Electrochemical polymerization of 1,3,4,6-tetraarylpyrrolo[3,2-b]pyrrole-2,5-dione (isoDPP) derivatives

Abstract

New π-conjugated polymers containing 1,3,4,6-tetraarylpyrrolo[3,2-b]pyrrole-2,5-dione (isoDPP) units in the main chain were prepared by electrochemical polymerization of isoDPP-based monomers M1–M3. Monomers M1 and M2 were previously obtained upon palladium-catalyzed Suzuki cross-coupling of dibromo-isoDPP derivatives 3,6-bis(5-bromothiophen-2-yl)-1,4-bis(4-tert-butylphenyl)pyrrolo-[3,2-b]pyrrole-2,5-dione 1a, or 3,6-bis(5-bromothiophen-2-yl)-1,4-bis(4-hexylphenyl)-pyrrolo-[3,2-b]pyrrole-2,5-dione 1b and 2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolanyl)-3-hexylthiophene. Monomer M3 was synthesized by Stille coupling of 1a and 2-(tributylstannyl)-4-octylthiophene. The yields were about 88%. M1–M3 were obtained as dark red powders with excellent solubility in organic solvents. The electrochemical polymerization was carried out under potentiodynamic conditions, i.e., the potential was repeatedly cycled between 0.0 (or −0.2) V and 1.2 V vs. ferrocene (FOC). The polymers P1–P3 precipitated as stable, insoluble, well adhering thin films on ITO-coated glass or glassy carbon electrodes. P1–P3 exhibit broad absorption spectra with maxima between 526 and 629 nm. They are non-fluorescent. Cyclovoltammetric studies indicate reversible oxidation and reduction behavior. Anodic cycling is accompanied with reversible colour changes of polymer films. The oxidation potentials are between 0.27 and 0.71 V vs. FOC, and the reduction potentials are between −1.21 and −1.69 V. The band gaps of the polymers are between 1.51 and 1.62 eV. Due to their optical and electrochemical properties the polymers might be useful as active materials in electrochromic displays.