Tunable Mechanical and Optoelectronic Properties of Organic Cocrystals by Unexpected Stacking Transformation from H- to J- and X‑Aggregation

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 transformation of a specific
molecule is an unmet challenge, and a priori prediction
of the performance in different stacking modes is extraordinarily
difficult to achieve. In particular, the existence of hybrid stacking
modes and their combined effect on physicochemical properties of molecular
crystals are not fully understood. Herein, unexpected stacking transformation
from H- to J- and X-aggregation is observed in the crystal structure
of a small heterocyclic molecule, 4,4′-bipyridine (4,4′-Bpy),
upon coassembly with N-acetyl-l-alanine
(AcA), a nonaromatic amino acid derivative. This structural transformation
into hybrid stacking mode improves physicochemical properties of the
cocrystals, including a large red-shifted emission, enhanced supramolecular
chirality, improved thermal stability, and higher mechanical properties.
While a single crystal of 4,4′-Bpy shows good optical waveguiding
and piezoelectric properties due to the uniform elongated needles
and low symmetry of crystal packing, the significantly lower band
gap and resistance of the cocrystal indicate improved conductivity.
This study not only demonstrates cocrystallization-induced packing
transformation between H-, J-, and X-aggregations in the solid state,
leading to tunable mechanical and optoelectronic properties, but also
will inspire future molecular design of organic functional materials
by the coassembly strategy.