Stability of polycyclic aromatic hydrocarbons on graphite: resistance to horizontal displacement

The stability of adsorbed molecules on surfaces depends on the magnitude of adsorbate-substrate interaction energies and the resistance of these molecules to horizontal displacement. Therefore, it is essential to analyse both interaction energy (E_int) and its change with horizontal displacement (ΔE_int). In physisorbed monolayers at a highly oriented pyrolytic graphite (HOPG)/solvent interface, molecular building blocks often contain both aromatic and alkyl chain moieties, as aromatic molecules without additional substituents are difficult to observe via scanning tunnelling microscopy. This suggests that aromatic molecules have less adsorption stability on the HOPG than n-alkanes, though the underlying reason remains unclear. In this study, we performed dispersion-corrected density functional theory calculations to evaluate E_int and ΔE_int of polycyclic aromatic hydrocarbons (PAHs) on a graphite model surface (C₉₆H₂₄). ΔE_int was analysed for PAHs with the number of carbon atoms (N_c) from 6 to 24. The maximum ΔE_int (ΔE_int(max)) is related to the barrier height for lateral migration. The ΔE_int(max) per N_c showed directional dependence and ranged from 0.015 to 0.20 kcal mol^-1, with the largest value for PAHs being about two-thirds that of n-alkanes (0.30 kcal mol^-1), indicating greater mobility of the former. These findings demonstrate that aromatic and alkyl chain moieties in two-dimensional assemblies exhibit distinct resistance against horizontal migration. The preferential role of alkyl chains suggests that molecular assemblies align with the graphite lattice, prioritising alkyl unit positioning over aromatic orientation.