Bistability in Organic Magnetic Materials: A Comparative Study of the Key Differences between Hysteretic and Non-hysteretic Spin Transitions in Dithiazolyl Radicals

Dithiazolyl (DTA)- based radicals have furnished many examples of organic spin- transition materials, some of them occurring with hysteresis and some others without. Herein, we present a combined computational and experimental study aimed at deciphering the factors controlling the existence or absence of hysteresis by comparing the phase transitions of 4- cyanobenzo- 1,3,2- dithiazolyl and 1,3,5trithia- 2,4,6- triazapentalenyl radicals, which are prototypical examples of non- bistable and bistable spin transitions, respectively. Both materials present low- temperature diamagnetic and high- temperature paramagnetic structures, characterized by dimerized (center dot center dot center dot A-A center dot center dot center dot A-A center dot center dot center dot) n and regular (center dot center dot center dot A center dot center dot center dot A center dot center dot center dot A center dot center dot center dot A center dot center dot center dot) n pi- stacks of radicals, respectively. We show that the regular pi-stacks are not potential energy minima but average structures arising from a dynamic inter-conversion between two degenerate dimerized configurations: (center dot center dot center dot A-A center dot center dot center dot A-A center dot center dot center dot)n <->(-A center dot center dot center dot A-A center dot center dot center dot A-)n. The emergence of this intrastack dynamics upon heating gives rise to a second-order phase transition that is responsible for the change in the dominant magnetic interactions of the system. This suggests that the promotion of a (center dot center dot center dot A-A center dot center dot center dot A-A center dot center dot center dot)n <->(-A center dot center dot center dot A-A center dot center dot center dot A-) n dynamics is a general mechanism for triggering spin transitions in DTA-based materials. Yet, this intra-stack dynamics does not suffice to generate bistability, which also requires a rearrangement of the intermolecular bonds between the pstacks via a first-order phase transition.