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Issue 12, 2020
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The importance of sulfonate to the self-doping mechanism of the water-soluble conjugated polyelectrolyte PCPDTBT-SO3K

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Abstract

The conjugated polyelectrolyte poly[2,6-(4,4-bis-potassium butanylsulfonate-4H-cyclopenta-[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBTSO3K, or CPE-K) is part of a unique class of organic semiconducting polymers that are soluble in water and become doped in the presence of a proton source. One of these proton sources is water, which CPE-K is exposed to during purification by dialysis, allowing for its immediate use without an additional doping step. Previous studies have suggested that the actual dopant is H+, and that the addition of sodium hydroxide to CPE-K reverses the process and de-dopes the polymer. Curiously, the majority of these water-soluble self-doped polymers in the literature contain a pendant sulfonate group. However, a detailed study to gain insight into the importance of the sulfonate group in water-soluble self-doped polymer systems is lacking, despite its widespread presence in the literature. In this work, using Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and ultraviolet-visible-near infrared (UV-vis-NIR) absorption spectroscopy, we are able to elucidate the role of the pendant sulfonate group with regards to the fundamental doping mechanism of CPE-K. Specifically, we found the CPDT unit can be protonated by water, which is due to both the stabilizing influence of the sulfonate group, and the relatively high oxidation potential of the CPDT unit, both of which are required for self-doped water soluble polymer systems. Furthermore, we estimate the doping efficiency of CPE-K using electrical measurements and X-ray reflectivity (XRR), and reveal several different methods for the doping and de-doping of this material, showcasing this particular material's superior versatility.

Graphical abstract: The importance of sulfonate to the self-doping mechanism of the water-soluble conjugated polyelectrolyte PCPDTBT-SO3K

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Supplementary files

Article information


Submitted
08 Feb 2020
Accepted
25 Mar 2020
First published
26 Mar 2020

Mater. Chem. Front., 2020,4, 3556-3566
Article type
Research Article
Author version available

The importance of sulfonate to the self-doping mechanism of the water-soluble conjugated polyelectrolyte PCPDTBT-SO3K

D. X. Cao, D. Leifert, V. V. Brus, M. S. Wong, H. Phan, B. Yurash, N. Koch, G. C. Bazan and T. Nguyen, Mater. Chem. Front., 2020, 4, 3556 DOI: 10.1039/D0QM00073F

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