Formation of Acene-Based Polymers: Mechanistic Computational Study

Understanding
the mechanism of linear acene decomposition and its
reactivity is a prerequisite for controlling the stability of acenes
and their future applications. Previously, we suggested that long
acenes may undergo polymerization since the polymerization product
is thermodynamically more stable than the dimerization product. However,
due to kinetic considerations, the most thermodynamically stable product,
the polymer, might not necessarily be formed. To elucidate the situation,
we investigated the mechanisms of acene polymerization computationally,
using pentacene, hexacene, and heptacene as representative examples.
Similarly to dimerization, acene polymerization follows a stepwise
biradical pathway. Structural and steric hindrance of the polymer
backbone forces acene polymerization to proceed via the less reactive
noncentral benzene rings. Consequently, dimerization is always kinetically
more favorable than polymerization, irrespective of acene length.
Although, for long acenes starting from hexacene, both polymerization
and dimerization are barrierless pathways relative to the reactants,
polymerization is thermodynamically preferred for hexacene and heptacene
and even more so for longer acenes (since polymerization forms four
new C–C bonds while dimerization forms only two). Indeed, reinvestigation
of available experimental data suggests that acene-based polymers
were probably obtained experimentally previously.