Threshold Reduction in Polymer Lasers Based on Poly(9,9-dioctylfluorene) with Statistical Binaphthyl Units

We introduce novel statistical copolymers of poly(9,9‐dioctylfluorene), PFO, which contain various concentrations of 6,6′‐(2,2′‐octyloxy‐1,1′‐binaphthyl) spacer groups. We demonstrate that, owing to the large dihedral angle (> 60°) between neighboring naphthalene units, we could hinder the formation of the highly ordered β‐phase in thin films of the copolymers. In low‐temperature photoluminescence measurements, the typical signature of the PFO β‐phase at 442 nm is no longer observed for copolymers with a binaphthyl concentration of about 12 %. Moreover, the optical properties of the copolymers resembled those of the glassy α‐phase PFO. Second‐order distributed feedback (DFB) lasers based on thin films of the homopolymer PFO showed a minimum lasing threshold of 11.7 μJ cm–2 (λmax = 452 nm, excitation at λ = 337 nm with 500 ps pulses). With increasing binaphthyl concentration in the copolymer backbone, the lasing threshold steadily decreased to 3 μJ cm–2 for a binaphthyl concentration of about 12 %. Therefore, our novel copolymers provide a vast improvement for PFO‐based optoelectronics. Novel poly(9,9‐dioctylfluorene) copolymers containing statistically distributed binaphthyl spacer groups hinder the formation of the highly ordered β‐phase in thin films of the copolymers, resulting in a dramatic reduction of the excitation density required to reach the threshold of distributed feedback lasers (see Figure). Thus, these copolymers may be useful as gain materials in organic solid‐state lasers.