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 (D2d 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–Cα)–π* hyperconjugation has a negative effect on spin–spin coupling.