This paper relates a comparative study of crystal structures of two isomeric compounds. They differ only through their polyaromatic group sequence: O—ø—CO2—ø—C ≡ C—ø—CO2on the one hand and O—ø—C ≡ C—ø—CO2—ø—CO2on the other, and give rise to very different polymorphisms. The chiral mesogenic (S) 1-methylheptyl 4-[(4-octyloxy-3-fluoro) benzoyloxy]tolan-4′ carboxylate (8FBTOlM7) crystallizes in the P212121space group, with two different molecules A and B per asymmetric unit (Z= 8) witha= 7.903(2),b= 11.324(1) andc= 80.576(6)Å. The polyaromatic central cores of both molecules are parallel and aligned in the same direction, giving dimers with a tilt angle close to 11°. The molecules give rise to sheets, of which the arrangement is smectic C and is antiferroelectric-like. The repetitive unit in the crystal phase is a bilayer one, where molecules are alternatively tilted from one layer to the other. The molecular arrangement is compared with that of the (S) 4-[l-methylheptyloxy-carbonyl] phenyl 4-heptyloxytolan-4′ carboxylate (7TBOlM7) which gives rise to a very rich polymorphism, including ferro-, antiferro- and ferri-electric phases. The binding energy within a sheet and between neighbouring sheets was evaluated by using molecular mechanics for both compounds. It seems that the relative positions of the benzyl and tolan groups inserted between the polar ester and ether groups in the polyaromatic central core, is of a prime importance in the molecular arrangement and consequently in the mesomorphic sequence.