Paramagnetic diradical switches based on polyacene systems

Abstract

Interrelations between the molecular structure and spin-transition energetics were revealed based on analysis of the literature data and on the results of systematic quantum chemical study of the effect of the size of the acene backbone and the nature of radical substituents on the electronic and magnetic properties of acene derivatives. The mechanism underlying switching of magnetic properties, which is based on the reversible transformation of a diamagnetic folded structure into a biradical form with planar acene backbone, was investigated. The key factors necessary for realization of spin transitions include localization of the spin density of radicals on the atoms bonded to the acene molecule and the presence of an energy barrier preventing the return to the diamagnetic state, which is provided by bulky radical groups. A decrease in the aromaticity of the central six-membered ring with increasing size of the acene backbone leads to destabilization of the high-spin states and makes spin transitions energetically unfavorable. The results obtained agree with the experimental data on organic diradicals and offer prospects for prediction of the spin-switchable properties in new compounds.