Theory of intermolecular exchange in coupled spin- <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mfrac><mml:mn>1</mml:mn><mml:mn>2</mml:mn></mml:mfrac></mml:math> nanographenes

Open-shell nanographenes can be covalently bonded and still preserve their local moments, forming interacting spin lattices. In the case of benzenoid nanographenes, Ovchinnikov-Lieb rules anticipate ground state superstructure thereby sign intermolecular exchange. Here we address underlying microscopic mechanisms for exchange in this type system. We find that, general, three different contribute. First, Hund's ferromagnetic that promotes interactions electrons overlapping orbitals. Second, superexchange driven by hybridization, identical to Anderson kinetic exchange, which is a decreasing function Hubbard-$U$ energy scale always antiferromagnetic. Third, Coulomb-driven superexchange, increases as $U$ involves virtual excitation excited molecular orbitals are extended over entire structure. either ferro- or antiferromagnetic, accounting rules. compute these energies coupled $S=1/2$ phenalenyl triangulenes, using multiconfigurational methods both with Hubbard models, addressing influence long-range Coulomb on interactions.