| Whereas pi-conjugated polymers hold an enormous potential for organic electronics and photonics, monodisperse conjugated oligomers are superior from the perspectives of both scientific investigation and practical application in view of the well-defined and uniform molecular structure in addition to high chemical purity. Furthermore, some pi-conjugated polymers form thermotropic liquid crystals thanks to the rod-like backbone structure, offering spontaneously aligned solid films capable of anisotropic light absorption, emission, and charge transport. In this regard, monodisperse conjugated oligomers have an added advantage of facile processing into macroscopically ordered solid films because of their relatively low melt viscosity. My thesis was motivated to design, synthesize, and characterize glassy-liquid-crystal oligofluorenes possessing all the essential features to the realization of highly efficient and strongly polarized organic light-emitting diodes (OLEDs). These devices are potentially useful as an efficient light source for liquid crystal displays, as electroluminescent displays with improved viewing quality, in projection displays and stereoscopic imaging systems. Major accomplishments are summarized in what follows.; Glassy-nematic films of monodisperse oligofluorenes consisting of varied central units were implemented for strongly polarized OLEDs to complete the color gamut of light emission. More efficient, full-color OLEDs emerged from the intermolecular Forster energy transfer in lightly doped emitter layers, where both the donor and the acceptor molecules were simultaneously aligned. In addition, standard white-light emission was accomplished through energy transfer to a desired extent in a binary emitter layer. To further improve device efficiency, hybrid materials were synthesized by chemically attaching monodisperse oligofluorenes to hole-, electron-, and non-conducting cores. Despite the presence of nonmesogenic cores, the mesogenic pendants retain a high degree of orientational order, thereby enabling polarized light emission while allowing for tunable molecular orbital energy levels and charge-carrier mobilities. To gain new insight into how the emitter layer's chemical composition affects device performance, both chemical modification and physical doping were performed to modulate charge injection and transport. The roles played by interfaces bounding the emitter layer were also elucidated in terms of the distribution of excitons and the extents of quenching at interfaces. In addition to improving efficiency, dispersing excitons from interfaces may be conducive to device lifetime. |
