Dielectric and electro-optic studies of a bimesogenic liquid crystal composed of bent-core and calamitic units

A bimesogen, BR1, composed of a bent-core and calamitic unit, linked laterally via a flexible spacer is investigated by dielectric and electro-optic techniques. X-ray results show the presence of clusters in the nematic phase, and the cluster size is of the order of the thickness of a single layer. The splitting of the small-angle scattering $\Delta \chi /2$ is about 50°, which indicates SmC like clusters with a significant tilt of the molecules in the quasilayers. The sign reversal of the dielectric anisotropy Δε′ is observed as a function of frequency; the behavior is rather similar to that exhibited by the conventional dual frequency nematics, composed of a calamitic mesogen, with the exception that it occurs at much lower frequencies in this material. Interestingly, as the bimesogen enters its nematic phase, the average permittivity decreases as the temperature is lowered. This indicates the onset of antiparallel association of some of the dipoles in the system, and this type of association is much more prominent in BR1 in comparison to other bent-core liquid crystalline systems composed of the same bisbenzoate core unit. The analysis of the dielectric spectra using the Maier-Meier model confirms the onset of an antiparallel correlation of dipoles occurring at the isotropic to nematic phase transition temperature. Additionally these results support a model of the cluster where the transverse dipole moments in the neighboring layers are antiparalleled to each other.

Figure 1

(a) Chemical structure and the transition temperatures of the dimesogenic material under study Iso $=$ isotropic, ${\mathrm{N}}_{\mathrm{cybC}}=$ cybotactic nematic phase composed of SmC clusters, Cr $=$ Crystalline [14]. Electric dipole components determined by molecular modeling for individual mesogens were estimated to be as follows: ${\mu }_{\parallel \mathrm{BC}}=3.12\mathrm{D},{\mu }_{\perp \mathrm{BC}}=5.18\mathrm{D},{\mu }_{\parallel \mathrm{RL}}=2.45\mathrm{D}$, and ${\mu }_{\perp \mathrm{RL}}=2.96\mathrm{D}$. (b) shows the sample geometry used in the measurements of the parallel component of permittivity, obtained by inducing a homeotropic alignment on application of a magnetic field (B) in a direction perpendicular to the director (n) of the originally planar aligned cell.

Figure 2

The dielectric anisotropy measured at a frequency 6 kHz is compared for cells with different cell thicknesses (∼5, 12.7, and 26.2 μm). The changes in $\Delta {\varepsilon }^{\prime }$ are higher as the cell thickness is increased. Although the dielectric permittivity between the planar and the apparent homeotropic states in the 5 μm cell is close to each other (indicating that the cell has not really switched to a homeotropic state), the change is much more noticeable in cells of 12.7 and 26.2 μm thicknesses. The difference in $\Delta {\varepsilon }^{\prime }$ between the 12.7 and the 26.2 μm cells is not as large as between 5.0 and 12.7 μm, and hence it can be safely assumed that the molecular alignment in the thicker cell of 26.2 μm has reasonably switched to a homeotropic state.

Figure 3

The real part of dielectric permittivity plotted as a function of reduced temperatures for frequencies 100 Hz and 6 kHz. The values of $\left({\varepsilon }_{\parallel }^{\prime }\right)$ and $\left({\varepsilon }_{\perp }^{\prime }\right)$ are obtained from the homeotropic and planar cell configurations, respectively, for a cell of thickness 26 μm. The average permittivity is defined as $\langle {\varepsilon }^{\prime }\rangle =\frac{1}{3}\left({\varepsilon }_{\parallel }^{\prime }+2{\varepsilon }_{\perp }^{\prime }\right)$.

Figure 4

(a) Frequency-temperature plots of the transmittance curve obtained in a 5 μm planar cell under an applied field of 2 V/μm. The contour line implies a constant value of transmittance. (b)–(d) Textures obtained between crossed polarizers for $T$/$T_{\mathrm{NI}}$ = 0.93; R (red arrow) in each of the figures denotes the rubbing direction, and this is set at angle ∼22.5° from the polarizer axis. (b) No field applied; (b) 6 ${\mathrm{V}}_{\mathrm{pk}}$, 50 Hz and (c) 15 ${\mathrm{V}}_{\mathrm{pk}}$, 50 Hz, the cell switches to an almost homeotropic state. The length of the white bar in (c) is 60 μm. All pictures were taken with a fixed exposure time of 130 ms.

Figure 5

Frequency plots of the relative dielectric permittivity obtained from (a) planar cell at a reduced temperature ($T$ − $T_{\mathrm{NI}}$ = −22 °C) and (b) homeotropic cell ($T$ − $T_{\mathrm{NI}}$ = −16 °C): green (upper-thicker) line denotes ${\varepsilon }^{\prime }$, blue (lower-thinner) line denotes ${\varepsilon }^{\prime \prime }$, □ symbol represents $d{\varepsilon }^{\prime }/d\left(lnf\right)$ [the derivative of ${\varepsilon }^{\prime }$ with respect to (ln $f$)]. In (a) and (b), the dotted, dashed, dashed-dotted, and dashed-dashed-dotted lines are the deconvoluted components of $d{\varepsilon }^{\prime }/d\left(lnf\right)$. P1, P2, P3, and ${\mathrm{P}}_{\mathrm{ITO}}$ are the relaxation peaks for the planar cell given in (a); H1, H2, and ${\mathrm{H}}_{\mathrm{ITO}}$ are the modes obtained in the homeotropic configuration shown in (b).

Figure 6

The dielectric strengths (δε) corresponding to the various relaxation processes P1, P2, and P3 (planar configuration) and H1 and H2 (homeotropic configuration) as a function of the reduced temperature. Peaks identified as: P1 and H1 correspond to the rotation around the short molecular axis of the rodlike molecules, P2 and H2 are due to rotation around the long molecular axis of the bent-core mesogen, and P3 is due to the rotation around the long molecular axis of the rodlike mesogen.

Figure 7

Relaxation frequency ($f_R$) for the various relaxation processes as a function of the reciprocal temperature (${\mathrm{K}}^{-1}$). The frequency scale on the right refers to the mode $P_3$ only

Figure 8

The dielectric strength corresponding to H1 (□) and H2 (□) (red); fit of the data to the M-M model shown by the solid lines.

Figure 9

The order parameter as a function of temperature calculated from the experimental data for the homeotropic cell. The solid red line represents the fit of $S$ to the power law ($1-T/T_{\mathrm{NI}}$)${}^{\gamma }$, γ is the critical exponent = 0.24. Note that $S$ for the bent-core system studied here though typical is generally lower than for the calamatics.

Figure 10

Temperature dependencies of the $g$ factors. $g_{\parallel }$ and $g_{\perp }$ are the anisotropic Kirkwood correlation factors for the director parallel and perpendicular to the electric field, respectively.

Figure 11

Possible arrangement of the mesogenic units in smectic layers. The system in the nematic phase has a local smecticlike structure.

Figure 12

X-ray diffraction patterns of an oriented samples of compound BR1 under a magnetic field reproduced from [14] with permission from The Royal Society of Chemistry; the arrow denotes direction of the magnetic field; (a) The ${\mathrm{N}}_{\mathrm{cybC}}$ phase at 45 °C (shows the pattern after subtraction of the scattering in the isotropic liquid state $T=80$ °C) and (b) χ scans over the diffused small-angle scattering (for 2θ = 1.5°– 4.0°) at 65, 55, and 45 °C, $I_{\mathrm{rel}}=I$($T$)/$I$(80 °C, Iso).