Probing the Polymorphic Landscape of ROY through Selective Deuteration

Abstract

ROY is an iconic polymorphic system, known for its red, orange, and yellow crystal forms that arise from subtle differences in molecular conformation and packing. Here, we examine how this well-established solid-state landscape responds to one of the smallest possible structural modifications: selective H/D substitution. Three selectively deuterated analogues, ROY-D1, ROY-D4, and ROY-D8, containing one, four, and eight deuterium atoms, respectively, were synthesized and subjected to systematic polymorph screening using solution crystallization, solvothermal methods, liquid-assisted grinding, and melt crystallization. The resulting forms reveal extensive isostructural relationships with known ROY polymorphs while preserving the characteristic correlation between molecular torsion angle and crystal color. Overall, the deuterated analogues largely preserve the ROY crystal-structure landscape, retaining known packing types and the torsion-angle−color relationship, while showing subtle differences in polymorph formation, thermal transitions, and storage-related thermal response.