σ-Dimerization of organic radicals as mechanism for strongly hysteretic magneto-strutural phase transitions

In this contribution, we present a few systems of σ-dimerizing organic radicals, which provide examples of magneto-structural phase transitions that occur with a large thermal or photochemical hysteresis window. In each case, the dimerization causes drastic structural changes that underlie a substantial energy barrier for the conversion between the diamagnetic σ-dimer phase and the paramagnetic π-radical phase. We demonstrate that in the case of dithiazolyl radicals the existence of such energy barrier allows photo-generation of paramagnetic state with remarkable thermal stability. In the case of N-oxyl radicals that exist as a rotationally disordered plastic crystalline phase at room temperature and dimerize into a long-range ordered crystalline phase upon cooling, the barrier to the intra-dimer bond breaking causes a large thermal hysteresis, which exhibits high sensitivity to applied pressure. We will also discuss general requirements for observing such transitions for other organic radicals.