| Summary: | 碩士 === 國立臺灣科技大學 === 化學工程系 === 88 === Abstract
Two series of disc-like molecules (schemeⅠ5 and schemeⅡ5) with a quinoxaline or a biquinoxaline hetrocyclic core and eight n-alkoxy side chains which exhibits a discotic mesophase, viz., Q8-n and BQ8-n (2,3,7,8-Tetra-[3,4-dialkoxyphenyl]-pyrazino-[2,3-g] quinoxaline and 2,3,2',3'-Tetra-[dialkoxyphenyl]-[ 6,6'] biquinoxaline, respectively] were synthesized. The homologous series of disc-like molecules, (schemeⅠ5 and schemeⅡ5) are easily accessible by condensation of1,2-Bis-(3,4-dialkoxy-phenyl)-ethane-1,2-dione with biphenyl-3,4,3',4'-tetraamine and benzene-1,2,4,5-tetraamine respectively, in the presence of acetic acid. The phase transition temperatures were determined by DSC and POM. The dependence of side chains density has also been observed. The disc-like molecules (schemeⅠ5 and schemeⅡ5) with eight alkoxy side chains exhibited discotic liquid crystalline behavior, whereas they were not liquid crystalline with four side chains. In the series of Q8-n (n=5~12) an odd-even fluctuation with n in the flexible part of the molecular skelton affects both melting temperatures and molar melting entropies, similar effect is also observed for isotropization temperatures and isotropization entropies. The mesophase range in this series appears to increase at first with n and then decreases after attaining a maximum value for Q8-10 (n=10). To evaluate the exact mass of the compounds, matrix—assited laser desorption time-of-flight mass spectroscopy (MALDI-TOF MS) was applied.
The combination of the condensation reactions with a selective cleavage of methoxy group next to octoxy groups (n=8) extends the versatility of the procedure and provides additional possibilities of the synthesis of unsymmetrically substituted discotics. In this report, the synthesis of dihydroxyhexaoctoxy side chains in the series of MQ8-a and MQ8-b 11 as shown in schemeⅤis of special interest, since it represents a central precursor molecule for difunctional discotic monomers. Selective cleavage of the methoxy group of 1-(3,4-bis-octyloxy-phenyl)-2 -(3-methoxy -4-octyloxy-phenyl)-ethane-1,2-dione 9 with 48% HBr in refluxing acetic acid led to 10, following condensation with benzene-1,2,4,5-tetraamine and purification by column chromatography affords two monomers 11. Subsequent polycondensation reaction in solution with various alkane-α, ω-dioic acid dichloride led to new polyesters, PQ8-a and PQ8-b with disc-shaped quinoxaline mesogenic groups in the main-chain, respectively. Both polymers PQ8-a and PQ8-b show discotic mesophase.
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