In-Plane Hydrogen Bonds and Out-of-Plane Dipolar Interactions in Self-Assembled Melem Networks

Melem (2,6,10-triamino-s-heptazine) is the building block of melon, a carbon nitride (CN) polymer that is proven to produce H₂from water under visible illumination. With the aim of bringing additional insight into the electronic structure of CN materials, we performed a spectroscopic characterization of gas-phase melem and of a melem-based self-assembled 2D H-bonded layer on Au(111) by means of ultraviolet and X-ray photoemission spectroscopy (UPS, XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. In parallel, we performed density functional theory (DFT) simulations of the same systems to unravel the molecular charge density redistribution caused by the in-plane H-bonds. Comparing the experimental results with the spectroscopic DFT simulations, we can correlate the induced charge accumulation on the N_aminoatoms to the red-shift of the corresponding N 1s binding energy (BE) and of the N_amino1s → LUMO+n transitions. Moreover, when introducing a supporting Au(111) surface in the computational simulations, we observe a molecule-substrate interaction that almost exclusively involves the out-of-plane molecular orbitals, leaving those engaged in the in-plane H-bonded network rather unperturbed.