Tuning intramolecular charge transfer in conjugated polyphenylenes through benzoquinone functionalization and post-polymerization modifications

Charge-transfer (CT) type π-conjugated polymers (CPs) comprising donor and acceptor aromatic units offer tunable optical and electrochemical properties, which are important for electronic and photonic applications. In this study, we synthesized CT-type polyphenylenes, P(Flu-BQ) and P(Ph-BQ), containing 9,9-dihexylfluorene or 1,4-dihexyloxybenzene as donor units and benzoquinone (BQ) as an acceptor. Reduction of BQ to hydroquinone (HQ) allowed systematic investigation of CT effects by comparison of optical and electrochemical properties before and after reduction. Polymers composed of BQ and HQ units, P(BQ-HQ), were also prepared via sulfuric acid-mediated polymerization, and their composition ratios were controlled by varying acid concentration. Subsequent reactions of HQ units yielded acetylated polymers, P(BQ-AcQ), and TCNQ-substituted polymers, P(TCNQ-AcQ), to modulate CT characteristics. UV–vis absorption, photoluminescence, fluorescence lifetime, and cyclic voltammetry studies revealed that CT along the polymer backbone strongly influences emission behavior and redox properties. Specifically, reduction of BQ suppressed intramolecular CT (ICT), leading to enhanced fluorescence in P(Flu-BQ) and P(Ph-BQ), while incorporation of TCNQ units enhanced CT in P(TCNQ-AcQ), as confirmed by formation of a 1:1 CT complex with 2,6-dimethyltetrathiafulvalene (DM-TTF). These results provide a clear relationship between CT and the optical/electrochemical properties of CPs, highlighting strategies to tune polymer functionality via donor–acceptor design.