A Novel Mesogen-Jacketed Liquid Crystalline Electroluminescent Polymer with Both Thiophene and Oxadiazole in Conjugated Side Chain

Mesogen-jacketed liquid crystalline polymers (MJLCPs) with both electron-transport oxadiazole and hole-transport thiophene in the side chain were reported for their promising electroluminescent property. Monomers of 2,5-bis(5-[(4-alkoxyphenyl)-1,3,4-oxadiazole]thiophen-2-yl}styrene (M-Cm, m is the number of the carbons in the alkoxy groups, m = 8,10) were synthesized and confirmed by H-1-NMR, mass spectrometry, and elemental analysis. The corresponding polymers were successfully obtained and characterized by thermal analysis, optical spectroscopy, cyclic voltammetry, electroluminescent analysis, polarized light microscopy (PLM), and wide-angle X-ray diffraction (WAXD). The polymers exhibited high decomposition temperatures reaching 382 degrees C and high T-g's reaching 184 degrees C. The absorption spectra indicated that both the monomers and polymers had little aggregation in film than that in solution, and the absorption spectra of the polymers showed an obvious blue-shift compared with those of the monomers. Both the monomers and the polymers had blue-green emission, and the photoluminescence spectra of the polymers in film suggested the formation of excimer or exciplex. The polymers showed lower HOMO energy levels and LUMO energy levels than those of the MJLCPs containing oxadiazole unit reported before. Electroluminescence study with the device configuration of ITO/PEDOT/PVK/polymer/TPBI/Ca/Ag showed maximum brightness and current efficiency of 541 cd/m(2) and 0.10 cd/A, which proved that the introduction of directly connected electron- and hole-transport units could greatly improve the EL property of side-chain conjugated polymers. The phase structures of the polymers were confirmed to be smectic A phase through the results of PLM and WAXD. The annealed samples emitted polarized photoluminescence at room temperature, which indicated potential utility for practical applications in display. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48:1502-1515, 2010