Origin of the Magnetic Bistability in Molecule-Based Magnets: A First-Principles Bottom-Up Study of the TTTA Crystal Caroline S. Clarke, Joaquim Jornet-Somoza, Fernando Mota, Juan J. Novoa,* and Merce ` Deumal* Departament de Quı ´mica Fı ´sica & IQTCUB, Facultat de Quı ´mica, UniVersitat de Barcelona, Martı ´ i Franque `s 1, Barcelona, Spain E-08028 Received June 30, 2010; E-mail: juan.novoa@ub.edu; merce.deumal@ub.edu Abstract: The magnetic bistability present in some molecule-based magnets is investigated theoretically at the microscopic level using the purely organic system TTTA (1,3,5-trithia-2,4,6-triazapentalenyl). The TTTA crystal is selected for being one of the best-studied molecule-based systems presenting magnetic bistability. The magnetic properties of the high- and low-temperature structures (HT and LT phases, respectively) are accurately characterized by performing a First-Principles Bottom-Up study of each phase. The changes that the magnetic exchange coupling constants (J AB ) undergo when the temperature is raised (LT f HT) or lowered (HT f LT) are also fully explored in order to unravel the reasons behind the presence of these two different pathways. The triclinic LT phase is diamagnetic due to the fact that the nearly eclipsed π dimer is effectively magnetically silent and not to formation of a covalent bond between two TTTA molecules. It is also shown that bistability in TTTA results from the coexistence of the monoclinic HT and triclinic LT phases in the temperature range studied. Introduction Bistability is the ability of a material to present two stable phases that can both exist within a given range of temperatures but above and below that range only one or the other phase exists. Bistability is a key potential property for the development of new devices, notably in data storage, and light or heat sensors. 1 Despite this fact, there is no quantitative description of the microscopic mechanism responsible for this behavior. Thus, a rational design of materials presenting this property and in particular of bistable molecule-based magnets, which are our target, is not yet fully attainable. An excellent example of experimentally well-characterized bistable molecule-based magnetic materials, 2,3 and thus a prototype of these materials, is provided by crystals of the neutral radical 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA, Figure 1). The TTTA crystal is an organic system presenting magnetic interactions that is bistable at room temperature: 3,4 on heating above 320 K only the ‘high-temperature’ (HT) paramagnetic phase (monoclinic P2 1 /c) is observed, and on cooling below 210 K only the ‘low-temperature’ (LT) diamagnetic phase is present (triclinic P1 j ) (Figure 2). The only spin-carrying units in these two crystal systems are the doublet TTTA radicals. The radical electron is formally ascribed to the deprotonated N atom in the five-membered dithiazolyl ring (Figure 1a). How- ever, UB3LYP/Aug-cc-pVTZ and UHF/Aug-cc-pVTZ calcula- tions indicate that the unpaired electron is partly delocalized over both heterocyclic rings (Figures 1b and 1c and Table 1), which is confirmed by EPR studies on dilute solutions. 3,7,8 This spin delocalization, also found for similar thiazyls in other calculations 3,5,8,9 and polarized neutron diffraction experiments, 9 allows the presence of multiple magnetic exchange pathways within the crystal. TTTA radicals also show polarization in the electron distribution (Figure 1d) since N atoms hold a net negative charge and S atoms a net positive charge. As a result, a strong dipole moment (0.7929 D) is generated, which has an important influence on the way these radicals arrange when forming crystals. The crystal packing in the HT paramagnetic phase (mono- clinic P2 1 /c) and the LT diamagnetic phase (triclinic P1 j ) is similar (see Figure 3), comprising 2D layers that stack in the third dimension. In both crystal structures, two consecutive rows within the same layer (Figures 3a and 3b) are aligned in opposite directions (top row, S leads right; bottom row, S leads left) and present numerous lateral S ··· N contacts that are shorter than the sum of the S and N van der Waals radii (S ··· N ) 3.20-3.63 Å). Neighboring 2D layers pile up one on top of the other with a π-π approach of molecules (Figures 3c and 3d). In the HT structure, the stacking of these 2D layers is regular, where all (1) Kahn, O. Chem. Br. 1999, 35, 24. (2) Barclay, T. M.; Cordes, A. W.; George, N. A.; Haddon, R. C.; Itkis, M. E.; Mashuta, M. S.; Oakley, R. T.; Patenaude, G. W.; Reed, R. W.; Richardson, J. F.; Zhang, H. J. Am. Chem. Soc. 1998, 120, 352. (3) McManus, G. D.; Rawson, J. M.; Feeder, N.; van Dujin, J.; McInnes, E. J. L.; Novoa, J. J.; Burriel, R.; Palacio, F.; Oliete, P. J. Mater. Chem. 2001, 11, 1992. (4) Fujita, W.; Awaga, K. Science 1999, 286, 261. (5) Fujita, W.; Awaga, K.; Matsuzaki, H.; Okamoto, H. Phys. ReV.B 2002, 65, 064434. (6) CCDC: (a) Allen, F. H. Acta Crystallogr. 2002, B58, 380. (b) Allen, F. H.; Motherwell, W. D. S. Acta Crystallogr. 2002, B58, 407. (7) Wolmershauser, G.; Johann, R. Angew. Chem., Int. Ed. Engl. 1989, 28, 920. (8) Chung, Y.-L.; Sandall, J. P. B.; Sutcliffe, L. H.; Joly, H.; Preston, K. F.; Johann, R.; Wolmershauser, G. Magn. Reson. Chem. 1991, 29, 625. (9) Campo, J.; Luzo ´n, J.; Palacio, F.; Rawson, J. M. In Carbon-Based Magnetism; Makarova, T., Palacio., F., Eds.; Elsevier: New York, 2006; Chapter 7. Published on Web 11/24/2010 10.1021/ja1057746  2010 American Chemical Society J. AM. CHEM. SOC. 2010, 132, 17817–17830 9 17817