Solution and solid-state spectra of quinacridone derivatives as viewed from the intermolecular hydrogen bond

Quinacridones are industrially important hydrogen-bonded pigments. The color in the solid state is vivid red, while it is pale yellow in solution, indicating evidently the involvement of intermolecular interactions in the solid state. The electronic structure has therefore been investigated with special attention to the role of intermolecular NH center dot center dot center dot O hydrogen bonds for three representative quinacridone compounds with different hydrogen-bond forming characteristics: unsubstituted gamma-quinacridone (gamma-QA) with two NH groups, mono-N- methylquinacridone (MMQA) with one NH and one CH3, and N,N'-dimethyl-quinacridone (DMQA) with two CH3 groups. The number of the NH center dot center dot center dot O hydrogen bonds per molecule is four, two, and zero for gamma-QA, MMQA, and DMQA, respectively. In solution, no significant difference in absorption maximum is recognized between gamma-QA, MMQA, and DMQA. However, in the solid state, the absorption maximum of gamma-QA appears at the longest wavelength, followed by MMQA and then DMQA, depending on the number of NH center dot center dot center dot O intermolecular hydrogen bonds. The role of the hydrogen bond is found to align transition dipoles in a "head-to-tail" fashion and to displace the absorption band toward longer wavelengths due to excitonic interactions. The extent of the spectral shift increases with increasing number of hydrogen bonds per molecule.