The role of the antenna in the process of the host sensitized luminescence of the DOTA cage coordinated with the Eu ion is investigated. The analysis of the optimal geometries of DOTA modified by several antennas is based on the results of density functional theory (DFT) calculations. The physical environment of the luminescence center (the lanthanide ion) is illustrated by charge density maps and described by the values of the crystal field parameters directly evaluated. The conclusions derived from this theoretical analysis support earlier observations that antennas attached to the cage play the sole role of harvesting and transferring the energy to the lanthanide ion, whereas the cage perturbs the symmetry of the environment of the lanthanide ion, giving rise to the sensitized luminescence. The implications of the separation of the two parts of the organic chelate, cage and antenna, are discussed within the theoretical models of the energy transfer and of forced f <-> f electric dipole transitions.