Synthesis and characterization of color tunable, highly electroluminescent copolymers of polyfluorene by incorporating the N-phenyl-1,8-naphthalimide moiety into the main chain

Abstract

A series of six new organic light emitting copolymers were prepared from 9,9′-dioctylfluorene (DOF) and N-phenyl-1,8-naphthalimide (NPN) using palladium catalysed Suzuki polymerization. The feed ratios of poly[2,7-(9,9′-dioctylfluorene)-co-N-phenyl-1,8-naphthalimide] (PFONPN) copolymers were 50 : 50, 65 : 35, 75 : 25, 90 : 10, 95 : 05 and 99 : 01 respectively. These copolymers have good thermal stability with an onset decomposition temperature (T_d) of ∼340–405 °C and a glass transition temperature (T_g) of 123–134 °C. All the copolymers are highly fluorescent and soluble in common organic solvents such as chlorobenzene, chloroform, dichloromethane, THF, toluene, etc. allowing their processing from desired solvent. The electroluminescence (EL) properties of the copolymers were also studied by fabricating single layer devices with ITO/PEDOT:PSS/PFONPN/Ca:Al configuration. The PL and the EL spectra of the copolymers reveal that by changing the content of the NPN moiety in the polyfluorene main chain from 1 to 50 mol%, the emission color of the polymers can be tuned from blue to green with Commission International de l'Echairage (CIE) coordinates being (0.17, 0.22) to (0.24, 0.49). This color tuning can be attributed to the strong energy transfer from the fluorene to NPN unit in the polymer backbone. The devices made using these copolymers are found to be very bright with PFONPN01 giving the highest brightness of 5236 cd m⁻² with a luminous efficiency of 3.52 cd A⁻¹.