Diketopyrrolopyrroles (DPP) are well-known organic pigments. The title compound (DPPP) is one of DPP derivatives that shows a high proton affinity because of the N atoms of the pyridyl ring. Protonation at the N-site brings about a variety of changes in color as well as in electrical and photoelectrical conductivities. Two crystal modifications have been found to exist: DPPP I (grown from vapor phase) and DPPP II (grown from solution). Structure analysis revealed that there is a striking difference in intermolecular hydrogen bonds between two modifications: DPPP I is characterized only by NH…O hydrogen bonds whereas DPPP II include two of the H-bond based on NH…N and NH…O.
Tomohiko Imoda, Hiroo Takahashi, Jin Mizuguchi, "Polymorph of 1,4-diketo-3,6-dipyridyl–pyrrolo-[3,4-c]pyrrole" in Proc. IS&T Int'l Conf. on Digital Printing Technologies (NIP20), 2004, pp 520 - 523, https://doi.org/10.2352/ISSN.2169-4451.2004.20.1.art00002_2