Published: July 29, 2011 r2011 American Chemical Society 6288 dx.doi.org/10.1021/ma201229g | Macromolecules 2011, 44, 6288–6302 ARTICLE pubs.acs.org/Macromolecules Disubstituted Liquid Crystalline Polyacetylene Derivatives That Exhibit Linearly Polarized Blue and Green Emissions Benedict A. San Jose, † Satoshi Matsushita, † Yasuyuki Moroishi, ‡ and Kazuo Akagi* ,† † Department of Polymer Chemistry, Kyoto University, Katsura, Kyoto 615-8510, Japan ‡ Institute of Materials Science, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan b S Supporting Information 1. INTRODUCTION Polymers are generally considered insulators; however, it has been demonstrated that conjugated polymers can become elec- trically conductive. Among these conjugated polymers, polyace- tylene (PA) has attracted much attention since the discovery of its metallic conductivity in doped form. When doped with iodine, pristine PA film with a metallic luster has a black surface and a high electrical conductivity on the order of 10 4 10 5 S/cm. 1 This discovery accelerated research on conjugated polymers and led to developments in polymeric light-emitting diodes (PLEDs), 2 plastic electronics, 3 polymer battery cells, 4 polymer photo- voltaics, 5 and other novel technologies. However, pristine PA film is insoluble in organic solvents and quickly loses its electrical conductivity when exposed to atmo- spheric conditions. Introducing an alkyl substituent into the polymer main chain increases solubility in organic solvents depending on the length of the alkyl chain. 6 However, the electrical conductivity of the substituted PA is significantly lower than that of the nonsubstituted PA. This phenomenon is due to less coplanarity of the main chain, which arises from steric repulsions between the substituents, a higher ionization poten- tial, and lower electron affinity. The main chain in the substituted PA is also still randomly oriented, which suppresses the electrical conductivity of the polymer. Adding a liquid crystalline (LC) moiety to the polymer main chain makes the polymer soluble in organic solvents and facil- itates alignment by the spontaneous orientation of the LC group. For example, monosubstituted liquid crystalline PA (mono- LCPA), prepared by introducing the LC moiety into the side chain of PA, shows liquid crystallinity and stability in air. 7 It is well-known that polymers with LC moieties spontaneously align to form multidomains by virtue of the orientation of the LC moieties. Further alignment of these polymers can be achieved by applying an external force such as shear stress or an electric or magnetic field 8 to construct a monodomain structure on a macroscopic level. The schematic in Figure 1 shows the forma- tion of multi- and monodomains through spontaneous orienta- tion and externally forced macroscopic alignment, respectively. Recently, we reported that mono-LCPA derivatives exhibit excellent macroscopic alignment under an applied magnetic field. Macroscopic alignment of the main chain led to both an electrical conductivity enhancement of two orders and an electrical anisotropy accompanied by the magnetically forced orientation of the LC side chain. 9 Received: May 31, 2011 Revised: July 7, 2011 ABSTRACT: We synthesized disubstituted liquid crystalline polyacetylene (di-LCPA) derivatives by polymerizing acetylene derivatives consisting of LC moieties either directly or indirectly attached to the main chain through flexible alkyl spacers. The di- LCPA derivatives show either enantiotropically thermotropic LC or lyotropic LC behavior. The origin of emission of substituted PAs, with respect to their substituents and structure, was elucidated. Depending on the substituents of the side chains, the polymers exhibit blue (470485 nm) and green (500540 nm) photoluminescence (PL) in chloroform and in cast film. The di-LCPA derivatives were macroscopically aligned using rubbing technique, and the aligned structures of the polymers are characterized in terms of main chain and side chain type alignments through XRD measurements. The emission color and alignment direction toward an external force in the di-LCPA derivatives are crucially determined by both the linkage forms (direct or indirect attachment) between the main chain and side chains and the molecular moieties (alkyl or aromatic moiety) of the side chains. The macroscopically aligned films of the polymers exhibit linearly polarized photoluminescence (LPL) with notable dichroic ratios. We fabricate multilayer electroluminescence (EL) devices using the polymers as the emissive polymer layer that emit 480 nm light with promising EL properties. We emphasize that although substituted PA derivatives are usually nonluminescent, the di- LCPA derivatives emit intense fluorescence with notable linear dichroism, and they could be promising for optically anisotropic luminescent materials.