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

The crystal structures of purpurin monohydrate and 1,4-dihydroxy-5,8-di-p-t oluidinoanthraquinone were determined from single-crystal and powder X-ray diffraction data, respectively, and the visible reflection spectra of 18 cr ystalline colorants, (anthraquinones and related compounds) were obtained. Twelve of the compounds showed moderate (440-1090 cm(-1)) bathochromic shif ts of their absorption maxima, compared to the solutions; other compounds e xhibited greater bathochromic (up to 2900 cm(-1)) or hypsochromic (up to -1 130 cm(-1)) shifts. For some of the compounds, the differences in colour be tween crystals and solutions are ascribable to the changes in molecular con formations or to the impact of hydrogen bonds. (C) 2000 Elsevier Science Lt d. All rights reserved.