From dispersion to orbital interactions: the nature of exceptionally strong π-stacked donor-acceptor bonding between...

Combined experimental and computational studies established that the antiaromatic hydrocarbon octamethylbiphenylene (OMB) forms strong π-stacked donor-acceptor complexes with a series of planar organic π-acceptors (trinitrobenzene, tetracyanopyrazine, tetrafluoro-p-benzoquinone, tetracyanoquinodimethane, tetrafluorotetracyanoquinodimethane, and 2,3-dichloro-5,6-dicyano-p-benzoquinone). UV-Vis-NIR spectroscopy demonstrated that 1 : 1 complexes of OMB with the stronger acceptors display formation constants of ∼10⁴-10⁵ M^-1 in dichloromethane and their charge-transfer bands extend beyond 1000 nm. X-ray structural analysis revealed that the co-crystals of OMB with the π-acceptors consist of alternating donor-acceptor π-stacks characterized by numerous intermolecular contacts shorter than the van der Waals separations. The systematic changes in the intramolecular geometries of OMB and acceptors in these stacks indicate partial ground-state charge transfer. DFT calculations reproduced the experimental trends and predicted binding energies of up to 27 kcal mol^-1 for the 1 : 1 complexes. Quantum theory of atoms in molecules and non-covalent interaction analyses revealed that the donor-acceptor bonding is delocalized over their entire interface, while natural bond orbital analysis indicated an unusually large ground-state charge transfer of up to 0.42 e in the strongest associations. Energy decomposition analysis shows that although dispersion provides the largest individual stabilizing contribution throughout the series, electrostatic and orbital interactions increase more rapidly with acceptor strength and become comparable to dispersion in the strongest complexes. These results demonstrate that the strong π-stacked donor-acceptor complexes occupy an intermediate region between conventional noncovalent π-stacking and weak intermolecular covalent bonding, where dispersion, electrostatic, and orbital (charge-transfer) interactions all make substantial contributions to the overall stabilization.