The amino-groups can manifest themselves in a large range of chemical shifts from -267 to -291 depending on their environment. Thus, we assume that the reduction of the NH2 peak contribution at -274 ppm might be due to the elimination of more volatile carbon nitride constituents or impurities during the thermal treatment. The 1 H spectra are composed of two main contributions at 9.7 and 4.1 ppm which correspond to the hydrogens of hydrogen-bonded amino-groups and adsorbed water molecules, respectively (Fig. ). The comparative analysis of R-CNx and Y-CNx presented above shows that the structural modifications produced by the high-temperature treatment are not easily detectable by conventional solid-state characterization approaches, although they clearly have a great impact on the light absorption properties of the materials (Figure ). Therefore, further attempts to characterize the R-CNx material were carried out starting by its dissolution in an appropriate solvent. The dissolution of PCN in KOH proceeds via hydroxyl attack on amino-groups of the polymer replacing them and forming water-soluble tri-potassium cyamelurate. Crucially, this approach enabling us to dissolve melon polymer has a potential to spectroscopically separate the contribution from species which possibly endow R-CNx with extended light-absorption in the visible range. Notably, the dissolution of the Y-CNx and R-CNx samples resulted in colourless and yellow-green coloured solutions, respectively (Fig. ). The evaporation and slow cooling of these solutions lead in both cases to the formation of needle-shaped single-crystals of different colour (Fig. ). In spite of that, the 13 C NMR analysis of the crystals dissolved in D2O shows identical signals at 158.2 and 168.7 ppm for both samples with the integrated areas ratio of ca. 1:1 and assigned to CN3 of the heptazine and N-C-O -species, respectively, which is typical for potassium salts of cyameluric acid (Fig. ). In order to evaluate the presence of minor components in the solutions, which might be responsible for the R-CNx coloration, the obtained solutes of the PCN samples in KOH were concentrated by evaporation, while the formed tri-potassium cyamelurate crystals were removed from the system. The solution of Y-CNx sample still showed the presence of a nearly pure potassium salt of cyameluric acid (Fig. ), having only one additional peak at 171.6 ppm, which we attribute to the C of potassium carbonate. In the 13 C NMR spectrum of the solution of R-CNx in KOH, on the other hand, the signal ascribed to carbon atoms in CN3 environment of the heptazine is absent, instead the intense peaks at the chemical shifts of 168.6-168.4 ppm suggest the presence of triazine species bearing fully deprotonated powder (R-CNx-KS) with the composition C1.00N1.90O0.90K0.01 according to EDX elemental analysis which is insoluble in water and DMSO. According to the XRD analysis (Fig. ) and considering the presence of only triazine species in the initial solution, the crystalline phase of the separated compound might be ascribed to the poly(triazine imide) phase. FTIR and Raman spectra reveal a wide absorption region in 1400-1660 cm -1 ascribed to C=N stretching of the heterocycle and a very intense Raman band at 704 cm -1 accompanied by a weaker band at 989 cm -1 (Fig. ) attributed to triazine breathing modes, thus corroborating the polytriazine nature of the material. The intense absorption peak at 1736 cm -1 coincides with the C=O bond stretching vibrations in alkali isocyanurates (Fig. ), while the absorption in the 3145-3340 cm -1 range and at ca. 2850 cm -1 is attributed to free and H-bonded NHx-moieties, respectively. This suggests that the fully deprotonated cyanurate mono or oligomeric species in the solution undergo tautomeric transformation producing insoluble polymerized isocyanurate fragments. A comparative analysis of UV Raman and FTIR spectra shows multiple strongly Raman active modes that are not visible in the IR spectrum 1578, 1389, 1285 and 589 cm -1 . These can be attributed to N=N stretching, N=N in trans-aromatic azo compounds, azoxy groups and C-N=N stretching combined with a C-N deformation vibrations, respectively. The observed bands at 1389 and 589 cm -1 are especially indicative of a trans-azo moiety inside the R-CNx-KS (i.e., in the solid obtained by slow evaporation of the R-CNx-K solution that was obtained after filtering off the tri-potassium cyamelurate crystals from the concentrated product solution of the R-CNx dissolution in KOH). Considering that it is very unlikely that azo groups are formed during KOH treatment of R-CNx, we conclude that they were a part of its structural units in the solidstate, linking the heptazine or triazine moieties and being responsible for the visible-light absorption. Indeed, the type of the linkage between the structural units of polymeric carbon nitride determines the materials optical properties to a greater extent than the substitutional doping of the heptazine itself, as it defines the symmetry, planarity or corrugation of the polymeric system. Notably, a band in the visible-light absorption region is still present in the UV-vis spectrum of the R-CNx-KS, though blue-shifted with respect to that observed for R-CNx (Fig. ).