Quantifying the Effect of Intermonomer Improper Angles on Electron Delocalization in Conjugated Polymers

Area of Science:

• Materials Science
• Computational Chemistry
• Polymer Science

Background:

• Electron delocalization in conjugated polymers is key for charge transport and mechanical properties.
• Coplanar arrangements maximize delocalization via π-orbital overlap, but precise geometric controls are unclear.
• Existing molecular mechanics (MM) models often overlook out-of-plane (improper) torsion effects.

Purpose of the Study:

• To develop a method for quantifying improper dihedral torsion effects on conjugated polymer electronic structure.
• To investigate these effects in representative polymers: P3HT, PTB7, and PNDI-T.
• To assess the impact of improper torsion on electron delocalization and compare with MM models.

Main Methods:

• Quantum mechanical calculations were performed on dimer or donor-acceptor units.
• A novel method was developed to isolate and quantify improper dihedral torsion.
• Energetic effects of improper and dihedral torsion coupling were analyzed.

Main Results:

• Improper torsion generally disrupts electronic delocalization in conjugated polymers.
• PNDI-T demonstrated significant conjugation even at 30° improper angles, attributed to its extended π-system.
• Coupling between improper and dihedral torsion had minimal energetic impact (<1 kcal/mol).

Conclusions:

• Existing MM models may be sufficient for local structure simulations without improper torsion parameters.
• Improper torsion's role in larger-scale polymer films warrants further investigation.
• An interactive visualization tool was created to aid in interpreting delocalization energy and electron density.