Fig. 4: The optical textures of compound 1 as observed between crossed polarizers on cooling from Col_hexΔ via Col_hexΔ/□ phases to Col_squ^T.

a–d The birefringent planar textures (columns parallel to the surface) and areas with an alignment of the columns perpendicular to the substrate surfaces (optically isotropic dark areas at the bottom). The optical indicatrices of the distinct phases are shown on the right. The scale bars are 100 μm. The insets show the planar textures with additional λ-retarder plate. In d the orientation of the polarizers (arrows) and the slow indicatrix axis orientation (dotted line) are shown, which are identical for a–d. In the Col_hexΔ and Col_hexΔ/□ phases (a, b) the birefringence is negative (the blue shifted fans with λ-retarder plate are parallel to the indicatrix direction), i.e. the main intramolecular π-conjugation pathway is on average perpendicular to the column long axis with no visible change in birefringence at the phase transition Col_hexΔ − Col_hexΔ/□ (a → b). At the first order phase transition to Col_squ^T the birefringence suddenly becomes Δn = 0 (c and inset) and then weakly positive (d the yellow shifted fans are now parallel to the indicatrix direction, see inset in d). This indicates that the π-conjugation pathway crosses a certain angle at which it changes from being perpendicular to the column direction to predominately parallel. On further cooling the birefringence in the Col_squ^T phase remains smaller than in the Col_hex phases, indicating that the π-conjugated rods retain a tilt by a certain angle with respect to the column long axis. The bright spots and lines in c are residues of the coexisting Col_hexΔ/□ phase at this strongly first order phase transition.