Structural isomerism-tuned magnetisation relaxation dynamics in lanthanide coordination complexes

The eigenspectrum and eigenvectors of the 2F7/2 ground multiplets of two structural isomer coordination complexes, Yb(trenovan) (H3trenovan = tris(((3-methoxysaliclidene)amino)ethyl)amine) and Yb(trenpvan) (H3trenpvan = tris(((5-methoxysaliclidene)amino)ethyl)amine), were determined by use of magnetometry and electron paramagnetic resonance and luminescence spectroscopies. The two 4f complexes crystallise in the trigonal 𝑃3̅ space group, have identical chemical formulas and differ only in the placement of the methoxy group on the aromatic ring of salicylaldehyde, with it being either ortho, Yb(trenovan), or para, Yb(trenpvan), to the phenoxo group. This structural isomerism is found to have a profound influence on the solid state static and dynamic magnetic properties of the complexes. In the bulk, both isomers display a combination of direct and raman magnetisation relaxation procceses, however, at vastly different rates. Magnetic dilution in an isostructural diamagnetic host supresses the occurrence of the direct procces suggesting that the direct process observed in the bulk originates from Yb(III) centres coupled by magnetic dipole interactions. The eigenspectrum, eigenvectors and spin lattice relaxation of Yb(trenpvan) are found to be closer to the ones found in Yb(trensal) (H3trensal = tris(((saliclidene)amino)ethyl)amine) where the methoxy group is substituted by an H atom, than to those of Yb(trenovan) where the methoxy group is in the ortho position. Thus, the position of chemical modifications has a tremendous influence on the static and dynamic solid state magnetic properties of 4f coordination complexes. Our detailed study on single crystals, demonstrates that the rarely studied effect of modifications of the position of second coordination sphere chemical groups on the dynamic magnetic properties can in fact be used to chemically tune the magnetisation dynamics of 4f spin systems.