No abstract

A series of new monomers of 2,5-bis[(4-tertbutyl-phenyl)-1,3,4-oxadiazole] styrene (M-Ct) and 2,5-bis[(4-alkoxy-phenyl)-1,3,4-oxadiazole]styrene (M−OCm, m is the number of the carbons in the alkoxy groups, m = 8, 10, 12, 14) were synthesized. Conventional radical polymerization of the monomers resulted in a series of new mesogen-jacketed liquid crystalline polymer (MJLCP) containing the 1,3,4-oxadiazole unit. The chemical structures of the monomers were confirmed by elemental analysis, mass spectrometry, 1H NMR, and IR. The molecular characterization of the corresponding polymers of P-Ct and P-OCms was performed with 1H NMR, gel permeation chromatography, and thermogravimetric analysis. Their phase structures and transitions were investigated by differential scanning calorimetry, wide-angle X-ray diffraction, and polarized light microscopy experiments. The P-Ct formed the hexatic columnar nematic (ΦHN) phase that is typical for MJLCPs, wherein the chain molecules were rodlike. The P-OCms exhibited a well-defined smectic A (SA) phase. As the mesogenic group is laterally jacketed to the polyethylene backbone through a single carbon−carbon bond, the P-OCm molecule in the SA phase should be more or less ribbonlike with the backbone squeezed by the parallel aligned side chains on both sides. The transition of the four P-OCms follows the sequence of SA ↔ N ↔ I. The comparison between P-Ct and P-OCms indicates that the flexibility of the side-chain tails is crucial to determine the LC structures. Namely, simply changing the chemical structures of small portion of the MJLCP may greatly vary the molecular packing behavior and thus the molecular shape in LC phase structures.

Styrene-based monomers with the first-and second-generation dendronized carbazoles were synthesized by a convergent strategy. Conventional free radical polymerizations were carried out to synthesize the dendronized jacketed polymers using these two monomers. The apparent molecular weights of the two polymers determined by gel permeation chromatography were 57 000 and 34 000 g/mol, respectively. The polymers had excellent thermal stability with their 5% weight loss temperatures all above 360 °C. Their photophysical properties in solutions and films were investigated. Different solvents had negligible effects on their UV-vis or photoluminescent (PL) spectra. A 10 nm blue shift of the emission peak was observed in the PL spectra of films compared with those of solutions. After the films were annealed, the emission peaks became narrower. X-ray diffraction techniques were utilized to examine the phase structures of the dendronized jacketed polymers. The first-generation polymer formed a hexatic columnar nematic phase, while the second-generation one exhibited a columnar nematic phase. This stiff main-chain conformation and the dendritic side-group structure could reduce the intramolecular interactions and aggregations of the carbazole side groups and prevent the formation of excimers, which would be beneficial to the optoelectronic properties. The two polymers showed similar electrochemical properties. Their HOMO levels were about -5.3 eV, which was quite close to that of PEDOT:PSS. Electroluminescent (EL) devices were fabricated in two configurations. A new emission peak was found in the EL spectra compared with the PL spectra. All the devices emitted blue lights and possessed low driving voltages. An introduction of an electron-transporting layer could greatly improve the device properties. The best device could reach a luminescence of 2195 cd/m 2 , a current efficiency of 0.240 cd/A, and an external quantum efficiency of 0.353%.