A fluorescent all-fluorene polyazomethine—towards soluble conjugated polymers exhibiting high fluorescence and electrochromic properties

Abstract

A conjugated all-fluorene polyazomethine (6) was prepared from the condensation of 9,9′-dioctyl fluorene dialdehyde and 9,9′-dihexyl fluorene diamine. The resulting polymer was soluble in common organic solvents courtesy of the solubilizing alkyl groups in the 9,9′ positions. Determination of the polymer's molecular weight was subsequently possible using standard characterization methods. Polymer formation was confirmed both by GPC and NMR with average number molecular weights ranging from 9 to 145 kg mol⁻¹. The absolute fluorescence quantum yield (Φ_fl) of 6 in dichloromethane was 0.19 while in thin films the value increased to 0.40. The fluorescence could further be restored to unity at 77 K while Φ_fl = 0.76 was possible by protonating 6 with trifluoroacetic acid. 6 underwent stark color changes from yellow to red upon doping with TFA and FeCl₃ and the resulting intermediates could be neutralized with Et₃N and hydrazine hydrate, respectively. Reversible protonation/deprotonation was possible both in solution and thin films. Meanwhile, electrochemical oxidation of 6 at ca. 1.5 V resulted in a 120 nm bathochromic change in the absorbance. The intermediate could be reduced at −0.2 V to regenerate the neutral 6 that absorbed at 425 nm. The Job method confirmed that the oxidized intermediate was the radical cation.