Influence of the molecular structure of compounds with terminal isothiocyanate group on the induction of the smectic A phase: Part II

ABSTRACT The influence of the structure of compounds having terminal isothiocyanate group in binary mixtures with compounds having terminal alkyl chains on the induction of the smectic A phase is shown. Compounds with a triple bond in the molecular core exhibit a weaker induction of a SmA phase than compounds without it. Molecular lengths, dipole moments and polarisabilities of investigated compounds were calculated. Dielectric spectroscopy studies were done for the chosen system to show that single compounds have the opposite signs of the dielectric anisotropy, while the mixture with maximum temperature of SmA-N phase transition in the nematic phase has a positive dielectric anisotropy. In addition, the dielectric spectroscopy results confirm that the SmA phase is indeed nucleated in the mixture. It has been concluded that there is a competition of different factors, like the shape of molecules, their polarity and polarisability affecting the induction of SmA phase. The shape of a molecule appears to be a crucial factor here since it determines the distance between molecules and influences the contribution of dispersion forces. GRAPHICAL ABSTRACT


Introduction
One observes the phenomenon of liquid crystal phases induction when a higher order phase appears for a mixture of mesogenic compounds exhibiting in their mesogenic behaviour the lower-order phases only.This is an example of a non-additive phenomenon occurring in liquid crystal mixtures [1][2][3].The existence of non-additive phenomena in mixtures results from the presence of intermolecular interactions other than in pure compounds.The stability of the liquid crystal phase depends on attracting intermolecular interactions, such as van der Waals forces [1][2][3], hydrogen bonds [1,4] and electron donor-acceptor interactions [5,6].Moreover, it also depends on intermolecular repulsive CONTACT Marzena Tykarska marzena.tykarska@wat.edu.plSupplemental data for this article can be accessed online at https://doi.org/10.1080/02678292.2023.2238655.
interactions [1].Various intermolecular interactions, such as dipole-dipole, dipole-induced dipole and induced dipole-induced dipole interactions [7][8][9][10], are responsible for the formation of higher ordered liquid crystal phases.In many cases steric factors also influence the phenomenon of induction in liquid crystal systems [11].The paper from 1975 [12] reporting non-additive behaviour in bicomponent mixtures as well as another one giving a theoretical explanation of such behaviour based on regular solution theory [13] are of great importance.Testing the miscibility of liquid crystal compounds is one of the basic methods of determining the suitability of compounds for use in mixtures [14] and composing usable mixtures [15,16].Such studies are undertaken not only for mixtures of calamitic compounds but also for mixtures of calamitic compounds with banana shaped compounds [17,18].Analysis of the specific interactions responsible for phase enhancement or induction led to the discovery of intercalated phase behaviour and a theory was developed to account for this [19,20]; and also more recently underpinned twistbend smectic phases reviewed in [21].
The induction of the SmA phase is observed in various types of compound systems: polar compounds [22,23], non-polar compounds [24,25], as well as systems consisting of both polar and non-polar compounds [22].
Fast response time is a highly desirable feature in technologies utilising liquid crystal as active medium, in display technologies mainly.The on and off switching times depend on many factors, mainly on the type of electro-optical effect.Here the materials properties are of crucial importance.The composition of liquid crystal mixtures is the key factor.The dual-frequency (DF) addressing technique is a method used to shorten the off-time of electro-optical switching [26,27] and allows for construction of fast operating devices [28].It is based on nematic liquid crystals which change the sign of the dielectric anisotropy (Δε) with the frequency of the applied electric field [29][30][31].Such LCs exhibit positive dielectric anisotropy below the so-called cross-over frequency while negative one above this parameter.Mixtures operating at the DF mode are obtained when a nematic mixture with positive dielectric anisotropy is added to a nematic mixture with the negative dielectric anisotropy.Many such mixtures and their components were obtained in our laboratory [32][33][34][35].Most of mentioned DF mixtures contain compounds with cyano (−CN) terminal group.The isothiocyanate (−NCS) group in the terminal position of the molecular core provides high polarity and low viscosity.Many isothiocyanates have been obtained, which form a nematic phase in a wide temperature range with high clearing point and low melting point [36][37][38][39][40][41].The isothiocyanate compounds are good candidates to formulate DF nematic materials as they play the role of compounds with positive permittivity [32,42].Moreover, the parallel component of the permittivity tensor ε k strongly depends on the frequency of the electric field [43,44].In liquid crystal systems with isothiocyanates the SmA phase is sometimes induced [45].This is an undesirable effect when we take into account the formulation of nematic DF mixtures.
This paper aims to determine the influence of various parts of the chemical structure of compounds with a polar -NCS group on the induction of the SmA phase in mixtures with compounds having terminal non-polar groups.The induction strength is determined based on the temperature-concentration area of the occurrence of the SmA phase in phase diagrams.Particular attention is paid to the maximum temperature of the SmA phase occurrence in investigated systems.The results are compared with previously obtained results for similar compounds terminated with polar -OCF 3 group [46].The influence of the alkyl chain length and of the structure of a rigid core, namely the presence of cyclohexyl ring and triple carbon bond as well as the presence and position of fluorine atoms on the induction strength, was presented there.

Experimental
Two sets of investigated compounds together with their acronyms, phase transition sequence and temperature ranges are presented in Tables 1 and 2. The numbering of compounds is common for this paper and two other parts (I [46] and III [51]).In Table 1 there are compounds with terminal polar -NCS group differing in a structure of the molecular core, lateral substitution of the molecular core with fluorine atoms and the length of alkyl chains.All these compounds form nematic liquid crystal phase only.Four-ring isothiocyanates are characterised by high temperatures of N-Iso phase transition (above 300°C).
The compounds from Table 2 are substituted with fluorine atoms in the lateral positions.They also differ with a structure of the molecular core and the length of alkoxy chain.Investigated compounds show nematic phase, the exception is compound D.5.2 which is an isotropic substance.
The compounds from Table 1 having polar terminal group -NCS were mixed with compounds from Table 2 characterised by terminal non-polar groups and substituted by fluorine atoms in different lateral positions.Mixtures of such bicomponent systems were tested with various methods.Their phase transition temperatures were determined based on observations of textures under polarising optical microscope (POM) Biolar (PZO, Warsaw, Poland) equipped with a heating stage THMS-600 (Linkam, Tadworth, UK) and temperature controller T95-STD (Linkam, Tadworth, UK).The material was placed between microscope slides without an alignment layer.The measurements were performed in a heating cycle with a heating rate of 1 deg/min near the phase transition region.The results are presented in the form of phase diagrams.
The change of the thickness of the smectic layers upon temperature was established based on the measurements of X-ray diffraction studies with the use of powder diffractometer D8 Discover (Bruker, Karlsruhe, Germany) with Cu lamp.It worked at the Bragg-Brentano geometry with a parallel beam.The K α2 line was eliminated by the use of a shutter made of copper ribbon placed right before the detector.Liquid crystal samples were placed on glass plates (15 × 15 × 0.7 mm) with homeotropic aligning layers in a form of semi-free sample and put in a thermostatic chamber HTK 1200N (Anton Paar, Graz, Austria) driven with temperature controller TCU 1000N (Anton Paar, Graz, Austria).The temperature stabilisation while measurement was better than 0.1 deg.
Dielectric Spectroscopy studies were done with the impedance analyser HP 4195A (Hewlet Packard, USA).This equipment allows for measurements at frequencies from 100 Hz to 10 MHz.The temperature was controlled by the Linkam TMS 92 (Linkam, Tadworth, UK) controller with the precision of 0.1 deg.The samples were placed in the heating stage TMSE 660 (Linkam, Tadworth, UK).The measuring process was controlled by custom made software using Agilent Vee software platform.The measuring cells with low-resistivity ITO electrodes (the sheet resistance was as low as 10 Ω/sq.) and cells gap of 3 µm were custom made.The planar alignment was induced with using of 60 nm of The length of the isolated molecules, their dipole moments and polarisability were calculated using the commercial program Scigress.The geometry optimisation of the molecules was made using DGauss B88-LYP DFT procedure with DZVP base [52,53].The complete set of results is given in Supplementary Materials.

Mixtures prepared with compound A.5.5
Each compound with a terminal polar -NCS group (Table 1) was mixed with the compound A.5.5 with non-polar terminal chains to show the influence of the molecular structure of terminally polar compounds on the induction of the SmA phase.For all bicomponent systems the formation of the SmA phase was observed.The texture of the nematic and induced smectic A phase is shown in Figure 1.
The influence of the presence of different parts of the rigid core of compounds with -NCS group is shown in Figure 2. In summary phase diagrams, the curves of phase transitions Cr-N and two-phase coexistence region N + Iso correspond to the system with the strongest induction of the SmA phase (in Figure 2(a) for system VII.NCS.5-A.5.5).For other systems only lines corresponding to SmA-N transition are presented.The four-ring compound with triple bond I.NCS.5 in a mixture with compound A.5.5 gives an induction of the SmA phase and the maximum temperature of this phase existence T max is 202.5°C.The removal of the triple bond from the structure I.NCS.5, giving the structure VII.NCS.5, causes the increase of T max up to 220°C.The substitution of the fluorine atoms in the lateral position to the structure I.NCS.5, giving the structure IV.NCS.5, causes the decrease of the T max to 185°C.The removal of cyclohexyl ring from the rigid core of the structure I.NCS.5, giving the structure XI.NCS.5, causes the decrease of the T max to 112°C.In Figure 2(b) the curve corresponding to SmA-N phase transition for this compound XI.NCS.5 is also marked to see better the difference in the induction strength.The removal of a triple bond and simultaneous introduction of the ethylene group between cyclohexyl and phenyl rings to the structure of XI.NCS.5 compound, giving the structure of compound XIV.NCS.5, cause the increase of the T max up to 136°C.The compound XV.NCS.5 which does not have any linking groups in the rigid core in a mixture with A.5.5 gives stronger induction of the SmA phase (the increase of the T max by 19 deg) than compound having a triple bond between phenyl rings (XI.NCS.5),but only a little smaller (the decrease of the T max by 5 deg) than compound having ethylene group between cyclohexyl ring and phenyl ring (XIV.NCS.5).The exchange of the cyclohexyl ring for the phenyl ring in the structure of XV.NCS.5 compound, giving the structure of compound XVI.NCS.3, causes the increase of the T max to 154°C in a mixture with A.5.5.For this system the induction of the SmA phase is the strongest in the group of all systems containing three-ring polar compounds.Additionally the concentration x max , corresponding to maximum temperature of existence of the induced SmA phase T max , is shifted to lower concentration of A.5.5 compound.For this system x max = 0.3 what suits the situation that one molecule from compound with negative dielectric anisotropy corresponds to two molecules from compound with positive dielectric anisotropy.
Comparing the influence of the same part of molecular structure in three-and four-ring compounds it is seen that for shorter molecules it is more pronounced.Because the addition of triple bond to the former structure causes the decrease of T max by 15% but to the latter structure only by 8%.
The influence of the number of fluorine atoms laterally substituted to the rigid core on the induction of the SmA phase is shown in Figure 3  the T max is smaller by 2 deg but the concentration range is narrower by 0.1 mole fraction.The x max is simultaneously shifted from 0.4 to 0.5 mole fraction of compound A.5.5.
The influence of the length of alkyl chain of compounds with -NCS terminal group on the induction of the SmA phase is shown on the example of two homologous series of compounds VII.NCS.n and IV.NCS.n,where n = 2-5, in mixtures with A.5.5 compound, Figure 4.The decrease of the alkyl chain length causes the decrease of the T max of the induced SmA phase for both series.When the length of alkyl chain is reduced from 5 to 4 carbon atoms it causes small decrease of T max , only by 1.9 deg (it is 0.5%) in case of VII.NCS.n(Figure 4(a)) and by 6.6 deg (it is 3.6%) in case of IV.NCS.n(Figure 4(b)).In the case of compounds with shorter alkyl chains (n = 2, 3) a bigger decrease of the induction with the decrease of the alkyl chain length is observed.

Induction of the SmA phase in systems with other non-polar compounds
In Part I [46] it was shown that in the case of compounds with terminal -OCF 3 group the removal of the triple bond from the four-ring structure (compare I. OCF3.n and VII.OCF3.n;structures of rigid cores are common for Part I and Part II) causes a decrease of the SmA phase induction.For compounds with terminal -NCS group the removal of the triple bond from the four-ring structure (compare I.NCS.n and VII.NCS.n)causes an increase of the SmA phase induction (Figure 2(a)).To check if this is not an accidental example, the removal of the triple bond from the structure of four-ring compounds (compare I.NCS.4 and VII.NCS.4) in a mixture with non-polar tolane B.5.4 and of three-ring compounds (compare XI.NCS.5 and XV.NCS.5) in a mixture with non-polar biphenyl D.5.4 was checked.The results of their miscibility, presented in Figure 5, show that in all cases the tendency is the same, namely the removal of the triple bond from the structure of isothiocyanate compounds causes the increase of the SmA phase induction.When a very short compound D.5.4 is added to isothiocyanate compounds the x max of the induced SmA phase is shifted to higher concentration of D.5.4 (0.6 mole fraction).It means that privileged is the situation when two non-polar molecules accompany one polar molecule.

X-ray studies
The studies of the layer thickness were performed for two mixtures: XI.NCS.5-A.5.5 (0.4 mole fraction of   d are in the range 2.80-2.84nm, while for the mixture of compound XI.NCS.5 with B.5.4 these values are in the range 2.49-2.50nm.The calculated length of compounds XI.NCS.5, A.5.5 and B.5.4 is 2.48 nm, 2.96 nm and 2.17 nm, respectively.In Figure 6(b) the calculated length with the assumption of its additive change versus concentration marked as a dashed line.For the mixtures containing 0.4 mole fraction of compounds with negative dielectric anisotropy, these values are as follows 2.67 nm and 2.36 nm, respectively.In addition the measured layer thickness of the SmA phase of both mixtures is added to Figure 6(b).For both systems the ratio is d=l calc = 1.06 at maximum measured value of d taken.Thus regardless of the l calc length of the compound with negative dielectric anisotropy, the induced phase is monolayer SmA, but a little bit enhanced.The compound with a longer core has a dominant role in the formation of the smectic layer.

Dielectric studies
Dielectric studies were made for two pure compounds XI.NCS.5 and A.5.5, and for their mixture containing 0.4 mole fraction of compound A.5.5.This mixture was chosen due to its widest temperature range of the induced SmA phase.The measurements were made at the cooling cycle from the nematic phase starting at 180°C.The measuring frequency varied from 100 Hz to 10 MHz.
The compound A.5.5 was placed in a cell with a planar (HG) alignment.The molecular structure of the A5.5 molecule suggested that this molecule does not have any permanent molecular dipole moment, hence the electric response at the radiofrequency range should be poor in planar alignment as well as in the homeotropic (HT) one.
The experimental results for A5.5 are shown in Figure 7(a), where the real part of permittivity ε 0 ?perpendicular to the director is presented vs. temperature for four frequencies (1 kHz, 10 kHz, 100 kHz, 1 MHz).We observed that in the thin cell the crystallisation temperature is ~40°C while the melting temperature found in POM observations is ~89°C, so it supercools at almost 50 deg.We found many times in dielectric spectroscopy of nematics or smectics that the crystallisation temperature shifts down when dielectric properties are measured in thin cells, at the low cooling rate.The cell walls forcing the planar alignment often stabilise mesophases.Moreover, the temperatures of nucleation of nematic phase from isotropic liquid shift often up in thin cells with planar anchoring.During measurements, we did not use DC field because the A.5.5 compound has a (low) negative electric anisotropy thus it cannot be reoriented by DC field from HG to HT orientations.The permittivity ε 0 ? is around 2.4 and it slightly decreases with temperature.
Compound XI.NCS.5 was also placed in an HG cell.The first measurement was made at the nematic phase without any DC field (Figure 7(b)) to determine ε 0 ? .The molecular structure of the XI.NCS.5 molecule suggests that this compound exhibits the positive electric anisotropy due to the significant longitudinal component of the molecular dipole moment related to the polar -NCS group and fluorine atoms.Moreover, the response in HG cell should be rather poor.The experimental observations confirm the above predictions.One cannot observe any strong molecular relaxation in the electric response in HG cells and the value of permittivity is low ( 3:4).
The 10 V DC field in the HG cell with XI.NCS.5 reorients the director to an HT orientation.Thanks to this effect it is possible to determine the value of the parallel component of permittivity ε 0 k .Moreover, we predicted strong molecular relaxation in such orientation.It should be a molecular S-mode (rotation around the short molecular axis).Figure 7(c) shows the temperature dependence of the permittivity ε 0 k vs. temperature for four frequencies of the measuring field.Additionally, the result for the HG cell is shown in the same figure.Full symbols show the permittivity ε 0 ?measured without a DC field (shown also in Figure 7(b)), while opened symbols correspond to ε 0 k component.The values of the permittivity at the frequency of 1 kHz at the temperature of 20°C are: ε 0 ?= 3.4 and ε 0 k = 13.3.Compound XI.NCS.5 has the positive dielectric anisotropy (Δε ¼ 9:9) due to its structure with the polar group -NCS in the terminal position.Relaxation frequency of S-mode changes with temperature since S-mode is Arrhenius-type relaxation [43,44].Figure 7 confirms that A.5.5 compound exhibit nematic phase with negative dielectric anisotropy while XI.NCS.5 compound exhibits nematic phase with positive dielectric anisotropy.
Dielectric studies of the mixture with a concentration of 0.4 mole fraction of compound A.5.5 were made in a cell with a planar alignment (HG) to confirm the existence of SmA phase.The results of measurements of permittivity ε 0 ?without applied DC voltage is presented in Figure 8(a).The arrows on the graph show the temperature of phase transitions of the mixture at the cooling cycle.They are respectively 115.5°C for the transition N-SmA and 38.5°C − 28.0°C for the whole crystallisation process.No dispersion in permittivity was observed in the HG cell (the lower value of permittivity for 1 MHz in Figure 8(a) is due to the parasitic effect in ITO cell [54]).Additionally the value of ε 0 ? is rather low.It means that the electric response does not contain any relaxation since the value of the transversal molecular dipole moment is small.The values of the permittivity slightly decrease at the nematic phase with decreasing temperature.This is typical for planarly aligned nematics of a positive dielectric anisotropy (see results for e.g.5CB [55]).At the transition to the SmA phase the drop of ε 0 ?values is observed.Within the SmA phase ε 0 ?slowly increases.The variation of ε 0 ?at SmA phase is due to the increase of the molecular order at decreasing temperature.The behaviour at the N phase suggests that the mixture is dielectrically positive.
The temperature dependence of the permittivity was measured with the applied DC voltage of 10 V to confirm the positive dielectric anisotropy (Figure 8(b)) at the SmA phase expected.One can observe that the nematic phase switches to the homeotropic orientation under the DC field (so-called Freederiksz transition) due to the positive dielectric anisotropy of investigated mixture.The reorientation of a director in nematic is detected as an increase of the permittivity value.After reorientation, one can see the dispersion in nematic phase due to the molecular S-mode -like in HT cell.At the nematic phase the appearance of dielectric dispersion was observed especially at proximity of the transition to the SmA phase.The SmA phase due to layered order is insensitive for DC field, hence we cannot see the Freeedericksz transition in SmA phase.The phase transition temperatures under DC field are respectively 115°C for N-SmA and 37°C-30°C for crystallisation.Dielectric spectroscopy of investigated XI.NCS.5-A5.5 system clearly shows that in the binary mixture of nematic compounds the SmA phase is induced.

Results of calculations
The following molecular parameters were calculated: the length of the molecular core, the length of the terminal chains, the molecular dipole moment μ and its longitudinal μ k and transverse μ ?components, the molecular polarisability α and its longitudinal α k and transverse α ?components, as well as the anisotropy of polarisability Δα ¼ α k À α ?À � .The results of calculations are presented in Figures 9-11 and complete set of results for all tested compounds is given in Tables SM3-SM8 in Supplementary Materials.The longitudinal and transverse components of calculated parameters are parallel and perpendicular to the main molecular axis of inertia.Some values show the odd-even effect for homologous series.Compounds with polar terminal -NCS group have higher values of longitudinal dipole moment and polarisability than of transverse ones.
From the mentioned results of calculations it is not obvious which parameter is dominant at the SmA phase induction phenomenon.The strongest induction is    observed neither for the longest compounds I-VI or for the most polar compounds I, IV nor for the compounds I-VI of highest polarisability.The competition between these parameters is essential for the formation of smectic order.The molecular length affects the attractive dispersion forces but the polarity and polarisability affect the role of the permanent dipole moment and the induced dipole moment, respectively, at intermolecular interactions.

Discussion about dependence of the SmA phase induction on the calculated parameters
The molecular length is an important parameter affecting the strength of the SmA phase induction.The removal of cyclohexyl ring from the structure of isothiocyanate compounds causes the decrease of the T max of the induced SmA phase (compare systems with I. NCS.n and XI.NCS.ncompounds for n = 5 in mixtures with A.5.5 in Figure 2(a), and for shorter homologue n = 4 in Table SM1).The decrease of T max of the induced SmA phase is from 202.5 to 112°C in a former case and from 200 to 97°C in the latter case.The decrease of the molecular length due to the decreasing number of carbon atoms in terminal alkyl chain also causes the decrease of the T max of the induced SmA phase (compare the systems in Figure 4).The decrease of the molecular length due to the removal of cyclohexyl ring is around 0.4 nm (Table SM3).It corresponds to the decrease of the alkyl chain from n = 5 to 2 carbon atoms.If one compares the influence of both factors on the SmA phase induction strength one can see that the length of a rigid core is more significant because its shortening gives stronger decrease of T max .It is well visible on the example of compound VII.NCS.5; the removal of the cyclohexyl ring (compound XV.NCS.5)causes the decrease of the T max as much as 89 deg (it is 40%) but the removal of three ethyl groups (compound VII.NCS.2) causes the decrease of the T max by 24.7 deg.only (it is 11%).The shortening of the compounds with terminal -NCS group due to the removal of cyclohexyl ring or the decrease of the alkyl chain influences the change of the dipole moment only slightly.For example, the change of the dipole moment due to the removal of cyclohexyl ring is smaller than 0.2 D (compare compounds I.NCS.5 (6.21 D) and XI.NCS.5 (6.10 D)) and due to the decrease of the alkyl chain is smaller than 0.1 D (for compounds VII.NCS.n for n = 5, 4, 3 and 2 the dipole moment is 5.62, 5.68, 5.63 and 5.61 D, respectively).Whereas, the shortening of the compounds with terminal -NCS group due to the mentioned actions influences the change of the electron polarisability to a much greater extent.For example, the decrease of the polarisability due to the removal of cyclohexyl ring is around 10 Å 3 (compare compounds I.NCS.5 (75.71 Å 3 ) and XI.NCS.5 (65.12 Å 3 )) and due to the decrease of the alkyl chain length is smaller than 10 Å 3 (for compounds VII.NCS.n,n = 5, 4, 3 and 2 the polarisability is 62.44, 60.20, 58.66 and 55.90 Å 3 , respectively).
The substitution of the rigid core by fluorine atoms at the lateral position changes the induction strength very much simultaneously not affecting the molecular length significantly.It was already discussed in [46,56] why compounds of the positive dielectric anisotropy and -OCF 3 terminal polar group in mixtures with compounds with the negative anisotropy of the permittivity provide an induction of SmA phase.Namely, the presence of two fluorine atoms at the ortho position to the terminal polar group make the bigger distance between molecules and the smectic ordering is not induced due to the weaker dispersion forces.The same is true for isothiocyanate compounds.Here the compound with a single fluorine atom in ortho position to terminal polar group (II.NCS.4) in mixture with compound A.5.5 provides the induction stronger than in the case of the compound with two fluorine atoms (I.NCS.4).The increase of T max of the induced SmA phase is from 200 Table 3.Comparison of the molecular width s and the ratio length to width l/s, molecular dipole moment μ and its longitudinal μ k and transverse μ ?components, molecular polarisability α, and the maximum temperature of the induced SmA phase existence T max. .Comp. 1 Cr 65.5 SmB cr 118.9 SmA 137.3 N > 250 Iso [47].
to 205°C (Figure 3(a)).It is worth to notice that the isothiocyanate compound (Comp.1) with bicyclohexylotolane core and without any fluorine atoms creates smectic phases by itself [47], see Table 3.The factor which is responsible for this behaviour is still the molecular shape but this time not the molecular length l but molecular width s is of crucial importance.The molecular width of the Comp. 1 without fluorine atoms at ortho position is 0.434 nm, for compound II.NCS.4,with the single fluorine atom is 0.455 nm and with two fluorine atoms at ortho position is 0.477 nm.The increase of the distance between molecules causes that the dispersion forces are weaker and the tendency for formulation of smectic layers decreases.Such relation is also confirmed by the compound III.NCS.4 which has one chlorine atom in ortho position to -NCS group.The exchange of the fluorine atom present at the ortho position (compound II.NCS.4) to the chlorine atom (compound III.NCS.4) does not change the molecular length (2.81 nm) and their dipole moment (5.60 D of II.NCS.4 and 5.59 D of III.NCS.4).But the increase of its molecular width from 0.455 nm to 0.494 nm is important.
The shift of two fluorine atoms from ortho position to -NCS group to the next phenyl ring to positions 3' and 5' causes even more significant decrease of the T max by 24.5% (compare compounds VII.NCS.3 and X.NCS.3 in Figure 3(b)).In this case the molecular length is the same and their width is the same, but their dipole moment is different.The dipole moment value decreases from 5.63 to 5.21 D and this decrease is mainly due to the decrease of its longitudinal component of the molecular dipole moment μ k from 5.59 to 5.14 D. The comparison of the strength of the SmA phase induction of two compounds having fluorine atoms in positions 3' and 5' (IX.NCS.3 and X.NCS.3)shows that it is comparable for both compounds, see Figure 3(b), regardless there is one fluorine atom at the ortho position to -NCS group or even none.It is another evidence that the steric shape of molecules is important for the induction phenomenon.The role of fluorine atoms in the ortho position to -NCS group in such situation becomes less important, even though the molecular dipole moment of compound IX.NCS.3 increases to 6.02 D and in case of compound X.NCS.3 decreases to 5.21 D in comparison to compound VII.NCS.3 (with two fluorine atoms at ortho position) of a value 5.63 D. The role of fluorine atoms placed at ortho position is now filled by fluorine atoms in positions 3' and 5'.They do not allow molecules of compounds with the negative dielectric anisotropy come closer and the smectic layers cannot be constituted so easy.
The introduction of fluorine atoms into positions 2' and 3' at the middle ring of the molecular core, for compounds with two fluorine atoms at ortho position to -NCS group, causes the decrease of the induction strength by 10% for compound IV.NCS.4 compare to I. NCS.4 and by 8.6% for longer homologue IV.NCS.5 compare to I.NCS.5 in mixtures with A.5.5 (Figure 3(a) and Table SM1).These additional lateral fluorine atoms cause the increase of the transverse μ ?component of dipole moment from 0.59 D (for compound I. NCS.4) to 1.97 D (for compound IV.NCS.4).The same influence of the introduction of additional fluorine atoms into positions 2' and 3' is observed for the pair of compounds II.NCS.4 and V.NCS.4.The T max of the induced SmA phase in mixture with compound A.5.5 decreases from 205 to 172°C (it is 16%).If we compare the ability of compounds IV.NCS.4 and V.NCS.4 to induce the SmA phase, we found out that is similar (T max are 178.4 and 172°C).It may be because in the case of compound V.NCS.4 the free rotation of phenyl ring causes that the width of the molecule can change from 0.46 nm to 0.48 nm depending on whether fluorine atoms have sync or async orientation.Thus for async orientation we obtain the same situation as for IV.NCS.4 compound when it is mixed with A.5.5.For the structure without fluorine atoms in ortho position (Comp. 1 in Table 3) introduction of fluorine atoms into positions 2' and 3', giving the structure VI.NCS.4,destabilises the SmA phase and the induction strength is the smallest one from the compounds presented in Figure 3(a).This compound has the smallest value of the dipole moment 4.78 D (longitudinal component 4.44 D) from all tested isothiocyanate compounds.From these observations one can conclude that when few parameters change simultaneously the most important for the induction strength is longitudinal component of the dipole moment, then the width of molecules and finally the transverse component of the dipole moment.It is true when the length of molecules does not change.
The decrease of the molecular length of isothiocyanate compounds due to the removal of the triple carbon bond (around 0.26 nm) causes unexpectedly the increase of the ability of the SmA induction.Such situation was observed in the systems with A.5.5 for four ring compounds I.NCS.5 and VII.NCS.5 (increase of T max by 17.5 deg, Figure 2 To explain the exceptional behaviour of removal of a carbon-carbon triple bond from the structure of isothiocyanate compounds the polarity and polarisability of compounds were calculated.They show that the removal of this bond from the structure of compound I.NCS.4causes the decrease of both: the dipole moment (from 6.25 to 5.68 D for VII.NCS.4) as well as polarisability (from 73.49 to 60.20 Å 3 for VII.NCS.4) of molecules.Such modification of these parameters together with the decrease of the molecular length should lead to the decrease of the induction strength.The analysis of the electrostatic potential surface of isothiocyanate molecules shows that the presence of a triple bond in the molecular core changes the distribution of electrons in the molecule (Figure 12) causing that the terminal -NCS group is less polar.Thus this compound has weaker electron-acceptor properties hence its tendency for the induction of the SmA phase is smaller than for biphenyls with the same terminal group.It is not observed for compound I.
For polar compounds the molecular shape and particularly its length are important for the induction strength of the SmA phase.The shortest compound among all tested, which still gives an induction of the SmA phase in a mixture with A.5.5, is compound XVI.NCS.3 with the molecular length l ¼ 1:95 nm.It consists of terphenyl core (1.16 nm) and short alkyl chain (0.48 nm).The Comp. 2 with the same alkyl chain length but with the core shorter by 0.18 nm (Table 4) does not cause the induction of the SmA phase in a mixture with A.5.5, B.5.4 and D.5.4 compounds.The induction is also not observed in mixtures with another two-ring polar Comp. 3 (Table 4) because its molecular length l ¼ 1:76nm is even shorter.Although compounds with the negative dielectric anisotropy can have only two rings at the rigid core and let the SmA phase appear (compound D.5.4,l ¼ 1:96 nm and l core ¼ 0:73 nm, in mixture with XI.NCS.[38]; l = 1.76 nm, l core = 0.72 nm, l alkyl = 0.74 = 35.2°C)but in the case of compounds with the positive dielectric anisotropy the core must contain at least three rings to let induce SmA phase.In the case of the core without linking groups the compound with terphenyl core causes the stronger induction (compound XVI.NCS.3)than the compound with cyclohexyl biphenyl core (compound XV.NCS.5 even with longer chain) in mixture with A.5.5 (the difference of T max is 23 deg).

Conclusions
The presented results concern the influence of the structure of compounds with the positive dielectric anisotropy having terminal -NCS group on the induction of the SmA phase in mixtures with non-polar compounds with the low-negative dielectric anisotropy.The induction of the SmA phase was confirmed by X-ray and dielectric studies.This work is a continuation of the previous work [46] where polar compounds having a terminal -OCF 3 group were tested.Current results confirm previous observations that the compounds with the positive dielectric anisotropy in mixtures with compounds with the low-negative dielectric anisotropy exhibit the increasing tendency for formulation of smectic order when the parallel component of molecular dipole moment increases and simultaneously the perpendicular component of molecular dipole moment decreases.Here it was shown that the molecular shape plays a very important role in the SmA phase induction phenomenon because it has an influence on the dispersion forces.When molecules are longer, due to the increase of the core or an elongation of the alkyl chain, the induction strength increases.It is worth noticing, that the increase of the core length is more essential than the increase of the chain (the contribution of the cyclohexyl ring to the length of molecule is the same as three methylene groups but the decrees of the SmA induction strength is bigger when cyclohexyl ring is removed from the molecular structure).When the molecular width increases after adding substituents the distance between molecules increases as well, thus the dispersion forces decrease and the induction strength decreases.The competition between the molecular polarity and the molecular shape is shown on the example of the position of lateral fluorine atoms.The substitution of the core with three fluorine atoms (at positions 3, 3' and 5', see compound IX.NCS.3)causes comparable induction as for substitution by two fluorine atoms (3' and 5', see compound X.NCS.3) even though the molecular dipole moment is smaller in the latter case, but the width of both structures remains the same and in the consequence the distance between molecules and dispersion forces remain the same.
Quite different is the influence of the triple bond in the structure of molecules.For compounds with -OCF 3 group introduction of this group to the rigid core between phenyl rings causes the increase of the SmA phase induction strength due to the elongation of the molecule.But in the case of compounds with -NCS terminal group the same modification of the structure causes the decrease of the induction strength even though it is connected with the decrease of the dipole moment and polarisability.In the latter case the polarity of a terminal group decreases thus the electron donoracceptor interactions decrease.Because the influence of the removal of triple carbon bond from the rigid core on the induction of the SmA phase depends on the kind of terminal polar group thus the influence of different polar terminal groups on the induction strength will be shown and discussed in Part III [51].The presented results allow to broaden the knowledge of intermolecular interactions in the formation of the smectic ordering and can be used in further design of the molecular structure of compounds intended for the dual-frequency addressing technique.
. Compounds I-VI.NCS.4 have the same structure of the molecular core (bicyclohexyl tolanes) and the same length of their alkyl chain (n = 4) but different lateral substituents.Compound I.NCS.4 in a mixture with A.5.5 gives the induction of SmA phase and the T max = 200°C, Figure 3(a).This compound has two fluorine atoms in the last phenyl ring in ortho position with respect to the terminal -NCS group.The removal of one of these fluorine atoms (compound II.NCS.4) causes the increase of the T max by 5 deg.It was also checked how the chlorine atom substituted to phenyl ring in ortho position to -NCS group influences the SmA induction strength.In contrary to compound II.NCS.4 with one fluorine atom which increases the SmA induction strength in relation to I.NCS.4,the compound with one chlorine atom (III.NCS.4)decreases the T max by 21 deg.The presence of additional two fluorine atoms substituted to the middle phenyl ring causes the decrease of the SmA induction strength.It was already shown in Figure 2(a) for longer compounds I.NCS.5 and IV.NCS.5 (the decrease of T max by 17.5 deg).Here for shorter homolog IV.NCS.4 the T max decreases even more (by 21.6 deg) compare to I.NCS.4.For laterally substituted compounds the removal of fluorine atoms from ortho position to -NCS group of compound IV.NCS.4 gives the decreases of the induced SmA phase strength by 6 deg when one fluorine atom is removed (V.NCS.4) and by 40 deg when both fluorine atoms are removed (VI.NCS.4).In Figure 3(b) the influence of the presence of fluorine atoms in the structures of bicyclohexyl biphenyls with terminal -NCS group and three carbon atoms in the alkyl chain is shown.Most of the tested polar compounds have two fluorine atoms in the ortho position to -NCS group.The shift of both fluorine atoms to the next aromatic ring causes the decrease of T max of induced SmA phase existence from 208°C for compound VII.NCS.3 to 157°C for compound X.NCS.3 in a mixture with A.5.5 compound.What is worth to notice that the addition of a single fluorine atom at the ortho position to the structure X.NCS.3, giving the structure of compound IX.NCS.3, causes the decrease of area of the induced SmA phase in the phase diagram.Although

Figure 6 .
Figure 6.(Colour online) The results of X-ray measurements obtained at the SmA phase upon cooling from the nematic phase, showing the smectic layer spacing versus temperature (a) and concentration (b) for mixtures XI.NCS.5 with a 0.4 mole fraction of A.5.5 ( ) and B.5.4 ( ); the length of molecules of compounds XI.NCS.5, A.5.5, B.5.4 is added with open symbols.Dashed lines correspond to the situation of additive change of the molecular length.The measurement of the layer spacing d error is not bigger than 0.01 nm.

Figure 8 .
Figure 8. (Colour online) The real part of permittivity vs. temperature for four frequencies (1 kHz, 10 kHz, 100 kHz, 1 MHz) for the mixture of XI.NCS.5 and a 0.4 mole fraction of A.5.5, measured a) without a DC field and b) with a 10 V DC field.

Figure 9 .
Figure 9. (Colour online) Comparison of molecular lengths of compounds with -NCS terminal group from different homologues series and of the A.5.5 compound.

Figure 10 .
Figure 10.(Colour online) Comparison of the dipole moment (a), its longitudinal μ k (b) and transverse μ ?(c) components of compounds with -NCS terminal group from different homologues series and of the B.5.4 compound.

Figure 11 .
Figure 11.(Colour online) Comparison of polarisability (a), its longitudinal α k (b) and transverse α ?(c) components, as well as anisotropy of polarisability Δα (d) of compounds with -NCS terminal group from different homologues series and of the A.5.5 compound.
(a)) as well as for three ring compounds XI.NCS.5 and XV.NCS.5 (increase of T max by 19 deg, Figure 2(b)).It is not an accidental example because the same effect is observed in mixtures with other compounds having non-polar terminal chains, namely B.5.4 and D.5.4.In the former case for four ring compounds I.NCS.4 and VII.NCS.4 the increase of T max is by 18 deg (Figure 5(a)) and in the latter case for three ring compounds XI.NCS.4 and XV.NCS.4 the increase of T max is by 17.3 deg (Figure 5(b)).It is an exception from the observed rule that the decrease of the length causes the decrease of the induction strength what was observed for removal of cyclohexyl ring or methylene groups from the alkyl chain.It is also in a contradiction to the results obtained in Part I [46] for trifluoromethoxy compounds, for which the removal of the triple bond causes the decrease of T max for all structures presented there.The removal of other linking group -CH 2 CH 2 -from the structure of isothiocyanate compound, leading to the reduction of the molecular length by around 0.22 nm, causes the decrease of the induction ability in a mixture with compound A.5.5 (compounds XIV.NCS.5 and XV.NCS.5 decrease of T max by 5 deg, Figure 2(b)).

Table 1 .
Structures and phase transition temperatures [°C] of the compounds with terminal polar group.

Table 2 .
Structures and phase transition temperatures [°C] of the compounds with terminal non-polar group.

Table 4 .
The molecular structures of compounds which do not form any liquid crystal phase and do not cause the induction of the SmA phase; l -molecular length, l core -molecular core length, l alkyl -molecular alkyl length.