Post-Functionalization of Thiophene-Fluorene Alternating Copolymers via Anodic Phosphonylation
Authors: Taniguchi Kohei , Kurioka Tomoyuki , Sato Kosuke , Tomita Ikuyoshi , Inagi Shinsuke 
Published: August 2024

Phosphonylated polymers are promising materials because of their application in sensor chemistry and their unique electronic properties. Among these polymers, conjugated polymers containing dialkylphosphonate at their side chains have been used in chemical sensors1 and exhibit unique electronic properties.2 However, there have been a few reports on phosphonylation into the conjugated polymer main chains except for poly(3-hexylthiophene) (P3HT)3 and polyaniline derivatives,4 demonstrating that the effect of phosphonate on their main chains has been unclear. To introduce the phosphonate into their main chains, electrochemical post-functionalization is an attractive approach because of the simple protocol and mild reaction conditions.5 Recently, anodic phosphonylation of P3HT at the main chains with trialkyl phosphite was reported.3 However, the applicable precursor polymer is limited to P3HT. Furthermore, the substitution degree was quite low (0.25), indicating that it is necessary to be improved for establishment.

Here, to expand applicable precursor polymers and improve the substitution degree, anodic phosphonylation of thiophene–fluorene alternating copolymers (P(Fl-Th)) was conducted (Figure 1), and then we obtained the phosphonylated conjugated polymer (P(Fl-Th-phos(Et)) (substitution degree: 0.26). The obtained polymer showed that phosphonate was introduced selectively into electron-rich thiophene rings. However, the substitution degree was still low. When a 2,6-lutidine was added to the reaction solution, the substitution degree was increased up to 0.83. Furthermore, the substitution degree could be controlled by tuning the amount of charge passed, and the optoelectronic properties of P(Fl-Th-Phos(Et)) were gradually changed through control of the substitution degree. In addition, it was found that various trialkyl phosphite were available in anodic phosphonylation.

To functionalize the fluorene units, we carried out anodic phosphonylation of P(Fl-EDOT) which was di-substituted at thiophene moieties, and then the degree of phosphonylation reached 0.75 under the constant-current condition. On the other hand, when we performed constant-potential anodic phosphonylation of P(Fl-EDOT), the substitution degree was improved up to 0.98, indicating that the use of constant-potential electrolysis enabled quantitative anodic phosphonylation.

These results demonstrate that anodic phosphonylation can be applied to versatile π-conjugated polymers and that the position of phosphonate introduction can be controlled through the design of appropriate precursor polymers. In addition, due to two alkyl chains, the phosphonate group could enable further modification such as side-chain engineering of conjugated polymers.

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Figure 1