Effect of Mechanical Properties and Photoluminescence in Elastic Crystals Triggered by Substituent Regioisomerism

Deformable organic photoluminescent crystals are a kind of fundamental material to exploit cutting-edge flexible optoelectronic devices. However, the realization that modulation of mechanical and photoluminescent properties is difficult due to structural properties, relationships for bendable molecular crystals remain unclear, especially between molecular packing, mechanical properties, and photoluminescence (PL). Herein, we report different supramolecular structures of intermolecular π···π stacking via the substituent regioisomerism effect (SRIE), based on three crystalline dicarbazol-9-yl-based pyrazine regioisomers of para-, meta-, and ortho-convergent substitutions (p-DCzP, m-DCzP, and o-DCzP, respectively). We discover that the elasticity of p-DCzP and m-DCzP crystals benefit from one-dimensional π···π stacking chains of carbazole (Cz) units, while the brittleness of o-DCzP crystals results from the multidirectional π···π stacking of Cz units. Molecular dynamics (MD) simulations suggest that the energy of one-dimensional π···π stacking supramolecular chains is beneficial for reducing noncovalent interaction energy and stabilizing the aggregation structures. In addition, such π···π stacking chains in elastic crystals of m-DCzP can vanish the aggregation-caused quenching effect to maintain higher photoluminescence quantum yields of blue emission from dilute solutions to crystals. This research reveals the SRIE on mechanical properties and photoluminescence in molecular crystals. We believe that this work will provide new perspectives for the development of flexible optoelectronic functional crystal materials.