Excited-State Intramolecular Proton- and Inter-Polymer Charge-Transfer of Semiconducting Redox Polymers for Fe(III), Cd(II), and Hg(II) Sensing

Herein, IPCT-ESIPT exhibiting nonconventional fluorescent semiconducting polymers (FSPs) are synthesized using poly(vinyl alcohol) (PVA), 2-acrylamido-2-methylpropanesulfonic acid, N-hydroxymethyl acrylamide, and in situ-generated 2-(3-(N-(hydroxymethyl)acrylamido)propanamido)-2-methylpropane-1-sulfonic acid. The IPCT- and ESIPT-mediated triple-light emission at different excitation wavelengths (λex); coexistence of FSP6-amide, FSP6-imidol, and FSP6-amide canonical; and ICT-/PET-assisted sensing capabilities of FSP6 are explored via spectroscopic analyses, computational calculations, electrochemical measurements, and impedance analyses. The π–π* transitions, IPCT–ESIPT, N-branching- and −CH3 group (side chain)-associated rigidity, alongside hydrophilic/hydrophobic balance are facilitated by −C(═O)NH/–C(═O)N, −C(═N)OH, −C(O–)N+, and −SO3H heteroatomic subluminophores. The enhancement in hydrogen bonds and stabilization of −C(O–)N+/–C(═N)OH in a polar-protic environment by the hydroxyl functionalities of semicrystalline PVA facilitate IPCT/ESIPT emission and electron/proton transport through the channels generated in the polymer matrix. The ICT-mediated reduction and oxidation of Fe(III) and Cd(II), respectively, and PET-mediated reduction of Hg(II) are explored via quenching/enhancement of photoluminescence in the presence of Fe(III) and Hg(II)/Cd(II), spectroscopic analyses, cyclic voltammetry, and impedance data. The detections of Fe(III), Cd(II), and Hg(II) by FSP6-amide, FSP6-amide canonical, and FSP6-imidol, respectively, are confirmed by low limits of detection of 1.225 (λex1), 5.241 (λex2), and 2.249 (λex3) ppb. The ac impedance spectroscopy and I–V data of semiconducting FSP6 indicate proton/electron conductivity = 3.37 × 10–5/2.80 × 10–5 S cm–1 (pH = 7.0/8.0).