Pancake Bonding: An Unusual Pi‐Stacking Interaction

Abstract

A category of parallel π-stacking interaction, termed pancake bonding, is surveyed. The main characteristics are: the interaction occurs among radicals with highly delocalized π-electrons in their singly occupied molecular orbitals (SOMOs), the contact distances in the π-stacking direction are shorter than the typical van der Waals distances, and the stabilization obtained by the bonding combination of the SOMO orbitals leads to direct atom-to-atom overlap with strong orientational preferences. These atypical intermolecular interactions contain a component of electron sharing between the radicals that can be viewed as covalent-like. Pancake bonded dimers characteristically have low-lying singlet and triplet states and show characteristic interlayer vibrational modes. Pancake bonded aggregates serve as molecular components in many conducting and other functional organic materials. The role of van der Waals (vdW) interactions in pancake bonded dimers, chains, and other aggregates is different from closed shell vdW aggregates: here the Pauli repulsions reduce the attractive dispersion interaction significantly. Fluxionality between π- and σ-bonded aggregates often occur in the context of pancake bonding. Both experimental and computational aspects are reviewed.

Recent developments in symmetry‐adapted perturbation theory

“…The latter one is exact for interactions involving a doublet state (as there is only one unpaired spin to flip) and much milder in general, as shown by comparisons to the existing nonapproximated high‐spin of Reference (an example comparison for the N–N interaction is presented in Figure ). The single spin‐flip treatment removes the short‐range artifacts of SF‐SAPT( S 2 ) for model systems, however, it does little to improve the splittings in important classes of complexes such as pancake‐bonded dimers of aromatic radicals . Clearly, one needs to go beyond first‐order perturbation theory, and the extensions of SF‐SAPT to the second‐order exchange induction and exchange dispersion corrections, as well as an extension from ROHF to CASSCF monomer wavefunctions, are in progress in my group.…”

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.…”

Photo/redox-responsive 2D-Supramolecular assembly involving Cucurbit[8]uril and a star-shaped porphyrin tecton

“…Our group has been focusing over the past few years on the development of tailor-made redoxcontrollable molecular or supramolecular systems involving electrogenerated viologen-based -cation radicals R1VR2 +• in Figure 1B) as key responsive and/or assembling elements, using their ability to self-assemble into sandwhich-like, dimeric entities called -dimers ([(R 1 VR 2 ) 2 ] 2+ in Figure 1B) [10][11][12][13][14][15][16][17][18][19][20]. These studies, and many others [16,[21][22][23][24][25][26][27][28][29], have served to establish that the effective formation of such multi-center bonded dimers in solution actually requires either very low temperatures and/or high concentrations in radicals. As an alternative, viologen-based -dimers can also be observed in standard temperature and concentration ranges, when using barrel-shaped cavitan ds known as cucurbit [8]urils (CB [8]), made of 8 glycoluril moieties connected by methylene bridges, whose inner cavity is ideally suited to the inclusion of two viologen-based radicals [30][31][32][33][34][35].…”