Synthesis of Intrinsically Dual-Emissive Aliphatic Conducting Polymer and ESIPT-Associated Switching of Amide-Aggregate to Imidol-Aggregate for Sensing of Cr(III) and Fe(III)

The design, synthesis, and optimization of excited-state intramolecular proton transfer (ESIPT)-associated dual-emissive aliphatic conductive polymer is one of the very challenging tasks, and has not been reported to date. Herein, aliphatic fluorescent conducting polymers (FCPs) are synthesized by polymerizing N-(monomethylol)acrylamide (MMA), acrylic acid (AA), and in situ-generated 3-N-(monomethylolacrylamido)propanoic acid (NMMAPA). Of different FCPs, the maximum population of heteroatomic nontraditional luminophores, i.e., secondary amide (−CONH), imidol (−CN(OH)), tertiary amide (−CON), and carboxylic acid (−COOH), in FCP4 is supported by the spectroscopic analyses, thermal profiles, fluorescence enhancements, and computational calculations. Thus, further investigations are made on FCP4 to explore the photophysical properties, check the suitability in dual metal ion sensing, and study the proton conductivity. The ESIPT-associated dual light emissions at 436 nm (λ_em1) and 573/617 nm (λ_em2) originate from FCP4 (amide)/FCP4 (amide)-aggregate and FCP4 (imidol)/FCP4 (imidol)-aggregate, respectively, and are supported by concentration-dependent emissions, time-correlated single photon counting studies, solvent polarity effects, and computational measurements. Regarding this, the high fluorescence quantum yields of 0.68 and 0.18 at λ_em1 and λ_em2, respectively, confirmed the ESIPT-associated strong dual emissions of FCP4. The UV spectrum within 264-300 nm, FTIR peak at 2165 cm^-1, binding energies of −CN(OH)/-CN(OH) at 399.0/533.4 eV, and computational studies indicate the coexistence of FCP4 (amide)/FCP4 (amide)-aggregate and FCP4 (imidol)/FCP4 (imidol)-aggregate forms of FCP4. In FCP4, −CONH/-CN(OH)/-CON/-COOH/-CH₂OH-associated dipolar and hydrogen-bonding interactions, n−π* transitions, and N-branching-associated rigidity contribute to ESIPT-associated amide-imidol phototautomerism, aggregation-enhanced emissions, dual light emissions, metal ion sensing, and conductivity. The strong coordinations of Fe(III) and Cr(III) with FCP4 (amide) and FCP4 (imidol), respectively, are supported by spectroscopic, thermal, and computational studies. The strong quenching efficiencies of Fe(III) and Cr(III) are indicated by the very low limits of detection of 0.1142 and 0.0534 ppb, respectively. The I-V and ac impedance spectroscopy data of FCP4 having 0.28 cm thickness and 1.72 cm² area indicate high proton conductivities of 3.53 × 10^-5 and 3.22 × 10^-5 S cm^-1 at pH = 7.0 and 8.0, respectively.