Synthesis and Light-Emitting Properties of Polyfluorene Copolymers Containing a Hydrazone Derivative as a Comonomer

Abstract

Poly[9,9-bis(2′-ethylhexyl)fluorene-2,7-diyl] (PBEHF) and a series of fluorene copolymers containing a hydrazone comonomer, poly 9,9-bis(2′-ethylhexyl)fluorene-2,7-diyl-co-[aza(2,7-fluorene-9-ylidene)methyl]diphenylamine [poly(BEHF-co-FDPA)], were synthesized through Ni(0)-mediated polymerization. The synthesized polymers were characterized and their photophysical, electrochemical, and electroluminescent properties were investigated. Interestingly, the copolymer containing only 1% FDPA, poly(99BEHF-co-1FDPA), was found to exhibit photoluminescence (PL) emission that is significantly red-shifted with respect to that of the PBEHF homopolymer. All the copolymers exhibit PL emission maxima in the yellow region of the spectrum in the range from 530 to 540 nm, whereas the PBEHF homopolymer film produces maximum PL emission in the deep blue region at 424 nm. Light-emitting devices were fabricated in an ITO/PEDOT:PSS (50 nm)/polymers (80 nm)/Ca (50 nm)/Al (200 nm) configuration. The electroluminescence (EL) spectra of these devices consist of similar wavelengths to those found in the PL results; however, the EL devices constructed from the copolymers exhibited device performances that were significantly better than that of the device using the PBEHF homopolymer. This enhancement of performance is due to a more efficient charge carrier balance in the devices, which arises because of the effective exciton confinement (or charge carrier trapping) in the FDPA groups.