Polymeric light emitting properties and structural relationships of fluorene-based conjugated copolymers containing various hole transporting derivatives

Abstract

A series of new fluorene-based alternating polymers (PF-CZ50, PF-DPA50, PF-PXZ50, PF-PZB50) composed of comonomers containing well-known hole-transporting moieties, namely carbazole, diphenylamine, phenoxazine, and phenothiazine, respectively, were synthesized via Suzuki coupling reactions. The molecular structures of the polymers were characterized with 1H NMR spectroscopy and elemental analysis, and their photophysical, electrochemical, and electroluminescence properties were investigated. The absorption and photoluminescence (PL) emission maxima of the copolymers in the solid state varied depending on which hole-transporting unit was present. Although the polymers’ LUMO levels are similar, the differences between their HOMO levels mean they have different hole-transportation properties. Light-emitting devices using the polymers as emitting layers were fabricated with ITO/PEDOT:PSS/polymer/Ca/Al configurations. The emitted light of these polymers ranged from yellowish green to red for the sequence PF-CZ50, PF-DPA50, PF-PXZ50, and PF-PZB50. Especially, PF-PXZ50 and PF-PZB50 emitted almost pure red with good brightness. The planarity of the hole-transporting moieties in the polymers is the main factor affecting the efficiency of these electroluminescence (EL) devices. The planarity of each hole-transporting moiety was calculated by computational analysis using the ab initio Hartree–Fock (HF) with the split-valence 6-31G∗ basis set. As the magnitude of the non-planarity of the hole-transporting moieties increased, it was found that the EL quantum efficiency of the polymers was enhanced. Compared to PF-CZ50 containing planar carbazole moieties, PF-DPA50 containing a non-planar diphenylamine moiety showed better EL performances without a significant change of the emitting color. In the case of PF-PXZ50 and PF-PZB50, their emission color was red-shifted, and the EL quantum efficiency of their LED devices was enhanced, with respect to the result for PF-CZ50, because of their extra heteroatoms (oxygen and sulfur respectively), which bend the molecules and strengthen their activities as electron donors.