Conjugation of Nitrogen-Linked Phthalein Polymers by Forming Ions in the Backbone upon Protic Acid Addition

Abstract

Polymers with amine linkages composed of the halochromic phthalein moiety are prepared from n-octylamine, aniline, 4-hexylaniline, and p-anisidine with a dibromofunctionalized phthalein monomer via Buchwald–Hartwig polyaminations. Characterization with thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) showed excellent thermal stability with onset degradation temperatures in air above 360 °C and high thermal transitions with T_gs up to 256 °C. Treating the polymers with a protic acid converts the nitrogen-linked phthalein structure into a conjugated form through the generation of ions in the backbone, which is supported by ultraviolet–visible (UV–vis) spectra that display evidence of carbocation species and reduced optical energy gaps, with the lowest energy gap observed at 1.45 eV. Cyclic voltammetry (CV) provides estimations of the energy levels for each polymer with deep highest occupied molecular orbital (HOMO) levels ranging from −5.37 to −5.47 eV. Additionally, electron paramagnetic resonance (EPR) spectroscopy detected air-stable unpaired electrons in the protic acid-treated polymers, which suggests resonance between a closed-shell ionic form and an open-shell radical form that is expected to yield intriguing electrical and magnetic properties.