Phenothiazine-based conjugated polymers: Synthesis, electrochemistry, and light-emitting properties

Two new phenothiazine-containing conjugated polymers, poly(10-hexylphenothiazine-3,7-diyl) (PHPT) and poly(10-hexylphenothiazine-3,7-diyl-alt-9,9-dihexyl-2,7-fluorene) (PPTF), were synthesized and characterized, and their photophysical, electrochemical, and electroluminescent properties were investigated. The optical band gaps of PHPT and PPTF were 2.69 and 2.76 eV, respectively. Both polymers showed greenish-blue photoluminescence (490 nm) in dilute solutions with a fluorescence quantum yield of 0.40. Identical solid-state and dilute solution absorption and emission spectra were observed, showing that excimers were not formed in PHPT or PPTF thin films. Ionization potentials (HOMO levels) estimated from cyclic voltammetry were 5.0-5.1 eV for the phenothiazine-based polymers, making them good candidates for hole transport materials in devices. Spectroelectrochemistry revealed that the observed multiple oxidation peaks in the cyclic voltammetry of PHPT have associated multiple absorption peaks due to the formation of radical cations (polarons) and dications (bipolarons). Greenish-blue electroluminescence with luminance of up to 320 cd/m(2) was observed for the PPTF organic light-emitting diodes. These results show that the phenothiazine ring is an excellent building block for lowering the ionization potential and for impeding pi-stacking aggregation and excimer formation in conjugated polymers.