Insights into the magnetism and phase transitions of organic radical-based materials

Organic radicals have been consistently regarded as promising building blocks for the next generation of applied materials. Multiple radical families have been developed and characterized in the last decades, fostered by the ever-growing capabilities of organic synthesis. Thiazyl-, spiro-biphenalenyl-, 1,2,4-benzotriazinyl-, and nitroxide-based radicals have furnished striking examples of metal-free switchable materials, whose phase transitions are accompanied by changes in the magnetic, optical and/or electrical response. While similar in origin, their actual mechanism, driving force(s), and spin state stabilities often depict a different landscape. Fundamental knowledge on such aspects, as well as on the underlying network of spin exchange couplings and non-covalent interactions (including pancake bonding), are key to understand their spin transition, and the tailored modification of their properties. These complex features cannot be extracted based solely on experimental input, but rely on a computational interpretation. In this Perspective article, we discuss the insight gained from computational modelling into the magnetism and phase transitions of organic radical-based materials. We focus on the key importance of dynamic effects due to the labile nature of π-stack interactions assembling those materials, the structural distortions driven by spin changes, the coupling between electronic structure and order-disorder transitions, and the dependence of spin correlation upon temperature. All these phenomena uncovered by simulations should assist in the rational design of new dynamic organic crystals