Abstract The widespread occurrence of Indian yellow on an early 17th-century wall painting in Rajasthan (India) was initially indicated by photo-induced luminescence imaging of the painted scheme in the Badal Mahal within the Garh Palace (Bundi). The presence of the organic pigment was subsequently confirmed by HPLC-ESI-Q-ToF. The results of a multi-analytical study focusing on two samples from the wall painting and two reference pigments from the British Museum and National Gallery (London, UK) are presented here. The research focused on the possible causes for the different yellow/orange hues observed in the painting samples. Analysis of cross-sections with SEM-EDS revealed similar elemental composition for the Indian yellow paint layers, but different underlying layers, indicating a variation in painting technique. The composition of the Indian yellow samples was investigated by HPLC-ESI-Q-ToF with both positive and negative ionisation. In addition to euxanthic acid and euxanthone, a sulphonate derivative of euxanthone was found to be present in all samples, while relative amounts of the three components varied. Flavonoid molecules—morin, kaempferol, quercetin and luteolin—were also detected in one wall painting sample (characterised by a brighter yellow colour) and not in the sample that was more orange. The optical properties of the samples were characterised by photoluminescence spectroscopy in both solid state and aqueous solution. The contribution of each organic compound to the emission spectrum of Indian yellow in solution was also investigated by time-dependent density functional theory (TDDFT) calculations. Small differences in terms of spectral shift were observed in solid state experiments, but not in solution, suggesting that the spectral differences in the emission spectrum were mostly due to different contributions of solid-state arrangements, most likely driven by π-π stacking and/or hydrogen bonds. However, a slight difference at high energies was observed in the spectra acquired in solution and TDDFT calculations permitted this to be ascribed to the different chemical composition of the samples. Time-resolved measurements highlighted di-exponential lifetime decays, confirming the presence of at least two molecular arrangements. Py(HMDS)-GC–MS was also used for the first time to characterise Indian yellow and the trimethylsilyl derivative of euxanthone was identified in the pyrograms, demonstrating it to be a suitable marker for the identification of the pigment in complex historic samples.

中文翻译：

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对印度黄的成分及其在拉贾斯坦壁画中的应用的新见解

摘要 17 世纪早期在拉贾斯坦邦（印度）的一幅壁画上广泛出现了印度黄色，最初是通过 Garh 宫（本迪）内的巴达尔玛哈尔（Badal Mahal）绘画方案的光致发光成像来表明的。随后通过 HPLC-ESI-Q-ToF 确认有机颜料的存在。此处介绍了一项多分析研究的结果，该研究侧重于来自大英博物馆和国家美术馆（英国伦敦）的壁画的两个样本和两种参考颜料。研究的重点是在绘画样本中观察到的不同黄色/橙色色调的可能原因。使用 SEM-EDS 对横截面的分析揭示了印度黄色油漆层的相似元素组成，但底层不同，表明绘画技术存在差异。印度黄样品的组成通过 HPLC-ESI-Q-ToF 与正电离和负电离进行了研究。除了丁子酮和丁子酮外，发现所有样品中都存在丁子酮的磺酸盐衍生物，而这三种成分的相对含量各不相同。黄酮类化合物分子——桑椹素、山奈酚、槲皮素和木犀草素——也在一个壁画样本中被检测到（以更亮的黄色为特征），而不是在更橙色的样本中。通过固态和水溶液中的光致发光光谱表征样品的光学性质。还通过时间相关密度泛函理论 (TDDFT) 计算研究了每种有机化合物对溶液中印度黄发射光谱的贡献。在固态实验中观察到光谱位移方面的微小差异，但在溶液中没有观察到，这表明发射光谱中的光谱差异主要是由于固态排列的不同贡献，最有可能是由 π-π 堆叠和/或氢键。然而，在溶液中获得的光谱中观察到高能量下的轻微差异，TDDFT 计算允许将其归因于样品的不同化学成分。时间分辨测量突出了双指数寿命衰减，证实了至少两种分子排列的存在。Py(HMDS)-GC-MS 也首次用于表征印度黄，并在热图中鉴定了丁酮的三甲基甲硅烷基衍生物，