Magneto-Optical Correlations and Optical Signatures of Low-Coordinated and Highly Symmetrical Tb(III) Complexes

This study investigates the magnetic and luminescent properties as well as magneto-optical correlations of three carefully selected low-coordinate Tb(III) and symmetrical molecules: the three-coordinated [Tb{N(SiMe₃)₂}₃] (1), its congeneric five-coordinated solvate [Tb{N(SiMe₂H)₂}₃(thf)₂] (2), and a three-coordinated derivative bearing a tri(aryloxide) ligand of the formula [Tb{O-C₆H₃-2,6-(^tBu)₂}₃] (3). While slow magnetic relaxation properties are absent, as anticipated because of the equatorial tris-monoanionic ligand coordination around the oblate Tb(III) ion, their optical properties are extremely rich and question the magneto-optical correlation approaches. In particular, the large ⁷F_J energy splitting in 1 and 3 allows for the observation of almost all crystal-field level contributions of each of the ⁵D₄ → ⁷F_J transitions. This is associated with a remarkable sharpness of the emission peaks that allows for the observation of subpeaks as possible signatures of vibronic coupling, provided that the temperature is low enough for each emission peak to be sufficiently narrow. This result is, as expected, observed on 1, blurred on 2 (because of coordinating thf solvent disorder), and enhanced on 3 (through the antenna effect of the tri(aryloxide) organic ligand). These findings imply great caution must be applied when extracting magneto-optical correlations out of luminescent measurements and confrontation with ab initio calculations. It is shown that the whole temperature dependence of each ⁵D₄ → ⁷F₆ transition needs to be considered to properly describe the Tb(III) ⁷F₆ ground-state splitting. Interestingly, particularly rich and original signals can be observed below the N₂ temperature region (4 K → 77 K), making such low-temperature investigation mandatory to fully understand the emissive properties. These results are a new addition to the body of data that shows that low-coordinate and highly symmetrical molecules are prime candidates for investigating, rationalizing, and deciphering the various contributions that govern lanthanide luminescence.