Donor-Acceptor Contributions to Ferromagnetic Exchange Coupling within Heterospin Biradicals

Abstract

The evaluation of new Donor-Acceptor heterospin biradicals is carried out within this thesis: SQ-Gal, SQ-Ph-Gal, SQ-E-NN, and SQ-TNN. A molecular orbital and spin density analysis is performed in order to predict the exchange coupling within these biradicals. A Valence Bond Configuration Interaction model is used to describe exchange coupling in a Donor-Acceptor-type Semiquinone-NitronylNitroxide (SQNN) biradical ligand. Within this framework, a SQ → NN charge transfer state is responsible for the experimentally determined ferromagnetic coupling in the ground state, and the intraligand exchange parameter is correlated with the single-site Coulomb repulsion integral (U), the singlet-triplet gap (2K) of the CT excited state, and the electronic coupling matrix element, Hab. This charge transfer transition is observed spectroscopically, and probed using resonance Raman spectroscopy. Importantly, if the crystal structure of the Donor-Acceptor biradical is known, this Hab can be directly linked to the molecular conformation determined by X-ray crystallography. We suggest how our methods could be extended to any bridge fragment for SQ-Bride-NN complexes. This model is useful for designing new strongly exchange coupled systems, for using electronic structure theory to study exchange in organic systems, and for correlating exchange coupling parameters with elements of electron transfer theory.