Investigation of high ∆ ε derivatives of the [ closo-1-CB 9 H 10 ]-anion for liquid crystal display applications †

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Introduction
Polar liquid crystals and polar compounds compatible with nematic materials are important for adjusting dielectric anisotropy, ∆ε, and hence modifying electrooptical properties of materials used in liquid crystal display (LCD) technology. 1,2n this context, we have been investigating zwitterionic derivatives of the [closo-1-CB 9 H 10 ] -cluster (A, Fig. 1) as potential additives to liquid crystalline hosts.Recently, we demonstrated that polar derivatives, 1 [6], 2[n] and 3 [6]a (CHART I), of type IA (Fig. 1) significantly increase ∆ε of a nematic host and have high virtual N-I transitions, [T NI ], although they themselves rarely are mesogenic. 3,4rtunately, these compounds have limited solubility in nematic materials, and their high ∆ε values were extrapolated from infinite dilutions.Subsequent investigation of zwitterionic esters of type IIA (Fig. 1), containing a sulfonium group (4 [5]a and 4[5]b, CHART I) or a pyridinium fragment (5, 5,6 CHART I) revealed that they form nematic phases, have satisfactory solubility in nematic hosts, and possess ∆ε between 30 and 40.
For the 4-cyanophenol ester in series 5 a record high ∆ε of 113 was measured in a nematic host. 5,6n continuation of our search for new polar compounds with improved mesogenic and dielectric properties, we investigated derivatives 3[n] and esters 4[n] (CHART I).Here, we report the synthesis and thermal and dielectric characterization of the two series of compounds in the pure form as well as in binary mixtures.in the presence of Pd 2 dba 3 and [HPCy 3 ] + [BF 4 ] -in a THF/NMP mixture (Scheme 1). 3 The iodides 6 were obtained from protected mercaptans 7 3 upon reactions with an 60 appropriate dibromides 8 7 under hydrolytic conditions as described before. 3romatic esters 4[n]a-4[n]l were prepared by reacting acid chlorides of sulfonium acids 9[n] with appropriate phenols 10 in the presence of NEt 3 (Scheme 2).was obtained using a ligand-free Suzuki coupling reaction 12 (Scheme 5).Phenols 10h, 13 10i, 14,15 and 10l 16 were obtained as reported in the literature.

Transition temperatures and enthalpies of compounds 3[n]
and 4[n] were determined by differential scanning calorimetry 125 (DSC).Phase structures were assigned by optical microscopy in polarized light, and the results are shown in Tables 1-3.The effect of alkyl chain extension at the thiane ring on thermal properties was investigated for select esters 4[3] (Table 3).Thus, extending the C 3 H 7 chain in

Binary mixtures
To assess the new materials for formulation of LCD mixtures, selected compounds were investigated as low 200 concentration additives to nematic host ClEster, which is an ambient temperature nematic characterized by a small negative ∆ε of -0.56.In addition, solutions of 3 compounds in CinnCN were prepared to establish their virtual clearing temperatures [T NI ].

Dielectric measurements
Analysis of selected binary mixtures in ClEster revealed linear dependence of dielectric parameters on concentration, 245 which, after extrapolation, established dielectric values for pure additives (Fig. 3).Analysis of the data in Table 4 demonstrates that for esters

Analysis of dielectric data
Dielectric parameters extrapolated for pure additives were 275 analyzed using the Maier-Meier relationship (eq 1), 18,19 which includes molecular and phase parameters. 20Using experimental ε ║ and ∆ε values and DFT-calculated parameters µ, α, and β (Table 5), equations 2 and 3 are used to calculate the apparent order parameter, S app , 20 and Kirkwood factor, g (Table 4).The 280 effect of the additive was ignored in the determination of field parameters F and h in equations 2 and 3; F and h were calculated using the experimental dielectric and optical data for pure ClEster host. 21,225 The molecular electric dipole moment, µ, and polarizability, α, required for the Maier-Meier analysis were obtained at the B3LYP/6-31G(d,p) level of theory in the dielectric medium of ClEster. 22While molecules in series 3[n] are essentially conformationally stable with a strong preference for the trans 295 isomer in the diequatorial form, 5 sulfonium esters 4[n] exist as a dynamic mixture of interconverting stereoisomers trans and cis in about 4:1 ratio (Fig. 4). 5 Therefore, their molecular parameters were obtained as a weighted sum of values calculated for the two stereoisomers     The longitudinal dipole moment in esters 4[n] was also increased by incorporation of a pyrimidine fragment; compounds possessing such a fragment are known to exhibit substantial dielectric anisotropies. 23Thus, the ester of 2-(4-350 hexylphenyl)pyrimidin-5-ol, derivative 4[3]h, has a calculated dipole moment µ = 13.14D, which is about 2.5 D higher than that of 4-pentylphenol 4[3]a.
Lateral fluorination has no effect on the magnitude of the longitudinal molecular dipole moment component, µ  .Thus, The effectiveness of these compounds as high ∆ε additives to ClEster was investigated using the Maier-Meier formalism.Following a frequently used approach in designing of polar liquid crystals, the analysis initially assumed the order parameter of the additives to be the same as for the ClEster host (S = 0.65), and Kirkwood factor (g) was set at 0.5. 24,25esults in Table 4  Esters of phenols with polar substituents are expected to have higher ∆ε values.Thus, OCF 3 , F, and additional COO groups enhance the longitudinal dipole moment, which results in ∆ε of about 50 (e.g.

Discussion
The centerpiece of polar materials presented here is the 425 sulfonium zwitterion 20 of the [closo-1-CB 9 H 10 ] -cluster with calculated ground-state electric dipoles of 16.  5).In contrast to 3[n], compounds 4[n] with several polar groups exhibit lower melting points, higher solubility in nematic hosts, and display mesogenic behavior.These qualities are quantified 450 in the Maier-Meier analysis and reflected in high order, S app , and Kirkwood, g, parameters, as shown for diesters 4[n]j and 4[3]k (Table 4).
Results in Table 4 indicate that small polar compounds are more effective additives due to their higher density of dipoles 455 in the unit volume (larger number density N).For instance, ester 4[3]c and its "extended" analogue diester 4[3]j have essentially the same experimental dielectric parameters (∆ε ≈ 70) in spite of a larger dipole moment in the latter by about 2 D (Table 5).Benzonitrile derivative 4[3]m, although not prepared 460 in this investigation, is expected to exhibit a relatively large dielectric anisotropy, on the basis of its small size and large dipole moment.
Finally, it should be emphasized that analysis of experimental dielectric data using the Maier-Meier formalism 465 provides informative insight into the behavior of additives in nematic solutions and has become an important tool in our investigation of polar compounds. 20inimize the error, the intercept in the fitting function was set as the peak T NI for the pure host.
Electrooptical measurements.Dielectric properties of solutions of selected esters in ClEster were measured by a Liquid Crystal Analytical System (LCAS -Series II, LC Vision, Inc.) using GLCAS software version 0.13.14, which implements literature procedures for dielectric constants. 28The instrument was calibrated using a series of capacitors.The homogeneous binary mixtures were loaded into ITO electrooptical cells by capillary forces with moderate heating supplied by a heat gun.The cells (about 10 µm thick, electrode area 1.00 cm 2 and anti-parallel rubbed polyimide layer) were obtained from LC Vision, Inc.The filled cells were heated to the isotropic phase and cooled to rt before measuring the dielectric properties.Default parameters were used for measurements: triangular shaped voltage bias ranging from 0.1-20 V at 1 kHz frequency.The threshold voltage, V th , was measured at a 5% change.For each mixture the measurement was repeated 10 times for two cells.The results were averaged to calculate the mixture's dielectric parameters.Results are provided in the ESI and extrapolated values for pure additives are shown in Table 4.

Fig. 1 .and 3 [ 6 ]
Fig. 1.The structure of the [closo-1-CB 9 H 10 ] -cluster (A) and its polar derivatives IA and IIA.Each vertex represents a BH fragment, the sphere is a carbon atom, and Q + stands for an 50

C 3 H 7 C 3 H 7
Compounds in series 3[n] display only crystalline 135 polymorphism and melt at or above 200 °C, which is consistent with behavior of the previously reported derivative 3[6]a. 3Extension of the sulfonium substituent in 3[6]a by the cyclohexylethyl fragment in 3[6]c increased the melting point by 44 K. Analogous comparison of 3[3]b and 3[5]b shows that 140 extension of the alkyl group at the B(10) position by two methylene groups lowered the melting point by 25 K. Thus, in contrary to expectations, elongation of the core in 3[6]a did not induce mesogenic behavior or reduce the melting point.
4[3]b-4[3]f, only the 4-butoxyphenol ester 4[3]b displays a 160 monotropic nematic phase and has the lowest melting point in the entire series (111 o C, Table 2).Extension of the phenol core by another ring generally increases the melting point, and also induces nematic behavior.The only exception is the 3,4,5trifluorophenol derivative 4[3]d, in which the addition of the 165 benzene ring does increase the melting point by 85 K in 4[3]g but fails to induce a mesophase.However, another biaryl derivative, phenylpyrimidinol ester 4[3]h with a terminal hexyl group, does exhibit a 44 K wide nematic phase.Insertion of a -C 6 H 4 COO-fragment into the 4-trifluoromethoxyphenyl ester 170 4[3]c only moderately increases the melting point (by 25 K) in 4[3]j and induces a wide-range enantiotropic nematic phase (T NI = 244 o C) along with rich crystalline polymorphism.Substitution of a lateral fluorine into the benzoate fragment of 4[3]j lowered the nematic phase stability by 10 K, and, contrary 175 to expectations, markedly increased the melting temperature in 4[3]k.Finally, insertion of a fluorophenylethyl fragment into the cyclohexyl ester 4[3]f increased the melting point by 38 K and induced a 26 K wide nematic phase in 4[3]i.Similar insertion of a fluorinated biphenylethyl fragment into 4[3]f 180 resulted in appearance of a nematic phase in 4[3]l (T NI = 278 o C).
4[3]b to C 5 H 11 in 4[5]b 185 lowered the melting point by 10 K, and had no impact on nematic phase stability.Further extension of the terminal chain to C 7 H 15 lowered T NI by 4 K in 4[7]b.The same alkyl chain extension in the phenylpyrimidinol ester 4[3]h had little effect on the melting temperature, however, it lowered T NI by 10 K in 190 the pentyl analogue 4[5]h and by an additional 22 K in the heptyl derivative 4[7]h.More significant melting point reduction, by about 25 K, is observed in derivatives 4[3]c and 4[3]j upon extension of C 3 H 7 to C 7 H 15 in 4[7]c and 4[7]j, respectively.In addition, the chain extension in 4[3]j lowered 195 the T NI by 18 K to 226 o C in 4[7]j.

205
Analysis of ClEster solutions demonstrated that most derivatives 3[n] and 4[n] dissolve in the isotropic phase in concentrations up to about 10 mol%.However, solutions stable at ambient temperature for at least 24 hr are limited to about 4-210 5 mol%.For instance, compound 4[3]c forms stable 5.5 mol% solutions in ClEster.On the other hand, compound 3[5]b and ester 4[3]g were found to be least soluble in ClEster, and the latter precipitates even from a 1.3 mol% solution at ambient

Fig. 2 .
Fig. 2. Plot of peak temperatures of the N-I transition vrs concentration in ClEster.
4[n]-trans and 4[n]-cis 300 and the composite numbers for 4[n] are shown in Table 5. 22 mole fraction in ClEster a  Values predicted for assumed  = 0.65 and g = 0.5.For details see text and the ESI.Typical error of experimental extrapolated dielectric parameters ±1.b Assumed value.c Experimental data from ref3 .d Experimental data from ref5 .e Not measured; see text.f ∆ε < 0 for a 3.0 mol% mixture.

355 results for 4 [ 3 ]
l show that µ  remains nearly the same as in the 4-pentylphenol ester 4[3]a.However, the transverse component, µ⊥, of the molecular dipole moment increases by 1.7 D, changing the orientation of the net dipole moment vector with respect to the main molecular axis from β = 13.1 o in 4[3]a to β = 21.3 o in 4[3]l.Analysis of the computational results for the 4[3]-trans isomers shows that, with the exception of 4[3]l, the net dipole moment is nearly parallel with the long molecular axis, and the angle β ranges from 2 o in 4[3]k to 14 o in 4[3]e (avrg 7.8 o ±3.7 o ).In the 4[3]-cis isomers the angle β is larger by an average of 4.6 o ±1.7 o relative to the trans analogues.The molecular shape also affects anisotropy of polarizability ∆α, which is larger for the linear 4[3]-trans molecules than for the bent 4[n]-cis analogues by an average of 4.6±0.5 Å 3 .

4 [ 3
]c, 4[3]d, and 4[3]j).A particularly large ∆ε of 81 is predicted for ester 4[3]m containing a CN group (CHART I).Surprisingly, the least effective dipole moment booster is the pyrimidine fragment in 4[3]h, with a predicted relatively low ∆ε of 35.Compounds in series 3[n] have predicted higher ∆ε values (~70) than those for esters 4[n].However, these values are observed only at infinitely low concentrations.At higher concentration (~2 mol%) molecular aggregation significantly reduces ∆ε.Experimental ∆ε for 4[n] are in general agreement with theoretical predictions, mainly due to fairly high and uniform S app values.Analysis of data in Table 4 demonstrates that experimental S app of 0.61±0.02for the compounds are comparable with the order parameter of ClEster (S = 0.65).The only exceptions are 4[3]e (S app = 0.51), 4[3]h (S app = 0.68), and 4[7]j (S app = 0.67).These outlying S app values for the first two compounds are consistent with the extreme virtual clearing temperatures, [T NI ] = -4 o C for 4[3]e and [T NI ] = 161 o C for 4[3]h, and demonstrate low compatibility of the former (4[3]e) and higher compatibility of the latter (4[3]h) with the host.The Kirkwood parameter, g, has a broader range for esters 4[n] between 0.55 for 4[3]e and 0.78 for 4[7]j and reflects different degrees of molecular association of the additive in solutions.In general, the observed values for g are higher than that initially assumed (g = 0.5).Perhaps most gratifying is that compounds 4[3]j, 4[7]j, and 4[3]k with the highest values of µ  show little association (g = 73, 78, and 67, respectively).Particularly interesting is the observed decreased association (increased g) upon alkyl chain extension in 4[n]j.This demonstrates that molecular structure containing several polar groups placed in the semi-rigid core provide a successful design for preparation of high ∆ε materials.On the other hand, analysis of compounds in series 3[n] gives low g values (e.g.g = 0.25 for 3[5]b), which shows that these materials are prone to 420 excessive aggregation in solutions.This is also consistent with their low solubility and non-linear dependence of dielectric parameters versus concentration.
3 D and 9.9 D for zwitterions 20-I and 20-II, respectively, in ClEster dielectric medium.The observed difference in the dipole moments originates from the strong polarization of electron density 430 towards the carbon atom in the boron cluster.Elongation of the molecular core by substitution in the antipodal positions of the [closo-1-CB 9 H 10 ] -cluster and the thiane ring in 20-I and 20-II helps to induce liquid crystalline behavior and increases compatibility with nematic hosts.In general, compounds with a 435 total of 2 or 3 rings in both series 3[n] and 4[n] do not form liquid crystalline phases; the only exception thus far is the 4butoxyphenol ester 4[n]b.440 The calculated longitudinal dipole moment, µ  , in compounds 3[n] is about 16 D and originates solely from 20-I.Esters 4[n] can achieve the same magnitude of µ  by combining the moderate dipole moment of 20-II with that of a polar substituent.Examples include diesters 4[n]j and 445 benzonitrile 4[3]m, in which the net dipole moments are calculated to be 16.2 and 17.3 D, respectively (Table
The acids were converted into esters 18 after which the protecting benzyl group was removed under reductive conditions (Scheme 4).

Table 1 .
Transition temperatures ( o C) and enthalpies (kJ/mol, in italics) for

Table 4 .
Extrapolated experimental (upper) and predicted (lower in italics) dielectric data and results of Maier-Meier analysis for selected compounds.a

Table 5 .
Calculated molecular parameters for selected Obtained at the B3LYP/6-31G(d,p) level of theory in ClEster dielectric medium.For esters 4[n] calculated for an average molecule at the equilibrium ([cis] = 21 mol%).For details see text and the ESI.b Angle between the net dipole vector µ and µ║. a demonstrate that esters 4[n] of non-polar phenols or alcohols exhibit expected ∆ε values of about 24 (4[5]a and 4[3]e).The lowest ∆ε value of 17.8 is predicted for 4[3]l, which is the largest molecule investigated in this series.This low value is due, in part, to the low number of molecules in the unit volume (low N number).