Synthesis, Optical, and Electroluminescent Properties of Alternating Copolymer Based on Phenothiazine and Fluorene with Oxadiazole Pendant

A new alternating copolymer based on phenothiazine-fluorene with aromatic 1,3,4-oxadiazole (OXD) as a side chain (poly(10-{4-[5-(4-tert-butyl-phenyl)-[1,3,4]oxadiazol-2-yl]-phenyl}-phenothiazine-3,7-diyl-alt-9,9-dihexyl-2,7-fluorene, PTOXDPF) was synthesized by the Suzuki coupling reaction. The maximum absorption wavelength of PTOXDPF solution showed at 410 nm which was dramatically blue-shifted than that of the polymer without OXD (poly(10-hexylphenothiazine-3,7-diyl-alt-9,9-dihexyl-2,7-fluorene, PTPF). This is due to that the presence of OXD pendant reduces the possibility of inter-chain interactions. The HOMO energy level of PTOXDPF estimated from the cyclic voltammetry was -5.13 eV, which was similar to the polymer without OXD pendant (PTPF). The OXD pendant does not affect HOMO energy level significantly. This is due to that oxidation potential of PTOXDPF dominated by the phenothiazine unit. The maximum efficiency and brightness of the electroluminescent (EL) device based on PTOXDPF (ITO/PEDOT/PTOXDPF/Al) were 45.4 cd/m2 and 5.21 × 10−2 cd/A, which were dramatically higher than PTPF (2.65 cd/m2 and 1.32 × 10−3 cd/A). This is presumably due to that the OXD pendant in PTOXDPF improves the electron transporting ability in the emissive layer.