Design of Single-Molecule Magnets: Insufficiency of the Anisotropy Barrier as the Sole Criterion

Determination of the electronic energy spectrum of a trigonalsymtnetry mononuclear Yb3+ single-molecule magnet (SMM) by highresolution absorption and luniinescente spectroscopies reveals that the first excited electronic doublet is placed nearly 500 cm(-1) above the ground one. Fitting of the paramagnetic relaxation times of this- SMM to a thermally activated (Orbach) model {tau = tau(0) X exp [Delta(Orbach)/(k(B)T)]} affords an activation barrier, Delta(Orbach), of only 38 cm(-1). This result is incompatible with the spectroscopic observations. Thus, we unambiguously demonstrate, solely on the basis Of experimental data, that Orbach relaxation cannot ci priori be considered as the main mechanism determining the spin dynamics of SM.Ms. This study highlights the fact that the general:synthetic approach of optimizing SMM behavior by maximization of the anisotropy barrier, intimately linked to the ligand field, 'as the sole paratneter to be tuned, is insufficient because of the complete neglect of the interaction of the magnetic moment of the molecule with its environment. The Orbach mechanism is expected dominant only in the cases in which the energy of the excited ligand field state is below the Debye temperature, which is typically low for molecular crystals and, thus, prevents the use of the anisotropy barrier as a design criterion, for the realization of high-temperature SM.Ms. Therefcire, consideration of additional design criteria that address the presence of alternative relaxation processes beyond the traditional double-well picture is required.