(204c) Continuum of Solute Dimers Drives Distinct Growth Modes on Anisotropic Faces of Etioporphyrin

One of the basic tenets of the classical theories of crystal nucleation and growth is the Szilard postulate, which states that molecules from the supersaturated phase join a nucleus or a growing crystal individually. Over the years, numerous crystals have been found to nucleate by assembly of monomers and grow by sequential association of monomers to growth sites. Recent experiments in bio- and geomimetic environments have spotlighted a partial deviation from the Szilard rule, a bimodal distribution of growth species that arises from the assembly of ordered or amorphous structures in the solution, which then incorporate into the crystal as a whole. Here we report a complete violation of the Szilard rule. In octanol solutions of etioporphyrin, a planar molecule whose crystals have attractive electronic and optical properties, the solute populates a continuum of quasi-parallel dimers that differ from the conformation in the crystal by shift and orientation. Molecular absorption and luminescence spectroscopies and all-atom molecular dynamics (MD) simulations reveal that the stabilities of the dimers are within the thermal energy and enforce free conversion between the distinct structures. The monomer concentration in the solution in below 0.1 %. Despite the structural complexity of the solute, the velocity of the steps on the (010) faces of etioporphyrin crystals (measured using time resolved in situ AFM) increases linearly with the total solute concentration. The observed linearity indicates that the molecular reaction at the growth sites in monomolecular and the majority of solute dimers seamlessly adopt the crystallographic conformation. By contrast, step growth on the other major face, (001), scales with the square of the solute concentration. We eliminate three potential mechanisms that may manifest as super-linear growth: inaccurate solubility determination, escalating kink density at elevated supersaturation, and adsorption of step pinners on the crystal surface. The disqualification of the three scenarios enforces the conclusion that the quadratic kinetic law indicates a bimolecular process of incorporation. We demonstrate that in contrast to numerous other solution grown crystals, solute molecules directly incorporate in the kinks and do not reach the growth sites after first adsorbing on the terraces between steps. The paucity of solute monomers precludes a collision between two monomers to form a crystallographic dimer as the mechanism of the crystallization reaction. MD evaluation of solute interactions with kinks on the (001) face reveal a molecular trap selective for solute dimers located on the approach to all kinks, which necessitates a collision with a second dimer to ease the captured solute towards the kink. The results with etioporphyrin demonstrate the lack of correlation between the solute state and the incorporation kinetics driven by the solute. In a broader context, these observations illuminate the immense diversity of unexplored crystallization scenarios that pave the long road to crystallization control.