Steric hindrance effect of Schiff-base ligands on magnetic relaxation dynamics and emissive behavior of two dinuclear dysprosium complexes

Abstract

The self-assembly reactions between mixed-ligand and tetrahydrate dysprosium acetate in the presence of mixed organic solvents lead to two structural similar dinuclear dysprosium complexes with composition formulas of Dy2(L1)2(L2)2(CH3OH)2·CH2Cl2·CH3OH (1) and Dy2(L1)2(L3)2(CH3OH)2·CH3CN (2), where L1, L2 and L3 represent the deprotonated form of 4-tert-butyl-2-(7-methoxybenzo[d]oxazol-2-yl)phenol, (E)-1-(((3,5-di-tert-butyl-2-hydroxyphenyl)imino)methyl)naphthalen-2-ol and (E)-2,4-di-tert-butyl-6-((2-hydroxybenzylidene)amino)phenol. The tiny difference of the core structure of 1 and 2 is derived from the steric hindrance of Schiff base ligands L2 and L3. Dynamic magnetic measurements reveal that 1 and 2 show frequency-dependent out-of-phase alternating-current susceptibility signal peaks at different temperatures under zero dc field, diagnostic of single-molecule magnet behavior. The experimental derived energy barrier to magnetization reversal for 1 and 2 is 108(1), 47(2) and 33(3) K. Ab initio CASSCF calculations performed on 1 and 2 suggest that the origin of the difference in magnetic properties originates from the variation in the single-ion anisotropy that arises due to minor structural variation. Further, the equation to calculate the effective energy barrier for Dy2 proposed earlier is found to yield an excellent agreement with the experimental results. Solid state fluorescence measurements performed on 1 and 2 demonstrate that both exhibit two ligands centered components of fluorescent emissive, in addition, with different emitting colors and chromaticity coordinates. The discrepancy of fluorescence and single molecule magnet behavior showed by 1 and 2 can be attributed to the steric hindrance effect of Schiff base ligands.