Extraordinary electrochemical stability and extended polaron delocalization of ladder-type polyaniline-analogous polymers

Abstract

Electrochemical stability and delocalization of states critically impact the functions and practical applications of electronically active polymers. Incorporation of a ladder-type constitution into these polymers represents a promising strategy to enhance the aforementioned properties from a fundamental structural perspective. A series of ladder-type polyaniline-analogous polymers are designed as models to test this hypothesis and are synthesized through a facile and scalable route. Chemical and electrochemical interconversions between the fully oxidized pernigraniline state and the fully reduced leucoemeraldine state are both achieved in a highly reversible and robust manner. The protonated pernigraniline form of the ladder polymer exhibits unprecedented electrochemical stability under highly acidic and oxidative conditions, enabling the access of a near-infrared light-absorbing material with extended polaron delocalization in the solid-state. An electrochromic device composed of this ladder polymer shows distinct switching between UV- and near-infrared-absorbing states with a remarkable cyclability, meanwhile tolerating a wide operating window of 4 volts. Taken together, these results demonstrate the principle of employing a ladder-type backbone constitution to impart superior electrochemical properties into electronically active polymers.

Fig. 1. (a) Synthesis of LPANI from polymer precursor P1, and the redox interconversion of its LLB, LEB, and LPB forms. (b) Size exclusion chromatogram, calculated M_n, and polydispersity (Đ) values. (c) UV-vis spectra of the oxidation process from LLB to LEB to LPB in THF; (inset) photographic images of LLB, LEB and LPB solutions. (d) Protonation of LPB with MSA and structural formulae of the closed-shell and open-shell resonance forms of the protonated LPS form.

Fig. 2. ¹³C NMR spectra of small molecule model SLB and SPB, and varied oxidation states of LPANI (d₈-THF at 298 K).

Fig. 3. Cyclic voltammograms of (a) the LEB film in acetonitrile with 0.1 mol L⁻¹ TBAPF₆. (b) LPANI solution in THF with 0.15 mol L⁻¹ MSA. (c) LPANI solution in THF with 1.2 mol L⁻¹ LiClO₄. In all cases, a Pt wire was used as the counter electrode and Ag/AgCl was used as the reference electrode.

Fig. 4. Cyclic voltammograms of (a) conventional PANI and (b) LPANI after applying +1.0 V potential for a variable amount of time (0–60 min). (c) Plot of the current intensities in (a) and (b) vs. the time of holding at +1.0 V. The experiment was performed on working electrodes (conventional PANI or LPANI solid deposited on carbon fabric cloth) in acetonitrile with 0.5 mol L⁻¹ sulfuric acid. A Pt wire was used as the counter electrode and Ag/AgCl was used as the reference electrode.

Fig. 5. (a) Visible-NIR absorption spectra of LPS in solution and in thin film. (b) HOMO/LUMO energy level diagrams of LLB, LEB, LPB, and LPS. (c) EPR spectra of LPB and the LPS solid. (d) Temperature-varied magnetic susceptibility of the LPS solid.

Fig. 6. (a) Architecture and photographic images of the electrochromic device at −2.0 V and +2.0 V, respectively. (b) Absorption spectra of the electrochromic device when applying −2.0 V or +2.0 V for 60 s. (c) Absorbance at 378 nm and 845 nm with voltage swept between −2.0 V and +2.0 V for 200 cycles. (d) Time-dependent absorbance changes at 845 nm; dots: experimental data spots; lines: pseudo-first-order fitting.