Ionic-Environment Control of Highest Occupied Molecular Orbital−Lowest Unoccupied Molecular Orbital Levels and Polaronic Transitions in Sulfonated Polyaniline Nanobelts: A Platform for Switchable Conductivity and Antibacterial Activity

Abstract The frontier molecular orbital energies and polaronic transitions of conjugated polymers are exquisitely sensitive to their ionic environment, yet this relationship has remained underexplored in sulfonated polyaniline systems. Herein, an environmentally friendly, template-free approach is introduced for preparing nanobelts of poly(aniline-co-m-aminobenzenesulfonic acid), designated as NPAABS. We demonstrate that exchangeable counterions control not only chain conformation but also the highest occupied molecular orbital−lowest unoccupied molecular orbital (HOMO−LUMO) gap, polaron formation, and antibacterial activity. Quantitative elemental analysis reveals that approximately 43% of all sulfonate species are exchangeable counterions—resolving a long-standing ambiguity in sulfonated aniline copolymer chemistry. Removal of these counterions induces a dramatic coil-to-expanded conformational transition, visualized by a 114% increase in nanobelt length, accompanied by significant modulation of the electronic structure. Solvent-gated polaronic transitions are fully reversible and correlate with a downshifted HOMO level in the doped state. The expanded, polaron-rich salt form exhibits potent antibacterial activity, surpassing standard antibiotics. This work establishes a unified framework linking ionic environment, chain conformation, HOMO−LUMO engineering, and biofunction.