Vinylene-versus azomethine-bridged carbazole-based polymers for light emission and sensor applications

Abstract

Two new carbazole-based soluble π-conjugated polymers were synthetized and investigated: P1 is a poly(p-phenylenevinylene) analogue and P2 is a poly-azomethine-type polymer. The macromolecular structures of P1 and P2 were confirmed by nuclear magnetic resonance and infrared spectroscopies. The effect of vinylene (in P1) and azomethine (in P2) π-linkers on thermal, optical and electrical properties of the two macromolecules was mainly studied. The differential scanning calorimetry and thermogravimetric analyses showed that both polymers are amorphous and better thermal stability was observed in the case of the azomethine containing polymer. The photophysical behavior of these π-conjugated systems were investigated by UV–visible absorption and photoluminescence spectroscopies. Schottky single-layer diodes and Organic Field-Effect Transistors (OFETs) using P1 and P2 as active layers were elaborated in order to study the electrical properties of these polymeric materials. P1 is a semi-conducting and fluorescent polymer, with a yellowish-green emission and a charge carrier mobility of 5.1 10−6 m²/V.s making it a good candidate for organic light emitting devices (OLED) applications. However, the poly-azomethine P2, in its neutral state, is of little interest when compared to the polyvinylene derivative, due to its very low charge transport capacity and negligible fluorescence intensity. We demonstrate in this study that the electrical and optical properties of the poly-azomethine P2 can be dramatically improved by nitrogen protonation of the azomethine linkers. Indeed, under exposure to trifluoroacetic acid (TFA) vapors, an emitted fluorescence that is a linear function of the amount of TFA added to the P2 solution was observed and a charge carrier mobility of 7 10−5 m²/V.s was achieved. This opens the way for these azomethine-containing carbazole-based polymers to promising applications in electrical and fluorescence turn-on sensors.