Nematic bent-core

Thermotropic liquid crystals, 15 86-98 bent-core, 15 98 discotic phases of, 15 96 frustrated phases of, 15 94-96 metallomesogens, 15 97 nematic liquid crystals, 15 86-92 smectic liquid crystals, 15 92-94 Thermotropic mesophases, 20 79 Thermotropic polycarbonates, 19 804 Thermotropic polyesters, liquid-crystalline, 20 34... 

The free energy density is written as a sum of the nematic-type contribution (/n), the general smectic contribution (fs), the energy due to the polarization selfinteraction (/,), and the term specific for the bent-core systems (/be) ... 

The free energy density terms introduced so far are all used in the description of the smectic phases made by rod-like molecules, the electrostatic term (6) being characteristic for the ferroelectric liquid crystals made of chiral rod-like molecules. To describe phases made by bent-core molecules one has to add symmetry allowed terms which include the divergence of the polar director (polarization splay) and coupling of the polar director to the nematic director and the smectic layer normal ... 

The first term in (7) describes the coupling between the polarization splay and tilt of the molecules with respect to the smectic layer normal. This coupling is responsible for the chiral symmetry breakdown in phases where bent-core molecules are tilted with respect to the smectic layer normal [32, 36]. The second term in (7) stabilizes a finite polarization splay. The third term with positive parameter Knp describes the preferred orientation of the molecular tips in the direction perpendicular to the tilt plane (the plane defined by the nematic director and the smectic layer normal). However, if Knp is negative, this term prefers the molecular tips to lie in the tilt plane. The last term in (7) stabilizes some general orientation (a) of the polar director (see Fig. 7) which leads to a general tilt (SmCo) structure. 

Sathyanarayana P, Mathew M, Sastry VSS, Kundu B, Le KV, Takezoe H, Dhara S (2010) Splay bend elasticity of a bent-core nematic liquid crystal. Phys Rev E (Rapid) 81 010702(R)-... 

Taushanoff S, Le KV, Williams J, Twieg RJ, Sadashiva BK, Takezoe H, Jakli A (2010) Stable amorphous blue phase of bent-core nematic liquid crystals doped with a chiral material. J Mater Chem 20 5893-5898... 

Kundu B, PratibhaR, MadhusudanaNV (2007) Anomalous temperature dependence of elastic constants in the nematic phase of binary mixtures made of rodlike and bent-core molecules. Phys Rev Lett 99 247802-1-4... 

Fig. 22 Nanoparticles decorated with pro-mesogenic dendronized or bent-core liquid crystal ligands nematic Fe304 mixed monolayer nanoparticles capped with dendronized cyanobiphenyl ligands and oleic acid (17) [132], and mixed monolayer, non-mesogenic gold nanoparticles decorated with bent-core liquid crystal and hexane thiolates (18) [547]...

The bent core molecules do not only exhibit spontaneous resolution in smectic phases. One achiral derivative resolves in a nematic phase in this fluid state [ 145], while a substituted oxadiazole which forms a biaxial nematic phase also segregates [ 146]. The bent core clearly has a special stereochemical influence as a result of the effects it induces beyond the molecule, at least for liquid crystals. 

Y. Wang, H. Yoon, H.K. Bisoyi, S. Kumar, Q. Li, Hybrid rod-like and bent-core liquid crystal dimers exhibiting biaxial smectic-A and nematic phases. J. Mater. Chem. 22, 20363-20367 (2012)... 

Fig. 1.1. Macroscopic polarization in the distorted nematic phase composed of bent-core molecules with transverse electric dipoles or cone-shaped molecules with longitudinal dipoles.

General expressions for the flexocoefiicients of nematic liquid crystals have been obtained in terms of the direct correlation function using the powerful density functional approach. These expressions have been used to obtain some interesting numerical results using the Perkus-Yevic approximation for the pair correlation function. The results from the density functional theory have also been used in computer simulations of flexoelectricity using model bent-core molecules interacting via the Gay-Berne potential. Alternative general expressions for the flexocoefiicients have... 

J. Harden, B. Mbanga, N. Eber, K. Eodor-Csorba, S. Sprunt, J.T. Gleeson and A. Jakli, Giant flexoelectricity of bent-core nematic liquid crystals, Phys. Rev. Lett. 97(15), 157802/1-4, (2006). doi 10.1103/PhysRevLett.97.157802... 

Giant flexoelectricity of bent-core nematics studied by the flexing... 

Instead of discussing these we have only noted those layered structures which are relevant for understanding the properties of bent-core nematics and especially the flexoelectricity of these materials. 

As described above, the kink in the molecular shape and the requirement to fill the space as effectively as possible are not compatible with a three-dimensional fluid order. In other words, when translating a bent-core molecule in the melt of the neighbouring bent-core molecules, it experiences a periodic potential with its periodicity determined by the length I of the molecules. To allow for fluidity at the macroscopic level, one needs to frustrate the bent-core structure so that they do not lock into smectic layers easily. Such a frustration can be introduced by some steric or electrostatic disturbance of the bare bent-core (or peeled banana ) shape, which has been seen in some modulated smectic phases.As discussed by Bailey and Jdkli,a steric or electrostatic inclusion in the core of the molecules leads to layer modulation, an SmCc structure and broken smectic layers that effectively correspond to a columnar phase, as the inclusions increase. Following this picture, here we postulate that bent-core nematics are probably more frustrated than the electrically unswitchable B7 (columnar) phases, in which the broken smectic ribbons are separated by melted fluid nematic regions. Such over-frustrated B7 materials are characterized... 

Fig. 3.5. Sketch of the cluster nanostructure of the nematic phase of bent-core molecules.

In the following sections of this chapter we will summarize the direct, as well as the converse, flexoelectric measurements in fluid and elastomeric (dry or swollen) bent-core nematic liquid crystals, and try to explain these observations using the structural model outlined above. 

The direct method introduced in the previous section was first employed for studying the flexoelectric response of a bent-core nematic liquid crystal. 

ClPbislOBB is not the only bent-core nematic that has a giant 63 flexo-coefficient for a few other BC compounds (whose chemical structures and phase sequences are shown in Fig. 3.11) and mixtures a similar magnitude of jesj has been obtained (see Table 3.1). 

Many bent-core molecules do not have nematic phases, but may be added to calamitic nematics to tune the flexoelectric behaviour. Several groups have reported measurement results in such guest-host systems ° using either HAN cells or the flexoelectro-optic effect mentioned above. It was found that some BC compounds used as dopants in small (< 10%) concentration may effectively increase ei —es of the host. ° Other, mainly asymmetric, BC compounds had hardly any influence on ei - - es of the host nematic. ... 

The presence of clusters in BC nematics is now well established from various measmements. Recent studies " have in fact indicated a ferroelectric or an antiferroelectric response to an applied electric field, and an unusual low-frequency (presumably collective) mode has been detected in the dielectric spectra of bent-core nematics, which might also be related to clusters. In spite of the intense studies, however, the exact structure and the physical properties of the clusters are still unknown. Therefore, not surprisingly, a precise physical model for the role of polar clusters in the flexoelectric response of BC nematics and a quantitative estimation of the resulting increment of the flexocoefiicients has not yet been worked out. 

Summarizing, experimental observations suggest that the giant (direct or converse) flexoelectricity of bent-core nematics is related to the polar smectic clusters occurring in them. In order to explore the exact mechanism for how clusters contribute to the flexoelectric response, further experimental and theoretical studies are needed. 

Finally, it is worth mentioning that a phenomenon analogous to the difference between the normal and giant flexoelectricity of calamitic and bent-core nematics, respectively, exists in crystals, ceramics and polymers too. The flexoelectric response (defined in Eq. (3.1)) of perovskite-type ferroelectrics, " of relaxor ferroelectric ceramics and polyvinylidene fluoride (PVDF) films are four orders of magnitude larger than the flexoelectricity of dielectric crystals. In those sohd ferroelectric materials the polarization induced by flexing is evidently of piezoelectric origin. 

Unfortunately swollen LCEs are still not robust enough for long-term operation. However, if the rod-like molecules of a conventional LCE are substituted by BC nematics, a bent-core hquid crystalline elastomer (BCLCE) can be created. BCLCEs might combine excellent flexoelectric properties with rubber elasticity. 

The frequency dependences of the bend fiexoelectric coefficients were also measured for the same BC nematic fluid monomer, BC nematic swollen in a calamitic liquid crystal elastomer (BCN-LCE) and for the bent-core nematic elastomer (BCLCE) as shown in Fig. 3.14. One can see that for each material the fiexoelectric effect was found to be zero below 1 Hz, then the response increases abruptly up to 2 Hz and then decreases slightly. The apparent absence of the response below 1 Hz is probably due to screening by free ions. The slow decrease of the fiexoelectric coefficient at higher / is not yet clear. We assume, however, that it is not a measurement error, because 5CB showed a constant value in this frequency range. 

O. Francescangeli, V. Stanic, S.l. Torgova, A. Strigazzi, N. Scaramuzza, C. Ferrero, S. Orlandi and C. Zannoni, Ferroelectric response and induced biaxiality in the nematic phase of bent-core mesogens, Adv. Func. Mater. 19(16), 2592-2600, (2009). doi 10.1002/adfm.200801865... 

S.H. Hong, R. Verduzco, J.C. Williams, R.J. Twieg, E. DiMasi, R. Pindak, A. Jakli, J.T. Gleeson and S. Sprunt, Short-range smectic order in bent-core nematic liquid crystals. Soft Matter 6(19), 4819-4827, (2010). 

A. Jakli, M. Chambers, J. Harden, M. Madhabi, R. TeeUng, J. Kim, Q. Li, G.G. Nair, N. Eber, K. Fodor-Csorba, J.T. Gleeson and S. Sprunt, Extraordinary properties of nematic phases of bent-core liquid crystals, (Emerging Liquid Crystal Technologies III, San Jose, January 20-24, 2008). Proc. SPIE 6911, 691105/1-10, (2008). doi 10.1117/12.768866... 

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