Synthesis and thermal behavior of triblock semifluorinated n-alkanes

A series of semifluorinated long-chain triblock alkanes of the general formula: H(CH2)n(CF2)6(CH2)nH, abbreviated as HnF6Hn, where n = 12, 14, 16, 18 and 20, have been synthesized in two steps, by radical addition of 1,6-diiodoperfluorohexane I(CF2)6I to two equivalents of an appropriate terminal alkene CH2 = CH(CH2)n−2H, followed by subsequent deiodination. This novel class of molecules has been subjected for the first time for thermal characterization. Although the structure of these compounds differs from that typical for liquid crystals, the investigated herein (alkyl)-fluoroalkyl-(alkyl) triblocks were found to exhibit thermotropic liquid crystalline phases. Phase polymorphism of the synthesized compounds was investigated upon both heating and cooling, in the temperature range of 270–360 K, using differential scanning calorimetry (DSC). Additionally, polarized optical microscopy was applied in order to identify the phases and characterize the phase transitions on the base of their textures. In the investigated temperature range, the measured compounds exhibit rich phase polymorphism, including different types of liquid crystalline phases (e.g., smectic A, smectic B and smectic E). Upon analyzing DSC results, it can be concluded that for compounds with n = 16, 18 and 20 all the detected phase transitions (crystalline ↔ smectic, between smectic phases, smectic ↔ isotropic) shift toward higher temperature with increasing in the hydrocarbon chain length. The phase situation for compounds with n = 12 and 14 is slightly different. However, for all the studied compounds, it can be observed that the longer hydrocarbon chain, the higher temperature of the phase transition to the isotropic phase. No metastable phases were observed.