DOI: 10.1002/chem.201100581 Frequency-Domain Fourier-Transform Terahertz Spectroscopy of the Single- Molecule Magnet (NEt 4 )ACHTUNGTRENNUNG[Mn 2 (5-Brsalen) 2 ACHTUNGTRENNUNG(MeOH) 2 Cr(CN) 6 ]** Jan Dreiser,* [a, b] Alexander Schnegg, [c] Karsten Holldack, [d] Kasper S. Pedersen, [e] Magnus Schau-Magnussen, [e] Joscha Nehrkorn, [a] Philip Tregenna-Piggott, [f] Hannu Mutka, [g] Høgni Weihe, [e] Jesper Bendix, [e] and Oliver Waldmann* [a] In memory of Philip Tregenna-Piggott Introduction Single-molecule magnets (SMMs) are exchange-coupled mo- lecular spin clusters that exhibit slow relaxation of magneti- zation. [1] Intriguing quantum effects, such as tunneling of the magnetization or quantum phase interference, have been observed in SMMs. [2] In the quest to create SMMs with ever longer magnetic relaxation times, many efforts have been undertaken to raise the barrier for magnetization reversal. To date values of up to 170 K have been realized. [3] Most SMMs are based on 3d transition-metal ions only. Recently, however, approaches have been made to introduce 4d, 5d, or lanthanide and actinide ions as magnetic centers. [4] Detailed information about ground-state and low-energy excitations is conveniently obtained from inelastic neutron scattering (INS) or electron paramagnetic resonance (EPR). In the case of spin values being sufficiently well defined, the selection rules pertinent to INS read DS = 0,  1 and DM S = 0,  1, which allows for both inter- and intramultiplet transi- tions. On the other hand, the selection rules in EPR, DS = 0 and DM S =  1, allow for intramultiplet transitions only. As a consequence of the fewer possible transitions in EPR as compared to INS, EPR data can strongly support the inter- pretation of complicated INS data: comparing EPR and INS spectra provides a means to infer the type of a transition being intramultiplet (DS = 0) or intermultiplet (DS ¼6 0). In addition, EPR delivers a much higher spectral resolution than INS. Conventional EPR is run in applied magnetic fields, whereas INS is a frequency-domain technique, and so magnetic parameters deduced by these techniques frequent- ly differ. [5, 6] To avoid difficulties, EPR spectra should hence also be taken without external magnetic field (e.g., using frequency-sweep techniques (such as frequency-domain magnetic resonance spectroscopy, or FDMRS), [7] time- domain spectroscopy (TDS), [8] or frequency-domain Fourier- [a] Dr. J. Dreiser, J. Nehrkorn, Prof. O. Waldmann Physikalisches Institut, Universität Freiburg 79104 Freiburg (Germany) Fax: (+ 49) 7612035707 E-mail : oliver.waldmann@physik.uni-freiburg.de [b] Dr. J. Dreiser Swiss Light Source, Paul Scherrer Institut 5232 Villigen PSI (Switzerland) Fax: (+ 41) 563105292 E-mail : jan.dreiser@psi.ch [c] Dr. A. Schnegg Helmholtz-Zentrum für Materialien und Energie Institut für Silizium-Photovoltaik, 12489 Berlin (Germany) [d] Dr. K. Holldack Helmholtz-Zentrum für Materialien und Energie Institut für Methoden und Instrumente der Forschung mit Synchrotronstrahlung, 12489 Berlin (Germany) [e] K.S. Pedersen, M. Schau-Magnussen, Dr. H. Weihe, Prof. J. Bendix Department of Chemistry, University of Copenhagen 2100 Copenhagen (Denmark) [f] Dr. P. Tregenna-Piggott Laboratory for Neutron Scattering, Paul Scherrer Institut 5232 Villigen PSI (Switzerland) [g] Dr. H. Mutka Institut Laue-Langevin, 38042 Grenoble Cedex 9 (France) [**] Brsalen = N,N’-ethylenebis(5-bromosalicylidene)iminato anion. Abstract: We have investigated the novel single-molecule magnet (NEt 4 )- ACHTUNGTRENNUNG[Mn 2 (5-Brsalen) 2 ACHTUNGTRENNUNG(MeOH) 2 Cr(CN) 6 ](1; 5-Brsalen = N,N’-ethylenebis(5-bromo- salicylidene)iminato anion) using spec- troscopic as well as magnetization and susceptibility measurements. Frequen- cy-domain Fourier-transform terahertz electron paramagnetic resonance (FDFT THz-EPR) based on the gener- ation of THz radiation from a synchro- tron in combination with inelastic neu- tron scattering (INS) allows for the dis- crimination between intermultiplet and intramultiplet transitions. Together with ac/dc magnetic susceptibility measurements the obtained set of data provides a complete characterization of the lowest energetic magnetic excita- tions. We find that the new compound 1 exhibits much weaker intermolecular interactions than found in the closely related compound: KACHTUNGTRENNUNG[Mn 2 (5-Brsalen) 2 - ACHTUNGTRENNUNG(H 2 O) 2 Cr(CN) 6 ](2). Furthermore, two phonon lines in the vicinity of the mag- netic excitations are detected. Keywords: chromium · EPR spec- troscopy · inelastic neutron scatter- ing · magnetic properties · manga- nese  2011 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim Chem. Eur. J. 2011, 17, 7492 – 7498 7492