Session E31: Design and Control of Molecular Magnets (QIS2)

The power of molecular chemistry in nanoscale materials research: from quantum physics properties to water oxidation catalysis

Molecular chemistry brings many powerful advantages to the study of nanoscale materials of various kinds, and this area of ‘molecular nanoscience’ is therefore a rapidly growing field. The advantages include monodisperse (single-size) products and a monolayer shell of organic ligands that imparts solubility and crystallinity, allowing structural characterization of molecular crystals to atomic resolution by X-ray crystallography. The ligands can usually also be modified as desired, allowing tuning of redox properties and atom/isotope labelling (²H, ¹⁹F, etc.) for studies in the solid state and solution, such as NMR spectroscopy. In the molecular nanomagnetism arena, these advantages have been absolutely crucial in the study of single-molecule magnets (SMMs), molecules that function as individual ultra-small nanomagnets. They have greatly assisted the synthesis and study of numerous SMMs, leading to discovery of new quantum physics phenomena important to new 21^st century technologies, such as exchange-biased quantum tunneling of the magnetization vector and quantum superposition states. Giant (~4 nm) SMMs have also bridged the gap between the ‘top-down’ world of traditional magnetic nanoparticles and the ‘bottom-up’ world of molecular nanomagnets. Recently we have developed controlled ways to form supramolecular [Mn₃]_n oligomers of 2 or more linked Mn₃ SMMs to study the resulting quantum properties, introduced by the weak inter-SMM exchange coupling, in more detail, including in solution for the first time. Some of our larger magnetic molecules, such as [Mn₁₂O₁₂(O₂CR)₁₆(H₂O)₄] and others, can be described as a small piece of metal oxide within an organic shell, and recent work has established that they can also function as homogeneous electrocatalysts for water oxidation to O₂ gas with low overpotentials. A selection of these materials and studies will be described.   

Controlling Anisotropy in the Presence of Magnetic Coupling in Molecular Magnets

The control of magnetic anisotropy and magnetic coupling are crucial to the design of devices utilizing molecular materials for both spintronic and quantum information processing. Generally, theoretical models and synthetic approaches exist for optimizing these parameters independently to a very high degree. However, design principles for either orthogonal or synergistic, predictable control of anisotropy and coupling are far more complex. This presentation will focus on our efforts to address the challenge of designing molecules that can effectively preserve the orientation and magnitude of their anisotropy upon introducing coupling interactions that necessarily alters the local crystal field.   

Magnetic properties of actinide complexes from first principle calculations.

Magnetic properties of actinides are often analyzed using models devoted to lanthanides. But due to the higher covalency of the former, those models are not adequate and a more precise description of the complexes is necessary in order to get the full understanding of those properties. The calculation of the magnetic properties of open-shell 5f molecules is a challenge for the methods of quantum chemistry: actinide complexes have many low lying configurations, spin-orbit effects are important and correlation effects must be taken into account. The first principle SO-CASPT2 method gives results that compare well to experimental data and permits to analyze the different contributions to the magnetic properties.

On monomers, calculations enable to
- determine crystal field parameters
- analyze the effect of covalency
- scrutinize the contributions above the LS coupling scheme.

On dimers, they permit to
- analyze the magnetic coupling between the two magnetic centers
- propose Spin Hamiltonians which permit to model the magnetic behavior of those complexes   

Theoretical Investigation of Actinides Based Single Molecule Magnets

Single molecule magnets (SMMs) are highly appealing for reducing the length scale of magnetic materials that have potential applications in information storage and spin electronics.¹ SMMs show slow relaxation of the magnetization of purely molecular origin. The actinides elements are promising for the design of SMMs and examples have emerged.²
In this contribution, the electronic structure of plutonium based SMMs³ have been computationally examined by means of multiconfigurational calculations. A modified ligand has also been studied in order to modify the electronic properties.⁴ We are currently investigating other actinide-based SMMs, namely [Np(COT)₂]⁵ and AmFe₂.⁶ We are also analyzing the inclusion of dynamical correlation by means of multiconfiguration pair-density functional theory (MC-PDFT),⁷ a new method developed in our group. These results will be discussed.

[1] Gatteschi, D. et al. Oxford Univ Press: New York, 2006; p 1-395.[2] Meihaus, K. R.; Long, J. R. Dalton Trans. 2015, 44, 2517.[3] Magnani, N. et al. Chem. Commun. 2014, 50, 8171.[4] Gaggioli, C. A.; Gagliardi L. Inorg. Chem. 2018, 57, 8098.[5] Magnani, N. et al. Angew. Chem. Int. Ed. 2011, 50, 1696.[6] Magnani, N. et al. Phys. Rev. Lett. 2015, 114, 5. [7] Gagliardi, L. et al. Acc. Chem. Res. 2016, 50, 66.   

Self-interaction effects in the density functional description of iron(II) spin-crossover molecules

Spin-crossover metal complexes are one of the paradigmatic examples of magnetic molecular materials showing switching and bistability at the molecular level. Octahedral Fe2+ molecules are particularly interesting as they often exhibit such a spin-crossover transition. Many efforts were made to assess the performance of density functional theory for such systems. However an exchange-correlation functional able to account accurately for the energetic of the various possible spin-states has not been identified yet.
We present results on different Fe2+ ions using self-interaction corrected semilocal exchange-correlation functionals [1,2,3] within the FLO-SIC DFT framework and we focus on the energy differences between the various spin states. Further we compare our data to high level quantum chemical calculations.


[1] M. R. Pederson, A. Ruzsinszky, and J. P. Perdew, J. Chem. Phys. 140, 121103 (2014).
[2] M. R. Pederson, J. Chem. Phys. 142, 064112 (2015).
[3] T. Hahn, S. Liebing, J. Kortus, and M. R. Pederson, J. Chem. Phys. 43, 224104 (2015).   

Molecular Geometries of Fe (II) Spin-Crossover Complexes

Spin-crossover (SCO) complexes are a class of inorganic compounds in which external stimuli facilitate a change in geometry and spin state from a diamagnetic low spin state to a paramagnetic high spin state or vice versa. Due to this bistability, they are an important class of molecules for new energy technologies and molecular electronics¹. The multiconfigurational nature and large number of electrons of these molecules make it difficult to provide accurate theoretical descriptions of the electronic structure. To date, density functional theory (DFT) has been the foundation for the study of these complexes’ geometries². Recent advances in computing has made it now possible to produce geometries with methods beyond DFT.
Octahedral Fe(II) (d⁶) molecules are among the most well studied of this class of compound. Here, an accurate description of the multiconfigurational electronic structure of several Fe(II) SCO complexes using the CASPT2 method is presented. The convergence of geometry with respect to active space and basis set size as well as differences with the traditional DFT approach and the refined CASPT2 structures are noted.
1. P. Gutlich, H. A. Goodwin (2004) Spin Crossover in Transition Metal Compounds I. Springer Berlin
2. K. P. Kepp, Inorganic Chemistry 55, 2717 (2016).   

Coupled-Cluster Valence Bond Theory for Exact Treatment of Spin-Fluctuation

We present a valence bond approach for treating spin-fluctuation called coupled-cluster valence bond (CCVB). In particular, we will discuss the three-pair extension of CCVB, CCVB+i3, and its application to spin-frustrated systems such as molecular magnets. CCVB+i3 can describe spin-fluctuation exactly without invoking neither an exponential wall nor spin contamination. We also discuss a future direction on including charge-fluctuation into this model.   

Molecular Magnetism and Quantum Information

Quantum Information Science and instrumentation for next-generation computing, information, and other fields is evolving quickly into an interdisciplinary field that intersects strongly with the missions, interests and portfolios of basic energy sciences.The fundamental research in chemical physics, quantum chemistry, inorganic chemistry, and magnetism is strongly synergistic with applications such as sensing, navigation, communications, simulation and computing. Researchers in these fields must play a role in enabling and understanding manipulation of uniquely quantum phenomena that arise from the entanglement of electrons, spins, and low-energy excitations in actual molecular systems. This talk will summarize emerging possibilities for chemical sciences based upon DOE Basic-Energy-Sciences rountable reports. It will focus on activities targeting the fundamental design, manipulation, or addressing of molecular-scale quantum-chemical qubits, the use of early quantum processors for chemical simulation, and synergistic prospects for the near-term quantum- systems and algorithms research supported by BES. To address the aims of this focused session, specific contact to the chemistry, magnetism, and optical response of molecular magnets and organic poly-radicals will be featured.