Formation of acene-based polymers: mechanistic computational study.

Area of Science:

• Organic Chemistry
• Materials Science
• Computational Chemistry

Background:

• Linear acenes are organic molecules with potential applications, but their stability and reactivity, particularly decomposition mechanisms, require thorough understanding.
• Previous hypotheses suggested polymerization as a thermodynamically favorable pathway for long acenes.

Purpose of the Study:

• To computationally investigate the mechanisms and kinetics of linear acene polymerization.
• To determine the factors influencing the formation of polymers versus dimers in acene systems.

Main Methods:

• Density Functional Theory (DFT) calculations were employed to model acene polymerization.
• Pentacene, hexacene, and heptacene were used as model compounds to study polymerization mechanisms.
• Reaction pathways, transition states, and energy barriers for polymerization and dimerization were analyzed.

Main Results:

• Acene polymerization proceeds via a stepwise biradical mechanism, similar to dimerization.
• Steric hindrance in the polymer backbone directs polymerization to less reactive noncentral benzene rings.
• Dimerization is consistently kinetically favored over polymerization for all investigated acene lengths.
• Despite kinetic favorability of dimerization, polymerization becomes thermodynamically preferred for hexacene and longer acenes.

Conclusions:

• Kinetic factors, specifically steric hindrance, favor dimerization over polymerization in linear acenes.
• Thermodynamic favorability of polymerization increases with acene length, suggesting potential for polymer formation in longer systems.
• Re-evaluation of experimental data supports the possibility of previously observed acene-based polymers.