Sigma‐ versus Pi‐Dimerization Modes of Triangulene

Abstract

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 . 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.

Pancake Bonding: An Unusual Pi‐Stacking Interaction

“…[82] Complex potentials urfaces can arise in multiply pancake bondeds ystems where pancake (p-bonded) dimer is found computationally to be competitive with singly and doubly s-bonded dimers as in the case of triangulenes. [83]…”

“…Fluxional behavior can be used to generate temperature dependent color and magnetism, a results to be viewed in the context of the search for “smart” π‐functional materials that are sensitive to and react to environmental stimuli . Complex potential surfaces can arise in multiply pancake bonded systems where pancake (π‐bonded) dimer is found computationally to be competitive with singly and doubly σ‐bonded dimers as in the case of triangulenes …”

Spin Delocalization, Polarization, and London Dispersion Forces Govern the Formation of Diradical Pimers

“…Examples of radicals that form sigma [13][14][15][16][17] and pi dimers. 16,[18][19][20][21] Understanding this divergent dimerization behavior would aid in the application of these radicals into useful materials, because the properties of the two types of radical dimers show remarkable differences. For example, sigma dimers usually absorb mostly in the UV region of the optical spectrum and have properties that are more consistent with "normal" closed-shell organic molecules.…”

Platforms for Stable Carbon‐Centered Radicals

“…[66] In recent years,M orita and co-workers have extensively investigated various derivatives of trioxotriangulenes and their application to battery materials. [21,67] 45R exhibits drastically altered spin distribution with the largest spin density at the central carbon atom. Notably, 45R showed excellent thermal stability up to 240 8 8Cinthe air without any bulky groups,and their half-lives were more than weeks.T he dicyanomethylene-substituted analogues 46 a,b…”