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

Determination of the electronic energy
spectrum of a trigonal-symmetry mononuclear Yb3+ single-molecule
magnet (SMM) by high-resolution absorption and luminescence 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
{τ = τ0 × exp­[ΔOrbach/(kBT)]} affords an
activation barrier, ΔOrbach, 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 a priori be considered
as the main mechanism determining the spin dynamics of SMMs. 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 parameter 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 SMMs. Therefore, consideration of additional design
criteria that address the presence of alternative relaxation processes
beyond the traditional double-well picture is required.