When monitoring the low-energy emission of R-CNx, a distinct new peak (centered at 540 nm) becomes apparent in the PL excitation spectrum. This suggests the presence of distinct emissive states below the band edge of R-CNx, which are clearly visible also in the PL spectra recorded for different excitation wavelengths (Fig. ). The subbandgap emission observed in R-CNx is likely associated with an increased number of tail states due to the higher density of localized planar-R-CNx units in the Y-CNx framework (Fig. ) produced by heptazine fragmentation and azo-linkages formation after elevated temperature processing.