Interplay between Hydrogen Bonding, Epitaxy, and Charge Transfer in the Self-Assembly of Croconic Acid on Au(111) and Ag(111)

The structure and ordering of two-dimensional sheets of croconic acid (CA) on Au(111) were studied with scanning tunneling microscopy and first-principles calculations. Extended porous honeycomb networks are formed that differ from those that were recently reported to form on Ag(111) and density functional theory (DFT) calculations are used to help rationalize how the two substrates, Au(111) and Ag(111), influence the CA networks. The CA network that is synthesized on Au(111) corresponds to the 2D structure that has the lowest ground-state DFT energy when modeled without the substrate and uses a different isomer of CA than the one that is found within the Ag(111) network and the bulk three-dimensional crystal; this is attributed to the weaker CA-CA and CA-substrate interactions on Au(111). The Au(111) network has no net dipole moment, but a hydrogen transfer mechanism which forms a metastable polar network seems plausible if the latter remains dynamically stable. The formation of the Ag(111) network is attributed to the larger, and varied, influence of the binding site on the electronic structure of CA.