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Re-thinking TTTA magnetism : moving from a static to a dynamical computational perspective Mercè Deumal Solé, Universitat de Barcelona (Barcelone, Espagne)

Mercè Deumal Solé, Universitat de Barcelona (Barcelone, Espagne)

Salle de séminaire IRSAMC - Jeudi 9 Avril, 14h - 15h

The neutral radical 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA) is a prototype of molecule-based bistable materials. TTTA crystals undergo a first-order phase transition between their low-temperature (LT) diamagnetic and high-temperature (HT) paramagnetic phases, with a large hysteresis loop that encompasses room temperature.

The standard approach for both the interpretation and simulation of magnetic properties of materials is based on a static perspective, where it is assumed that these properties can be readily obtained with a single static configuration (usually an X-ray resolved structure or, alternatively, an optimized structure). However, in a recent work,[1] we have revealed that the HT phase of TTTA is the result of a dynamical conversion between two dimerized structures, while the LT is a conventional ordered crystal. This discovery poses a most intriguing question : which is the impact of the large-amplitude fluctuations accompanying the pair-exchange dynamics of HT on the physical properties of the TTTA material ? This is a crucial issue because one could intuitively think that, if the HT phase is a dynamical conversion between two dimerized structures and a dimerized structure is not magnetic, there is no reason for the HT phase to behave as a paramagnet. Besides addressing this issue, it will also be shown that the magnetic properties of TTTA cannot be properly understood without taking the thermal fluctuations into consideration.

By means of the combined use of ab initio molecular dynamics and extensive DFT calculations, we have included the effect of the vibrorotational degrees of freedom on the evolution of the magnetic exchange interactions along the fast pair-exchange dynamics of TTTA, in what we call a ‘dynamic perspective’.[2] By doing so, we have uncovered the key concept to obtain a quantitative description of its magnetic properties, that remained elusive up to know despite the numerous efforts devoted to it.[3-6] In view of our results, we question the validity of the common calculation of magnetic exchange couplings from ‘static’ X-ray structures when dealing with labile materials such as TTTA. Indeed, the physical meaning of the JAB values in those cases has not yet percolated into the field of molecule-based materials.

[1] Vela, S. ; Mota, F. ; Deumal, M. ; Suizu, R. ; Shuku, Y. ; Mizuno, A. ; Awaga, K. ; Shiga, M. ; Novoa, J.J. ; Ribas-Arino, J. Nat. Commun. 5, 4411 (2014).

[2] Vela, S. ; Deumal, M. ; Shiga, M. ; Novoa, J.J. ; Ribas-Arino, J. Chem. Sci. 6, 2371 (2015).

[3] Furuya, M., Kawazoe, Y., Ohno, K. Sci. Tech. Adv. Mater. 5, 689 (2004).

[4] Ohno, K., Noguchi, Y., Yokoi, T., Ishii, S., Takeda, J., Furuya, M. ChemPhysChem 7, 1820 (2006).

[5] Clarke, C.S., Jornet-Somoza, J., Mota, F., Novoa, J.J., Deumal, M. J. Am. Chem. Soc. 132, 17817 (2010).

[6] Domingo, A., Vérot, M., Mota, F., de Graaf, C., Novoa, J. J., Robert, V. Phys. Chem. Chem. Phys. 15, 6982 (2013).