Enabling Control over Mechanical Conformity and Luminescence in Molecular Crystals: Interaction Engineering in Action.

Molecular crystals of π-conjugated molecules are of great interest as the highly ordered dense packing offers superior charge and exciton transport. However, integration into optoelectronic devices remains a major challenge due to its inherent brittle nature. Herein, we report the control over mechanical conformity in the single crystals of pyridine appended thiazolothizaole derivative by modulating the molecular packing through interaction engineering. Two polymorphs were prepared by achieving control over the thermodynamic/kinetic factors of crystallization, one of the polymorphs exhibits elastic bending while the other is brittle. The control over the bending ability was achieved by forming co-crystals with hydrogen/halogen bond donors. A seamless extended criss-cross pattern with respect to the bend plane through ditopic H-bonding motif showed the highest compliance towards elastic bending, while the co-crystals with a layered criss-cross arrangement with the segregated layers of co-formers exhibit slightly lower bending conformity. Our results update the rationale behind the plastic/elastic bending in molecular crystals. The co-crystals of ditopic halogen bond co-crystals are particularly appealing for waveguiding applications as the co-crystals blends high mechanical flexibility and luminescence properties. The hydrogen bond co-crystals are non emissive in nature due to excited state proton transfer dynamics. The rationale behind the fluorescence properties of these materials were also established from DFT calculations in a QM:MM framework.