Topologically Frustrated Bonding in [2]Triangulene Chains

Carbon nanomaterials offer advantages for advanced computing devices such as atomically precise design and low spin-orbit. In this work, the smallest open-shell alternant polyaromatic hydrocarbon, phenalenyl, was chosen to target a 1-dimensional all-carbon spin-1/2 chain. A dibromo-phenalene monomer was synthesized and sublimed onto and polymerized on a Au (111) surface under ultra-high vacuum conditions and characterized using low temperature scanning tunneling microscopy. A lone phenalenyl radical (n = 1 system) on the surface exhibits Kondo resonance expected from an open-shell impurity. However, dimer structures (n = 2 system) appear to hybridize neighboring non-bonding modes, opening a gap around the fermi level (E_F). The magnitude of the opened gap is lower than expected for hybridized molecular orbitals, at ~100 meV. Further, the phenalenyl dimer has a non-Kekulé resonance structure in which the two radicals cannot be paired into a π-bond, a property known as topological frustration. This behavior is rationalized using a one-site tight-binding model with effective hopping parameter (τ). Including the 3^rd-nearest-neighbor coupling in the atomistic model is the minimum level of theory necessary to capture the observed bonding behavior. The bonding interactions occur exclusively by virtue of the 3^rd-nearest-neighbor coupling between molecular units connected solely by a σ-bond between atoms with no density of the molecular orbitals comprising the interaction.