Electronic transitions of the purpurin dye. UV–vis polarization spectroscopy and quantum chemical predictions

Purpurin (1,2,4-trihydroxy-9,10-anthraquinone) is investigated by Synchrotron Radiation Linear Dichroism (SRLD) spectroscopy using stretched polyethylene as an anisotropic solvent, covering the range 58,000–15,000 cm⁻¹ (172–670 nm). The observed wavenumbers, intensities, and polarization directions are compared with theoretical predictions using Time-Dependent Density Functional Theory (TD–DFT), leading to assignment and characterization of all major spectral features in the investigated region. The results demonstrate that the spectrum of purpurin is closely related to that of quinizarin (1,4-dihydroxy-9,10-anthraquinone), indicating that the additional hydroxy group in the 2-position in purpurin can be considered as a minor perturbation of the chromophore.