Visible spectra of crystalline anthraquinone colorants: the impact of crystal packing

The crystal structures of purpurin monohydrate and 1,4-dihydroxy-5,8-di-ptoluidinoanthraquinone were determined from single-crystal and powder X-ray diffraction data, respectively, and the visible reflection spectra of 18 crystalline colorants, (anthraquinones and related compounds) were obtained. Twelve of the compounds showed moderate (440¯1090 cm-1) bathochromic shifts of their absorption maxima, compared to the solutions; other compounds exhibited greater bathochromic (up to 2900 cm-1) or hypsochromic (up to -1130 cm-1) shifts. For some of the compounds, the differences in colour between crystals and solutions are ascribable to the changes in molecular conformations or to the impact of hydrogen bonds.