Novel rod-like fluorene-trimers exhibiting smectic LC mesophases

Two novel fluorene-trimers were synthesised following a Suzuki-type cross-coupling reaction. Methyl pendant chains in 9-position of the fluorene building blocks ensured a rod-like backbone of a small molecular diameter while the introduction of terminal, flexible n-octyl and n-hexadecyl alkyl chains in the 2- and 7″-positions causes the occurrence of two different liquid-crystalline (LC) mesophases for both trimers. The terminal chain length was found to strongly affect the phase transition temperatures but not the number and type of LC phases formed. By using several analytical techniques (differential scanning calorimetry DSC, polarizing microscopy, and X-ray diffractometry) the exact nature of the mesophases was determined. The n-hexadecyl- and n-octyl-substituted fluorene-trimers characteristically form smectic LC phases at transition temperatures of 108 and 171 °C, respectively, which when heated further are transformed into lower ordered nematic LC mesophases at 140 and 187 °C, respectively.