Rethinking the Terms “π-Stacking” and “π–π Stacking” Again: A Proposal to Clarify the Language of Aromatic Interactions

The terms "π-stacking" and "π-π stacking" are widely used to describe attractive interactions between aromatic molecules, yet they are often interpreted as implying a distinct, π-orbital-based attractive force and a preference for a perfectly face-centered stacking geometry. Over the last three decades, structural and theoretical studies have shown that neutral, closed-shell aromatic systems instead favor T-shaped and parallel-displaced geometries and that their interaction energies can be rationalized entirely in terms of the same noncovalent forces that govern molecular interactions in general. In typical ground-state aromatic complexes, there is negligible π-orbital mixing and no demonstrable additional attractive force that is unique to π electrons. Consequently, the mechanistic content of the terms "π-stacking" and "π-π stacking" is, in most cases, ill-defined and potentially misleading when interpreted as describing a special class of interactions. Here, we advocate replacing such usage with a description in terms of Electrostatic, Dispersion, Desolvation, Induction, and Exchange-repulsion contributions (EDDIE). We further discuss those specific situations─such as charge-transfer absorptions, excitonically coupled H- and J-aggregates, and true charge-transfer salts─in which π-orbital mixing is appreciable and terminology invoking "π-stacked" systems remains physically meaningful, though still not required to describe the underlying binding forces.