Radical site-dependent exchange interactions in acridane-based bisnitroxides

Abstract

To investigate the radical site-dependence of exchange coupling in bisnitroxides, we have synthesized novel three acridane-based diradicals, tert-butyl nitroxide groups at the 2,7-, 2,6- and 3,6-positions of 10-benzyl-10′-methyl-9,9′(10H,10′H)spirobiacridine (1-pp, 1-mp and 1-mm, respectively). Theoretical and experimental magnetic studies indicate that 1-pp and 1-mm exhibit a singlet ground state, while 1-mp exhibits a triplet ground state. These spin states can be explained by the spin polarization mechanism. Quantitative analysis revealed a difference in the magnitude of the exchange coupling constants between 1-pp and 1-mm owing to their local spin structures corresponding to 1,4- and 1,3-phenylene diradical motifs (Kekulé and non-Kekulé structures, respectively).