In the present work, outcomes from a density functional theory investigation on the effect of lanthanides (Lns)-containing 1,10 phenanthroline complexes are discussed. Starting from the recently synthetized [Sm(L1)3]3+ (L1 = 1,10 phenanthroline), the effect of metal center replacement and of functionalization in position C5 and C6 of the ligand is examined. In detail, the comparative analysis of electronic and structural properties proceeds testing the presence of Gd3+ and Tm3+ instead of Sm3+, in combination with 1,10 phenanthroline-5,6 dione (L2), 5,6 dimethyl 1,10 phenanthroline (L3) and 1,10 phenatroline-5,6 diamine (L4) ligands. From deeper analysis, it turns out that the lanthanides do not affect the geometrical structure of the complexes, since slight variations of metal–ligand distances are observed. On the other hand, combination of ligands and metals lead to an improvement of the calculated electronic absorption for all the compounds with respect to the available experimental data, linked to the different nature of the frontier orbitals. In particular, for all complexes expected [Gd(L1)3]3+ a near-IR absorption is observed and discussed, thus opening to the possible adoption of such complexes for future photoinduced medical applications.