Organic Crystal Growth: Hierarchical Self-Assembly Involving Nonclassical and Classical Steps

Organic crystal nucleation and growth are complex processes that often do not fit into the framework of the existing crystallization theories. We investigated a crystal growth mechanism of an organic dye, perylene diimide, using high -resolution cryogenic transmission electron microscopy and optical spectroscopy. The elucidated mechanism involves classical (monomer attachments) and nonclassical pathways, exhibiting a self-assembly sequence where all steps are interconnected. It starts from the assembly of molecular pi-stacks that are initially disordered. They gradually optimize their structure, rigidify, and interact to form crystalline domains. The latter further evolve via the addition of individual molecules, and crystal fusion (via oriented attachment). All the observed supramolecular trans-formations are connected and follow a clear hierarchy starting from the molecular-scale interactions. The elucidation of the complex pathway of organic crystallization as a series of coordinated supramolecular transformations at multiple scales conceptually advances the understanding of order evolution in organic matter.