Donor-σ-Acceptor Dyad-Based Polymers for Portable Sensors: Controlling Photoinduced Electron Transfer via Tuning the Frontier Molecular Orbital Energies of Acceptors

In this study, we synthesized three novel aromatic imide-based conjugated donor­(D)-σ-acceptor (A) dyad-based polymers showing different photophysical properties. These D-σ-A dyad-based polymers (e.g., BDTBT-NI, BDTBT-NDI, and BDTBT-PDI) consist of the same conjugated donor main chains (benzodithiophene-bithiophene (BDTBT)) and side chains bearing different imide acceptors (naphthalimide (NI), naphthalene diimide (NDI), or perylenediimide (PDI)). The photophysical and electrochemical characteristics of the three polymers were studied using various methods, including UV–visible absorption, steady-state fluorescence, and time-resolved fluorescence experiments as well as cyclic voltammetry. As the acceptor strength of the side chain was increased, photoinduced electron transfer from the donor backbone to the acceptor side-chain moiety was found to occur more effectively. Among the three polymers, BDTBT-PDI was found to be used as an efficient extractor and portable naked-eye sensing probe for Fe²⁺ ions; notably, it exhibited a “turn-on” fluorescence response with a detection limit at a picomolar concentration. The BDTBT-PDI:Fe²⁺ complex was also used for ultrasensitive detection of spermine (with a Stern–Volmer constant of 6.7 × 10⁶ M^–1) in urine samples and was established as an efficient portable test kit for the real-time detection of spermine in the vapor released from fermented food samples. Finally, the “on–off” fluorescence behavior of BDTBT-PDI in the presence of Fe²⁺ and spermine enabled us to develop molecular logic gates using this polymer.