Pancake Bond Orders of a Series of π‐Stacked Triangulene Radicals

Abstract

Conjugated radicals are capable of forming π-stacking "pancake-bonded" dimers. Members of the family of triangulene hydrocarbons, non-Kekulé neutral multiradicals, can utilize more than one singly occupied molecular orbital (SOMO) to form multiple pancake-bonded dimers with formal bond orders of up to five. The resulting dimer binding energies can be quite high and the intermolecular contacts rather small compared to the respective van der Waals values. The preferred configurations are driven by the large stabilization energy of overlapping SOMOs.

Pancake Bonding in π‐Stacked Trimers in a Salt of Tetrachloroquinone Anion

“…Also, a( 3, + 3) local minimum exists between the rings within at rimer,i ndicating ac age-like electronic structure. [12a] In this case, however,w eh ave two electronss hared by three rings, rather than just two, thust he formal bond order [15] of these pancake bonds is about 0.707, which will be discussed below (they extend across 3r ings, whereas those in radical dimers are shared between two rings). [12] Crystals of the studied compound are diamagnetica nd insulators (their electrical conductivity is approximately 6.4 10 À13 Scm À1 at 20 8C), similar to Peierls-distorted stacksc omprising pancake-bonded dimers of radicals.…”

Sigma‐ versus Pi‐Dimerization Modes of Triangulene

“…Although the σ‐dimers discussed above are interesting on their own, the more important part of this study is to explore and elucidate the possible connections between σ‐dimers and the π‐dimers for 6‐TRL. The π‐dimer of 6‐TRL features a 4e/mc (4 electrons, multicenter) double pancake bond (Figure b) . Based on our previous report, the slip‐6 and slip‐2 π‐dimers of 6‐TRL are very stable with E int of −19.58 and −16.40 kcal mol −1 without ZPE correction.…”

“…M05‐2X includes middle range dispersion, a very important factor that affects the stability of the π‐dimers and that needs to be described accurately for obtaining energy differences along the π to σ isomerization process. This DFT is proven to be successful in several previous studies on phenalenyls and triangulenes . We adopted the Pople type 6–311G(d, p) basis set and the basis set evaluation can be found in the Supporting Information.…”

“…in which n is the number of π‐electrons and π‐centers in the monomer PAH. The sum of the Mulliken spin population of a monomer in the dimer was calculated to characterize the open‐shell character of some dimers . All geometry optimizations and frequency calculations were done using the Gaussian 09 package (Revision D.01) .…”

“…The driving force in each case is provided by electron pairing, and the interpretation of their differences is the main target of this paper. The radical/diradical character and their formal pancake bond orders when forming the π‐dimers are indicated in Figure …”