Electrochemical C–H Arylation of Polythiophenes By Using Aryldiazonium Salts
Authors: Koiso Satoshi , Sato Kosuke , Inagi Shinsuke 
Published: December 2024

Electrochemical post-functionalization is a polymer reaction method in which a nucleophile (or electrophile) attacks the doping state of a π-conjugated polymer generated by applying a potential to the polymer film1,2). This reaction scheme faces various challenges, including the limitation that only conducting polymers can be used as a precursor in the solid state. In order to solve this problem, development of a liquid-phase electrochemical post-functionalization is required. However, it is also known that electron transfer between electrodes and dissolved polymers does not proceed efficiently. Therefore, in this study, we developed a liquid-phase electrochemical post-functionalization that does not rely on direct electron transfer between polymers and electrodes. This method is expected to broaden the application of the electrochemical post-functionalization.

Based on the report on the electrochemical C–H arylation of heteroaromatic rings by using the aryl radicals produced by the reduction of aryldiazonium salts3), we investigated to apply the reaction for the C–H arylation of poly(3-hexylthiophene) (P3HT). This approach allows for direct C–H bond functionalization without the need for introducing a functional group beforehand. Moreover, the environmentally friendly electric energy is used as the reaction driving force, avoiding need of a metal catalyst required in conventional methods4,5).

Constant current electrolysis of P3HT dissolved in chlorobenzene with an aryl diazonium salt was conducted, and the introduction of an aryl group into the 4-position of the thiophene ring was confirmed by 1H NMR. However, the 1H NMR spectrum of the product (P3HT-Ar) suggested that an unsubstituted phenyl group derived from chlorobenzene was also introduced. Subsequent solvent investigation showed that using a mixed solvent (CHCl3/CH2Cl2 = 2:1) suppressed this side reaction. In addition, the formation of aryldiazonium salts in-situ, by adding 4-nitroaniline and tBuONO, yielded products with the higher degree of functionalization compared to the direct addition of aryldiazonium salts. Current density studies showed that at low current density conditions suppressed side reactions and improved the degree of functionalization. Furthermore, the degree of functionalization could be controlled to a certain range from 0% to 30%, by adjusting the charge passed. It was also possible to introduce 4-trifluoromethylphenyl group in this protocol. As a reference condition, when unsubstituted aniline without an electron-withdrawing group was used as a substrate, no arylation reaction proceed.

Finally, the photophysical and electrochemical properties of P3HT-Ar were investigated. CV measurements of P3HT and P3HT-Ar with different degree of functionalization showed the oxidation potential shifted positively with increasing the degree of functionalization. The reduction peak, which did not appear in P3HT, was observed for P3HT-Ar. Furthermore, UV-vis absorption spectroscopy revealed that the maximum absorption wavelength (λmax
abs) shortened as the degree of functionalization increased. These are possibly induced by the electron-withdrawing property of the 4-nitrophenyl group and the decrease in the effective π-conjugation length due to the main chain twisting. The fluorescence quantum yield significantly decreased with increasing the degree of functionalization, attributed to the fluorescence quenching effect of the 4-nitrophenyl group.

Reference

1) S. Inagi, T. Fuchigami, Macromol. Rapid Commun., 2014, 35, 854.

2) T. Kurioka, S. Inagi, Chem. Rec., 2021, 21, 2107.

3) D. Hata, M. Tobisu, T. Amaya, Bull. Chem. Soc. Jpn.,
2018, 91, 1749.

4) Y. Li, G. Vamvounis, J. Yu, S. Holdcroft, Macromolecules,
2001, 34, 3130.

5) H. Goto, B. Ochiai, Y. Matsumura, Electrochemistry, 2023, 91,112004.

6) D. Giri, S. N. Islam, S. K. Patra, Polymer, 2018, 134, 242.

Figure 1