posted on 2023-11-13, 18:05authored byKyung
Sun Park, Xuyi Luo, Justin J. Kwok, Azzaya Khasbaatar, Jianguo Mei, Ying Diao
Understanding the
solution-state aggregate structure and the consequent
hierarchical assembly of conjugated polymers is crucial for controlling
multiscale morphologies during solid thin-film deposition and the
resultant electronic properties. However, it remains challenging to
comprehend detailed solution aggregate structures of conjugated polymers,
let alone their chiral assembly due to the complex aggregation behavior.
Herein, we present solution-state aggregate structures and their impact
on hierarchical chiral helical assembly using an achiral diketopyrrolopyrrole-quaterthiophene
(DPP-T4) copolymer and its two close structural analogues wherein
the bithiophene is functionalized with methyl groups (DPP-T2M2) or
fluorine atoms (DPP-T2F2). Combining in-depth small-angle X-ray scattering
analysis with various microscopic solution imaging techniques, we
find distinct aggregate in each DPP solution: (i) semicrystalline
1D fiber aggregates of DPP-T2F2 with a strongly bound internal structure,
(ii) semicrystalline 1D fiber aggregates of DPP-T2M2 with a weakly
bound internal structure, and (iii) highly crystalline 2D sheet aggregates
of DPP-T4. These nanoscopic aggregates develop into lyotropic chiral
helical liquid crystal (LC) mesophases at high solution concentrations.
Intriguingly, the dimensionality of solution aggregates largely modulates
hierarchical chiral helical pitches across nanoscopic to micrometer
scales, with the more rigid 2D sheet aggregate of DPP-T4 creating
much larger pitch length than the more flexible 1D fiber aggregates.
Combining relatively small helical pitch with long-range order, the
striped twist-bent mesophase of DPP-T2F2 composed of highly ordered,
more rigid 1D fiber aggregate exhibits an anisotropic dissymmetry
factor (g-factor) as high as 0.09. This study can be a prominent addition
to our knowledge on a solution-state hierarchical assembly of conjugated
polymers and, in particular, chiral helical assembly of achiral organic
semiconductors that can catalyze an emerging field of chiral (opto)electronics.