Three new sets of mononuclear LnIII complexes of general formulas [LnL3]·CH3OH (LnIII = Yb (1), Er (2), Dy (3), Gd (4) and Eu (5)), [LnL2(tmh)(CH3OH)]·n H2O·m CH3OH (LnIII = Yb (1b), Er (2b), Dy (3b), Gd (4b)) and [LnL2(tta)(CH3OH)]·CH3OH (LnIII = Yb (1c), Er (2c), Dy (3c), Gd (4c) have been prepared from the reaction of Ln(CF3SO3)·nH2O salts with the tridentate ligand 2-(tetrazolate-5-yl)-1,10- phenanthroline (HL). For the two latter sets, additionally with the respective β- diketonate ligands 2,2,6,6-tetramethylheptanoate (tmh) or 2-thenoyltrifluoroacetonate (tta). In the [LnL3]·CH3OH complexes the LnIII ions are coordinated to three phenanthroline-tetrazolate ligands showing a LnN9 coordination sphere. Dynamic ac magnetic measurements for 1 – 3 reveal that these complexes only exhibit single molecule magnet (SMM) behaviour when an external dc magnetic field is applied, with Ueff values of 11.7 K (1), 16.0 K (2) and 20.2 K (3). When the tridentate phenanthrolinetetrazolate ligand is replaced by one molecule of methanol and the β-diketonate ligand tmh (1b – 3b) or tta (1c – 3c), a significant increase in Ueff occurs and, in the case of the DyIII derivates 3b and 3c, out-of-phase χ’’ signals below 15 K and 10 K, respectively, are observed under zero-dc magnetic field. CASSCF+RASSI ab initio calculations performed on the DyIII derivates support the experimental results. Thus, for 3 the ground Kramers’ doublet is far from being axial and the first excited state is found to be very close in energy to the ground state so the relaxation barrier in this case is almost negligible. Conversely, for 3b and 3c, the ground Kramers’ doublet is axial with a small quantum tunneling of the magnetization (QTM) and the energy difference between the ground and first Kramers’ doublets is much higher, which allows these compounds to behave as SMMs at zero-field. Moreover, these calculations support the larger Ueff observed for 3b compared to 3c. Additionally, the solid-state photophysical properties of 1, 2, 4 and 5 show that the phenanthroline tetrazolate ligand can act as an effective antenna to sensitize the characteristic YbIII, ErIII and EuIII emissions through an energy transfer process.