Abstract
Indigo is composed of two pairs of NH group (electron donor) and C=O group (acceptor) which form strong intermolecular hydrogen bonds based on NH…O. Quinacridone (QA) and diketopyrrolopyrrole (DPP) are also classified into the same kinds of hydrogen-bonded pigments. While the color of QA and DPP in solution is pale-yellow, indigo exhibits a vivid blue color in solution. However, crystallization of QA and DPP induces a large bathochromic shift (about 1,500-1,600cm-1) to give a brilliant red color in the solid state, whereas the spectral shift in indigo amounts only to about 1,100cm-1. This mechanism has been tackled from the standpoint of molecular and crystal structures as well as intermolecular interactions. The transition dipole of indigo is found to point the long-molecular axis, while it is along the short-molecular axis in QA and DPP. Therefore, in indigo, the electrons can well be delocalized upon photo-excitation along the long-molecular axis, leading to an optical absorption in the visible region (“blue color”). On the other hand, the electron delocalization is limited only along the short-molecular axis in QA and DPP,giving a pale-yellowish color. In the solid-state, the interaction between transition dipoles is found to play an important role. The intermolecular hydrogen bonds in QA and DPP align the transition dipoles (“head-to-tail” arrangement), thus inducing a large bathochromic shift on crystallization. However, the “head-to-tail” arrangement is not available in indigo. So the bathochromic shift is relatively small on going from solution to the solids state.
