Morphology-dependent AEE performance of conjugated poly(tetraphenylethene)s

Abstract

The relationship between aggregation-enhanced emission (AEE) features and aggregation morphologies of AEE-active conjugated polymers in solvents of weakened quality is of fundamental importance to their optoelectronic, sensing or imaging applications. To address this correlation, we synthesized three conjugated poly(tetraphenylethene)s, o-PTPE, m-PTPE and p-PTPE, via Sonogashira cross-coupling polymerization. Among them, the 1,2-bis(4-ethynylphenyl)-1,2-diphenylethene (TPEE) units are linked at the ortho, meta and para positions of the phenyl groups, respectively. They show typical AEE behaviors in solvents of weakened quality, but with different spectral trends in different solvent mixtures. Concomitantly, these conjugated polymers self-assemble to form free-standing sheets with a thickness of ca. 10 nm and a micrometric plane in the toluene/methanol mixture solvents with moderate methanol contents. Solid nanospheres form when the solvent quality was further weakened with the increasing methanol content. o-PTPE and p-PTPE self-assemble into hollow nanostructures in chloroform/hexane mixtures. These aggregates are typically mesostable and dynamic in solution and can be further transformed into solid nanospheres. However, in the chloroform/methanol solvents, the conjugated poly(tetraphenylethene)s form solid nanospheres. These aggregation morphologies and evolutions can be directly correlated to the AEE spectral behaviors. In the solvents of weakened quality, o-PTPE, m-PTPE and p-PTPE, which are completely hydrophobic semiflexible homopolymers, can be packed into ordered nanostructures such as nanospheres, hollow nanoassemblies and free-standing sheets. This self-assembly mode has transcended the classical amphiphilicity paradigm of micelle-like nano-objects fabricated by amphiphilic (macro)molecules in selective solvents.