The title compound (BP-DPP), an industrially important red pigment, is characterized by an extremely large bathochromic shift upon crystallization as compared with other DPP analogues. The electronic structure of BP-DPP has therefore been investigated from the standpoint of crystal structure and exciton coupling effects. BP-DPP is found to crystallize in a fashion “bricks in a brick wall”, just like J-aggregates in cyanine dyestuffs. On the molecular plane of BP-DPP, there are chains of intermolecular hydrogen bonds between the NH group of one molecule and the O atom of the neighboring one. The hydrogen bond aligns the transition dipoles in a fashion “head-to-tail”, leading to a bathochromic displacement. Additionally, there are diagonal pairs of the brick wall structure whose exciton coupling again induces a bathochromic shift. Both the hydrogen bond on the molecular plane and the brick wall structure along the molecular stack are found to significantly displace the absorption band toward longer wavelengths on going from solution to the solid state.
J. Mizuguchi, "J-aggregate-Like Structure and its Influence on the Electronic Spectra in 1,4-diketo-3,6-bis(4-biphenylyl)pyrrolo[3,4-c]pyrrole" in Proc. IS&T Int'l Conf. on Digital Printing Technologies (NIP17), 2001, pp 760 - 763, https://doi.org/10.2352/ISSN.2169-4451.2001.17.1.art00072_2