Aminyl tetraradicals with planar tetraazanonacene backbones have quintet (S = 2) ground states and do not show any detectable thermal population of the low-spin excited states up to the highest temperature investigated (100 K) in the 2-methyltetrahydrofuran (2-MeTHF) matrix. This indicates that the nearest electronic excited state (triplet) is at least ∼0.3 kcal mol-1 higher in energy, that is, the triplet-quintet energy gap, ΔETQ > 0.3 kcal mol-1, which is consistent with the broken-symmetry-DFT-computed ΔETQ of about 5 kcal mol -1. In concentrated (ca. 1-10 mM) solutions of tetraradical 4 in 2-MeTHF at 133 K, a fraction of tetraradicals form a dimer (association constant, Kassoc ≈ 60 M-1), with a weak, antiferromagnetic exchange coupling, J/k ≈ -0.1 K ∼ 0.2 cal mol -1, between the S = 2 tetraradicals. This weak intradimer exchange coupling is expected for two tetraradicals at the distance of about 6 Å. The most sterically shielded tetraradical 5 in 2-MeTHF has a half-life of 1 h at room temperature; the product of its decay is the corresponding tetraamine, suggesting that the hydrogen atom abstraction from the solvent is primarily responsible for the decomposition of the tetraradical.
ASJC Scopus subject areas
- Colloid and Surface Chemistry