Unexpected Stacking Transformation from H- to J- and X-Aggregated Co-Crystals Leading to Tunable Mechanical and Optoelectronic Properties

Abstract

Molecular stacking modes, generally classified as H-, J-, and X-aggregation play a key role in determining the optoelectronic properties of organic crystals. However, the control of stacking transitions of a specific molecule is an unmet challenge and a priori prediction of performance in different stacking modes is currently not possible. Herein, unexpected stacking transition from H- to J- and X- aggregation is observed in the crystal structure of a minimalistic aromatic molecule, 4,4’-bipyridine, upon co-assembled with N-acetyl-L-alanine, a non-aromatic amino acid derivative. This stacking transition improves the physicochemical properties of the co-crystals, including a large red-shifted emission, enhanced chirality, improved thermal stability and mechanical properties. While the single crystal of 4,4’-Bpy shows good optical waveguiding and piezoelectric properties, the co-crystal has a significantly lower band-gap and improved conductivity. This demonstrates the first example of H- to J- and X- packing transition of a minimalistic model by co-crystal aggregation and provides a proof-of-concept strategy for organic optoelectronics design.

Keywords: Organic co-crystals, molecular stacking, supramolecular chemistry, mechanical property, optoelectronic materials