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 ( FSP s) 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 (λ ex ); 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 −CH 3 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 −SO 3 H 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 (λ ex1 ), 5.241 (λ ex2 ), and 2.249 (λ ex3 ) 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).