Experimental consideration of covalent‐bonding interactions in stacks of singlet biradicals having Kekulé structures

Abstract

Kekulé singlet biradicals stabilized by phenalenyl rings afford one‐dimensional stacks in the solid state, in which covalent‐bonding interactions exist not only within a molecule but also between molecules. Through the covalent‐bonding interactions spreading over the stack, the lowest‐energy absorption band shifts to a lower‐energy region in the solid state relative to the solution band, and an electro‐conductive property in a single component state is also observed. Copyright © 2011 John Wiley & Sons, Ltd.

Pancake Bonding: An Unusual Pi‐Stacking Interaction

“…couplings are comparable in magnitude as wast he case for Kubo's chain. [87,108,109] The situation in Haddon's SBPLYm etallic conductors (Figure 15 c) is different because the number of electrons available for pancakeb ondingi sh alf (1/4 band filling) of that of Kubo's chain corresponding to PBO = 1/2; [16d, 110] still the couplings within and between the radicals upplying units are similari nm agnitude and produce wide conductions bands. [111] Opportunities for Pancake Bonding in Graphene Nanoflakes Figure 16 illustrates the relationship between ag raphene sheet and various triangular multiradicals, such as PLY( 1), and two other triangulene shaped graphene flakes.I th as been well established [112][113][114][115] that as the size of hydrogen terminated highly conjugated nanoflakes increases, their edge atoms develop significant p-type polyradical character depending on the size, the shape and topology of the flakes.W hile PLY [37] and trioxotriangulene ( Figure 1c)a nd its derivatives [99] are radicals and form single pancake bonds,d iradicals and multiradicals can form multiple pancake bonds or multiple single pancake bonds as discussed in this review.T he XRD of the trioxotriangulene molecules demonstrated pancake bondinga lso confirmed by DFT analysis.…”

“…The larger blue ellipses in Scheme point to antiferromagnetic coupling within the molecular unit, while the smaller red ellipses indicate antiferromagnetic coupling between the molecules via pancake bonding . A reasonable hypothesis consistent with the experiments is that these two couplings are comparable in magnitude as was the case for Kubo's chain . The situation in Haddon's SBPLY metallic conductors (Figure c) is different because the number of electrons available for pancake bonding is half (1/4 band filling) of that of Kubo's chain corresponding to PBO=1/2; still the couplings within and between the radical supplying units are similar in magnitude and produce wide conductions bands …”

Substituents Destabilize the Molecule by Increasing Biradicaloid Character and Stabilize by Intramolecular Charge Transfer in the Derivatives of Benzobis(thiadiazole) and Thiadiazolothienopyrazine: A Computational Study

“…Conjugated biradical systems can be broadly divided into two types, namely Kekule and non-Kekule systems. [19,21] Both these categories can be drawn as having resonance between the valence bond structures with some of the structures having a closed-shell form. Kekule-type biradicals are characterized by having all the atoms' valencies satisfied.…”

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

“…[4] The observedi ntrastack contacts cannot be described by classical van der Waals p-stacking, but rather by the p-stacking aggregates drivenb yt he formation of ab ond (called pancake bonding) resultingf rom the presence of singlyo ccupied molecular orbitals (SOMOs) in the constituting monomers. [6] This strong p-electron coupling provides ap lausible path for electron or hole transport in materials exhibiting unusuale lectrical conductivity, [1,7] and varied magnetic [8] or optical propertiesd ue to their low lying triplet states. [6] This strong p-electron coupling provides ap lausible path for electron or hole transport in materials exhibiting unusuale lectrical conductivity, [1,7] and varied magnetic [8] or optical propertiesd ue to their low lying triplet states.…”

“…[5] Ac ovalent-like through-space p-stacking multi-electron/multicentre interaction termed as pancake bonding is observed in various radical dimers. [6] This strong p-electron coupling provides ap lausible path for electron or hole transport in materials exhibiting unusuale lectrical conductivity, [1,7] and varied magnetic [8] or optical propertiesd ue to their low lying triplet states. [9] In this paper, we describe the first instanceo fp ancake bonding extending beyond as ingle pair (dimers) of semiquinone radicals.…”