Density functional theory/time-dependent density functional theory investigations on the color-structure relationship of biopigment molecules

The study of molecular derivatives of biological dyes is of great importance for the green transformation of the printing, dyeing, and textile industries. In this study, B3LYP density functional methods are used to optimize the geometric configuration of the selected molecules and to explore the relationship between the structure and color of biological dye molecules and their color mechanism. This study focuses on the analysis of polyenes, quinones, indoles, and azide biological dyes in colors commonly used in the textile industry: blue, yellow, purple, red, green, and so on. Quantum chemical investigations show that the conjugate structure of a biological dye is directly related to its color and that the group or structure affecting the conjugate structure will in many cases cause a change in color. In addition, time-dependent density functional theory spectral calculations with the CAM-B3LYP functional for UV-Vis spectra show that in the visible band, the color of the remaining band after subtracting the absorption wavelengths was exactly the same as the color of the pigment. These results indicate that the color of the pigment is exactly complementary to the light absorption color of the material. Our study provides theoretical guidance for the design of molecular derivatives of biological dyes and is expected to promote the green transformation of the textile, printing, and dyeing industries to a certain extent.