Grand Canonical Quantum Mechanical Study of the Effect of the Electrode Potential on N‑Heterocyclic Carbene Adsorption on Au Surfaces

Increasing interest has been focused on using N-heterocyclic carbenes (NHCs) as surface ligands to replace thiols in the preparation of self-assembled monolayers (SAMs) on gold due to their larger adsorption energies. However, one of the drawbacks of these NHC-based SAMs is that they are unstable under electrochemically reducing conditions. In this study, grand canonic quantum mechanics (GC-QM) were used to study the effect of the electrode potential (U) on the adsorption of NHC on Au(111). The NHC adsorption energies were significantly weaker (∼0.92 eV) under constant U conditions compared to those under constant charge conditions, demonstrating the importance of using GC-QM for studying electrochemical systems. Consistent with experiments, the results from our calculations indicated that the adsorption energy decreased as U became more negative but increased as U became more positive. These results were rationalized using the frontier orbital theory. Importantly, based on the same analysis, when NHCs or their analogues with a smaller gap between the singlet ground and triplet first excited states (ΔE_S–T < 1.1 eV) were employed as molecular anchors, the adsorption energy was much less affected by U. The same results were obtained for other common SAM substrates (i.e., Ag(111), Cu(111), and Pt(111)). Therefore, based on our GC-QM calculations, we propose that the key to developing a stable NHC-based SAM under electrochemical reducing conditions is to focus on NHCs or their analogues as surface ligands with small ΔE_S‑T’s.