Lockable Multiple Twisting in Donor–Acceptor Molecules for Emergent Crystalline Structures and Optical Properties

In comparison to incorporating different functional groups into rigid molecules to generate diverse interaction competitions, achieving lockable multiple twisting conformations in the molecular backbone can provide a similar capability. This approach also helps streamline the synthesis process and reduces the likelihood of unintentional property impairments. However, this strategy remains largely unexplored in small molecules. Here, we report the development of a donor–acceptor (D–A) molecule with a D–D–A–D–D backbone structure and sophisticated side-chain design that enables lockable multiple twisting conformations in its molecular backbone. Through heteroseeded self-assembly, orange-emitting two-dimensional (2D) platelets can be formed. These platelets consist of a unique D-D-A-D-D conformer with a benzothiadiazole group and neighboring fluorene groups arranged in almost the same plane but with a significant twisting relative to the next attached fluorene groups. The resulting 2D platelets exhibit a pronounced redshift in their optical properties compared to the monomer in solution, along with a high fluorescence quantum yield of approximately 73%. Furthermore, this molecule can adopt two intertwined conformations with large twisting angles to form one-dimensional (1D) microribbons, which emit green light. The packing of these two conformations results in slightly red-shifted optical spectra compared to the monomer in solution and a high fluorescence quantum yield of around 87%.