Elucidating the Hue Shift and Metal Coordination Mechanism of Traditional Sappanwood Mordant Dyeing on Silk via UPLC–MS/MS and DFT

Understanding the chromogenic and dye-metal chelation mechanisms in sappanwood ( Caesalpinia sappan L. ) dyeing is fundamental to the precise conservation of silk heritage and the optimization of modern botanical dyeing. Herein, three distinct hues of sappanwood-dyed silk were reconstructed via traditional techniques, and brazilein coordination behaviors with two mordants were systematically investigated. It was found that the introduction of alum and ferrous sulfate mordants induced a significant hue transition of the dyed silk from the native orange to vivid red and deep purple-black, respectively. Density Functional Theory (DFT) calculations further revealed that brazilein exists as the ring A-deprotonated monoanion in neutral solution. Based on this, thermodynamic analysis indicated that the Al 3+ -mordanted system exhibits the coexistence of trans- like and cis- like configurations, while the Fe 3+ -mordanted system is dominated by a single trans- like configuration. Hole-electron analysis demonstrated that the vivid red color arises from the electrostatic field effect of closed-shell Al 3+ , while the deep purple-black originates from the mixed charge transfer of open-shell Fe 3+ causing spectral broadening. Furthermore, the proposed coordination models and excited-state analysis successfully explained the trends observed in the experimental UV-vis absorption spectra upon mordanting. In summary, this study elucidates the metal chelation and chromogenic mechanisms of sappanwood dyeing at the molecular level, providing a theoretical basis for conserving textile heritage and advancing modern botanical dyeing.