Reactivity of acenes: mechanisms and dependence on acene length

Abstract

Acenes are polycyclic aromatic hydrocarbons consisting of linearly fused benzene rings. These compounds are currently the subject of great interest from both fundamental and applied perspectives, particularly for use in organic electronics. This review highlights the computational studies carried out to understand acene reactivity, their reaction mechanisms, and the relationship of the latter to acene length. Generally, as acene length increases, the reactivity of acenes increases, and a greater tendency towards a biradical mechanism is observed. For example, an interesting change in mechanism (from concerted to biradical) is observed for the reaction of acenes (benzene through pentacene) with molecular oxygen. A computational study of the addition of HCl (which behaves as an electrophile) and water (which behaves mostly as a nucleophile) to acenes revealed that acene reactivity increases along the series up to hexacene and remains constant from hexacene and above because of the biradical character of the ground state of higher acenes. Although the exothermicity of the addition of water and HCl to acene is similar, the activation barrier for the addition of water is significantly higher than that for HCl. There is a substantial substituent effect on the energy barriers for electrophilic and nucleophilic reactions. Phenyl substitution at the most reactive meso‐carbon atoms of the central ring of acene blocks dimerization through this ring but does not efficiently prevent dimerization through other rings. With respect to the self‐reactivity (such as dimerization, etc.) of acenes, a computational study showed that the preferred pathways are the formation of acene dimers via a central benzene ring and the formation of acene‐based polymers. Interestingly, longer acenes are predicted to self‐react to form polymers, rather than acene dimers, as the thermodynamically preferred product. These findings invite experimentalists to reinvestigate the dimerization of long unsubstituted acenes. Given that computational studies directly relate to the gas phase reactivity of acenes, whereas long acenes find application in organic semiconductor devices as solids, the next research challenge is to obtain an understanding of the solid state reactivity of long acenes. Copyright © 2012 John Wiley & Sons, Ltd.