Phase behaviour and crystal structures of 2',3'-difluorinated p-terphenyl derivatives

The crystal structures of difluorine derivatives of p-terphenyls (nTm) have been determined by single-crystal X-ray diffraction. For the unsymmetrical substituted compounds 2',3'-difluoro-4-methyl-p-terphenyl (1T0, C₁₉H₁₄F₂) and 4-ethyl-2',3'-difluoro-4''-methyl-p-terphenyl (1T2, C₂₁H₁₈F₂), the crystal structure is disordered, with molecules statistically entering the crystal in up and down orientations, with full superposition of all the atoms, except for those of the terminal groups (H/methyl for 1T0 and methyl/ethyl for 1T2). For triclinic 2',3'-difluoro-4,4''-dimethyl-p-terphenyl (1T1, C₂₀H₁₆F₂), with the space group P-1, the two crystallographically independent molecules have the same conformation, which is different from monoclinic 1T0 (space group C2) and 1T2 (space group C2/c). A common feature of the conformation of the three compounds is the noncoplanar twisted arrangement of the three rings of the p-terphenyl moiety. Two-dimensional (2D) Hirshfeld fingerprint plots are consistent with H...H and C...H contacts in the crystal packing. For the three compounds, the phase behaviour has been investigated by POM (Petra/Osiris/Molinspiration) and differential scanning calorimetry (DSC) analysis. 1T2 is mesogenic, with enantiotropic nematic behaviour.