Synthesis, Liquid Crystalline Properties And Fluorescence Of Hexacatenar 1,3,4-oxadiazole Derivatives

Phasmids and polycatenar liquid crystals have been known since 1985. They consist of a long rod-like rigid core ending in two half-disc moieties. Originally, the term'phasmid'referred to rod-like mesogens ending in two three-chain phenyl groups because of the similarity of the molecules to a six-legged stick-like insect called a phasma. The core contains in general, five rings and each'half-disc'is usually composed of two or three paraffinic chains grafted on both terminal phenyl rings in meta and/or para positions. The molecular architecture of such hybrid mesogens, situated between rod-like and disc-like mesogenic compounds, gives rise to rich polymesomorphism, especially in the case of biforked mesogens, which may present nematic (N), lamellar, cubic (Cub) and columnar mesophases in the same series or in a pure compound. During the past two decades, research of polycatenar mesogens has been extended from traditional type to many new types,including polycatenar metallomesogens, polycatenar mesogens containing bent-core, hydrogen-bonded polycatenar mesogens and functional polycatenar mesogens. They are particularly noteworthy in the theoretical and practical aspects even though they are on the fringe of the above-mentioned concept of polycatenar mesogens.Since the discovery of electroluminescence(EL) in poly(1,4-phenylene vinylene) (PPV) by Burroughes et al. in 1990 there has been tremendous interest in polymer light-emitting diodes (LEDs). The research concentrated on improving of external quantum efficiency (EQE) of LEDs. EQE was defined as the ratio between photons emitted and electrons injected. Because of its hole-transporting feature, the velocity of hole-injecting in PPV is much faster than that of electron-injecting. This results in that photons and electrons recombine near the interface between PPV and cathode of metal, which caused quenching of many excitons and furthermore, lowed the brightness and EQE of devices. To overcome the above-mentioned shortcoming, electron-transporting materials were introduced into bilayer or multilayer devices to balance the highly hole-injection. 1,3,4-oxadiazole derivatives, such as 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD), are widely used as electron-transporting layer in OLED, due to their thermal and chemical stability, electron-affinity and unique optical properties.Since the discovery of columnar liquid crystals, many research efforts have been devoted to the study of their structure and physical properties. Columnar liquid crystals often display a high degree of electronic communication between mesogenic cores within a column, and such systems have been generally viewed as efficient charge transport and energy transport conduits. Discotic molecules, e.g., tripheylene, hexabenzocoronene and perylene diimide derivatives can self-organize into columns, which then located into different lattice and formed the columnarmesophase. Polycatenar mesogens usually exhibit stable columnar mesophase. Introducing 1,3,4-oxadiazole group into columnar liquid crystals might result in highly efficient electron-transport materials. At present, reports about this kind of materials are still limited. Based on the above-mentioned reasons, we designed and synthesized a series of hexacatenar mesogens based on 1,3,4-oxadiazole, named 1,4-bis{5-(3,4,5-trialkoxylphenyl)-1,3,4-oxadiazole-2-yl}benzene (abbreviated as P-P-OXD-n, n=8,10,12,14). Liquid crystalline properties of P-P-OXD-n were investigated by polarized light optical microscopy, differential scanning calorimetry and X-ray diffraction. Fluorescent properties were investigated by photoluminescence spectrum. The results were:(1) P-P-OXD-14 and P-P-OXD-12 exhibited rectangular and hexagonal columnar mesophase in a wide temperature range respectively. Crystallization of the compounds were slow even at room temperature, which is favor of the practical application as electron-transporting materials. P-P-OXD-10 also exhibited columnar mesophase in a wide temperature range. Assuming the density of compounds was 1g/cm3 and according to the results of XRD, number of molecules per slice of columns were calculated, which are 2.07 and 1.82 in rectangular and hexagonal columnar mesophase, respectively. The configuration of the molecules energy-optimized by MM2 showed that rigid cores are bent, so we proposed that the two molecules arranged in a manner of back to back in a stratum of columns. P-P-OXD-n showed rectangular columnar mesophase when n is long, while hexagonal with small n due to the change of molecular shape with the decrease of n .(2) P-P-OXD-n showed highly-efficient fluorescent properties, the wavelength of emission peak (λmax) was 416nm.The fluorescence emission weaken andλmax red-shifted with the increase of solvent polarity .Our present results confirmed that P-P-OXD-n combined the liquid crystalline property and the electron-transporting ability of 1,3,4-oxadiazole derivatives and might be potential to be electron-transporting materials used in organic light-emitting diodes to improve the brightness and external quantum efficiency (EQE) of devices.