Lithium Adsorbtion on Polyacenes $\&$ Zig-zag-edge Graphene Strips

TL;DR

This work addresses how lithium adsorption modifies the electronic and structural properties of polyacenes and zig-zag-edge graphene strips, with the goal of linking molecular resonance concepts to extended carbon nanostructures. The authors combine three approaches—classical chemical resonance theory, Hückel tight-binding MO analysis, and ab initio density functional theory (DFT) computations—to predict adsorption sites, charge transfer, and edge localization. For finite polyacenes, Li donates electrons preferentially to central radical-like carbon sites, producing bond weakening and local $sp^3$-like distortion, with the simple MO picture placing the relevant states at the Fermi energy $E_F=0$, and for extended zig-zag systems the edge-localized nonbonding MOs accommodate the donated charge, yielding approximately $ frac{1}{3}$ unpaired electron per edge unit cell that can couple ferromagnetically. DFT calculations show near-complete charge transfer per Li (e.g., $ ear ~0.82$–$0.87$ e) and reproduce symmetry-breaking tendencies such as double adsorption on central rings, while larger systems favor bulk-like adsorption with Li on both sides to minimize Coulomb repulsion and enhance interlayer binding in graphite. The results connect classical molecular concepts to nanoscale carbon materials and suggest practical implications for Li storage and 2D carbon functionalization, including edge-engineered polarity and reactivity.

Abstract

The effect of increased electron-density (from adsorbed Li atoms) in polyacenes and in nano-ribbons with zig-zag edge is discussed in terms of resonance theoretical considerations and in terms edge-localized frontier molecular orbitals. The argumentation from simple pictures is finally using the density functional theory (DFT) for anthracene, polyacene polymer and graphene strips. Some discussion is made for zig-zag edge graphene.

Figures (7)

• Figure 1: First line: The $\pi$-network for anthracene. Second line: The $\pi$-network for semi infinite polyacene
• Figure 2: Neighbor-paired resonance structures for (neutral) anthracene in $(a)$. Resonance structures for Lithium coordinated anthracene in central ring in $(b)$ and in end ring in $(c)$. Benzene-like local conjugated 6-cycles are indicated with a small circle in the center of the associated ring.
• Figure 3: The LUMO density for anthracene in the first line. And in the second line, the amplitude for a non-bonding MO of infinite polyacene. Different Colors represent different phases.
• Figure 4: Optimized configurations of two Li atoms adsorbed on anthracene obtained using the B3LYP/6-311G* functional and basis set.
• Figure 5: Charge distribution of $(a)$ antracene, $(b)$ one lithium atom adsorbed to anthracene, $(c)$ two lithium atoms adsorbed to anthracene. The lithium atom in the up-center corresponds to the Lithium above the anthracene as in Fig. \ref{['liad']} and the lithium atom in the down-center corresponds to the lithium below the anthracene.