(152br) Concentration-Driven Transition between Classical and Nonclassical Modes in Organic Crystallization.

One of the most consequential assumptions of the classical crystal growth theories is the Szilard postulate, which states that molecules join a growing crystal individually. Myriad examples of classical crystallization by sequential addition of single molecules exist. Recent studies have documented numerous violations of the Szilard rule, whereby amorphous or crystalline precursors form in the solution and incorporate into a crystal surface contributing to a fast growth mode. The found transition to nonclassical crystal growth requires chemical modifications that range from solution pH and ionicity shifts to the addition of auxiliary compounds. Here we demonstrate that crystals of etioporphyrin I transition from classical to nonclassical growth driven only by increasing solute concentration. Etioporphyrin I crystals carry appealing optical and electronic properties due to their low symmetry. At moderate supersaturations, etioporphyrin I crystals grow classically by the association of solute molecules to dislocation-generated steps. At intermediate driving forces, oval amorphous particles of about 100 nm size associate to the surface, do not contribute to growth, and immerse under advancing crystal layers to creating major three-dimensional defects in the crystal. At elevated supersaturations, numerous particles of similar size land on the crystal surface and contribute to a fast non-classical growth mode. These particles merge with the crystal lattice and transform into stacks of about 20 crystal layers, which then spread along the surface and coalesce with other layer stacks. Light scattering characterization of etioporphyrin I solutions reveals the presence of mesoscopic solute-rich clusters which exhibit the four signature behaviors of this phase: their average diameter, about 100 nm, is steady in time and independent of the solute concentration, their number increases exponentially with solute concentration, and the solute concentration, with which they equilibrate, correlates with the initial solute concentration. These properties of the clusters distinguish them from domains of other condensed phases, such as crystals or dense liquids. The size similarity suggests that the particles that land on the crystal surface originate as mesoscopic solute rich clusters which may land on the surface after ageing transforms them into amorphous domains or while still liquid. Crystal quality assessment by polarized microscopy reveals that crystals grown nonclassically are defect-rich and may be less useful as electronic elements. The observations with etioporphyrin I crystallization suggest that greater crystal perfection derives from classical growth and nonclassical crystallization may be avoided either by lowering the supersaturation or by employing additives that suppress cluster formation.