(424a) The Elementary Reactions for Incorporation into Crystals

Crystals endow natural and synthetic materials with essential structures and functions. The crystallization rates and pathways are largely dictated by the slow ingress of solute molecules into specifically structured sites on the crystal surfaces, the kinks, but the mechanism that administers incorporation into the kinks remains elusive. Here we show that the incorporation of a solute molecule into a kink divides into two elementary reactions. First, the solute forms an intermediate complex, in which it binds, but only partially, to the molecules that comprise the kink; the solute relocates to the kink in the second step. We combine time-resolved in situ atomic force microscopy with all-atom molecular dynamics simulations to examine the crystal growth of etioporphyrin I, which represents a class of materials with appealing electronic properties. We employ solvents with distinct functionalities as reporters on the molecular structures and dynamics along the incorporation pathway. Violating the classical models of solution crystallization, the measured activation barriers for crystallization from four solvents disconnect from the respective strengths of the solute-solvent interactions. Experiments and simulations resolve this controversy to reveal an intermediate state, stabilized by solute-solvent contacts, that breaks incorporation into two elementary reactions. The spatial constraints on solvent access to the open solute sites rank how much the distinct solvents stabilize the intermediate state and set the height of the barrier to be surmounted for incorporation. The proposed two-step scheme of molecular incorporation presents a new paradigm for solution crystallization that may contribute to understanding crystallization in nature and expedite the selection of solutes and solvents in the crystallization process design of organic pharmaceuticals and advanced materials.