Crystal structure of cholesteryl 4-[4-(4-n-hexylphenylethynyl)-phenoxy]butanoate – a liquid-crystalline unsymmetric dimer

Abstract

Cholesteryl 4-[4-(4-n-hexylphenylethynyl)-phenoxy]butanoate, which exhibits the phase sequence: Cr 119.3°C (42.4 J g−1) SmA 196.4°C (1.1 J g−1) TGB–N* 202.4°C (5.4 J g−1) I, crystallizes in the triclinic space group P1 with unit cell parameters: a = 10.527(1), b = 13.151(2), c = 16.991(2) Å, α = 86.13(1)°, β = 98.96(1), γ = 105.43(1)°, Z = 2. The crystal structure has been solved by direct methods using single-crystal X-ray diffraction data and refined to R = 0.0618. There are two crystallographically independent molecules, I and II, in the asymmetric unit. In both the molecules the phenyl rings are planar. The dihedral angle between the two phenyl rings is 12.16° and 18.14° for molecules I and II, respectively. In both the molecules, the six-membered rings of the cholesterol moiety are conformationally very similar. However, pronounced differences are observed in the conformation of the five-membered ring, which is intermediate between half-chair and envelope in molecule I, and half-chair in molecule II. The packing of molecules in the crystalline state is found to be a precursor to the Smectic A phase structure. The molecules in the crystal are held together by van der Waal's interactions.