We determined the optical gap of nano-crystals of PTI-materials using the onset values from reflectance spectra (Tauc plot, Figure , a), and valence band-EELS spectra of the conduction band (Figure ). The DFT calculations show that the valence band of a monolayer is doubly degenerate at the Γ-point, and that it is composed of states that contain contributions from the nitrogen lone-pairs (Figure ). Group-VI semiconductors with a lone-pair upper most valence band are referred to as "lone-pair semiconductors". The nonbonding character of the VB has been theoretically predicted for other graphitic carbon nitride materials. In CN-containing molecules like melamine, both the highest occupied orbital (HOMO) and the lowest occupied orbital (LUMO) have π-character; thus, the HOMO-LUMO transition is allowed. However, in PTI layers the electronic VB → CB transition is forbidden because there is no orbital overlap between the non-bonding orbitals of the VB and the π*-like orbitals of the conduction band (Figure ). We observe a similar behavior in the bulk assembly of PTI layers, corrugation only lifts the degeneracy at the Γ-point (Figure ). The forbidden VB → CB transition prevents direct estimation of the band gap by optical methods or EELS as the experimentally observed absorption corresponds to the π → π* transition between the VB-2 and the CB. The energetic difference between the forbidden VB → CB and the observed VB-2 → CB transitions is the reason for the observed discrepancy between the DFT bandgap and the experimental absorption onset obtained from the Tauc plot and EELS measurements (Figure ). The VB-2 → CB transition is a π → π* transition of 3.6 eV that corresponds more closely with the experimentally derived gap by Tauc plot and aloof-EELS (Figure ). We summarize the results of DFT, EELS, and UPS considerations in an energy level diagram (Figure ).