Hans‐Ulrich Güdel

A detailed investigation of the unusual dynamics of the magnetization of (Et4N)3Fe2F9 (Fe2), containing isolated [Fe2F9]3- dimers, is presented and discussed. Fe2 possesses an S=5 ground state with an energy barrier of 2.40 K due to an axial anisotropy. Poor thermal contact between sample and bath leads to a phonon bottleneck situation, giving rise to butterfly-shaped hysteresis loops below 5 K concomitant with slow decay of the magnetization for magnetic fields Hz applied along the Fe--Fe axis. The butterfly curves are reproduced using a microscopic model based on the interaction of the spins with resonant phonons. The phonon bottleneck allows for the observation of resonant quantum tunneling of the magnetization at 1.8 K, far above the blocking temperature for spin-phonon relaxation. The latter relaxation is probed by AC magnetic susceptibility experiments at various temperatures and bias fields. At H=0, no out-of-phase signal is detected, indicating that at T smaller than 1.8 K Fe2 does not behave as a single-molecule magnet. At 1 kG, relaxation is observed, occurring over the barrier of the thermally accessible S=4 first excited state that forms a combined system with the S=5 state.

Yb³⁺‐doped MnCl₂ and MnBr₂ crystals exhibit strong red upconversion luminescence under near‐infrared excitation around 10 000 cm⁻¹ at temperatures below 100 K. The broad red luminescence band is centred around 15 200 cm⁻¹ for both compounds and identified as the Mn^{2+ 4}T_1g→⁶A_1g transition. Excitation with 10 ns pulses indicates that the upconversion process consists of a sequence of ground‐state and excited‐state absorption steps. The experimental VIS/NIR photon ratio at 12 K for an excitation power of 191 mW focused on the sample with a 53 mm lens is 4.1 % for MnCl₂:Yb³⁺ and 1.2 % for MnBr₂:Yb³⁺. An upconversion mechanism based on exchange coupled Yb³⁺–Mn²⁺ ions is proposed. Similar upconversion properties have been reported for RbMnCl₃:Yb³⁺, CsMnCl₃:Yb³⁺, CsMnBr₃:Yb³⁺, RbMnBr₃:Yb³⁺, Rb₂MnCl₄:Yb³⁺. The efficiency of the upconversion process in these compounds is strongly dependent on the connectivity between the Yb³⁺ and Mn²⁺ ions. The VIS/NIR photon ratio decreases by three orders of magnitude along the series of corner‐sharing Yb³⁺‐Cl⁻‐Mn²⁺, edge‐sharing Yb³⁺‐(Cl⁻)₂‐Mn²⁺ to face‐sharing Yb³⁺‐(Br⁻)₃‐Mn²⁺ bridging geometry. This trend is discussed in terms of the dependence of the relevant super‐exchange pathways on the Yb³⁺–Mn²⁺ bridging geometry.

We report the syntheses, structures and magnetic properties of two decametallic Ni(II) clusters with unprecedented supertetrahedral cores, stabilised by the (hitherto unobserved) μ₆-coordination modes of the tris-alkoxides {MeC(CH₂O)₃}³⁻ and {C₆H₉O₃}³⁻.

We review our recent work in the field of molecular spin clusters and single‐molecule magnets, showing how inelastic neutron scattering (INS) can be used to determine magnetic exchange interactions and anisotropy splittings. A general introduction to neutron scattering precedes selected examples, building upon the first determination of exchange coupling in a transition metal complex using INS, through anisotropic exchange in cobalt(II) spin clusters to the determination of exchange interactions in a dodecanuclear nickel(II) wheel. The strength of INS for the accurate determination of anisotropy splittings in single‐molecule magnets is revealed. Not only can one determine the axial zero‐field splitting parameter D, which plays a key role in single‐molecule magnet behavior, but also higher‐order terms important in understanding the quantum tunneling behavior. Finally, we review two of our synthetic approaches towards new single‐molecule magnets based on nickel, manganese, and iron.