Semi-fluorinated p-terphenyl liquid crystals

p-Terphenyls carrying two or four fluorine atoms in the central ring were synthesized via Suzuki-Miyaura cross-coupling reaction using phenylboronic acids and fluorinated dibromobenzenes. By calorimetric and X-ray diffraction analyses it was found that the p-terphenyls carrying two fluorine atoms develop nematic and/or smectic (A and C) phases. On the contrary, the p-terphenyls substituted with four fluorine atoms are not liquid crystals. Differences found in the thermal behavior are explained in terms pi-pi(F) interactions arising from the electron withdrawing ability of the fluorine atom. Such pi-pi(F) interactions let the p-terphenyls stack in a staggered configuration that explains the stability of the smectic C phase. On the other hand, the semi-fluorinated p-terphenyls absorb and emit in the blue region of the electromagnetic spectrum; a measured large Stokes shift indicates that these compounds are nearly transparent to their own emitted light. This optical characteristic, combined with liquid crystals properties of low order, makes these new p-terphenyls potential candidates for linearly polarized blue-emitting layers in electroluminescent devices.