Electrochemical C–H Phosphonylation of Dissolved Conjugated Polymers via Direct Electron Transfer at the Electrode

π-Conjugated polymers (CPs) exhibit attractive optoelectronic properties conferred by an extended π-conjugation system along their main chains. CPs containing polar phosphonate groups have attracted special interest because their exceptional affinity to metal ions and electrodes can be exploited in electrochemical devices. A favorable strategy for postfunctionalizing CPs with phosphonate groups is electrochemical phosphonylation, which requires only a simple protocol under mild reaction conditions. However, this established method requires a CP film for the electrolytic process. A postfunctionalization method based on the conventional electrolysis of CPs in solution is expected to improve the versatility and scalability of electrochemical phosphonylation. Herein, we demonstrate that CPs dissolved in an electrolyte can be electrochemically phosphonylated via direct electron transfer at the anode. In particular, we show the quantitative phosphonylation of poly(9,9-dioctylfluorene) (PFO) at low current densities, despite the small diffusion coefficient of PFO. Stirring-induced convection facilitated polymer diffusion, thereby improving the efficiency of electrochemical PFO phosphonylation. Moreover, the degree of functionalization could be precisely controlled by adjusting the amount of passed charge. Interestingly, a moderate functionalization degree improved the fluorescence quantum yield to 0.82 in phosphonylated PFO and the reaction proceeded from various trialkyl phosphites. Finally, the reaction mechanism of anodic phosphonylation was determined through density functional theory calculations.