The origin of the bistability in the thiazyl radical 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA): A first principles bottom-up investigation of the magnetic properties of its high temperature polymorph

Abstract

A first principles bottom-up study is performed on the high temperature polymorph of the thiazyl radical 1,3,5-trithia-2,4,6-triazapentalenyl, TTTA, a purely organic molecule-based magnet that shows thermal bistability, with the aim of getting a quantitative understanding at the molecular level the mechanism of thermal bistability and the factors that control the existence of a hysteresis loop. Experimental studies on crystals of TTTA have shown that it is a thermally bistable material that exhibits a broad hysteresis loop encompassing room temperature (the high temperature phase is paramagnetic, while the low temperature phase is magnetically silent). Both structures present 1D π-stacks and the changes in the magnetic properties up to now have been solely attributed to changes within these stacks, which in the high temperature phase are composed of evenly-spaced radicals and in the low temperature phase the radicals are separated by alternately long and short spacings. Our study shows that the magnetic topology of the high temperature polymorph is clearly 3D with important interactions between the stacks, however a 1D magnetic model gives a qualitative reproduction of the macroscopic magnetic properties. This suggests that a proper investigation into the origin of the bistability of this material requires consideration of the inter-stack interactions as well as looking at the difference in packing within the stacks.