Conformational\nTuning of Magnetic Interactions in\nCoupled Nanographenes

Phenalenyl (C<sub>13</sub>H<sub>9</sub>) is an open-shell\nspin-1/2\nnanographene. Using scanning tunneling microscopy (STM) inelastic\nelectron tunneling spectroscopy (IETS), covalently bonded phenalenyl\ndimers have been shown to feature conductance steps associated with\nsinglet–triplet excitations of a spin-1/2 dimer with antiferromagnetic\nexchange. Here, we address the possibility of tuning the magnitude\nof the exchange interactions by varying the dihedral angle between\nthe two molecules within a dimer. Theoretical methods ranging from\ndensity functional theory calculations to many-body model Hamiltonians\nsolved within different levels of approximation are used to explain\nSTM-IETS measurements of phenalenyl dimers on a hexagonal boron nitride\n(h-BN)/Rh(111) surface, which exhibit signatures of twisting. By means\nof first-principles calculations, we also propose strategies to induce\nsizable twist angles in surface-adsorbed phenalenyl dimers via functional\ngroups, including a photoswitchable scheme. This work paves the way\ntoward tuning magnetic couplings in carbon-based spin chains and two-dimensional\nlattices.