Molecular orientation of crystalline and glassy polyfluorene layers induced by photo-aligned polyimide films

We have investigated the alignment of poly (9,9-dioctylfluorenyl-2,7-diyl) (PFO) induced by a photo-aligned polyimide film, which is optically, thermally, and chemically stable. The ∼30 nm-thick PFO layer was spin-coated onto a photo-aligned polyimide film. Slow cooling and quenching of the sample from the liquid crystalline phase of PFO formed uniaxially-aligned crystalline and glassy layers of PFO, respectively. For comparison, we also investigated the alignment of PFO induced by a rubbed polyimide film. For the crystalline PFO layer on the photo-aligned film, an extremely high polarization ratio was observed in photoluminescence (PL), which was ∼30. It was much larger than that of the crystalline PFO layer formed on the rubbed film (∼20). Indeed, such a high PL polarization ratio has not previously been reported in crystalline PFO layers formed by any other alignment method. For the glassy PFO layer formed on the photo-aligned film, the PL polarization ratio was 10.8. It was slightly larger than that of the glassy PFO layer on the rubbed film (9.5). These results show that the photo-aligned polyimide film used in this study is an excellent alignment film for PFO layers, especially for crystalline layers.