2D elastic fluorinated donor–acceptor type π-conjugated molecular crystals and their optical crystal–polymer hybrid films

Abstract

Robust and mechanically deformable organic molecular crystals present a promising alternative to conventional inorganic and polymeric solids as flexible materials. Here, we report the design of elastic crystals based on asymmetric fluorinated donor–acceptor–donor molecular structures involving 2D planes of heteroatom interactions and pitched π-stacking arrangements. 4,7-Bis(2-thienyl)-5,6-difluoro-2,1,3-benzothiadiazole in crystals exhibited a planar structure with minimized torsion angles (within 2.04°) due to intramolecular heteroatom interactions and hydrogen bonds between the fluorinated benzothiadiazole acceptor and thiophene donor groups. The crystal also formed a two-dimensional noncovalent network along the ac-axis, stabilized by F–H hydrogen bonds between the fluorinated benzothiadiazole-thiophene units and CH–π interactions between thiophene moieties. Furthermore, introducing heteroatoms along the molecular short-axis leads to the formation of pitched π-stack structures. These crystals show macroscopic elasticity for applying stress in each (100) and (001) plane, respectively; thus, the material has 2D elasticity. Micro- and nano-indentation tests revealed that the mechanical robustness (elasticity and hardness) of the crystal surpasses that of common polymeric solids. In addition to their mechanical properties, the crystal exhibited yellow-photoluminescence due to charge-transfer interaction (a maximum emission wavelength of 563 nm). This design of donor–acceptor–donor promotes inter- and intra-molecular charge-transfer interactions. The 2D elastic crystals were easily hybridized with polyimide films and exhibited 3D active optical waveguide properties. This molecular-supramolecular strategy for tuning the mechanical and optical properties provides new opportunities for the engineering of crystal functionalities.