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 (<i>E</i>_int) and its change with horizontal displacement (Δ<i>E</i>_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 <i>via</i> scanning tunnelling microscopy. This suggests that aromatic molecules have less adsorption stability on the HOPG than <i>n</i>-alkanes, though the underlying reason remains unclear. In this study, we performed dispersion-corrected density functional theory calculations to evaluate <i>E</i>_int and Δ<i>E</i>_int of polycyclic aromatic hydrocarbons (PAHs) on a graphite model surface (C₉₆H₂₄). Δ<i>E</i>_int was analysed for PAHs with the number of carbon atoms (<i>N</i>_c) from 6 to 24. The maximum Δ<i>E</i>_int (Δ<i>E</i>_int(max)) is related to the barrier height for lateral migration. The Δ<i>E</i>_int(max) per <i>N</i>_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 <i>n</i>-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.