Journal of Applied Polymer Science, Vol.74, No.14, 3351-3358, 1999
New light emitting materials: Alternating copolymers with hole transport and emitting chromophores
A new type of novel high-efficiency light-emitting nitrogen-containing poly(phenylene vinylene) (PPV)-related copolymers, which have hole-transfer moieties such as triphenylamine (TPA) and conjugated aromatic units such as 4,4＇-biphenylene, 1,4-phenylene, 2,5-dimethyl-1,4-phenylene, 1,4- or 1,5-naphthylene, and 9,10-anthrylene, was designed and synthesized by the well-known Wittig-Hornor reaction. The resulting alternating copolymers were highly soluble in common organic solvents. They can spin-cast onto various substrates to give highly transparent homogeneous thin films without heat treatment. The introduction of TPA units in the PPV backbone improved processibility and limited the pi-conjugation length. Furthermore, the additional pi-electron delocalization between the lone-paired electron in the nitrogen atom and pi-electrons in the conjugated units contributed to the improvement of the fluorescence quantum yields of these copolymers. All these alternating copolymers except TPA-PAV have high-efficiency photoluminescence and they are Very promising for light-emitting diodes (LEDs). It is very promising that TPA-PAV will emit white light when used in LED device due to the broad emission spectra. The origin of the broad spectrum is contributed by the charge-transfer complex formation, which can be proved by the absorption and emission spectra of TPA-PAV solutions. When the aromatic units were 1,4-phenylene, 1,4- or 1,5-naphthylene, 4,4＇-biphenylene, and 9,10-anthrylene, respectively, with increase of the capability to accept electrons in aromatic units, the charge transfer from TPA to aromatic units occurred; consequently, the fluorescence quantum yield decreased. The introduction of the alkoxy-substitute group on the aromatic units in the polymer backbone caused the red shift of the absorption and emission spectra of the copolymers due to the stronger delocalization of the ir-conjugated system.