Lessons on the Discovery and Assignment of Polymorphs, Highlighted by the Case of the Latent Pigment DPP-Boc

The latent pigment 1,4-diketo-2,5-di-t-butoxycarbonyl-3,6-diphenylpyrrolo[3,4-c]pyrrole (DPP-Boc) has three well-characterized polymorphic forms (denoted α, β, and γ) in the solid state, although some confusion exists in the literature concerning the assignment of these polymorphs. This paper clarifies the assignment of the three polymorphs of DPP-Boc, discusses various issues concerning the polymorphism of this material, and highlights several general issues regarding the characterization, discovery, and assignment of polymorphic systems.