE family show partial hydrolysis in water, which limits their use as photocatalysts. However, the 2D [Cu₂X₂(BPEE)]_n (X= Cl, Br, and I) CPs are more insoluble and stable in water. Following the DFT results, the study of [Cu₂I₂(BPEE)]_n as photocatalysts with water-persistent organic dyes (methylene blue (MB), methyl orange (MO), and tartrazine (Trz)) has been done. This approach allows us to assess whether the experimental synthesis of novel compounds with improved optical properties is worthwhile for their potential use as heterogeneous photocatalysts for water remediation.