Accessing Selective Crystallization of ROY Polymorphs Using Directional Crystallization from the Melt

A substance can crystallize in more than one crystal form. This phenomenon, known as polymorphism, occurs particularly often for organic compounds because of the comparable lattice energies of the different molecular conformations or packing arrangements. The reproducible obtaining of some specific polymorphs remains an open scientific question because polymorphism is dramatically sensitive to experimental conditions. 5-Methyl-2-[(2-nitrophenyl) amino]-3-thiophenecarbonitrile (ROY) is an ideal molecular system to study polymorphism since it crystallizes into at least 13 different forms. Directional crystallization under defined dissipative thermodynamic conditions was applied to control the selective growth of ROY polymorphs. It is demonstrated that one of the thirteen polymorphs (OP) can reproducibly be obtained. The reasons for this selectivity are discussed in the frame of nonequilibrium thermodynamics.