The influence of molecular shape and electronic properties on the formation of the ferroelectric nematic phase

ABSTRACT The synthesis and characterisation of two series of ferroelectric nematogens based on RM734 having an additional methoxy group on the central phenyl ring are reported, the 3-methoxy-4-((4-nitrophenoxy)carbonyl)phenyl 2-alkoxy-4-alkoxybenzoates (7-m-n) and the 3-methoxy-4-((3-fluoro-4-nitrophenoxy)carbonyl)phenyl 2-alkoxy-4-alkoxybenzoates (8-m-n). In order to compare the behaviour of these series to those of the corresponding materials that do not contain the methoxy group on the central phenyl ring, we also report the synthesis and characterisation of 4-[(4-nitrophenoxy)carbonyl]phenyl 4-methoxybenzoate (11-0-1), 4-[(3-fluoro-4-nitrophenoxy)carbonyl]phenyl 4-methoxybenzoate (12-0-1) and 4-[(3-fluoro-4-nitrophenoxy)carbonyl]phenyl 2,4-diethoxybenzoate (12-2-2). Two compounds in which a lateral ethoxy chain is attached to the central ring, 3-ethoxy-4-[(4-nitrophenoxy)carbonyl]phenyl 2,4-dimethoxybenzoate (18-2-1) and 3-ethoxy-4-[(3-fluoro-4-nitrophenoxy)carbonyl]phenyl 2,4-dimethoxybenzoate (19-2-1), are also described. The behaviour of these materials shows that the relative stabilities of the ferroelectric nematic, NF, and conventional nematic, N, phases are governed by a subtle interplay of steric and electronic factors. Furthermore, the electronic factors are better understood in terms of isolated regions of electron density rather than by a single large longitudinal dipole moment. In terms of molecular shape, to observe the NF phase it appears that the molecular structure must include one or more lateral substituents that enhance molecular biaxiality and destabilise the N phase. GRAPHICAL ABSTRACT


Introduction
The conventional nematic phase, N, underpins the multi-billion dollar liquid crystal display, LCD, industry [1,2] and although in recent years alternative technologies such as OLED displays have acquired significant market share, LCDs have continued to develop and improve, including, for example, the introduction of quantum dot backlights [3,4].This has ensured that LCDs remain the prevalent display technology.It is the case, however, that LCDs are a mature technology, and it is imperative that novel phases exhibiting new electro-optic properties are discovered to underpin the next generation LCD.In this regard, the recent discovery of the ferroelectric nematic phase is highly relevant and topical, and has quickly become the hottest topic in the field [5,6].
In the conventional N phase, the long axes of the rodlike molecules align more or less in a preferred direction known as the director, n, but their centres of mass are randomly distributed as in an isotropic liquid.The director has inversion symmetry such that n = -n, Figure 1, and the phase is non-polar.Over a century ago Born speculated that a polar fluid should exist if the dipoles of the constituent molecules were sufficiently large such that the dipole-dipole interactions between them could overcome thermal fluctuations [7].Some 90 years later, computer simulations of tapered Gay-Berne particles with a longitudinal dipole moment revealed ferroelectric nematic, N F , behaviour [8,9].The N F phase remained experimentally elusive until 2017, when two groups independently reported materials that are now understood to exhibit polar nematic phases.The compounds in question are referred to as RM734 [10] and DIO [11] and were shown to exhibit the N F phase [12] in which the director does not possess inversion symmetry, n ≠ -n, and the phase is polar, Figure 1.The N F phase has huge application potential and underpinning this are properties including ease of alignment [13,14], strong non-linear optical response [15][16][17][18][19], high polarisation values [11,12,20,21], large dielectric permittivities [22][23][24][25][26][27][28][29][30], and switching at very low electric fields [11,12,24,[31][32][33][34][35][36].
In order for the huge application potential of the N F phase to be realised, new ferroelectric nematogens are required having tailored properties and to achieve this requires a better understanding of the structure-property relationships for this class of materials.This endeavour will lead to the discovery of new phase behaviour such as the recently reported ferroelectric smectic A phase [34,37,38].We have previously reported a range of ferroelectric nematogens based on RM734 in which the length of the lateral alkoxy chain was varied in addition to its position in the molecule [18,36,39,40].In these materials, it was found that the N F phase was observed even for molecules with long lateral chains, although both the N and N F phases were destabilised as the lateral chain length increased, as reported by Mandle et al. [10].These compounds contained a single lateral substituent.Here, we extend this design approach and report compounds having two lateral alkoxy groups with the expectation that direct N F -I transitions may be seen [18,23,25,28,29,36,40,41]. Specifically, we report the synthesis and characterisation of two series of ferroelectric nematogens based on RM734 but that have an additional methoxy group on the central phenyl ring, the 3-methoxy-4-((4-nitrophenoxy)carbonyl)phenyl 2-alkoxy-4-alkoxybenzoates (7-m-n) and the 3-methoxy-4-((3-fluoro-4-nitrophenoxy)carbonyl)phenyl 2-alkoxy-4-alkoxybenzoates (8-m-n), Figure 2. In the acronyms used to refer to these compounds, m is the length of the lateral alkoxy chain and n is the length of the terminal alkoxy chain.The structural difference between these series is the addition of a fluorine atom ortho to the terminal nitro group in the 8-m-n series, and this allows for the effects of molecular polarity and polarisability on the N F phase to also be considered.In order to compare the behaviour of these series to those of the corresponding materials that do not contain the methoxy group on the central phenyl ring, we also report the synthesis and characterisation

Synthesis
The synthetic routes used to prepare the 7-m-n and 8-m-n series are shown in Scheme 1, for 11-0-1, 12-0-1 and 12-2-2 in Scheme 2, and for 18-2-1 and 19-2-1 in Scheme 3.These syntheses are based on methods which have been reported by Li et al. [22,23].A detailed description of the preparation of both series, including the structural characterisation data for all intermediates and final products, is provided in the Supplementary Information.

Optical studies
Phase characterisation was performed by polarised light microscopy, using an Olympus BH2 polarising light microscope equipped with a Linkam TMS 92 hot stage or a Zeiss AxioImager A2m equipped with a Linkam THMS600 hot stage.The untreated glass slides were 0.17 mm thickness.The cells treated for planar alignment were purchased from INSTEC with a cell thickness between 2.9 and 3.5 μm or with homeotropic alignment from WAT with a cell thickness of 1.6 μm.

Differential scanning calorimetry
The phase behaviour of the materials was studied by differential scanning calorimetry performed using Mettler Toledo DSC1 or DSC3 differential scanning calorimeters equipped with TSO 801RO sample robots and calibrated using indium and zinc standards.Heating and cooling rates were 10°C min −1 , with a 3-min isotherm between either heating or cooling, and all samples were measured under a nitrogen atmosphere.Transition temperatures and associated enthalpy changes were extracted from the heating traces unless otherwise noted.The entropy changes associated with the transitions observed were reported scaled by the universal gas constant, R, using a value of 8.314 J K −1 mol −1 .

Molecular modelling
The geometric parameters of the compounds of interest were obtained using quantum mechanical DFT calculations with Gaussian09 software [42].Optimisation of the molecular structures was carried out at the B3LYP/6-31 G(d) level of theory.A frequency check was used to confirm that the minimum energy conformation found was an energetic minimum.The relative orientations of the ester groups were varied, and it was found that inverting either, such that they were no longer parallel, reduced the molecular dipole moment by ≈ 1.7 D, but that these conformations were unfavourable.Visualisations of electronic surfaces and ball-andstick models were generated from the optimised geometries using the GaussView 5 software.The electronic surfaces were found with the cubegen utility in GaussView by generating a total density cube using a SCF density matrix and course grid, overlayed by an ESP surface map.Visualisations of the space-filling models were produced post-optimisation using the QuteMol package [43].

Spontaneous electric polarisation measurements
Spontaneous electric polarisation was determined by integration of the current peaks recorded during Ps switching upon applying a triangular voltage (22 Hz, 140 V pp ).The 5-µm-thick cell with ITO electrodes and no polymer aligning layers were used, and the switching current was determined by recording the voltage drop at the resistivity of 2 kΩ in serial connection with the sample.It was checked that under such conditions the saturated values of polarisation were obtained.

Dielectric spectroscopy
The complex dielectric permittivity, ε*, was measured using a Solartron 1260 impedance analyser, in the 1 Hz  −1 MHz frequency (f) range, with the probe voltage 20 mV.The material was placed in a 5-µm-thick glass cell with ITO electrodes.The electrodes were not covered with polymer aligning layers, as the presence of the thin (~10 nm) polyimide layers at the cell surfaces acts as an additional high capacitance being in a series circuit with the capacitor filled with the LC sample, and for the materials studied here with very high values of permittivity, this may strongly affect the measured permittivity of the LC phases.Lack of a surfactant layer resulted in a random configuration of the director in the LC phase.

Results
The transitional properties of the 7-m-n series are reported in Table 1.For comparative purposes, the data for 7-0-1 extracted from the literature have also been included [36].The transitional properties of 7-1-1 were in good agreement with those reported previously [23].All the new members of the 7-m-n series exhibited a ferroelectric nematic phase which formed directly from the isotropic liquid.The phase assignments were made using polarised optical microscopy with the transition to the N F phase being marked by the emergence of highly birefringent circular droplets which grew on cooling as described elsewhere [22,28,36,40,44].These droplets coalesced, Figure, and, in untreated glass slides, domains formed separated by domain boundaries, Figure 5(b).These domains are thought to be indicative of areas in which the director has differing orientations and thus the orientation of the polarisation also differs.In cells treated for planar alignment, the domain structure is more distinct, Figure 5(c).This texture has been described as 'banded' due to the differing areas of birefringence within the domains, and appears to be characteristic of the N F phase particularly when viewed in cells treated for planar alignment [22,29,36,39,40].The values of the scaled entropy change associated with the N F -I transition, ΔS N F I /R, listed in Table 1 are several times larger than ΔS NI /R for 7-0-1, and similar to those reported elsewhere for the N F -I transition [36,40].This additional entropic contribution is thought to be associated with the ordering of the dipoles in the N F phase.
The transitional properties of the 8-m-n series are reported in Table 2 and also included are the corresponding data for 8-0-1 extracted from the literature [36].As with the 7-m-n series, all the members of the 8-m-n series except 8-0-1 showed direct N F -I transitions.The N F phase in the 8-m-n series was assigned based on the observation of similar textures to those described earlier, with representative textures obtained for samples viewed in cells treated for planar alignment as shown in Figure 6(a,b).For samples viewed between untreated glass slides the textures are not so characteristic of the N F phase, Figure 6(c), and more reminiscent of the schlieren texture seen for the conventional nematic phase.The values of the entropy change associated with the N F -I transition are large, see Table 2, and similar to those reported for the 7-m-n series.The N F phase assignment was confirmed for compound 8-2-2 by measurements of spontaneous electric polarisation (Ps) and by dielectric spectroscopy, Figure 7.The measured value of the spontaneous electric polarisation was 6.4 µC cm −2 (10°C below the Iso-N F phase transition), and this is typical for the ferroelectric nematic phase.The value of Ps was nearly temperature independent.The polar character of the N F phase was manifested by the appearance of a strong dielectric relaxation mode [18,[23][24][25]28,29,36], with   the relaxation frequency ≈500 Hz directly below the I-N F phase transition; the relaxation frequency decreased to a sub-Hz regime 40°C below the transition, due to the strongly increasing viscosity of the material.The dielectric strength, Δ ɛ ≈ 7000, was almost temperature independent and this mode has been assigned to the collective movement of the polarisation vector direction, the phason mode [30].
The transitional properties of 18-2-1 and 19-2-1 are reported in Table 3, and both exhibit N F -I transitions.The N F phase was assigned again by the observation of a banded texture, with a representative example shown in Figure 8.The high values of ΔS N F I /R are consistent with these phase assignments.
The transitional properties of the 11-m-n and 12m-n series, see 3, are listed in Table 4.We note that the extensively studied ferroelectric nematogen RM734 corresponds to 11-1-1 using our numbering system.For comparative purposes, we have also listed the transitional data for 11-1-2, 11-2-2, 12-1-1 and 12-1-2 extracted from the literature [10,45].The data listed for 11-1-1, 11-2-1 and 12-2-1 have been reported previously by ourselves [18,25,40].The transition temperatures reported for 11-0-1 are in good agreement with those reported in literature albeit we report higher values [46].12-0-1 showed only the conventional nematic phase assigned by the observation of a characteristic schlieren texture.12-2-2 exhibited both the ferroelectric and conventional nematic phases.On cooling the isotropic phase, a characteristic schlieren texture containing two-and four-brush point defects formed in untreated glass slides and a uniform planar texture if viewed in a cell treated for planar alignment, Figure 9(a).In addition, the sample flashed when subjected to mechanical stress.The ferroelectric nematic phase was assigned by the observation of textures described earlier, and a representative example is given in Figure 9(b).

Discussion
We now turn our attention to how the changes in molecular structure affect the liquid crystalline behaviour in this group of materials.Compound 11-0-1 contains no lateral methoxy groups and only shows a conventional N phase with an associated T NI of 284°C.The addition of a methoxy group in a meta position to the terminal methoxy group gives 11-1-1 (RM734) and T NI falls by 96°C with an N F -N transition now seen at 131°C.If instead, the methoxy group is added to the central ring giving 7-0-1, then T NI falls by essentially the same amount, 95°C, but T N F N is a little lower, 126°C.The addition of both methoxy groups to the structure of 11-0-1 to give 7-1-1 sees a decrease in T NI of at least 180°C, and a N F -I transition is now seen at 104°C.The lowest value of the dipole moment for this group of molecules is seen for 11-0-1, 10.33 D, those of 11-1-1 and 7-0-1 are rather similar, 11.36 D and 11.59 D, respectively, and 7-1-1 shows the highest value, 12.73 D. The decrease in T NI on addition of a lateral methoxy group to 11-0-1 reflects the decreased structural anisotropy, see Figure 10, and the essentially identical values of T NI shown by 7-0-1 and 11-1-1 imply that the effect on molecular shape of the substituents at these two different positions must be similar.Surprisingly, the fall in T NI between 11-0-1 and 7-1-1 of at least 180°C is comparable to the sum of the decreases associated with the addition of the two individual methoxy groups of 191°C indicating that in terms of shape, the first group does not, to any large extent, shield the second, Figure 10.It is striking that the addition of the lateral methoxy groups gives rise to the ferroelectric nematic phase.Attempts to measure a virtual value of T N F N for the unsubstituted 11-0-1 using binary mixtures of 11-0-1 and 11-1-1 were not successful.These mixtures tended to phase separate and crystallise such that reliable data were not obtained and therefore a value of T N F N for 11-0-1 could not be extrapolated.Although the addition of a methoxy group to the central phenyl ring in 11-1-1 (RM734) to give 7-1-1 reduces T NI by at least 84°C, the  value of T N F I seen for 7-1-1 is just 27°C lower than T N F N for 11-1-1.We will return to this issue later.
Extending the lateral methoxy group in 7-1-1 to ethoxy in 7-2-1 sees a drop in T N F I of 23°C, whereas increasing the terminal group instead giving 7-1-2 is accompanied by a smaller reduction in T N F I of 7°C.Making both changes to give 7-2-2 sees T N F I fall by 25°C revealing that these effects are not simply additive.Increasing the length of the terminal substituent on the more biaxial molecule has a smaller relative effect such that T N F I falls by just 2°C between 7-2-1 and 7-2-2 but by a larger amount between 7-1-2 and 7-2-2 of 18°C.It should be noted that the dipole moments of these molecules are rather similar.Similar trends in T N F I are observed for the corresponding fluorinated materials, the 8-m-n series, although the magnitude of the differences between the corresponding members differ.This will be discussed in terms of the fluorine substituent later.It appears that the effects of increasing the terminal or lateral chain length on the relative stability of the N F phase depends on how the shape of the molecule is changed, and that the N F phase is favoured for a given shape or biaxiality either side of which it becomes less stable.The electrostatic potential surfaces of these materials show, Figure 10, that they all have regions of high and low electron density that alternate along the molecular long axis and are separated by the ester groups.It was suggested by Madhusudana [47], that such an arrangement stabilises the N F phase.
This suggestion is supported by the behaviour of the 11-m-n and 12-m-n series, Table 4.These series contain a single lateral substituent, and increasing its length, i.e., from 11-1-1 to 11-2-1 sees reductions in T N F N and T NI by 25°C and 57°C, respectively, whereas increasing the terminal chain length from 11-1-1 to 11-1-2 sees a much larger decrease in T N F N of 45°C but a much smaller decrease in T NI of just 6°C.The larger decrease in T NI    on increasing the length of the lateral methoxy group in 11-1-1 reflects the reduction in shape anisotropy.By comparison, however, the increase in molecular biaxiality results in a smaller effect on T N F I .The opposite behaviour is evident on increasing the length of the terminal methoxy group.On moving from 11-1-2 to 11-2-2, the increase in the length of the lateral methoxy group reduces the shape anisotropy resulting in a large fall in T NI but increases T N F N suggesting that the increase in biaxiality is now more favourable to the formation of the N F phase.As would be expected, the reverse is found moving from 11-2-1 to 11-2-2, and increasing the length of the terminal chain has little effect on T NI but reduces T N F N by 15°C.Again, the trends in the transition temperatures of the corresponding fluorinated materials are the same although the magnitudes of the temperature differences change.The differences between the members of the 11-m-n and 12m-n series on changing the values of m and n are larger than seen for the 7-m-n and 8-m-n series and this reflects the larger relative effect on molecular shape for the compounds containing a single lateral methoxy group.
Increasing the length of the methoxy group in the central ring in 7-1-1 to give 18-2-1, Table 3, sees T N F I fall by 32°C compared to 23°C moving from 7-1-1 to 7-2-1.Swapping the lateral groups in 7-2-1 to give 18-2-1 decreases T N F I by 9°C.Increasing the length of the chain has the greatest impact on T N F I if the chain is on the central ring although interchanging the ethoxy chain between the two rings has a much smaller effect.The extension of the methoxy to an ethoxy group clearly has a deleterious effect on the molecular shape but surprisingly, it is less important in which position the ethoxy group is in.This suggests that the optimum shape for the formation of the N F phase is not necessarily a teardrop-like shape but rather depends on the biaxiality of the molecule.Again, the trends are the same for the corresponding fluorinated compounds, 19-2-1, 8-1-1 and 8-2-1, although the differences between their values of T N F I are smaller.Presumably this reflects the smaller relative difference in shape arising from these structural changes given the effect of the lateral fluorine atom.
In Figure 11 we compare the transition temperatures of the materials containing lateral alkoxy groups on both the terminal and central phenyl rings, the 7-m-n and 8-m-n series, to those of the corresponding materials with only a single alkoxy group on either the terminal, the 11-m-n and 12-m-n series, or central phenyl ring, the NT3.m and NT3F.m series [36].The addition of the second alkoxy group extinguishes the N phase but has a significantly weaker effect on the stability of the N F phase.This reflects the greater sensitivity of the stability of the N phase to the reduction in shape anisotropy arising from the addition of the second lateral group.The stability of the N F phase is also decreased by the addition of the second lateral group in each of the materials implying that the greater molecular biaxiality exceeds some optimum value as suggested earlier.The single exception to this observation is that the value of T N F I of 7-1-2 is 11°C higher than T N F N for 11-1-2.The molecular dipole moment of the former is around 1.3 D greater.In our discussion so far, we have focussed on steric effects but as we will see electronic effects also play a significant role in determining the formation of the N F phase and will return to this apparently anomalous observation later.
We now turn our attention to the effect of fluorine substitution on the stability of the N F and N phases.The effects of adding a fluorine atom are twofold; specifically, the shape anisotropy of the molecules decreases, and the electronic properties of the molecule will change much more significantly than when extending a methoxy to an  charge density waves and these interact inhibiting the formation of antiparallel structures [47].The model reveals that parallel alignment is enhanced by reducing the amplitude of the charge density wave at either end of the molecule.The addition of the lateral fluorine substituent spreads electron density more evenly around the terminal ring as shown by the electrostatic potential surfaces, Figure 13, and this reduces the amplitude of the charge density wave which accounts for the increases in T N F I observed.As we noted earlier, however, the addition of the fluorine atom also changes the shape of the molecule, Figure 13.We have seen that this may either promote or destabilise the N F phase, and so a competition exists between the steric and electronic effects.The two pairs of compounds in which T N F I decreases on adding the fluorine atom both have lateral ethoxy groups and terminal methoxy groups.These compounds have the least anisometric shapes of this collection, and the addition of the fluorine atom presumably reduces this further leading to a greater decrease in the stability of the N F phase which counteracts the positive electronic effect.
For the corresponding pairs of the 11-m-n and 12m-n series, the addition of a fluorine atom ortho to the nitro group always decreases T NI as would be expected based on the decrease in shape anisotropy and increases  T N F N .Indeed, for 11-2-1 the addition of the fluorine atom sees the nematic phase extinguished and 12-2-1 shows a direct N F -I transition.These series have just a single lateral methoxy or ethoxy group and so the effect on the molecular shape by the addition of a fluorine atom differs from that for the 7/8-m-n and 18/19-2-1 materials.The largest difference in T N F N is between 11-1-2 and 12-1-2 of 31°C, and the smallest is between 11-2-1 and 12-2-1 of just 3°C.This is consistent with our previous discussion such that 11-1-2 has the most anisometric shape of this collection and so the addition of a fluorine atom leads to the smallest overall change in biaxiality.As suggested earlier, this change in shape may promote the formation of the N F phase complementing the electronic effect.By comparison, 11-2-1 has the least anisometric shape of this group and the addition of a fluorine atom presumably exacerbates this giving a shape that now destabilises the N F phase.This counteracts the electronic effect of the fluorine atom leading to an overall smaller increase in T N F N .
We return now to the observation that value of T N F I for 7-1-2 is 11°C higher than T N F N for 11-1-2 and this was the only example for which a compound with two lateral groups shows a more stable N F phase than the corresponding material with just a single lateral alkoxy group.The addition of a methoxy group to the central ring increases the charge density of this fragment which according to Madhusudana's model sees an increase in the stability of the N F phase [47].This will, of course, be true for each of these pairs of molecules but 11-1-2 has the most anisometric structure and so the addition of the methoxy group sees less of an increase in the molecular biaxiality.This combination of the electronic and shape considerations gives rise to the overall increase in the stability of the N F phase.Finally, we commented earlier that the addition of a methoxy group to the central ring in 11-1-1 (RM734) to give 7-1-1 reduces T NI by at least 84°C but the stability the N F falls by just 27°C.These changes are consistent with the view that the electronic effect of adding the methoxy group to the central ring helps to offset the associated deleterious shape change and so the decrease in the stability of the N F is less than may otherwise have been expected.

Conclusions
The relative stabilities of the N F and N phases are governed by a subtle interplay of steric and electronic factors.The latter appear to be well described, at least qualitatively, within the framework of a molecular model developed by Madhusudana that describes the formation of the N F phase in which the molecules possess longitudinal surface charge density waves and these interact inhibiting the formation of antiparallel structures [47].This approach is certainly more instructive than simply considering a molecular dipole moment.In addition to the electronic factors, it is clear that molecular shape plays a vital role in the formation of the N F phase, and furthermore, to observe the N F phase it appears that the molecular structure must include one or more lateral substituents that enhance molecular biaxiality and destabilise the N phase.From a molecular design viewpoint, the incorporation of lateral groups is a key factor in stabilising the N F phase.There appears to be an optimum shape for the formation of the N F phase and much work, both experimental and theoretical, is now required to understand this.

Figure 1 .
Figure 1.(Colour online) Schematic representations of the conventional nematic, N, phase (left) and the polar ferroelectric nematic, N F , phase (right).

Figure 2 .
Figure 2. Molecular structures of the 7-m-n and 8-m-n series; m is the number of carbons in the lateral alkoxy chain and n is the number of carbons in the terminal alkoxy chain.For m = 0 the lateral group is a H atom.

Figure 3 .
Figure 3. Molecular structures of the 11-m-n and 12-m-nseries; m is the number of carbons in the lateral alkoxy chain and n is the number of carbons in the terminal alkoxy chain.For m = 0 the lateral group is a H atom.

Scheme 1 .
Scheme 1. Synthetic route for the 7-m-n and 8-m-n series.

Figure 5 .
Figure 5. (Colour online) Polarised optical microscope textures observed for the 7-m-n series: (a) coalescence of droplets at the N F -I phase transition for 7-1-1 in untreated glass slides (T = 104°C), (b) banded texture of the N F phase in 7-2-1 in untreated glass slides (T = 80°C), (c) banded texture of the N F phase in 7-2-2 in a cell treated for planar alignment (T = 70°C).

Figure 6 .
Figure 6.(Colour online) Polarised optical microscope textures observed for the 8-m-n series: (a) banded texture of the N F phase in 8-2-1 in a planar aligned cell (T = 80°C), (b) banded texture of the N F phase in 8-1-2 in a cell treated for planar alignment (T = 80°C), (c) uncharacteristic texture of the N F phase in 8-2-2 in a cell treated for planar alignment (T = 75°C).

Figure 7 .
Figure 7. (Colour online) (a) The switching current (blue line) associated with polarisation reversal under an applied triangular wave voltage (black line) for 8-2-2 at 65°C.Measurements were performed in a 5-μm-thick cell with ITO electrodes and no alignment layer.(b) Real (left) and imaginary (right) parts of the complex dielectric permittivity measured vs temperature and frequency for 8-2-2, in a 5-µm-thick cell with ITO electrodes and no alignment layer.

Figure 8 .
Figure 8. (Colour online) Polarised optical microscope texture observed for 19-2-1 in a cell treated for homeotropic alignment showing the banded texture of the N F phase (T = 74°C).
from DSC cooling traces.b Measured using polarised light microscopy.c Literature value.

Figure 9 .
Figure 9. (Colour online) Polarised optical microscope textures observed for 12-2-2: (a) schlieren texture of the N phase in untreated glass slides (T = 108°C) and (b) banded texture of the N F phase in a cell treated for planar alignment (T = 102°C).

Figure 11 .
Figure 11.(Colour online) Comparison of the transition temperatures of the materials with a single alkoxy group (left bar) with the corresponding materials with two alkoxy groups (right bar).The top of the open bar shows the N-I transition and of the filled bar the N F -N/I transition.Compounds with X = H (top) and with X = F (bottom).The shaded areas mark the temperature range in which the samples had crystallised on cooling as measured by DSC.The transition temperatures reported for 12-2-2 and NT3F.2 were measured using polarised optical microscopy.

Figure 12 .
Figure 12.Comparison of the values of T F I of the 7-m-n series (open bars) and 8-m-n series (filled bars), with the top of the bars showing the transition to the N F phase.

Figure 13 .
Figure 13.(Colour online) The space-filling models (top), electrostatic potential surfaces (middle) and ball-and-stick models of (a) 7-1-2, and (b) 8-1-2, calculated at the B3LYP/6-31(d) level of theory.The arrow indicates the direction of the calculated dipole moment, with the head representing positive charge moving to the base which is negative.

Table 1 .
[36]sition temperatures and associated entropy changes for the 7-m-n series.The calculated dipole moments, µ, are also listed.The data for 7-0-1 have been extracted from Tufaha et al.[36].

Table 2 .
[36]sition temperatures and associated entropy changes for the 8-m-n series.The calculated dipole moments, µ, are also listed.The data for 8-0-1 have been extracted from Tufaha et al.[36].
m n T CrI /°C T N F I /°C *T N F N /°C T NI /°C ΔS CrI /R ΔS N F I /R *ΔS N F N /R ΔS NI /R µ/D 0 1 180 a *142 a 157 13.7 a *0.34 a 0.28 12.a Values extracted from DSC cooling traces.

Table 3 .
Transition temperatures and associated entropy changes for
aValues extracted from DSC cooling traces.

Table 4 .
Transition temperatures and associated entropy changes for the 11

-m-n and 12-m-n series
. The calculated dipole moments, µ, are also listed.