Substituent Effects in Parallel‐Displaced π–π Stacking Interactions: Distance Matters

Abstract Host–guest systems with Rebek imide type receptors and a 2,6‐di(isobutyramido)pyridine ligand were employed to investigate substituent effects in parallel‐displaced π–π stacking interactions. Changing the intermolecular distance between the para substituent on the aromatic platform of the receptor and the pyridine ring of the guest results in a strongly different substituent effect. With a short ethyne‐1,2‐diyl spacer between the Rebek imide and the aromatic platform, partial overlap of substituent and guest stabilizes the π–π stacking interactions independent of the electronic nature of the substituent (Wheeler–Houk model). When the substituent is shifted further away by using a buta‐1,3‐diyne‐1,4‐diyl spacer, direct, through‐space interactions between substituent and guest are prevented. A linear correlation between logK a (K a =association constant) and the Hammett substituent constant σ para is observed, confirming predictions by the Hunter–Sanders model experimentally.