Molecular Designs for Enhancement of Polarity in Ferroelectric Soft Materials

The racemic oxovanadium(IV) salmmen complexes, [VO((rac)-(4-X-salmmen))] (X = C12C10C5 (1), C16 (2), and C18 (3); salmmen = N,N′-monomethylenebis-salicylideneimine) with “banana shaped” molecular structures were synthesized, and their ferroelectric properties were investigated. These complexes exhibit well-defined hysteresis loops in their viscous phases, moreover, 1 also displays liquid crystal behaviour. We observed a synergetic effect influenced by three structural aspects; the methyl substituents on the ethylene backbone, the banana shaped structure and the square pyramidal metal cores all play an important role in generating the observed ferroelectricity, pointing the way to a useful strategy for the creation of advanced ferroelectric soft materials.

bulky alkoxy substituents (X) at the 4-positions of the aromatic rings (Fig. 1). These vanadium(IV) complexes each showed wide P -E hysteresis loops for their viscous phases. We have probed the roles of their respective structural features on the corresponding ferroelectric properties by comparing the results with those for analogs that have rod shape and a square planar coordination geometries.

Results
The new banana shaped racemic VO compounds [VO(4-X-salmmen)] (X = C 12 C 10 C 5 (1), C 16 (2) and C 18 (3)) incorporating branched and single alkyl chains were synthesized by complexation between VOSO 4 •nH 2 O and (rac)-4-X-salmmen [30][31][32] . Final products were obtained after purification by column chromatography (SiO 2 , CHCl 3 ). Compound 1 is viscous oil while 2 and 3 are solids at room temperature. Powder X-ray diffraction spectroscopy (PXRD) results demonstrated that 1 is in a liquid crystal (LC) state at 298 K for which sharp peaks in the low angle region and broad scattering halos in the wide angle region were observed; this corroborated its LC nature for which the d spacing value was 33.0 Å (2θ = 2.68°) (Fig. 2). In contrast, 2 and 3 are crystalline phases at 298 K. The branched alkyl chains in 1 likely play a role in decreasing the phase transition temperature of 1.  The phase transition behaviour of 1-3 has been investigated by differential scanning spectroscopy (DSC). 1 shows a phase transition from a crystalline state to the LC state at 208.3 K on heating, with no transition from the LC to an isotropic liquid (IL) state being observed in the DSC curve (Fig. S1). Compounds 2 and 3 also exhibited multiphase natures, with phase transitions accompanied by translocation of the alkyl chains at 314.6 K and 379.1 K for 2 and at 329.6 K, 358.7 K and 367.4 K for 3, with melting to IL states at 412.2 K for 2 and 413.5 K for 3, respectively. However, their LC properties were not able to be obtained, as shown by the results from variable temperature PXRD in which we observed remaining sharp peaks in the middle and wide angle regions at 410 K after annealing treatments at 443 K and sequential slow cooling in each case (Fig. S2).
We measured the dielectric constants of 1-3 within the frequency range 100 Hz to 1 kHz by inductance capacitance and resistance (LCR) measurements in order to investigate the electric field responses to the phase transitions (Fig. S3). During the heating of 1 (starting from 300 K), the dielectric constant remained unchanged up to 350 K. Subsequently, an abrupt elevation of the dielectric constant occurred above 350 K in 1 in accord with a transformation (melting) occurring from the LC of 1 then to its IL state (even though an endothermic peak was not observed in the DSC curve). In the case of 2 and 3, anomalies were observed at 356 K, 390 K and 418 K for 2, while, for 3, anomalies were observed at 356 K, 366 K, 370 K, and 404 K, respectively, with these transitions being consistent with the results of the DSC analysis mentioned above. A dielectric response at 1 kHz showed a similar trend with lower peak values, thus demonstrating that the dipoles of the complexes are insensitive to an electric field oscillating at the above frequency.
The ferroelectric properties of each of the phases observed for 1, 2, and 3 were studied by analysing polarization vs. electric field (P -E) curves using a TF Analyzer1000. All compounds failed to show ferroelectric behaviour at 298 K. Ferroelectricity of 1 was developed at 338 K, while 2 and 3 at 363 K showed well-defined hysteresis behaviour (Fig. 3). In the case of 1, the spontaneous polarization (Ps) is 1.08 μ C cm −2 under a coercive 100 kV cm −1 field with E c at 50 Hz. A strong frequency relaxation occurred at − 194 kV cm −1 , which is characteristic of ferroelectric behaviour. Compounds 2 and 3 exhibited almost identical (ferroelectric) behaviour, with a Ps of 1.03 μ C cm −2 and 1.09 μ C cm −2 , respectively, under a coercive 100 kV cm −1 field with E c at 150 Hz and a relaxation at − 194 kV cm −1 . Hysteresis loops were not observed above the melting points.

Discussion
We have investigated the origins of the above ferroelectricity in terms of structural aspects of (a) the methyl substituent on the ethylene backbone, (b) the banana shaped structure and (c) the square pyramidal coordination geometry of VO by synthesizing five 'control' compounds of the R and S derivatives 1X (X = R or S) (Fig. S4) Table 1). Both 1R and 1S showed very similar ferroelectric properties to 1 (Fig. S5); however, 4 without a methyl substituent as a chiral centre on the ethylene backbone showed only weak spontaneous polarization (67 nC cm −2 ) despite its banana shaped molecular structure. On the other hand, the rod shaped 5 bearing a methyl substituent also showed weak ferroelectricity, demonstrating that a coexistence of the factors (a) and (b) are important for observing the ferroelectricity. Moreover, we found that the presence of (c) the square pyramidal coordination geometry of the VO centre also plays important role in the generation of ferroelectricity. The ferroelectricity of the square planar derivative 6 incorporating both a methyl substituent and a banana shaped structure was also weak (53 nC cm −2 ). From these results, we conclude that synergism arising from the three principal structural features given by (a), (b) and (c) is important for the development of the ferroelectricity observed in 1-3 (Table 1).
Ferroelectricity for 1 developed at 338 K, a lower temperature than for 2 and 3, reflecting its phase transition occurring at lower temperature and in accord with the presence of branched alkyl chains. Compound 1 is in the LC state at both 298 K and 338 K, however, hysteresis was not observed at 298 K. This result indicates that a dipole inversion of the banana shaped 1 requires higher thermal energy than available at 298 K on application of the electric field. Although 2 and 3 do not show liquid crystal properties, their temperature-dependent softness and viscosity nature contribute to the generation of their ferroelectricity. These findings clearly open doors for the design and synthesis of the next generation of multifunctional ferroelectric soft materials.

Conclusions
We have synthesised the banana shaped ferroelectric VO soft materials 1-3 employing racemic salmmen ligands bearing branched/linear long alkyl chains. These compounds exhibit ferroelectric properties associated with large spontaneous polarisations. Moreover, the substitution of branched alkyl chains in the banana shaped structure led to both development of liquid crystallinity at room temperature as well as a decrease of the ferroelectric phase transition temperature. The synthesis of metallomesogens exhibiting liquid crystallinity at room temperature, such as 1, are anticipated to contribute not only to the development of optoelectronic display devices in applied electronics but also to the generation of soft materials exhibiting ferroelectricity at room temperature. Compounds 2 and 3 also show significant promise for exhibiting liquid crystallinity, and their possible simultaneous liquid crystallinity and ferroelectricity are under investigation. Clearly the construction of banana shaped square pyramidal metal complexes that also incorporate long alkyl chains provides a powerful strategy for generating novel multifunctional molecular soft materials displaying enhanced ferroelectricity.  (rac)-4-C 5 C 12 C 10 -salmmen. This compound was synthesized by the same method as described for (rac)-4-C 16 -salmmen using 4-C 5 C 12 C 10 -salicylaldehyde; yield 0.32 g (31%), 1  [VO((rac)-4-C 16 -salmmen)] (2). This complex was synthesized by the same method as employed for 1 using (rac)-4-C 16  [VO(rac)-4-C 18 -salmmen] (3). This complex was synthesized by the same method as employed for 1 using (rac)-4-C 18 -salmmen. Green powder, yield 0.34 g (81%), Anal. Physical measurements. 1 H NMR spectra were recorded on a JEOL (500-ECX) instrument (500 MHz) in deuterated solvents using TMS as internal reference. Elemental analyses (C,H,N) were carried out on a J-SCIENCE LAB JM10 at the Instrumental Analysis Centre of Kumamoto University. Differential scanning calorimetry (DSC) thermal analysis was carried out at 5 K min −1 on a SHIMADZU DSC50. Powder X-ray diffraction (PXRD) measurements were performed on a Rigaku X-ray diffract meter RAD-2A with a 2.0 kW Cu Ka X-ray. Dielectric constants were measured by an inductance capacitance and resistance (LCR) meter on a Wayne Kerr 6440B LCR meter. The determination of polarization was performed on an aixACT TF analyser 1000. Circular dichroism (CD) spectra of KBr pellets containing samples were measured by JASCO J-820 at RT.