Hydrogen Bonding Networks Formed by Hydroxyl-Functionalized N-Heterocyclic Carbenes on Ag(111)

Abstract

N-Heterocyclic carbenes (NHCs) have emerged as robust alternatives to thiols for metal-surface functionalization, offering strong σ-donor character, high thermal stability, and tunable molecular design. Despite advances in NHC surface engineering, the formation of ordered hydrogen-bonded NHC networks remains challenging. Such networks are of high interest, as they can modulate adsorbate orientation, charge distribution, and catalytic properties. Here, we investigate a hydroxyl-functionalized NHC, IMes–OH (4-(1-(4-hydroxy-2,6-dimethylphenyl)-1H-imidazole-3-ium-3-yl)-3,5-dimethylphenolate), on Ag(111) to achieve controlled adsorption and extended hydrogen-bonding networks. Using scanning tunneling microscopy, noncontact atomic force microscopy with O-terminated copper tips, and X-ray photoelectron spectroscopy, supported by density functional theory calculations, we reveal the formation of linear phenol–phenolate hydrogen-bonded chains at low coverage and extended two-dimensional networks at higher coverage. Within our measurements, an upright adsorption geometry and coexisting binding modes, direct surface bonding and adatom-mediated bonding, are observed. Our results demonstrate that hydroxyl-functionalized NHCs offer opportunities for molecular surface ordering and functional interface design.