This investigation demonstrates the heavy atom effect (HAE) concept in developing new organic phosphors and engineering the excited-state energy levels in lanthanide metal ion suprastructures. This was accomplished by coupling two independent energy-transfer photophysical processes: enhancing the electronic population in the excited triplet state through intersystem crossing (ISC) and transferring the triplet energy to the excited state of the lanthanide ions. A new series of iodo-substituted carboxylic ligands were synthesised through a tailor-made approach and complexes with Eu3+ ions to give one- and three-dimensional metal–organic frameworks (MOFs). Single-crystal structures of the europium complexes revealed the formation of a 1D linear coordination polymer for the monocarboxylate ligand and 3D MOFs for the dicarboxylate ligand. The HAE quenches the S1→S0 transition (self-fluorescence) in these ligands and promotes S1→T processes for building enhanced excited triplet electronic states. Single-crystal structures of iodo-substituted complexes proved that the ligand molecules were held together by strong π stacking. The π stack restricted vibration relaxation and, as a result, these ligands turned into white or yellowish solid-state organic phosphors. In Eu3+ ion complexes, the solid-state phosphorescence of the ligands was completely quenched and the triplet excitation energy was channelled into ligand-to-metal energy transfer. Thus, the current approach opens up a new strategy for designing luminescent MOFs based on the HAE principle by controlling the excited-state energy levels.