The electron paramagnetic resonance of polyradicals

The chemical synthesis and the characterisation of the organic polyradicals build up a representative part of the research in the field of magnetic molecular materials. For such single molecule units bearing two or more radical spins submitted to magnetic exchange interaction, the knowledge of the spin multiplicity of the magnetic ground state is of crucial interest. The various ways for the EPR techniques, e.g. CW-EPR, FT-EPR (Transient Spin Nutation), and ENDOR, to answer this question and others are reviewed. The basic contributions to the EPR response of an organic polyradical are summarised. Some results have been selected in the literature as illustrating examples, from the primary example of the biradical up to the (semi)infinite systems (dendrimers, polymers) through the oligoradical molecules (triradicals, tetraradicals,...).