Classical and Nonclassical Growth Modes of Etioporphyrin I Crystals

Etioporphyrin I is a model for the poorly understood crystallization of organic materials that is a part of device manufacturing, pharmaceutical production, and numerous physiological and pathological processes. Single crystals of porphyrins and their metal derivatives find application as transistors, solar cells, and field-effect elements, owing to their electronic and optical properties. Here we examine the molecular mechanisms employed during the growth of etioporphyrin I crystals. Atomic force microscopy, absorption spectroscopy, and molecular modeling reveal a unique combination of classical and nonclassical growth modes and distinct incorporation mechanisms of two adjacent crystal faces (010) and (001). At low supersaturations, the (010) faces grow strictly classically, by association of solute molecules to dislocation-generated steps, where the step velocity scales linearly with the solute concentration and the step density, with the chemical potential excess of the solute over that in the crystal. By contrast, the (001) face displays a bimolecular growth mechanism, where step velocity scales quadratically with concentration, likely due to the incorporation of solute dimers found to preform in the solution. We tentatively attribute the distinct growth behaviors to the different surface structures of the two faces, with the smoother (010) face facilitating monomer incorporation and the furrowed relief of the (001) face favoring dimer incorporation. At elevated supersaturations, growth on the (010) face transitions to another nonclassical growth mode, in which mesoscopic solute-rich clusters of etioporphyrin I, assembled in the solution, contribute strongly to growth by landing on the crystal surfaces and transforming into stacks of up to 50 layers that spread laterally. Both nonclassical growth pathways rely on weak attraction between solute monomers that hold together the dimers and are crucial to cluster assembly. Such attraction is typical in organic solute–solvent systems in which intermolecular forces are dominated by van der Waals interactions. Thus, our findings suggest that similar deviations from classical growth scenarios may be common in organic materials synthesis.