Spiro-centre substitution effects in the intramolecular spin–spin interactions of spirobiacridine diradicals

Abstract

Spirobiacridine diradicals exhibit intramolecular ferromagnetic coupling owing to their unique cruciform structure (D_2d molecular symmetry). In this study, novel spirodiradicals 2,2′,7,7′-tetra(tert-butyl)-9,9′(10H,10′H)-spirobisilaacridine-10,10′-dioxyl (1-Si) and a Ge-centred derivative (1-Ge) were synthesized. These compounds exhibited a ground triplet spin state (S = 1), which was confirmed by spectroscopic and magnetic studies. Interestingly, the exchange coupling constant of 1-Ge (+19.3(2) K) was larger than that of 1-Si (+14.2(3) K), although the distance between intramolecular radical centres was increased owing to its larger bonding lengths. This result implies that σ*(Si–C_α)–π* hyperconjugation has a negative effect on spin–spin coupling.