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

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 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.

Density functional theory predictions of the mechanical properties of crystalline materials

“…Using this methodology, we have calculated to a high accuracy versus experiment the elastic constants of amino acid 90,[98][99][100] and peptide crystals, 101,102 co-crystals, 103,104 and biominerals 105 (Fig. 2).…”

Recent Advances in Organic and Organic–Inorganic Hybrid Materials for Piezoelectric Mechanical Energy Harvesting

“…In 2020, the same research group demonstrated the molecular stacking mode interactions between rigid aromatic 4,4′‐bipyridine (4,4′‐Bpy) and a nonaromatic amino acid derivative of N‐acetyl‐L‐alanine (AcA) (see Figure a). [ 85 ] Transitions between the three known types of molecular stacking modes are known, namely, H‐aggregation (face‐to‐face stacking), J‐aggregation (staggered stacking) and X‐aggregation (crossed stacking), were observed in the hybrid system (Figure 7b,c). Due to the difficulties in studying each stacking mode in single molecules, hybrid systems were selected to study their physicochemical properties.…”

Ab-Initio Predictions of the Energy Harvesting Performance of L-Arginine and L-Valine Single Crystals

“…Initially the methodology was benchmarked with respect to three well-known inorganic piezoelectric materials; namely aluminium nitride (AlN), zinc oxide (ZnO) and α-quartz (SiO 2 ). This was then extended to the proteinogenic amino acids (Guerin et al, 2018a), biominerals (Guerin et al, 2018b), co-crystals (Ji et al, 2020) and peptides (Bera et al, 2021), with deviations from experiment ranging from 1-20%, which is highly accurate for identifying highperformance materials. The upper limit is observed in highly flexible materials with individual stiffness constants of less than 5 GPa.…”