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

Abstract

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 α and β 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 α and β 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 α 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 α phase by Rietveld refinement from powder X-ray diffraction data. The structure of the β 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 α and β phases of DPP-Boc is presented.