Why triple bonds protect acenes from oxidation and decomposition.

Area of Science:

• Organic Chemistry
• Materials Science
• Computational Chemistry

Background:

• Higher acenes are crucial organic semiconductors but suffer from poor oxidation stability.
• Functionalization is key to improving acene stability for electronic applications.

Purpose of the Study:

• To investigate the impact of functional groups, specifically alkynyl substituents, on the oxidation stability and reactivity of higher acenes (anthracenes, tetracenes, pentacenes).
• To elucidate the mechanisms of photooxygenation and thermal decomposition of functionalized acenes.
• To guide the design of oxidation-stable acene-based materials.

Main Methods:

• Synthesis of functionalized higher acenes with peripheral substituents.
• Identification of photooxygenation and thermolysis products and kinetic measurements.
• Computational analysis using Density Functional Theory (DFT) to predict reaction energies, molecular orbitals, and radical stabilization.

Main Results:

• Alkynyl groups significantly enhance oxidation stability and protect endoperoxides from thermal decomposition.
• Substituents increase regioselectivity in photooxygenation of tetracenes and pentacenes.
• Alkynylpentacenes undergo direct oxidation with singlet oxygen ((1)O(2)) to form a single regioisomer, albeit with a slow rate constant attributed to (1)O(2) deactivation.
• Photooxygenation likely proceeds via a concerted mechanism for alkynyl acenes, while thermolysis involves radical intermediates.

Conclusions:

• Alkynyl functionalization is a promising strategy for enhancing the stability and controlling the reactivity of higher acenes.
• The findings provide valuable insights for developing robust acene-based semiconductors for organic electronics.