Lanthanide Tetrazolate complexes combining single-molecule magnet and luminescence properties: the effect of the replacement of Tetrazolate N3 by β-Diketonate ligands on the anisotropy energy barrier

Abstract

Three new sets of mononuclear Ln(III) complexes of general formulas [LnL(3)]CH3OH [Ln(III)=Yb (1), Er (2), Dy (3), Gd (4), and Eu (5)], [LnL(2)(tmh)(CH3OH)]nH(2)OmCH(3)OH [Ln(III)=Yb (1b), Er (2b), Dy (3b), Gd (4b)], and [LnL(2)(tta)(CH3OH)]CH3OH [Ln(III)=Yb (1c), Er (2c), Dy (3c), Gd (4c)] were prepared by the reaction of Ln(CF3SO3)nH(2)O salts with the tridentate ligand 2-(tetrazol-5-yl)-1,10-phenanthroline (HL) and, for the last two sets, additionally with the -diketonate ligands 2,2,6,6-tetramethylheptanoate (tmh) and 2-thenoyltrifluoroacetonate (tta), respectively. In the [LnL(3)]CH3OH complexes the Ln(III) ions are coordinated to three phenanthroline tetrazolate ligands with an LnN(9) coordination sphere. Dynamic ac magnetic measurements on 1-3 reveal that these complexes only exhibit single-molecule magnet (SMM) behavior when an external dc magnetic field is applied, with U-eff values of 11.7K (1), 16.0K (2), and 20.2K (3). When the tridentate phenanthroline tetrazolate ligand is replaced by one molecule of methanol and the -diketonate ligand tmh (1b-3b) or tta (1c-3c), a significant increase in U-eff occurs and, in the case of the Dy-III complexes 3b and 3c, out-of-phase signals below 15 and 10K, respectively, are observed in zero dc magnetic field. CASSCF+RASSI ab initio calculations performed on the Dy-III complexes 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, 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 U-eff 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 Yb-III, Er-III, and Eu-III emissions through an energy-transfer process.