posted on 2021-08-25, 20:15authored bySaadia Chaudhry, Yukun Wu, Zhiqiang Cao, Shi Li, Jodie L. Canada, Xiaodan Gu, Chad Risko, Jianguo Mei
While it is known that the chain
length strongly affects the properties
of π-conjugated polymers, the effects of chain length on the
molecular structure, chain conformation, and oxidation state properties
in donor–acceptor-type conjugated structures remain unclear.
This limits our understanding of how the polymer molecular weight
impacts material properties. Here, a discrete and monodisperse oligomer
series (n = 3, 5, 7, 9, 15, and 21) and polymers
(nPB), composed of the donor 3,4-propylenedioxythiophene
(ProDOT), the acceptor benzothiadiazole (BTD), and methylthio end-capping
groups, are synthesized by C–H-activated cross-coupling. The
molecular structure, molecular weight, and dispersity of each oligomer/polymer
are thoroughly characterized by nuclear magnetic resonance spectroscopy,
mass spectrometry, and gel permeation chromatography. This series
reveals a rod-to-coil transition at n = 15 and coil
formation at polymer length scales of ∼28 units via solution small-angle neutron scattering characterization. The oxidation
states are deciphered via cyclic voltammetry, differential
pulse voltammetry, spectroelectrochemistry, and density functional
theory calculations. Oligomers 3–9 undergo successive one-electron
oxidation steps, while 15 and higher undergo multielectron oxidations
per step in CH2Cl2–TBAPF6 at
a Pt ultramicroelectrode. The electronic transition of each oxidation
state (1+, 2+, 3+, etc.) is tracked by absorption
spectroscopy, revealing a “bipolaron to di-polaron”
transition at n = 7 at which shorter oligomers prefer
bipolaron formation and longer oligomers prefer di-polaron formation
in their dication states. Furthermore, oxidized 15 has a lower electronic
transition energy compared to its polymer homologue, as shown by spectroelectrochemistry,
revealing a synergy between the chain length and the oxidation state
properties. This study proves that the convergence limit between small
molecule and polymer behavior occurs at approximately 15 units and
highlights the property transitions that occur as a function of chain
length for a donor–acceptor class of conjugated organic materials.