Sigma‐ versus Pi‐Dimerization Modes of Triangulene

We show that the diradicaloid triangulene, a graphene nano‐flake molecule, can aggregate in a variety of dimerization modes. We found by density functional theory modeling a number of triangulene dimers including six doubly bonded σ‐dimers in addition to the previously reported six pancake bonded π‐dimer isomers. The σ‐dimers display a wide range of stabilities: the interaction energy of the most stable σ‐dimer is −25.17 kcal mol−1. Besides the doubly bonded σ‐dimers with closed shell ground states, we also found an open‐shell singly σ‐bonded diradicaloid dimer. We found an interesting isomerization route between a doubly bonded σ‐dimer, a singly bonded σ‐dimer with a low‐lying triplet state and two π‐bonded dimers with low‐lying quintet states. Derivatives of triangulene, trioxo‐triangulenes (TOTs) have been previously characterized experimentally. Here, we show the reasons why so far only the π‐dimer but not the σ‐dimer was experimentally observed for all TOTs.