Beyond the active-electron approximation: Origin of ferromagnetic exchange in donor–acceptor heterospin biradicals

Abstract

The complex, ( NNSQ)CuTp Cum,Me, is comprised of three different spin carrying units. These three S = 1/2 sites are comprised of Cu(II), nitronyl nitroxide ( NN), and 5- tert-butyl-semiquinone ( SQ). Both the Cu(II)– SQ and SQ– NN pairwise interactions are strongly exchange coupled resulting an S T = 3/2 ground state due to ferromagnetic interactions. Low-temperature EPR spectroscopy confirms the S T = 3/2 ground state determined from magnetic susceptibility studies and indicates that the quartet is zero-field split by | D| ∼ 0.5 cm −1. We have probed the origin of the strong ferromagnetic exchange within the NNSQ heterospin biradical ligand by a combination of electronic absorption, and low-temperature magnetic circular dichroism spectroscopies. The origin of the strong intraligand NNSQ ferromagnetic exchange has been suggested to result from an intraligand charge transfer (ILCT) transition transition that possesses appreciable SQ π → NN π* charge transfer character. The magnitude of the ferromagnetic exchange has been related to the energy of this ILCT transition, the magnitude of the NN single site exchange integral, and the magnitude of the electronic coupling matrix element that connects the SQ SOMO with the NN LUMO.