Multi-coloration of polyurethane derivatives through click postfunctionalization, electrochemical oxidation, and Ag+ ion complexation

Abstract

A colorless polyurethane derivative bearing electron-rich alkynes substituted by the dialkylaniline donors at both sides was prepared by polyaddition between a diol monomer and tolylene-2,4-diisocyanate. Click chemistry-type postfunctionalization of this polyurethane was accomplished by a [2 + 2] cycloaddition followed by retro-cyclization with the electron-accepting tetracyanoethylene (TCNE) or 7,7,8,8-tetracyanoquinodimethane (TCNQ), yielding new orange or green colored polyurethane derivatives with the intramolecular charge-transfer (CT) band centered at 476 and 698 nm in CH₂Cl₂, respectively. These polymers were comprehensively characterized by GPC, ¹H NMR and IR spectroscopies, and elemental analysis. Despite its sufficient thermal stability, the precursor polyurethane was readily oxidized in air under mild heating conditions. Inspired by this result, the electrochemical oxidation of the precursor polyurethane was performed. Application of potentials higher than the first oxidation potential of 0.25 V (vs. Ag/AgCl) reasonably produced the blue colored aromatic amine-based cation radical. Although the cyclic voltammogram of the polyurethane films was irreversible, the blue color was almost persistent under an inert atmosphere. The colors of the postfunctionalized polyurethanes were further changed by Ag⁺ ion complexation with cyano groups, which modulated the strengths of the intramolecular donor–acceptor interactions. Thus, the solution color of the TCNE-adducted polyurethane changed to magenta and that of the TCNQ-adducted polyurethane to a yellowish-green upon the Ag⁺ ion addition. The plots in the 1976L*a*b* color space revealed the production processes of each color.