π-Conjugated Fluorescent Azomethine Copolymers: Opto-Electronic, Halochromic, and Doping Properties

A series of new conjugated copolyazomethines consisting of alternating fluorene and thiophenes were prepared. Different thiophenes were incorporated for examining their effect on the polymer's opto-electronic properties. Blue, red, and green emissions were possible contingent on the thiophene comonomer and the placement of the azomethine nitrogen. Most notably, the polyazomethines were fluorescent both in solution and thin films with measured absolute fluorescent quantum yields (Phi(fl)) ca. 10%. Their fluorescence could be enhanced to near unity at 77 K. High Phi(fl) were also measured with the addition of trifluoroacetic acid (TFA). The polyazomethines' color could also be reversible altered with protonation/neutralization with TFA/triethylamine. Similar to the halochromic effect, the polyazomethines could be reversibly oxidized/neutralized with FeCl3/hydrazine, resulting in reversible color changes between red and blue. The oxidation potentials and the reversibility of the anodic process were contingent on the type of thiophene incorporated into the copolymer. The stability of the electrochemically produced radical cation and the azomethine's resistance toward hydrolysis were also investigated with a model thiophene-fluorene-thiophene bisazomethine.