Abstract
Organic solvents host the synthesis of high-value crystals used as pharmaceuticals and optical devices, among other applications. A knowledge gap persists on how replacing the hydrogen bonds and polar attraction that dominate aqueous environments with the weaker van der Waals forces affect the growth mechanism, including its defining feature, whether crystals grow classically, by association of monomers, or nonclassically, by integration of precursors. Here we demonstrate a rare dual growth mode of etioporphyrin I crystals, enabled by liquid precursors that associate with crystal surfaces to generate stacks of layers, which then grow laterally by incorporating solute molecules. We combine time-resolved in situ atomic force microscopy to monitor the evolution of crystal surfaces with microfluidics to measure crystal growth rates; scattering microscopy to characterize the precursors; density functional theory, absorption spectroscopy and molecular simulations to characterize the molecular interactions in the solution; and quantitative optical birefringence to assess crystal quality. Our findings reveal the precursors as mesoscopic solute-rich clusters, a unique phase favored by weak bonds such as those between organic solutes. The lateral spreading of the precursor-initiated stacks of layers crucially relies on abundant solute supply directly from the solution, bypassing adsorption and diffusion along the crystal surface; the direct incorporation pathway may, again, be unique to organic solvents. Clusters that evolve to amorphous particles do not seamlessly integrate into crystal lattices but incorporate as gross defects. Crystals growing fast and mostly nonclassically at high supersaturations are not excessively strained. Our findings demonstrate that the weak interactions with solutes typical of organic solvents promote nonclassical growth modes by supporting liquid precursors and enabling the spreading of multilayer stacks.