Pancake π–π Bonding Goes Double: Unexpected 4e/All-Sites Bonding in Boron- and Nitrogen-Doped Phenalenyls

Abstract

Chemical bonding interactions are the main driving force for the formation of molecules and materials from atoms. The two-electron/multicenter pancake π-π bonding found in phenalenyl (PLY, 1) radical π-dimers is intriguing due to its unconventional nature of covalent bonding for molecular aggregations and its propensity to induce unique optical, electronic, and magnetic properties. By using high-level quantum chemistry calculations, we show that the B- or N-doped PLYs (2 and 4), usually considered closed-shell and therefore trifling, can be rendered open-shell singlet by proper edge substitutions (3 and 5). The resulting two unpaired valence electrons on each molecular unit contribute to the formation of a genuine pancake-shaped 4e/all-sites double π-π bonding upon intermolecular π-dimerization, in contrast to the 2e/half-sites single π-π bonding in the parent PLY π-dimers. The unusual double π-π bonding motif discovered in these PLY analogues may broaden the landscape of, and find new applications for, intermolecular covalent bonding interactions.

Constructing Stable π‐Dimers: Two Parallel Pancake π–π Bonds

“…Tian et al. investigated B/N‐doped PLY π‐dimer derivatives with appropriate edge substitutions, which exhibited double pancake bonding, and were able to show that the stabilization of substituted PLY π‐dimer derivatives was due to the formation of double pancake π–π bonding . The 1,3‐dithia‐2,4,6‐triazine π‐dimer has been extensively studied .…”

Pancake Bonding: An Unusual Pi‐Stacking Interaction

“…Much insighti nto the nature of pancake bonding has come from the prototypical neutral radical dimer of phenalenyl, PLY. [17,51] Ap otentiale nergy scan as af unctiono ft he parallel stacking distance, D, in the staggered configuration, is presented in Figure 4f or the dimer of PLY( 1)i nt he singlet and triplet ground states as well as for the isoelectronic p-dimers of B-PLY (2)a nd N-PLY( 3). The SOMO orbital of PLYb ecomes the doubly occupied HOMO for N-PLY, while it is an unoccupied orbital forB -PLY.…”

Towards Magnetic Carbo‐meric Molecular Materials

“…The hS 2 i values calculated at the PBE/6-311G* or B3LYP/6-311G** level, 0.78 and 1.05, respectively,a re in reasonable agreement with the theoretical value of 0.75 ( Figure S1 in the Supporting Information). In contrast, Hartree-Fock (HF) calculations or functionals including large amounts of HF exchange, strongly overestimate the hS 2 i values, and thus the (poly)radical character.M oreover,i nt his benchmark, only the PBE and B3LYP functionals are yielding as pin delocalization restricted to the C 12 outer-crown of carbo-phenalene, similarly to what is found for the parent phenalenyl radical, C 13 H 9 C. [18] The performance of the B3LYP and CAM-B3LYP functionals was further investigated in the calculation of the triplet-to-singlet energy gap of three organic diradicals, for which accurate values have been reported at the CASPT2 level of calculation (Table 1). [19] Whereas mxylylene and 2,7-dimethylene naphthalene are ferromagnetic triplet diradicals, p-ditolylene acetylene (CCH 2 ÀC 6 H 4 ÀCCÀC 6 H 4 ÀCH 2 C)e xhibits an open-shell singlet ground state.…”

“…Moreover, in this benchmark, only the PBE and B3LYP functionals are yielding a spin delocalization restricted to the C 12 outer‐crown of carbo ‐phenalene, similarly to what is found for the parent phenalenyl radical, C 13 H 9 . . The performance of the B3LYP and CAM‐B3LYP functionals was further investigated in the calculation of the triplet‐to‐singlet energy gap of three organic diradicals, for which accurate values have been reported at the CASPT2 level of calculation (Table ) .…”