| Abstract
| - Scanning tunneling microscopy (STM) images of 1,10-phenanthroline (PHEN) and dipyrido[3,2-a:2‘,3‘-c]phenazine (DPPZ) on Au(111) are recorded using both in situ and ex situ techniques. The images of PHENdepict regimes of physisorption and chemisorption, whereas DPPZ is only physisorbed. All physisorbedstructures are not pitted and fluctuate dynamically, involving aligned (4 × 4) surface domains with short-range (ca. 20 molecules) order for PHEN but unaligned chains with medium-range (ca. 100 molecules) orderfor DPPZ. In contrast, the chemisorbed PHEN monolayers remain stable for days, are associated with surfacepitting, and form a (4 × √13)R46° lattice with long-range order. The density of pitted atoms on large goldterraces is shown to match the density of chemisorbed molecules, suggesting that gold adatoms link PHENto the surface. For PHEN, chemisorbed and physisorbed adsorbate structures are optimized using plane-wavedensity-functional theory (DFT) calculations for the surface structure. Realistic binding energies are thenobtained adding dispersive corrections determined using complete-active-space self-consistent field calculationsusing second-order perturbation theory (CASPT2) applied to cluster-interaction models. A fine balance betweenthe large adsorbate−adsorbate dispersive forces, adsorbate−surface dispersive forces, gold ligation energy,and surface mining energy is shown to dictate the observed phenomena, leading to high surface mobility andsubstrate/surface lattice incommensurability. Increasing the magnitude of the dispersive forces through useof DPPZ, rather than PHEN, to disturb this balance produced physisorbed monolayers without pits and/orsurface registration but with much longer-range order. Analogies are drawn with similar but poorly understoodprocesses involved in the binding of thiols to Au(111).
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