University of Groningen
Rationalization of the Mechanism of Bistability in Dithiazolyl-based Molecular Magnets
Francese, Tommaso
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Publication date: 2019
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Francese, T. (2019). Rationalization of the Mechanism of Bistability in Dithiazolyl-based Molecular Magnets. University of Groningen.
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Chapter 7
Outlook and Future Challenges
The aim of this theoretical and computational work herein presented is to characteri-ze and study the physical-chemical properties of two dithiazolyl-based molecular magnetic systems, namely PDTA[1, 2] and TDPDTA[3], with the intent to rationa-lize the displayed bistable character and the correlated dynamical process. The work is carried out by also accounting, for sake of comparison, the prototype TTTA mate-rial. Some interesting aspects based on the presented research may prompt further the exploration in the field, as well as the design of new DTA-based materials with selected properties. Some of these possible routes are indicated here.
The study of the key forces acting within the two experimentally resolved poly-morphs of the TTTA[4–12] prototype system, as reported in Chapter 4, can be fur-ther extended to the dynamical case. The use of the Energy Decomposition Analy-sis[13, 14] (EDA) technique gave the chance to understand which, among the differ-ent energy contributions, is the most relevant in the two polymorphs. In particular, the ⇡ interactions and the four- and six-centers S· · ·N bridges contacts between ⇡-stacks are found to be the energy contributions governing the crystal packing. The interplay between these main factors, without omitting the non-negligible influence of the other minor inter-stack interactions (see Chapter 4), is at the origin of the pro-cess that drives the spin and structural transition between the two phases (LT!HT). Thus, it may be interesting for future research, to assess, by monitoring the dyna-mical evolution in time of the systems, how the aforementioned energy contribu-tions change, specifically outlining the one responsible for the “jump” between the LT and HT phases. This would convey to the experimentalist an extremely useful parameter to tune when engineering DTA-based materials. Moreover, hypotheti-cally, by controlling the energetic terms that favor one polymorph of the other, it might be possible to control the same LT!HT transition. In the spectrum of
sibilities in which this phenomenon would find practical application, the most rel-evant would be in the data processing/storage, where the LT/HT phases alterna-tion, coupled with their respective magnetic responses, might be exploited as 0 and 1 signal, thus avoiding loss of information. There are still open issues regarding the stability[4–9] of the DTA-based systems, that depends upon the environmental conditions. Nevertheless, the DTA-based material displays promising features that require further investigation.
Additional analyses from the static perspective have been performed in Chapter 5[15] of this work. Both the LT and HT polymorphs of the PDTA and TDPDTA ma-terials were analyzed in detail, and compared to the reference TTTA and 4-NCBDTA systems. In particular, the last one does not show bistability, but only spin transi-tion (see Chapter 3). The magnetic investigatransi-tion performed by accounting for the JABvalues, computed from the Heisenberg Hamiltonian for a couple of spin centers
A and B, and the subsequent computed susceptibility curves, highlighted a mis-match between experimental and theoretical curves in the HT-PDTA, HT-TDPDTA and LT-TDPDTA. While in the HT cases the origin of the mismatch is known and ascribed to the neglect of thermal fluctuations in the static study of the magnetic in-teractions, in the LT-TDPDTA case it was not clear. The subsequent employment of magneto-structural correlation maps, that helped us to outline the key role played by the geometrical arrangement based on the lateral slippage (dSL) and interplanar
distance (dIP) between monomers to favor or not ferromagnetic (FM) interactions,
facilitated the identification of the longitudinal slippage (dLG) as additional variable
to account when dealing with the characterization of the TDPDTA material. Particu-lar interesting is the fact that, the use of two-cluster models is sufficient to depict in qualitative terms the best geometrical arrangements which can favor the presence of FM dispositions. Moreover, the energy required to reach a FM configuration can also be inferred by the corresponding map, suggesting for the actual possibility to have a valid tool to design in silico the material with desired properties and later on, in the laboratory, to synthesize it, minimizing the trials and waste disposal, which is a major cost affecting many laboratories. The construction of these maps, whose trends have been validated also at Difference Dedicated Configuration Interaction (DDCI) level (see Appendix A.4), might straightforwardly be implemented in an automatic deep-learning, or similar algorithm, for data prediction, screening huge sets of DTA-based materials for a fast and effective material design.
The fact that we discovered a complementary process to trigger spin transition might help to define not only a way to design new materials, but also to have a certain range of control over their properties, ultimately required for practical
ap-plication.
Summarizing, to employ the Pair-Exchange Dynamics (PED)[12] mechanism in a newly designed material, two degenerate states should be present, separated by an energy barrier. The interconversion between the two degenerate configurations should be temperature dependent, ultimately providing a stable average structure that, in turn, belongs to the corresponding minimum of the free energy surface. On the other hand, if in the engineering process of a DTA-based material the comple-mentary spin transition process is targeted, in order for it to feature a stable HT phase, than the energy separation between the two minima, coinciding with the dimerized (LT) and regular (HT) structures, needs to be properly taken into consid-eration.
Extra contributions to the characterization of the PDTA and TDPDTA materials, and DTA-based materials in general, might be the investigation of their properties from a spectroscopic point of view. This approach might help to outline if a specific finger-print of the two dynamical processes is present and reflected in their corresponding power spectrum, as a function of the temperature. This would eventually help to design new photo-switchable[16, 17] DTA-based molecular magnets, which, in turn, would meet a potential widespread use for technological purposes.
The work herein reported and discussed is a new starting point for further prompt-ing the study and investigation of the DTA-based materials, but also of other or-ganic molecular-based materials. The improved and refined definition of the key geometrical descriptors, thus dIP, dSLand the dLG parameters, as well as the setting
of computational tools to be employed in the validation/study of the mechanisms characterizing the DTA-based compounds, can now play a strategic role in the fu-ture development of the field.
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