Azo or Not: Continuing the Crystallographic Investigations of β- Naphthol Reds
Azo or Not: Continuing the Crystallographic Investigations of β- Naphthol Reds
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是否含有偶氮：继续β-萘酚红的晶体学研究
DOI：
10.1021/acs.cgd.2c00079
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发表时间：
2022
影响因子：
3.8
通讯作者：
Runčevski, Tomče
中科院分区：
文献类型：
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作者：
Heaney, Matthew P.;Unruh, Daniel K.;Runčevski, Tomče
β-萘酚红是一类广泛使用的颜料，具有重要的历史、商业和文化意义。在工业上，特别是在艺术和遗产界，它们被称为偶氮颜料。然而，β-萘酚通常不是偶氮色素。由于1-芳基腙-2-萘酚骨架的烯醇/酮异构化，这些色素经常结晶为腙（酮）。因此，对其进行适当的表征是了解其固有的物理化学性质和固体化学反应性以及稳定性和耐光性的必要条件。晶体学研究表明，β-萘酚红在固体状态下倾向于采用腙形式。在这里，我们继续这些结构的研究，我们重点研究了两个突出的β-萘酚红，色素红40 （PR40）和色素红4 （PR4）。利用单晶x射线衍射，我们提供了决定性的证据，证明这两种颜料都是固态的酮/腙。因此，应避免经常错误地将其命名为偶氮颜料。为了确认体结构，我们进行了粉末衍射实验，然后进行了Rietveld细化。我们用光谱（IR, Raman, UV-vis）和热分析（TGA， DSC）来补充衍射实验。此外，我们还研究了发色团在溶液和固体状态下的耐光性。虽然固态颜料在实验过程中是稳定的，但溶液的紫外线照射会导致降解，这是通过色谱和质谱技术研究的。我们希望这项研究将揭示β-萘酚红以及整个色素的适当固态表征的必要性。
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.