Layered molecular dimers with biphenylene units: effect of the length of spacers and tails on smectogenic properties

CLAUDIA M. HEGUILUSTOY; ROSANA MONTANI; BERTRAND DONNIO; DANIEL GUILLON; RAÚL O. GARAY

Buenos Aires-Argentina

Simposio; 15th International Symposium on Metaestable, Amorphous and Nanostructured Materials; 2008

ISMANAM Steering Committee and International Advisory Board

<!-- /* Style Definitions */ p.MsoNormal, li.MsoNormal, div.MsoNormal {mso-style-parent:""; margin:0cm; margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:12.0pt; font-family:"Times New Roman"; mso-fareast-font-family:"Times New Roman"; mso-ansi-language:PL; mso-fareast-language:PL;} p.MsoBodyText3, li.MsoBodyText3, div.MsoBodyText3 {margin:0cm; margin-bottom:.0001pt; text-align:justify; mso-pagination:widow-orphan; font-size:12.0pt; font-family:"Times New Roman"; mso-fareast-font-family:"Times New Roman"; mso-ansi-language:EN-GB; mso-fareast-language:PL;} @page Section1 {size:595.3pt 841.9pt; margin:70.85pt 70.85pt 70.85pt 70.85pt; mso-header-margin:36.0pt; mso-footer-margin:36.0pt; mso-paper-source:0;} div.Section1 {page:Section1;} --> Two rigid mesogenic units connected by a conformationally flexible alkyl spacer constitutes a liquid crystal dimer (LCD) [1] whose mesomorphic properties are quite different compared to the ones showed by the structurally related monomeric liquid crystal. In fact, LCD behaviour resembles that observed in polymeric systems. Thus, LCDs have proven their usefulness as model compounds for the polymers. The unique liquid crystalline behavior of LCDs include the strong dependence of their transitional properties on the length and parity of the flexible spacers and the occurrence of alternating smectic phases where even- and odd-membered spacers respectively originate smectic A (SmA) and smectic C phases (SmC) [2]. The SmA phase posesses an intercalated structure while the anticlinal SmCA phase could have either an intercalated or a single layer structure. Here we compared the smectogenic properties of three homologous series of symmetric dimers CnCmCn with m = 5,6,7 and various terminal chains lengths [3]. The mesophases were investigated by differential scanning calorimetry, optical polarizing microscopy and X-ray diffraction. The X-ray results clearly confirm that a smectic E phase (SmE) exists in the whole CnC5Cn series. Though the dimers with n = 4,6 exhibit the usual alternation between intercalated SmA and intercalated SmCA phases, the mismatch in length between the terminal chains and spacers in the dimers with n = 8,10,12,14 unexpectedly leads to SmA phases or to a loss of the liquid crystallinity. As in the SmE and SmC case, the layer spacing d of all the SmA phases corresponds to the half dimer periodicity indicating that there is only one type of aliphatic sublayer of average thickness formed by both, the tails and the spacers. Probably, the unexpected SmA phases have a single layer structure in which the molecules adopt a U-shaped conformation with the aromatic units overlapping. References: [1]  C. Imrie, and P. Peterson, Curr. Opinion in Colloid Interface Sci. 7 (2002) 298. [2]  J. Watanabe et al., Mol. Cryst. Liq. Cryst. 346 (2000) 77. [3]  Dimers C4CnC4 have been already reported in ref. 2 and were reinvestigated.