Direct Formation of Large-Area 2D Nanosheets from Fluorescent Semiconducting Homopolymer with Orthorhombic Crystalline Orientation

Semiconducting
polymers have been widely investigated due to their
intriguing optoelectronic properties and their high crystallinity
that provides a strong driving force for self-assembly. Although there
are various reports of successful self-assembly of nanostructures
using semiconducting polymers, direct in situ self-assembly
of these polymers into two-dimensional (2D) nanostructures has proven
difficult, despite their importance for optoelectronics applications.
Here, we report the synthesis of a simple conjugated homopolymer by
living cyclopolymerization of a 1,6-heptadiyne (having a fluorene
moiety) and its efficient in situ formation of large-area
2D fluorescent semiconducting nanostructures. Using high-resolution
imaging tools such as atomic force microscopy and transmission electron
microscopy, we observed the solvent-dependent self-assembly behaviors
of this homopolymer; the identical starting polymer formed 2D nanosheets
with different shapes, such as rectangle, raft, and leaf, when dissolved
in different solvents. Furthermore, super-resolution optical microscopy
enabled the real-time imaging of the fluorescent 2D nanosheets, revealing
their stable and uniform shapes, fluorescence, and solution dynamics.
Notably, we propose an orthorhombic crystalline packing model to explain
the direct formation of 2D nanostructures based on various diffraction
patterns, providing important insight for their shape modulation during
the self-assembly.