Structural understanding of a polymorphic system by structure solution and refinement from powder X-ray diffraction data: the alpha and beta phases of the latent pigment DPP-Boc

The latent pigment 1,4-dioxo-2,5-di-tert-butoxycarbonyl-3,6-diphenyl-1,2,4,5-tetrahydropyrrolo[3,4-c]pyrrole (abbreviated DPP-Boc) has been found to exist in the solid state in two polymorphic forms, denoted the alpha and beta phases. Above ambient temperature, a thermal decomposition reaction occurs in both polymorphs to produce the commercially important pigment DPP. Significant differences have been observed in kinetic aspects of this reaction in the alpha and beta phases, and such differences in reactivity must ultimately originate from structural differences between the two polymorphs. In this paper we focus on establishing a structural understanding of these materials. The structure of the alpha phase of DPP-Boc has been determined previously from single crystal X-ray diffraction data, although certain features of the reported structure are unsatisfactory. Here we report a revised structure determination of the alpha phase by Rietveld refinement from powder X-ray diffraction data. The structure of the beta phase of DPP-Boc, which was previously unknown, has been solved directly from powder X-ray diffraction data using our Monte Carlo technique and Rietveld refinement. A detailed description and comparison of the structural properties of the alpha and beta phases of DPP-Boc is presented.