Two liquid crystalline partially fluorinated compounds: (S)-(+)-4-(1-metyloheptyloxycarbonylphenyl) 4'-(6-perfluorobutanoyloxyhex-1-oxy) biphenyl-4-carboxylate, in short MHPP(9F)HBC and (S)-(+)-4-(1-metyloheptyloxy-carbonylbiphenyl) 4'-(6-perfluorobutanoyloxyhex-1-oxy) benzoate, in short P(9F)HPMHBC, exhibiting para-electric, ferroelectric and antiferroelectric phases have been studied by dielectric spectroscopy and spontaneous polarization measurements. These two compounds have the same molecular weight but their terminal chains at para positions are interchanged. Influence of molecular structure on the phase diagram and dielectric behaviour is presented. Two relaxation process revealed in the antiferroelectric phase are interpreted as collective modes: anti-phase and in-phase fluctuation of phase and amplitude. The reorientation around the short molecular axis does not contribute to the dielectric spectra.