Small Changes With Big Consequences: Swapping Two Atoms In Side Chains Changes Phenylene-Ethynylene Packing And...

Area of Science:

• Materials Science
• Organic Electronics
• Supramolecular Chemistry

Background:

• Engineering conjugated materials for organic optoelectronics relies on understanding solid-state properties governed by non-covalent interactions.
• Current design often separates the conjugated 'main chain' from solubility-enhancing 'side chains', assuming the latter have minimal impact on optoelectronic behavior.

Purpose of the Study:

• To investigate how minor structural modifications in side chains of phenylene-ethynylene (PE) molecules affect their solid-state optical properties.
• To elucidate the role of non-covalent interactions, specifically fluoroarene-arene stacking, in determining molecular packing and optoelectronic performance.

Main Methods:

• Synthesis and characterization of anisyl-terminated three-ring PE derivatives with differing side chain fluorine substitution (2,4,6-trifluoro vs. 2,3,6-trifluoro).
• Analysis using X-ray crystallography to determine solid-state structures.
• Computational modeling via electronic structure calculations.
• Optical spectroscopy to measure fluorescence emission and force-induced shifts.

Main Results:

• Switching side chain arene substitution from 2,4,6-trifluoro to 2,3,6-trifluoro induced a >100 nm shift in fluorescence emission spectra.
• 2,4,6-trifluorinated side chains promoted cofacial fluoroarene-arene stacking, leading to twisted PE backbones and reduced conjugation.
• 2,3,6-trifluoro side chains did not engage in stacking, resulting in more coplanar PE backbones and intermolecular aggregation.

Conclusions:

• Minor alterations to typically ancillary side chains can drastically influence the solid-state optical properties of conjugated materials.
• Non-covalent interactions involving side chains are critical for controlling molecular packing and optoelectronic behavior.
• This work highlights opportunities for rational design and materials discovery in organic optoelectronics by tuning side chain interactions.