Phase transfer Pd(0)/Cu(I) catalysed polymerization reactions 7. Synthesis and thermotropic behaviour of 1,4-bis[2-(3′,3′'-difluoro-4′,4′'-di-n-alkyloxyphenyl)-ethynyl]benzene dimers

Abstract

The symmetrically difluorinated aryl-acetylene dimers, 1,4-bis[2-(3′,3′'-difluoro-4′,4′'-di-n-alkyloxyphenyl)ethynyl]benzenes (n = 7–12), were prepared by a one pot phase transfer. Pd(0)/Cu(I) catalysed, three step coupling of 1,4-diiodobenzene with 2-methyl-3-butyn-2-ol and the appropriate 1-bromo-3-fluoro-(4-n-alkyloxy)benzene. All members of the series display enantiotropic nematic and smectic C phases as well as an additional smectic mesophase in the sequence C[sbnd]S[sbnd]S_C[sbnd]N[sbnd]I. The textures observed by polarized optical microscopy strongly support identification of the second smectic phase as a S_E mesophase. The S_C temperature window increases as the length of the n-alkyloxy substituent increases, whereas the stability of the nematic and S_E temperature windows decrease. In these fluorinated dimers, destabilization of mesomorphic behaviour by lateral fluorine substitution is compensated by stabilization due to the large length to breadth ratio of the mesogen, such that melting and crystallization are depressed more than isotropization when compared to the non-fluorinated dimers.