The photo-active MIL-125 and MIL-125-NH2 metal-organic frameworks (MOFs), despite having a very similar crystalline structure, exhibit different optical behaviors. Luminescence in MIL-125 decays in about 1 ns, while for its amino counterpart, the lifetime of charge carriers is at least 1 order of magnitude larger. The origin of this difference is the key element for understanding the photocatalytic behavior of MIL-125-NH2 when associated with active nanoparticles, a behavior that is completely absent in MIL-125. By performing advanced ab initio electronic structure calculations, we find that charge carriers interact differently in the two MOFs with subsequent effects on the luminescence lifetimes and their catalytic performances. To confirm the predictions of our model, we synthesized a novel material in the MIL-125 family, MIL-125-NH2-10%(2), and confirm that our theory correctly predicts a faster decay compared to MIL-125-NH2.