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Spectroscopic fingerprints of carbon nitride functional groups locked-up in hydrogen bonding interactions

In this work we have investigated the effect of intermolecular H-bonding interactions on the local electronic state of functionalities (amino and priydine-like N) usually found in graphitic carbon nitride materials (gC3N4). For doing this, we have performed a combined X-ray photoemission (XPS) and absorption (NEXAFS) spectroscopy characterization of the melamine molecule (one of the g-C3N4 building blocks) both in the gas phase (non-interacting system) and in the solid state (H-bonded system). As supported by theoretical simulations of XPS and NEXAFS spectra, we have found that H-bonds mainly affects the N 1s level of the amino group, leaving almost untouched that one of the N inside the aromatic ring. This is responsible for a reduction of the intrinsic chemical shift existing between the two XPS N 1s levels. The lowering of the amino N ionization energy, also influences the energy position of the corresponding resonances of the N K-edge NEXAFS spectrum; i.e. the N 1s (NH2) --> LUMO shifts to lower photon energies. Solid state absorption spectra also show the quenching of resonances related to in-plane antibonding orbitals entirely localized on the amino functionalities. In their place we found a new kind of resonance which can be associated with a new anti-bonding orbital now extended over multiple amino groups belonging to adjacent molecules.