How delocalized are the polyacenes?

Area of Science:

• Computational chemistry
• Organic chemistry
• Quantum chemistry

Background:

• Polyacenes are aromatic hydrocarbons with potential applications in organic electronics.
• Understanding electron delocalization is crucial for predicting their chemical and physical properties.
• Accurate theoretical models are needed to study large polyacene systems.

Purpose of the Study:

• To quantify electron delocalization in polyacenes up to 50 carbon atoms.
• To assess the energetic cost of localizing pi electrons in polyacenes.
• To evaluate the accuracy of highly localized molecular orbital (HILO) models for predicting polyacene properties.

Main Methods:

• Self-consistent field (SCF) calculations were performed.
• Pi electrons were constrained to highly localized molecular orbitals (HILOs) on 2, 6, or 10 adjacent carbon atoms.
• Analogous calculations were conducted on hydrogen-only polyacene models.

Main Results:

• The energetic cost of localizing pi electrons decreased significantly with larger HILO sizes (60, 5, and 0.1 kJ/mol per ring atom for 2-, 6-, and 10-atom HILOs, respectively).
• Localized models showed minimal overestimation of hydrogenation energies (50%, 4%, and 0.1% for 2-, 6-, and 10-atom HILOs, respectively).
• The accuracy of HILO models improved substantially with increasing localization size.

Conclusions:

• Highly localized descriptions of pi electrons are sufficient for modeling polyacene chemistry.
• The use of HILO models provides a computationally efficient and accurate approach for studying large polyacenes.
• These findings simplify the theoretical investigation of polyacene properties and potential applications.