Density-Functional Theory Shows 2 <i>H</i> -Tetraphenylporphyrin Prefers Physisorption over Chemical Bonding on Ag(111)

Conformational changes upon adsorption can significantly influence a molecule’s behavior at surfaces. In this study, we employ density functional theory (DFT) with the r2SCAN+rVV10 functional to investigate the adsorption characteristics of 2H-tetraphenylporphyrin (2H-TPP) on a Ag(111) surface. We find that 2H-TPP physisorbes readily on all adsorption sites, but chemisorption is rare and involves large molecular distortion. The most stable configuration is physisorbed and occurs above the bridge site with an adsorption energy of −6.35 eV, which is 0.95 eV lower in energy than the most stable chemisorbed configuration. Thus, on Ag(111) physisorption is more stable than chemisorption. These findings, supported by electron localization function (ELF) analysis, are contrary to what has been found for the Cu(111) surface, for which chemisorption is the most stable binding. This is further corroborated by potential energy surface calculations using the climate image-nudged elastic band (CI-NEB) method along two reaction pathways, which reveal a 1.2 eV reaction barrier from the physisorbed to the chemisorbed configuration. We have found a barrier of only 0.024 eV between adjacent physisorbed sites, which is large enough to render it immobilized at room temperature. The results provide compelling evidence that 2H-TPP physisorbs on Ag(111) and will not chemically bond under normal circumstances.