Colloids and Surfaces A: Physicochem. Eng. Aspects 284–285 (2006) 635–639 Molecular orientation of crystalline and glassy polyfluorene layers induced by photo-aligned polyimide films Kenji Sakamoto a,∗ , Kiyoaki Usami a,b , Yoichi Uehara b,c , Sukekatsu Ushioda b,d a Nanomaterials Laboratory, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan b RIKEN Photodynamics Research Center, 519-1399 Aramaki-aoba, Aoba-ku, Sendai 980-0845, Japan c Research Institute of Electrical Communication, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan d Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan Received 21 June 2005; received in revised form 7 October 2005; accepted 28 October 2005 Available online 2 December 2005 Abstract 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. © 2005 Elsevier B.V. All rights reserved. Keywords: Polyfluorene; Photo-alignment; Polyimide; Azobenzene; Liquid crystalline polymers 1. Introduction Solvent soluble polyfluorenes, bearing suitable substituents in the carbon 9-position of the fluorene repeat unit [1], have currently received much attention as attractive materials for polymer-based light emitting diodes (LEDs) and thin film tran- sistors (TFTs). This is due to their high luminescence efficiency, high charge carrier mobility, excellent thermal stability, and ease of thin film formation by spin-coating. In addition to these prop- erties, the important nature of polyfluorenes is their thermotropic liquid crystallinity [2]. Uniaxial alignment of polyfluorenes can be obtained easily by performing a suitable thermal treatment on thin polyfluorene layers deposited on proper alignment films. The alignment of fully conjugated main-chain polymers, such as polyfluorene homopolymers and copolymers, enhances the charge carrier mobility [3,4]. Thus, the self-organization of liq- ∗ Corresponding author. Tel.: +81 29 860 4729; fax: +81 29 860 4718. E-mail address: SAKAMOTO.kenji@nims.go.jp (K. Sakamoto). uid crystalline polymers provides an attractive way to improve the switching frequency of polymer-based TFTs. Since polar- ized light is emitted from aligned light-emitting polymers [5], the self-organization of liquid crystalline polymers is useful in the fabrication of polarized LEDs. Polarized LEDs have potential for use as low-power-consumption backlights in liquid crystal (LC) displays. Rubbed films of polyimides [2–7] and poly (p- phenylenevinylene) (PPV) [8] are often used as the alignment films for polyfluorenes. However, mechanical rubbing creates dust particles and produces scratches at the alignment film surface. Since several tens nm-thick films are used in polymer- based LEDs and TFTs, the scratches are expected to severely affect the alignment of polyfluorenes. Thus, the development of non-contact alignment techniques is desirable for further improvements in polymer-based device performance. Photo-alignment is a promising non-contact alternative to the mechanical rubbing technique. Various photo-alignment meth- ods have been proposed and developed for LC displays [9–15]. Different alignment materials are employed, depending upon 0927-7757/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2005.10.076