Amending the anisotropy barrier and luminescence behavior of heterometallic trinuclear linear M-II-Ln(III)-M-II (Ln(III)=Gd, Tb, Dy; M-II=Mg/Zn) complexes by change from divalent paramagnetic to diamagnetic metal ions

Abstract

The sequential reaction of a multisite coordinating compartmental ligand LH4 [2-(2-hydroxy-3-(hydroxymethyl)-5-methylbenzylideneamino)-2-methylpropane-1,3-diol] with appropriate lanthanide salts followed by the addition of Mg(NO3)2·6H2O or Zn(NO3)2·6H2O in a 4:1:2 stoichiometric ratio in the presence of triethylamine affords a series of isostructural heterometallic trinuclear complexes containing [Mg2Ln]3+ [Ln = Dy (1), Gd (2) and Tb (3)] and [Zn2Ln]3+ [Ln = Dy (4), Gd (5) and Tb (6)] cores. The formation of 1-6 is demonstrated by X-ray crystallography as well as ESI-MS spectra. All complexes are isostructural possessing a linear trimetallic core with a central lanthanide ion. In this article we have discussed the comprehensive studies, involving synthesis, structure, magnetism and photophysical properties on this family of trinuclear [Mg2Ln]3+ and [Zn2Ln]3+ heterometallic complexes. Complexes 1 and 4 show slow relaxation of the magnetization below 12 K under zero applied direct-current field, but without reaching a neat maximum which is due to the overlapping with a faster quantum tunnelling relaxation mediated through dipole-dipole and hyperfine interactions. Under a small applied direct-current field of 1000 Oe the quantum tunneling was almost suppressed and temperature and frequency dependent peaks were observed, thus confirming the SMM behavior of complexes 1 and 4. The fit of the high-temperature relaxation times to the Arrhenius equation affords an effective energy barrier for the reversal of the magnetization of Ueff =72(2) K with o = 8 x 10-9 s for the SR process and Ueff = 61(2) K with o = 4 x 10-7 s for the FR process for 1 whereas for 4, an effective energy barrier for the reversal of the magnetization Ueff = 67(3) K with o = 4.5 x 10-8 s. To rule out the involvement of intermolecular collaborative interactions in the dynamic of relaxation, we have performed ac susceptibility measurements on 1:10 Dy:Y magnetic diluted samples of of 1 and 4, named as 1' and 4'. Interestingly, the diluted compounds 1' and 4' exhibits SMM behavior under zero magnetic field, thus suggesting that their relaxation processes are single molecular in origin and arise from the M-Dy-M unit. Ab initio CASSCF+RASSI calculations carried out on 1 and 4 confirm that the magnetic anisotropy is axial along the M-Dy-M axis and that the relaxation process occurs through the first excited energy level. Furthermore, the chromophoric [LH3]2- ligand is able to act as an 'antenna' group which was found to be effective in the selective sensitization of the emissions of TbIII-based complexes 3 and 6. The emission quantum yields and the luminescence lifetimes at room temperature are 11.7 % and 0.606 ms for 3, 22.7 % and 0.799 ms for 6.