2,6-Azulene-based Homopolymers: Design, Synthesis, and Application in Proton Exchange Membrane Fuel Cells

Abstract

Azulene-based homopolymers are of great interest from the point view of chemistry and material science. Herein, by means of Friedel–Crafts acylation to introduce solubilizing chains on the 1-position of azulene, we designed and synthesized two examples of 2,6-azulene-based homopolymers RP­(Az-AC16) and P­(Az-AC16). The arrangement of 2,6-azulene units is irregular for RP­(Az-AC16), while P­(Az-AC16) has head-to-head/tail-to-tail arranged 2,6-azulene units. Proton-responsive studies demonstrate that RP­(Az-AC16) and P­(Az-AC16) show reversible proton responsiveness in both solution and thin film. To utilize the dynamically reversible proton-responsive property of these polymers in thin films, RP­(Az-AC16) and P­(Az-AC16) were incorporated into a Nafion matrix as proton exchange membranes, wherein the Nafion/P­(Az-AC16) composite membrane exhibits significant increases in proton conductivity relative to the Nafion membrane at different temperatures of each relative humidity (RH), which further results in a 64% improvement in hydrogen fuel cell output power under 30% RH at 80 °C. Our studies have realized the first solution synthesis of 2,6-azulene-based homopolymers and the first application of azulene-based π-systems in hydrogen fuel cells.

Poly(2,6-azuleneethynylene)s: Design, Synthesis, and Property Studies

“…Azulene-based polymers are promising candidates for electrochromic devices, , thermoelectrics, and electrolyte membranes ,, due to their proton-responsiveness. We evaluated the stimuli-responsiveness of PAzE-1 and PAzE-2 to trifluoroacetic acid (TFA) by UV–vis absorption spectroscopy.…”

“…24,26 Recently, we realized the first solution synthesis of 2,6-polyazulenes and disclosed the impact of azulene orientations on proton conductivity of Nafion/2,6polyazulene composite membranes. 25 2,6-Azulene units in these reported polymers are oriented either irregularly or in head-to-head/tail-to-tail arrangement. 24−28 However, the conjugated polymers in which 2,6-azulene units are oriented in the same direction are still lacking due to synthetic challenges.…”

“…Hence, introducing solubilizing chains in the two positions may be a feasible strategy to ensure a good solubility of a conjugated polymer. We have proved that the C–Br bond located at 6-position of the electron-deficient seven-membered ring of azulene preferentially undergoes the oxidative addition to palladium (0). ,, Therefore, the selective 6-position alkynylation of the dialkyl-substituted 2,6-dibromoazulene derivative may afford a difunctional monomer that can be performed homopolymerization. Herein, we design and synthesize the first two examples of poly­(2,6-azuleneethynylene)­s ( PAzE-1 and PAzE-2 ) with different orientations of 2,6-azulene units in the polymer backbone (Figure ).…”

“…Azulene-containing molecules present tunable physicochemical properties, field-effect transistor (FET) performances, − proton-responsiveness, and conductivities , through connection positions along the C 2 v axis. In 2019, our group synthesized two 2,6-azulene-2,7-fluorene-based conjugated polymers with different orientations of 2,6-azulene units in polymer backbones and demonstrated that the one with random azulene orientations exhibits higher electrical conductivity in thin film when doped by trifluoromethanesulfonic acid. , Recently, we realized the first solution synthesis of 2,6-polyazulenes and disclosed the impact of azulene orientations on proton conductivity of Nafion/2,6-polyazulene composite membranes . 2,6-Azulene units in these reported polymers are oriented either irregularly or in head-to-head/tail-to-tail arrangement. − However, the conjugated polymers in which 2,6-azulene units are oriented in the same direction are still lacking due to synthetic challenges.…”

“…In 2019, our group synthesized two 2,6-azulene-2,7-fluorene-based conjugated polymers with different orientations of 2,6-azulene units in polymer backbones and demonstrated that the one with random azulene orientations exhibits higher electrical conductivity in thin film when doped by trifluoromethanesulfonic acid. , Recently, we realized the first solution synthesis of 2,6-polyazulenes and disclosed the impact of azulene orientations on proton conductivity of Nafion/2,6-polyazulene composite membranes . 2,6-Azulene units in these reported polymers are oriented either irregularly or in head-to-head/tail-to-tail arrangement. − However, the conjugated polymers in which 2,6-azulene units are oriented in the same direction are still lacking due to synthetic challenges. The most electron-rich 1- and 3-positions of azulene can readily and preferentially react with electrophiles .…”