Laterally substituted symmetric and nonsymmetric salicylideneimine-based bent-core mesogens

Bent-core mesogens have gained considerable importance due to their ability to form new mesophases with unusual properties. Relationships between the chemical structure of bent-core molecules and the type and physical properties of the formed mesophases are relatively unknown in detail and differ strongly from those known for calamitic liquid crystals. In this paper symmetric and nonsymmetric five-ring salicylideneaniline-based bent-core mesogens are presented, and the effect of lateral substituents attached at the outer phenyl rings (F, Cl, Br) or the central phenyl ring (CH3) on the liquid-crystalline behaviour and on the physical properties is studied. Corresponding benzylideneaniline-based compounds were additionally prepared in order to study the influence of the intramolecular hydrogen bond. The occurring mesophases were investigated by differential scanning calorimetry, polarising microscopy, X-ray diffraction and dielectric and electro-optical measurements. The paper reports on new findings with respect to the structure–property relationships of bent-core mesogens. On one hand, the disruptive effect of laterally substituted halogen atoms, F, Cl and Br, on the mesophase behaviour of three isomeric series was much lower than expected. On the other hand, an increase of the clearing temperature by 34 K was observed, caused by small lateral substituents. The electro-optical behaviour, especially the type of polar switching and corresponding molecular movements, is sensitive to variations in the molecular structure.


Figure S1
Molecular arrangement of bent-core molecules and their electro-optical switching in the SmC a P A phase Additional X-ray data Table S1: X-ray data for the layer reflections from Guinier film measurements (9 compounds) Table S2: X-ray data for the 2D modulated phases (compounds OH 4b, OH 5e-g) Table S3: Estimation of the number of molecules in the cross section of the 2D unit cells in the modulated smectic phases (compounds OH 4b, OH 5e-g) Figure S2: a) Small angle X-ray diffraction pattern for a partially surface-aligned sample of o-OH 4b; b) reciprocal and real lattice; c) one possible schematic model of the undulated SmCP phase of compound OH 4b.  Figure S1: The molecular arrangement of bent-core molecules considering the tilt and the polarity in adjacent layers of the SmCP phase. The smectic layers are perpendicular to the drawing plane. The different molecular symbols correspond to opposite bent directions perpendicular to the drawing plane: Symbol having a circle in the centre means front view; symbols with a cross means back-side view. Both directions of the bend mean also opposite polar axes. The red and blue symbols indicate layers of different chirality. In the upper part, on the left, the racemic antiferroelectric state is sketched, which can be switched to the ferroelectric state on application of an electric field, shown on the right. In the lower part, the homochiral antiferroelectric states are sketched on the left, in which the tilt direction alternates. The field-induced arrangement in the homochiral ferroelectric state is shown on the right. Polar smectic C phases (SmCP) are designated with the following symbols: P stands for polar, the suffixes A or F, added to P, indicate antiferroelectric or ferroelectric states (SmCP A and SmCP F , respectively). A synclinic or an anticlinic interlayer correlation is indicated by the suffixes s or a after C (i.e., SmC s P, SmC a P). In the so-called homochiral (homogeneous chiral) structures all layers have the same handedness, whereas in the racemic state the chirality alternates from layer to layer.

X-ray diffraction measurements
Additional X-ray data   S1   Table S3: Estimation of the number of molecules in the cross section of the 2D unit cells in the modulated smectic phases (T temperature, V M,cr molecular volume in the crystal calculated by the incremental method of A. Immirzi, B. Perini, Acta Cryst. 1977, A33, 216 -218, V M,l molecular volume in the isotropic liquid derived from V M,cr by the ratio of the average packing coefficients in both phases V M,l = V M,cr k cr /k l with k cr = 0.7 and k l = 0.55 according to A. I Kitaigorodski, "Molekülkristalle", Akademieverlag Berlin, 1979, V U ... volume of a hypothetical 3D unit cell calculated from the lattice parameters and a height h of about 0.52 nm corresponding to the assumed stacking distance of the molecules in bend direction by V U = abhsin, n cr number of molecules in the cross section of the unit cell for a crystal-like packing density, n l number of molecules in the cross section of the unit cell for a liquid-like packing density, n lc number of molecules in the cross section of the unit cell in the liquid-crystalline state, estimated as the average of n cr and n l ).  Figure S2: a) Small angle X-ray diffraction pattern for a partially surface-aligned sample of OH 4b at 115 °C, indexed on the basis of an oblique lattice with the parameters a = 6.6 nm, b = 4.8 nm, γ = 111.5°; b) reciprocal (green) and real lattice (black) with the possible orientation of the molecules in the undulated SmCP phase; c) one possible (schematic) model of the undulated SmCP phase with an orientation of the molecular long axis along lattice direction b (corresponding to the orange "molecule" in b).
One possible interpretation of the X-ray data for OH 4b in agreement with the other measurements is the following: The additional reflections may be indexed as a 11 reflection of an oblique 2D lattice.
Assuming the -11 reflection to be superimposed by the very strong ring-like 01 layer reflection, lattice parameters can be determined to a = 6.6 nm, b = 4.8 nm and  = 111.5°. Comparing the volume V U = 15.32 nm 3 of a hypothetical 3D unit cell with the molecular volume in the liquid crystalline phase V M,lc = 1.49 nm 3 , about 10 molecules occupy the cross section of the unit cell (cp . Table S3). From the azimuthal distribution of the wide angle scattering along (-scan), the tilt angle  of the molecules with respect to the layer normal can be determined to be ~20°, which is in good agreement with the optically determined value. From this tilt and the d-value of the 01 reflection (layer thickness 4.5 nm) an effective molecular length of L eff = 4.8 nm can be calculated according to L eff = d/cos. Using CPK models and with the assumptions that the bending angle amounts to nearly 120° and that all chains are in an all-trans configuration, a length L calc of 5.46 nm would be obtained. The difference (L eff = 4.8 nm and L calc = 5.5 nm) indicates that the molecular configuration assumed in this calculation is not correct for compound OH 4b. Another bending angle, a high portion of gauche conformers, or an interdigitation of the terminal chains could be the reason for the difference. The relatively large bromine atom at the central part of the molecule requires an increased lateral space, which could make easier an interdigitation of hydrocarbon chains of adjacent layers.

Synthesis of the compounds: Experimental procedures and analytical data
The preparation of the final compounds is sketched in the Schemes 2-5 (main paper). Because several reactions were performed by using similar experimental conditions, three general procedures are given here. In the second part new intermediates are described. In the third part the preparation of the final products is reported, the melting temperature and the mesophase behaviour are summarised in the corresponding Tables 2-5 (main paper).

General procedure 1 (GP1): Esterification of benzoic acids with phenols by the carbodiimide method
The substituted benzoic acid (0.005 mol), the corresponding phenol (0.005 mol) and a catalytic amount of DMAP were dissolved in 50 mL of dry dichloromethane. Dicyclohexylcarbodiimide (1.14 g, 0.0055 mol) was added and the mixture was stirred at rt for about 24 h. The reaction process was controlled by TLC. After completion the precipitated dicyclohexylurea was separated, the solvent evaporated and the crude material purified by recrystallization.

Condensation of anilines with benzaldehydes giving Schiff bases
To a slurry of the substituted benzaldehyde (0.005 mol) and the corresponding aniline (0.005 mol) in 30 mL ethanol, a catalytic amount of acetic acid was added and the mixture heated under reflux for about 10 min. Stirring was continued for 24 h at rt. The precipitate was filtered off and the crude compound was purified by recrystallization. For the preparation of the final products OH 5 and OH 6 only 0.0025 mol of the substituted diamine was used.

General procedure 3 (GP3)
Acylation of 2,4-dihydroxybenzaldehyde using acid chlorides To a slurry of the substituted benzoic acid (0.01 mol) in 50 mL of dry dichloromethane and one drop of pyridine, oxalyl chloride (0.04 mol as 2 M solution in dichloromethane) was added and the mixture refluxed for 2 h. The solvent was evaporated, 50 mL dichloromethane was added again to remove the residue of oxalyl chloride under vacuum together with the solvent. The crude acid chloride was dissolved in 30 mL of dry toluene. After addition of 2,4dihydroxybenzaldehyde (0.005 mol) and DMAP (catalytic amount) 0.011 mol triethylamine was slowly added dropwise. Stirring was continued for 6 h at 65 °C and afterwards for 10 h at rt. After cooling, the reaction mixture was filtered to remove the triethylamine hydrochloride, and the solvent evaporated in vacuum. The crude compound was purified by recrystallization.

Final compounds OH 1-6 and H 1-6
The compounds are reported as listed in Tables 1-6.