Distinct Folding Behavior of ortho-Phenylenes and 2,3-Pyrazinylenes

Area of Science:

• Polymer Chemistry
• Supramolecular Chemistry
• Organic Chemistry

Background:

• Ortho-phenylenes and 2,3-quinoxalinylenes are structurally similar foldamers and helical polymers.
• Ortho-phenylenes form compact helices with stacked rings.
• 2,3-Quinoxinylenes typically adopt extended helical structures without backbone stacking.

Purpose of the Study:

• To investigate the conformational differences between ortho-phenylenes and 2,3-quinoxalinylenes.
• To understand the factors driving the distinct helical behaviors of these related polymer systems.

Main Methods:

• Crystallography
• Variable-temperature NMR spectroscopy
• Density Functional Theory (DFT) calculations
• Study of short ortho-arylenes with varying benzene and pyrazine sequences.

Main Results:

• Pyrazine units in ortho-arylene architectures exhibit weaker arene-arene interactions, disfavoring cofacial ring packing.
• Bipyrazine units increase vibrational entropy for extended conformers.
• Extended helical conformations are favored for pyrazine-containing systems at higher temperatures, including room temperature.

Conclusions:

• The conformational preference for extended helices in 2,3-quinoxalinylenes is attributed to reduced inter-ring interactions and enhanced vibrational entropy.
• Temperature plays a crucial role in dictating the helical conformation of these systems.