Pedersen Kasper S, Dreiser Jan, Weihe Høgni, Sibille Romain, Johannesen Heini V, Sørensen Mikkel A, Nielsen Bjarne E, Sigrist Marc, Mutka Hannu, Rols Stephane, Bendix Jesper, Piligkos Stergios
†Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark.
‡CNRS, CRPP, UPR 8641, F-33600 Pessac, France.
Inorg Chem. 2015 Aug 3;54(15):7600-6. doi: 10.1021/acs.inorgchem.5b01209. Epub 2015 Jul 22.
Determination of the electronic energy spectrum of a trigonal-symmetry mononuclear Yb(3+) 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.
通过高分辨率吸收和发光光谱法测定三角对称单核Yb(3+)单分子磁体（SMM）的电子能谱，结果表明，第一激发电子二重态比基态高出近500 cm(-1)。将该SMM的顺磁弛豫时间拟合到热激活（奥巴赫）模型{τ = τ0 × exp[ΔOrbach/(kBT)]}中，得到的激活势垒ΔOrbach仅为38 cm(-1)。这一结果与光谱观测结果不一致。因此，我们仅基于实验数据明确证明，奥巴赫弛豫不能先验地被视为决定SMM自旋动力学的主要机制。这项研究突出了这样一个事实，即通过最大化与配体场密切相关的各向异性势垒来优化SMM行为的一般合成方法，作为唯一要调整的参数是不够的，因为完全忽略了分子磁矩与其环境的相互作用。预计奥巴赫机制仅在激发配体场态的能量低于德拜温度的情况下占主导地位，而对于分子晶体来说，德拜温度通常较低，因此，这使得不能将各向异性势垒用作实现高温SMM的设计标准。因此，需要考虑额外的设计标准，以解决传统双阱图景之外的替代弛豫过程的存在问题。
