Transition metal complexes of donor–acceptor biradicals

The nature of the electronic coupling in transition metal Donor–Acceptor and Donor–Bridge–Acceptor systems has become a subject of interest in the last decade due to the importance of electronic coupling in molecular electronics. Here we review the synthesis, structure, magnetism, and spectroscopy of donor–acceptor biradicals as they pertain to understanding the electronic origins and magnitude of the electronic coupling matrix element that figures prominently in electron transfer (transport). Stable D–A systems may be thought of as ground state analogues of charge separated states generated in photoinduced electron transfer processes and as model systems for understanding single molecule mediated electron transport between biased nanoelectrodes. The strong coupling (adiabatic) regime will be covered in this review as well as the methodology for probing the magnitude of the electronic coupling in this limit. D–A biradicals can facilitate long-range ferromagnetic exchange between localized spins mediated by delocalized electrons, and this is a new phenomenon in molecular systems. The use of the VBCI approach to develop a complete electronic structure description of strong electronic coupling in D–A and D–B–A biradicals, the origin of long-range biradical ferromagnetic exchange, and the relationship of these biradicals to molecular electronic materials will form the basis of the review.

► Molecular, conformational and electronic structure of biradical ligands presented.
► Valence Bond Configuration Interaction (VBCI) model presented.
► VBCI model used to relate magnetic exchange to electronic coupling.
► Bridge effects on electronic coupling of DBA biradical ligands reviewed.
► Mixed-valent DBA paramagnetic ligands promote long-range electron correlation.