Thermally and Magnetically Robust Triplet Ground State Diradical

High spin (S = 1) organic diradicals may offer
enhanced properties with respect to several emerging technologies,
but typically exhibit low singlet triplet energy gaps and possess
limited thermal stability. We report triplet ground state diradical 2 with a large singlet–triplet energy gap, ΔEST ≥ 1.7 kcal mol–1, leading to nearly exclusive population of triplet ground state
at room temperature, and good thermal stability with onset of decomposition
at ∼160 °C under inert atmosphere. Magnetic properties
of 2 and the previously prepared diradical 1 are characterized by SQUID magnetometry of polycrystalline powders,
in polystyrene glass, and in other matrices. Polycrystalline diradical 2 forms a novel one-dimensional (1D) spin-1 (S = 1) chain of organic radicals with intrachain antiferromagnetic
coupling of J′/k = −14
K, which is associated with the N···N and N···O
intermolecular contacts. The intrachain antiferromagnetic coupling
in 2 is by far strongest among all studied 1D S = 1 chains of organic radicals, which also makes 1D S = 1 chains of 2 most isotropic, and therefore
an excellent system for studies of low-dimensional magnetism. In polystyrene
glass and in frozen benzene or dibutyl phthalate solution, both 1 and 2 are monomeric. Diradical 2 is thermally robust and is evaporated under ultrahigh vacuum to
form thin films of intact diradicals on silicon substrate, as demonstrated
by X-ray photoelectron spectroscopy. Based on C–K NEXAFS spectra
and AFM images of the ∼1.5 nm thick films, the diradical molecules
form islands on the substrate with molecules stacked approximately
along the crystallographic a-axis. The films are
stable under ultrahigh vacuum for at least 60 h but show signs of
decomposition when exposed to ambient conditions for 7 h.