Azo or Not: Continuing the Crystallographic Investigations of β‑Naphthol Reds

β-Naphthol reds are a group of widely used pigments with prominent historical, commercial, and cultural significance. In industry, and especially within the art and heritage community, they are known as azo pigments. However, β-naphthols, very often, are not azo pigments. Because of enol/keto tautomerization of the 1-arylhydrazone-2-naphthol skeleton, these pigments oftentimes crystallize as hydrazones (keto). Therefore, proper characterization is necessary for understanding their intrinsic physicochemical properties and chemical reactivity in the solid state, as well as their stability and lightfastness. Crystallographic studies have indicated that β-naphthol reds tend to adopt the hydrazone form in the solid state. Here, we continue these structural investigations, and we focused on two prominent β-naphthol reds, pigment red 40 (PR40) and pigment red 4 (PR4). Using single-crystal X-ray diffraction, we provide decisive proof that both of these pigments are keto/hydrazones in the solid state. Therefore, the frequent yet erroneous designation as azo pigments should be avoided. To confirm the bulk structure, we performed powder diffraction experiments, followed by Rietveld refinement. We complemented the diffraction experiments with spectroscopic (IR, Raman, UV–vis) and thermal (TGA, DSC) analyses. Furthermore, we studied the lightfastness of both chromophores in solution and the solid state. While the solid-state pigments were stable over the course of the experiment, UV irradiation of solutions resulted in degradation, which was studied by chromatographic and mass-spectrometry techniques. We hope that this research will bring to light the necessity of proper solid-state characterization of β-naphthol reds, as well as pigments as a whole.