Electronic Ground State of Higher Acenes

TL;DR

This study uses density functional theory to analyze the electronic ground state of higher acenes, revealing an antiferromagnetic state with edge-localized spins in acenes with more than seven fused benzene rings.

Contribution

It demonstrates that higher acenes and polyacenes share an antiferromagnetic ground state driven by zigzag edges, extending understanding of their electronic structure.

Findings

Acenes with more than seven rings are antiferromagnetic.

Edge-localized spins increase with acene size.

Zigzag edges dictate pi-electronic structure.

Abstract

We examine the electronic ground state of acenes with different number of fused benzene rings (up to 40) by using first principles density functional theory. Their properties are compared with those of infinite polyacene. We find that the ground state of acenes that consist of more than seven fused benzene rings is an antiferromagnetic (in other words, open-shell singlet) state, and we show that this singlet is not necessarily a diradical, because the spatially separated magnetizations for the spin-up and spin-down electrons increase with the size of the acene. For example, our results indicate that there are about four spin-up electrons localized at one zigzag edge of 20-acene. The reason that both acenes and polyacene have the antiferromagnetic ground state is due to the zigzag-shaped boundaries, which cause pi-electrons to localize and form spin orders at the edges. Both wider graphene ribbons and large rectangular-shaped polycyclic aromatic hydrocarbons have been shown to share this antiferromagnetic ground state. Therefore, we demonstrate that the pi-electronic structure of higher acenes and ployacene are still dictated by the zigzag edges, and our results provide a consistent description of their electronic ground state.