Reverse Spin-Transition-Like Behavior in a Single-Molecule Organic Diradical

A spin transition in the organic diradical PyPBN is associated with a structural phase transition around 330 K, producing thermochromic and thermomagnetic responses, including a pronounced decrease in χ_mT upon heating that resembles a reverse spin-transition phenomenon. The exchange coupling parameters were experimentally determined as 2J/k_B ≫ +300 K and +121(4) K for the α and β phases, respectively, and estimated by DFT calculations to be +1448 K and +79 K. These results indicate drastic quenching of ferromagnetic coupling in the high-temperature phase. Crystallographic analysis reveals that the two phases differ primarily in the nitroxide-benzene torsion. Increased torsion suppresses π-delocalization, thereby destabilizing the triplet state. The resulting enthalpic penalty is partially compensated by the spin-entropy gain associated with the phase transition. A triplet state competes with two independent doublet spins (each S = 1/2), and the system possesses an intrinsic spin-entropy difference of R ln(4/3).