Substituent Effects in Parallel-Displaced π-π Stacking Interactions: Distance Matters.

Area of Science:

• Supramolecular Chemistry
• Organic Chemistry
• Physical Chemistry

Background:

• Host-guest systems are crucial for understanding molecular recognition.
• Rebek imide receptors and pyridine ligands offer tunable platforms for studying non-covalent interactions.
• π-π stacking interactions are fundamental in molecular assembly and biological processes.

Purpose of the Study:

• To investigate substituent effects on parallel-displaced π-π stacking interactions.
• To experimentally validate theoretical models (Wheeler-Houk and Hunter-Sanders) for π-π stacking.
• To elucidate the role of intermolecular distance in modulating substituent influence.

Main Methods:

• Synthesis and characterization of host-guest systems incorporating Rebek imide receptors and 2,6-di(isobutyramido)pyridine ligands.
• Utilizing ethyne-1,2-diyl and buta-1,3-diyne-1,4-diyl linkers to vary substituent-guest distances.
• Determination of association constants (K_a) via binding studies and correlation with Hammett substituent constants (σ_para).

Main Results:

• A short ethyne-1,2-diyl spacer resulted in stabilization of π-π stacking independent of substituent electronics, consistent with the Wheeler-Houk model.
• A longer buta-1,3-diyne-1,4-diyl spacer prevented direct through-space interactions, leading to a linear correlation between logK_a and σ_para, supporting the Hunter-Sanders model.
• Intermolecular distance was identified as a key factor determining the nature and magnitude of substituent effects in parallel-displaced π-π stacking.

Conclusions:

• The study experimentally demonstrates how varying intermolecular distance modulates substituent effects in parallel-displaced π-π stacking.
• Findings confirm the applicability of both Wheeler-Houk and Hunter-Sanders models depending on the specific spatial arrangement of interacting aromatic systems.
• This research provides valuable insights into the design principles for controlling molecular interactions through precise spatial positioning.