J O U R N A L O F C H E M I S T R Y Materials An assessment of carborane-containing liquid crystals for potential device application Andrew G. Douglass, Krzysztof Czuprynski,†Michelle Mierzwa and Piotr Kaszynski* Organic Materials Research Group, Department of Chemistry, Vanderbilt University, Nashville, TN 37235, USA Received 8th June 1998, Accepted 30th July, 1998 Two 4-alkoxyphenyl 12-pentylcarborane-1-carboxylate nematic liquid crystals have been synthesized. The materials are found to exhibit ideal mixing of nematic phases in their binary mixtures with analogous bicyclo[2.2.2]octane derivatives and with the polar nematogen 4-(4-isothiocyanatophenyl )-1-(trans-hexyl )cyclohexane.The smectic phases for the bicyclo[2.2.2]octanes are destabilized by addition of the carborane derivative.For carborane compound 5BC5 the extrapolated dielectric anisotropy and measured optical anisotropy are-1.3 and 0.057 respectively at 20 °C. The refractive indices have been correlated with the calculated electronic polarizabilities and the low birefringence measured for 5BC5 can, at least in part, be attributed to the carborane cylindrical symmetry. Introduction Current liquid crystal display applications rely on nematic, smectic A or C materials.1,2 Molecules forming such liquid crystalline phases typically comprise a rigid core with flexible substituents attached in such a way as to produce an extended rod-like shape.3,4 While the chains reduce the melting point of a compound, the mesogenic rigid cores provide the anisotropic interactions necessary for the occurrence of the liquid crystal phase and, to a large extent, dictate the properties of the bulk materials.Our studies have focused on inorganic boron closo-clusters5 as rigid core structural elements and their role in modifying thermal, dielectric and optical properties of liquid crystalline materials.6–11 p-Carborane (1) shown in Fig. 1 appears to be an excellent candidate for use in the mesogenic core of a calamitic liquid crystal.It is a three-dimensional, s-aromatic ring system12 which readily undergoes C-substitution with a variety of organic groups13,14 and hence can easily be incorporated into typical organic molecules. Previously, we demonstrated that mesogens containing 1 are good nematogens and have a tendency to destabilize smectic phases.6,9,11 This desirable property prompted us to study in detail two-ring esters containing 1 since analogous hydrocarbons possess low negative dielectric anisotropies and are used as additives to improve the performance of nematic devices.1 Here we provide synthesis and miscibility studies and describe mesogenic, dielectric and optical properties of two H H CnH2n+1 O OCmH2m+1 O CnH2n+1 O OCmH2m+1 O C6H13 NCS n = 5, m = 5 6CHBT n = 5, m = 5 n = 5, m = 10 5BC10 1 n = 5, m = 10 5BO5 5BO10 n = 6, m = 10 6BO10 5BC5 carborane esters, 5BC5 and 5BC10, and compare them with Fig. 1 p-Carborane (1), liquid crystal compounds and designations. the analogous bicyclo[2.2.2]octane esters 5BO515 and 5BO10 For carborane each vertex represents a B–H fragment and each filled (Fig. 1). The esters have been studied in pure states and as circle a carbon atom. binary mixtures with their analogs and also with the polar nematogen 4-(isothiocyanatophenyl )-1-(trans-4-hexyl )cyclohexane (6CHBT).16 This provides an extensive assessment of alkoxyphenol (Scheme 1). The preferred reagent for the prepthe potential for these carborane-containing liquid crystalline aration of chloride 2 is PCl5,17 while the 4-pentylbicyclo[2.2.2]- esters for use in display devices.octane-1-carbonyl chloride (3) was prepared using SOCl2.18 Formation of esters with 3 required 48 h reflux for complete reaction15 whereas the apparently more reactive carborane Results carbonyl chloride (2) was reacted at room temperature Synthesis overnight. The carborane esters 5BC5 and 5BC10 were synthesized from Thermal analysis and miscibility studies carbonyl chloride 2 and the bicyclo[2.2.2]octane esters 5BO5 and 5BO10 from carbonyl chloride 3 using the appropiate 4- The transitional data for the five compounds 5BC5, 5BC10, 5BO5, 5BO10 and 6BO1019 are presented in Table 1.Both carborane-containing materials, 5BC5 and 5BC10, exhibit low †The 1997 COBASE fellow on leave from Military University of Technology, Warsaw, Poland.clearing temperatures and their supercooled nematic phases J. Mater. Chem., 1998, 8(11), 2391–2398 2391extent, and the smectic behavior is only extinguished above 55 mol% 5BO5 [Fig. 2(b)]. Fig. 3 presents the binary phase diagrams for 5BC10-6BO10 (a) and 5BO10-6BO10 (b). In common with data presented in Fig. 2 nematic phases are shown to exhibit perfect miscibility. Increasing the proportion of 5BC10 to 6BO10 depresses the smectic transition temperatures and SB and SA phases are not formed above 20 mol% and 60 mol% 5BC10, respectively [Fig. 3(a)]. The phase diagram for 5BO10-6BO10 exhibits normal behavior [Fig. 3(b)] and as the concentration of 6BO10 is increased then the smectic B phase is stabilized in preference OCmH2m+1 HO X H11C5 O O X H11C5 O Cl OCmH2m+1 3, X = 2, X = Benzene, Et3N to the smectic C. Scheme 1 The phase diagrams for corresponding carborane and bicyclo[2.2.2]octane homologues 5BC5-5BO5 and 5BC10- are stable at room temperature on the order of weeks. 5BO10 are presented in Fig. 4(a) and (b), respectively. For Increasing the terminal chain length from five to ten methylene both diagrams nematic phases exhibit ideal mixing and for units increases the nematic phase range for these materials mixtures of 5BC10-5BO10 the smectic behavior is extinguished from 2 °C for 5BC5 to 14 °C for 5BC10.The nematic–isotropic above 40 mol% of 5BC10. transition temperatures for the bicyclo[2.2.2]octyl compounds, Binary mixtures 5BC5-6CHBT and 5BO5-6CHBT (Fig. 5) 5BO5 and 5BO10, are significantly higher than those for the exhibit ideal mixing of the nematic phases. For mixtures of analogous carboranes. The enthalpies for the nematic– 5BO5-6CHBT an induced smectic A phase is observed in isotropic transitions for the carborane derivatives are equal addition to the nematic. This smectic induction is such that whereas that for 5BO10 is larger than that for 5BO5.Of the the nematic range is reduced to 25 °C for mixtures containing five compounds only 5BO10 and 6BO10 are smectogenic 50–80 mol% 5BO5. forming enantiotropic smectic A and either monotropic smectic C or hexatic B20 phases. The binary phase diagrams for 5BC5-6BO10 and 5BO5- Dielectric anisotropy 6BO10 (Fig. 2) demonstrate ideal mixing behavior for the The dielectric anisotropies of solutions of 5BC5 in 6CHBT nematic phases.The smectic phases for 6BO10 are, however, were measured for four diVerent concentrations and the results strongly destabilized upon addition of the carborane derivative are plotted in Fig. 6. Extrapolation of the values obtained for [Fig. 2(a)]. Indeed, at 30 mol% of 5BC5 no smectic behavior the four solutions to pure 5BC5 gave a value of De= could be observed to -20 °C.Similarly, the addition of 5BO5 destabilizes the smectic phases for 6BO10 although to a lesser -1.3±0.1. Table 1 Phase transition temperatures and transitional enthalpies for the BC and BO materialsa. Compound C1 C2 SB SC SA N I 5BC5 T/ °C · 34.1 · 36.1 · DH/kcal mol-1 · 4.27 · 0.18 · 5BC10 T/ °C · 29.2 · 42.9 · DH/kcal mol-1 · 5.90 · 0.18 · 5BO5b T/ °C · 49.5 · 93.5 · DH/kcal mol-1 · 5.20 · 0.10 · 5BO10 T/ °C · 55.5 · 58.4 (· 30.5)c · 71.0 · 92.5 0 DH/kcal mol-1 · 0.94 · 5.30 · 0.03 · 0.33 0 6BO10d T/ °C · 55 (· 35) · 77.8 · 89.5 0 aObserved phases are denoted by bullets and monotropic transitions in parentheses. bLit.C·50.5 N·93.5·I, G. W. Gray and S. M. Kelly, Mol. Cryst. Liq.Cryst., 1981, 75, 95. cTemperature recorded by optical microscopy. dR. Dabrowski, J. Szulc and B. Sosnowska, Mol. Cryst. Liq. Cryst., 1992, 215, 13. Fig. 2 Binary phase diagrams for (a) 5BC5-6BO10 and (b) 5BO5-6BO10. The lines are guides to the eye. 2392 J. Mater. Chem., 1998, 8(11), 2391–2398Fig. 3 Binary phase diagrams for (a) 5BC10-6BO10 and (b) 5BO10-6BO10. The lines are guides to the eye.Fig. 4 Binary phase diagrams for (a) 5BC5-5BO5 and (b) 5BC10-5BO10. The lines are guides to the eye. Fig. 5 Binary phase diagrams for (a) 5BC5-6CHBT and (b) 5BO5-6CHBT. The lines are guides to the eye. Optical anisotropy to its high clearing point. The ne and no values (ne>no) obtained for the carborane derivative 5BC5 are higher than those measured for the bicyclo[2.2.2]octane analog 5BO5 The refractive indices for 5BC5 and 5BO5 have been measured for a range of temperatures (Table 2) and the data are plotted while the resulting birefringence is slightly lower for the former.Table 3 compares the refractive indices for four analogous in Fig. 7 as a function of the shifted temperature T-TNI. The isotropic refractive indices for 5BO5 were not measured due phenyl esters at the same shifted temperature, T-TNI= J.Mater. Chem., 1998, 8(11), 2391–2398 2393Table 3 Experimental refractive indices measured at T-TNI= -12.5 °C T/ °C ne no n navg a 5BC5 22.8 1.571 1.514 0.057 1.533 5BO5 80.5 1.533 1.471 0.062 1.492 5CH5b 63.6 1.527 1.466 0.061 1.486 4PH6c 36.0 1.608 1.497 0.111 1.534 anavg=(ne+2no)/ 3. bS. Takahashi, S.Mita and S. Kondo, Mol. Cryst. Liq. Cryst., 1986, 132, 53. cI. H. Ibrahim and W. Haase, J. Phys. (Paris), 1979, 40, 191. model24 [eqn. (1)] which assumes that the internal field is isotropic even in an anisotropic medium. 3 Vm 4 p NA · n2e,o,avg-1 n2 avg+2 =ae,o,avg where n2 avg=(n2 e+2n2 o)/3 (1) Fig. 6 Plots of e, ed and e) vs. concentration for mixtures of 5BC5- The parameters ae and ao represent the electric vectors 6CHBT.Line fit for e is R=0.999. parallel and perpendicular to the optical axis. The molar volumes (Vm) have been obtained from the experimental Table 2 Measured refractive indices for 5BC5 and 5BO5 as a function specific densities for 5CH523 and 4PH6.22 These molar volumes of temperature and those for 5BC525 and 5BO526 have been estimated using 5BC5 5BO5 group additivity to molar volume27 and are provided in Table 4 for comparison.For self-consistency of the data the estimated T/ °C ne no T/ °C ne no molar volumes have been used in eqn. (1) to derive the experimental polarizability data collected in Table 4. 39.4 1.526 83.5 1.530 1.471 37.7 1.527 82.5 1.531 1.471 Calculations 36.7 1.527 81.3 1.532 1.471 35.7 1.556 1.514 80.5 1.533 1.471 The average molecular polarizability (aavg), polarizability 34.7 1.559 1.514 79.5 1.534 1.472 anisotropy (a) and dipole moments for each of the four 33.8 1.562 1.513 78.5 1.535 1.472 30.9 1.564 1.513 74.5 1.538 1.472 analogues 5BC5, 5BO5, 5CH5 and 5PH5 have been calculated 28.7 1.566 1.513 70.6 1.542 1.473 using the MNDO method (Table 4).A plot of experimental 27.7 1.568 1.514 66.6 1.545 1.474 versus calculated average polarizabilities (aavg) is given in 26.2 1.570 1.514 63.3 1.547 1.475 Fig. 8. The molecular coordinates used in the calculations have 22.8 1.571 1.514 58.5 1.550 1.476 been chosen in such a way that the X-axis is defined by a line connecting the two terminal carbon atoms of the core and the phenyl ring lies in the XY plane.The molecular geometry of each compound was optimized with conformational constraints consistent with those found by X-ray analysis of analogous compounds. Thus the alkoxy group was constrained to be coplanar with the benzene ring in all cases.28 The carbonyl group was set to be coplanar with the benzene ring of the benzoate,28 perpendicular to the cyclohexyl ring of 5CH529 and staggered for 5BO5.The alkyl chains were constrained to be staggered in all cases. 28,29 Electronic absorption spectra The UV absorption spectrum for 5BC5 in ethanol exhibits similar intensities in its absorption maxima to those for 5BO5 which are about half those reported for p-methoxyphenyl benzoate30 (Fig. 9). The carborane derivative 5BC5 shows a small hypsochromic shift compared to the bicyclo[2.2.2]octane analog and exhibits a shoulder absorption at about 240 nm.Absorption spectra for both 5BC5 and 5BO5 are blue-shifted Fig. 7 Dependence of refractive indices (ne, no and niso) on temperature for 5BC5 (%) and 5BO5 ($). Table 4 Comparison of experimental (T-T NI=-12.5 °C) and calculated (MNDO) molecular polarizabilities (A° 3) -12.5°.21 The refractive indices and birefringence for 4- hexyloxyphenyl 4-butylbenzoate (4PH6)22 are greatest whilst VM a aavg aavg (calc.) ae-ao Da (calc.) S the values for 4-pentylcyclohexane-1-carboxylic acid 4-pentyl- 5BC5 396 —b 48.7 54.5 6.3 22.0 0.29 oxyphenyl ester (5CH5)23 are comparable with those for 5BO5 382 —b 43.9 48.6 6.7 18.6 0.36 5BO5.Carborane 5BC5 has the lowest birefringence despite 5CH5 360 (382) 41.0 45.5 6.2 17.9 0.34 having an average refractive index [navg=(ne+2no)/3] equal to 5PH5 325 (346)c 40.1c 46.0 10.2c 26.0 0.39 that of 4PH6.aEstimated based on group additivity, see ref. 25–27; experimental The molecular electronic polarizabilities ax have been data are in parentheses. bNot available. cValues are for analog 4PH6. calculated from the observed refractive indices nx using Vuks 2394 J.Mater. Chem., 1998, 8(11), 2391–2398from this comparison that in order to have stabilization of smectic phases in mixtures comparable with that for bicyclo[2.- 2.2]octanes the carborane derivatives require a significant increase in terminal chain length. Further studies are necessary to test this hypothesis. The excellent miscibility demonstrated for the carboranyl compounds in the low polarity bicyclo[2.2.- 2]octyl hosts is also observed for mixtures in the polar nematogen 6CHBT.It is interesting that 5BO5 actually induced smectic behavior in its mixtures with 6CHBT as bicyclo[2.2.2]octane derivatives are generally regarded as being nematogenic rather than smectogenic compounds.15 Obviously, the formation of smectic phases within the operating temperature range of a nematic device is undesirable although we note that certain devices have improved performance if there is a neighbouring smectic transition.1 The small, negative value of De for 5BC5 (-1.3) is consistent with a value of -1.1 reported for cyclohexane close analog 4CH6.31 It also consistent with expectation as the major contributors to the dielectric anisotropy are the outboard dipoles due to lone electron pairs on oxygen.Indeed, the Fig. 8 Plot of calculated (MNDO) vs. experimental average molecular MNDO calculations show that the transverse vector consti- polarizabilities. Open circles represent data obtained using estimated molar volumes and filled circles experimentally determined molar tutes the major component of the molecular dipole moment volumes.The slope and line fit (open circles only) are 0.93 and 0.986, which is about 1.8 D for 5BC5 and 5BO5, 1.7 D for 5CH5 respectively. and 2.1 D for 5PH5 (D=Debye, 1 D#3.33564×10-30 Cm). The refractive indices for 5BC5 are greater than those for 5BO5 and 5CH5 as a result of the highly polarizable electrons in the carborane cage.32 However, because of the threedimensional spherocylindrical symmetry of the carborane cage, cf.two-dimensional planar symmetry of a benzene ring, the resultant birefringence is expected to be lower in carboranecontaining mesogens than in aromatic analogs. Indeed, calculations on biphenyl, 1,1¾-bicarborane and 1-phenylcarborane clearly demonstrate that the carborane imparts a high average polarizability but low anisotropy of polarizability (Table 5).The same trend is observed in the data collected in Table 4; 5BC5 is predicted to have the highest average polarizability and a polarizability anisotropy intermediate between the phenyl and alicyclic analogues. Comparing the calculated and experimental values for aavg it can be seen that the experimental values tend to be 10–15% lower than the theoretical predictions (Table 4).Part of this overestimation by theory can be attributed to the use of estimated rather than experimental molar volumes which tend to be underestimated by about 5%. In view of the fact that we are comparing results from calculations Fig. 9 Plot of molar absorptivity against wavelength for 5BC5 (solid on a single conformer in the gas phase to those obtained for line) and 5BO5 (dashed line).The literature lmax values (228 nm, log e=4.27; 274 nm, log e=3.67) for p-methoxyphenyl benzoate are conformationally mobile molecules in an anisotropic, conmarked for comparison. densed phase the correlation is reasonable (R=0.986 in Fig. 8). The polarizability anisotropy values obtained for an idealized, gas phase molecule diVer from those measured in the relative to that of p-methoxyphenyl benzoate30 (vertical lines in Fig. 9). nematic phase due to intermolecular (orientational ordering) and intramolecular dynamics (conformational mobility) in the latter. The orientational ordering is generally approximated Discussion and conclusions by the second rank orientational order parameter S where b is the angle between the long molecular axis and the director The carborane-containing compounds 5BC5 and 5BC10 both form nematic phases with lower clearing temperatures than [eqn.(2)].33 For an axially symmetric molecule the order those for the analogous bicyclo[2.2.2]octyl compounds. This is in accord with our general observation that clearing points Table 5 Calculated (MNDO) polarizabilities (A° 3) for benzene and are lowered when p-carborane is substituted into a mesogenic carborane compounds core.6,9–11 Previous studies have also noted a tendency for carborane-containing compounds to destabilize smectic aavg Da phases6,9,11 which has been rationalized on the basis of the greater breadth of the carborane cage cf.bicyclo[2.2.2]octane or benzene.11 The results from this study are consistent with this observation as 5BO10 is smectogenic whilst 5BC10 is not.Further, the binary phase diagrams with 6BO10 demonstrate that for equal homologues smaller percentages of the carboranes are required to suppress smectic behavior than for analogous bicyclo[2.2.2]octanes. In fact, the binary phase aExperimental value obtained from molar refraction and density at diagram for 5BC10-6BO10 [Fig. 3(a)] is similar to that for the 77 °C is 20.8 A° 3, A. L. von Steiger, Chem. Ber., 1922, 55, 1968. 5BO5-6BO10 binary mixture [Fig. 2(b)]. It could be inferred J. Mater. Chem., 1998, 8(11), 2391–2398 2395parameter can be expressed as a ratio of experimental and microscope with a HCS250 Instec hot stage. Thermal analysis was obtained using a Mettler DSC 30 instrument.Transition theoretical birefringences or anisotropies of molecular polarizabilities [eqn. (3)].22 temperatures for pure materials were measured using small samples (1–2 mg) and a heating rate of 1 °Cmin-1, while for S=(3<cos2b>-1)/2 (2) the transition enthalpies large samples (10–15 mg) and fast S=(ae-ao)/Da (3) heating (10 °Cmin-1) was used.The uncertainties in the transition temperatures and transitional enthalpies are esti- Thus, if the assumptions made in determining (ae-ao) are mated as ±0.1 °C and ±5% respectively. Mixtures were reasonable then the results suggest that 5BC5 has a lower prepared by evaporation of dichloromethane solutions. For degree of orientational ordering than its analogs (Table 4) at mixtures the transition temperature was taken as the upper the same shifted temperature and that the lowered orientational limit of the biphasic region as observed by optical microscopy. ordering is contributing to lowering the observed birefringence.The phase diagrams were determined by the single concen- Unfortunately the assumptions made in obtaining order partration method.NMR spectra were obtained on a Bruker ameters (in particular estimation of the molar volume for 300 MHz instrument in CDCl3 and referenced to the solvent 5BC5) prevent these data from being considered fully reliable. (1H and 13C NMR). 11B NMR spectra were obtained at Our intention was rather to assess whether semiempirical 64.2 MHz using a Bruker 200 MHz spectrometer and refer- methods can reproduce the experimental data and conseenced to B(OMe)3.IR spectra were recorded using an ATI quently can be used for making predictions. Certainly the Mattson Genesis FTIR by deposition of a thin film from average polarizabilities are overestimated but reproduced solution onto sodium chloride disks. Mass spectrometry reasonably consistently by calculations (Fig. 8). Calculations was performed using a Hewlett-Packard 5890 instrument also suggest that the anisotropy of polarizability should be (GC–MS). Elemental analysis was provided by Atlantic lower for mesogens containing carborane instead of phenyl. Microlab, Norcross, Georgia. Dielectric anisotropies were This is supported by experiment as the birefringence is low measured using an APT III Automated Polarization Testbed for 5BC5 cf. 4PH6. At present it is not possible to assess the at room temperature and version 4.12b software (Displaytech, relative contributions of the orientational ordering and the Inc). The error in measurement of the dielectric constant is molecular polarizability to this low birefringence for 5BC5. estimated to be ±0.1. Refractive indices for 589 nm radiation Answering this question is important because low birefringence were determined using a Leitz Abbe Mark II refractometer has been recognized as a desirable property for twisted nematic connected to a thermostated circulating-water bath.Samples device applications.34 In order to resolve this debate and to were aligned by pretreatment of the prism surfaces with lecithin further investigate these carboranyl mesogens we hope to and rubbing unidirectionally.The errors in readings are measure the temperature dependence of the density and the ±0.001 and ±0.002 for the extraordinary and ordinary rays temperature dependent order parameters via deuterium NMR respectively. The temperatures recorded by the refractometer spectroscopy. were scaled to those of the DSC by comparison of TNI.The contribution of the s-aromatic carborane to the UV–VIS spectra of solutions in absolute ethanol were recorded electronic absorption spectrum for 5BC5 is more comparable using a Hitachi U-3000 spectrophotometer. Benzene and with that of an aliphatic ring than with that of a p chromodichloromethane was dried by distillation from calcium phore. Molar absorptivities for the carborane and bicyclohydride and triethylamine by standing over potassium hydrox- [2.2.2]octane derivatives are very similar and they are essenide.Quantum mechanical calculations were performed tially half of those for the compound with two phenyl rings using AMPAC 6.0 package. Polarizabilities were obtained (Fig. 9). As evident from Fig. 9, both 5BC5 and 5BO5 show in the user-defined molecular coordinates by using the benzene E band at about 220 nm and the B band double BRUTEKPOLAR Keyword.absorption at about 280 nm. The E band for 5BC5 is blueshifted by 3 nm with respect to that for 5BO5 and 6 nm with respect to p-methoxyphenyl benzoate.30 The spectrum of 5BC5 Syntheses exhibits a broad shoulder absorption at about 240 nm which 12-Pentyl-1,12-dicarbadodecaborane(10)-1-carboxylic acid.is absent in the spectrum of the bicyclo[2.2.2]octane analog. p-Carborane (2.0 g, 13.8 mmol) was placed in a dry 100 ml Since UV spectra for the corresponding carboxylic acids are three-neck flask equipped with a condenser, stopper and suba- similar to each other the origin of this shoulder absorption is seal. After flushing with nitrogen, dry THF (50 ml ) was added not clear.and the solution cooled to -78 °C. n-Butyllithium (1.92 M in In summary, the carborane-containing compounds are mis- THF, 7.18 ml, 13.8 mmol) was added via syringe in a dropwise cible with all-organic mesogens, have clearing points about manner causing a white precipitate to form. The mixture was 50 °C lower than analogous bicyclo[2.2.2]octane derivatives allowed to warm (redissolution occurs) and stir at room and appear to be strong suppressants of smectic phases.temperature for 20 min after which n-pentyl iodide (1.79 ml, Substitution of carborane for bicyclo[2.2.2]octane into the 13.8 mmol) was added. After stirring a further 3 h the reaction mesogen does not appreciably increase the UV absorption or was recooled to -78 °C and n-butyllithium (1.92 M in THF, birefringence whilst refractive indices are increased markedly. 7.18 ml, 13.8 mmol) was added dropwise via syringe. The This unique eVect of the carborane cage on bulk properties reaction was allowed to stir at room temperature for 20 min can be rationalized on the basis of symmetry arguments and and then CO2 was bubbled through it for a further 1 h.The is supported by quantum mechanical calculations. The expersolvent was removed on a rotavapor and KOH added (2 M, imental birefringence for 5BC5 is excessively low compared to 30 ml ). The mixture was extracted with hexanes (3×30 ml ) calculations suggesting a low order parameter. Dielectric which were discarded. The aqueous phase was acidified with properties are not significantly aVected by substitution with conc.HCl (pH 1) causing a white precipitate to form. Diethyl carborane. The data collected thus far suggest that the carborether (30 ml ) was added eVecting dissolution and the organic ane-containing compounds satisfy the criteria for nematic phase was separated. The aqueous phase was again extracted devices and that more detailed studies of this class of with ether (3×30 ml ) and the combined organics dried over compounds are warranted.sodium sulfate. The ether was removed and the white solid stirred in refluxing hexanes to extract the 12-pentyl-1,12- Experimental dicarbadodecaborane-1-carboxylic acid which was purified by sublimation (120–124 °C, 1 Torr) yielding a white solid (1.57 g, The phase transition points of the compounds and their mixtures were determined using a PZO ‘Biolar’ polarized 44% yield): mp. 139–143 °C; 1H NMR ([2H6 ]acetone), d 0.81 2396 J. Mater. Chem., 1998, 8(11), 2391–2398(t, J=7.1 Hz, 3H), 1.02–1.28 (m, 6H), 1.67 (t, J=7.8 Hz, 4-Pentylbicyclo[2.2.2]octane-1-carboxylic acid 4-decyloxyphenyl ester (5BO10). 4-Pentylbicyclo[2.2.2]octane-1-carbonyl 2H), 10.06 (br s, 1H), 1.2–3.4 (br m); 13C NMR, d 14.12, 22.80, 29.80, 31.74, 38.83, 76.31, 84.57, 163.40; 11B NMR, d chloride (3, 220 mg, 0.91 mmol, formed from the carboxylic acid and thionyl chloride (bp 150 °C/ 1 Torr, 76% yield) and -13.5 (d, JBH=165 Hz); IR 2952, 2929, 2606, 1716, 1415, 1282 cm-1.Anal. Calc. for C8H22B10O2: C, 37.19; H, 8.58. 4-decyloxyphenol (210 mg, 0.91 mmol) were dissolved in dry benzene (5 ml ) and dry pyridine (2.5 ml ) was added dropwise.Found: C, 37.43; H, 8.47%. The reaction was stirred and refluxed for 48 h and the volatiles removed by rotary evaporation. The crude product was then 12-Pentyl-1,12-dicarbadodecaborane(10)-1-carboxylic acid 4- passed through a 3 cm layer of silica eluted with 100 ml of pentyloxyphenyl ester (5BC5). 12-Pentyl-1,12-dicarbadodeca- dichloromethane–hexanes (1/4 v/v). Chromatographic separaborane( 10)-1-carboxylic acid (1.0 g, 3.87 mmol) and phos- tion (same eluant) and recrystallization from ethanol yielded phorus pentachloride (841 mg, 4.04 mmol) were placed in a 322 mg (70%): mp 58.4 °C; 1H NMR (400 MHz), d 0.86 (t, dry 25 ml flask and dry benzene (10 ml ) added. The reaction J=7.1 Hz, 6H), 1.08–1.50 (m, 28H), 1.72–1.77 (m, 2H), 1.88 was stirred under nitrogen for 20 min at 40 °C forming a clear (t, J=7.9 Hz, 6H), 3.90 (t, J=6.5 Hz, 2H), 6.82 and 6.89 solution.The solvent was removed yielding a colorless oil and (AB d, J= 9.0 Hz, 4H); 13C NMR, d 14.07, 14.09, 22.66, after flushing with nitrogen dry benzene (10 ml ) was added to 23.34, 26.00, 28.57, 29.24, 29.30, 29.37, 29.54, 30.36, 30.44, dissolve acid chloride 2.This solution was added dropwise to 31.88, 32.78, 39.19, 41.31, 68.31, 114.87, 122.14, 144.31, 156.56, a stirred solution of 4-pentyloxyphenol (729 mg, 4.04 mmol) 177.05; EIMS, m/z 456 (10), 180 (14), 179 (100%); IR 2957, and triethylamine (0.56 ml, 4.04 mmol) in dry benzene (10 ml ) 2921, 2854, 1747, 1506, 1457, 1227, 1197 cm-1.Anal. Calc. and the reaction stirred overnight at room temperature under for C30H48O3: C, 78.90; H, 10.59. Found: C, 79.19; H, 10.79%. nitrogen. The solution was then filtered through a silica gel plug eluted with benzene and the solvent removed. The product was purified by chromatography (dichloromethane–hexanes, Acknowledgments 154) giving 1.3 g (80% yield) followed by repeated recrystalliz- This project has been funded in part by the National Research ation from pentane, decolorization with charcoal in diethyl Council under the Collaboration in Basic Science and ether and finally distillation (194–196 °C, 0.3 Torr) to yield Engineering Program(COBASE).Support for this project has 847 mg colorless, opaque liquid (52% yield): mp 34.1 °C; 1H also been provided by the NSF CAREER grant (DMR- NMR, d 1.0–3.4 (br m), 0.84 (t, J=7.1 Hz, 3H), 0.92 (t, J= 9703002).We are grateful to the Organic Chemistry 7.0 Hz, 3H), 1.05–1.50 (m, 10H), 1.59–1.65 (m, 2H), 1.70–1.79 Laboratory of the Military University of Technology, Warsaw (m, 2H), 3.88 (t, J=6.5 Hz, 2H), 6.80 and 6.84 (AB d, J= for the gift of 4-pentylbicyclo[2.2.2]octane-1-carboxylic acid, 9.3 Hz, 4H); 13C NMR, d 13.83, 14.00, 22.18, 22.41, 28.11, 6CHBT and 6BO10.We would also like to thank Mr. J. E. 28.86, 28.98, 31.12, 38.36, 68.31, 74.19, 84.37, 114.89, 121.46, Harvey for the diectric anisotropy measurements and Professor 143.73, 157.16, 161.56; 11B NMR, d -14.1 (d, JBH=162 Hz); D. Demus for bringing to our attention ref. 27. EIMS, m/z 423–417 (max.at 420, 58%, M), 244–237 (max. at 241, 100%); IR 2956, 2932, 2865, 2614, 1762, 1505, 1298, 1241 cm-1; UV, lmax/nm ( log emax) 222 (3.95), 277 (3.36), References 283 (3.24). Anal. Calc. for C19H36B10O3: C, 54.26; H, 8.63. 1 I. Sage, in Thermotropic Liquid Crystals, ed. G. W. Gray, John Found: C, 54.31; H, 8.71%. Wiley and Sons, 1987, pp. 64–98. 2 D. Coates, in Thermotropic Liquid Crystals, ed.G. W. Gray, John Wiley & Sons, New York, 1987, pp. 99–119. 12-Pentyl-1,12-dicarbadodecaborane(10)-1-carboxylic acid 4- 3 D. Demus, Mol. Cryst. Liq. Cryst., 1988, 165, 45. decyloxyphenyl ester (5BC10). Prepared by the method used 4 K. J. Toyne, in Thermotropic Liquid Crystals, ed. G. W. Gray, for 5BC5 and purified by distillation (220 °C, 0.1 Torr, 93 mg, John Wiley and Sons, New York, 1987, pp. 28–63. 52% yield) followed by recrystallization from ethanol: mp 5 Boron Hydride Chemistry, ed. E. L. Muetterties, Academic Press, New York, 1975. 29.2 °C; 1H NMR, d 1.0–3.5 (br m), 0.82 (t, J=7.2 Hz, 3H), 6 A. G. Douglass, M. Mierzwa and P. Kaszynski, SPIE, 1998, 0.86 (3H, J=6.6 Hz, 3H), 1.05–1.40 (m, 20H), 1.57–1.68 (m, 3319, 59. 2H), 1.70–1.77 (m, 2H), 3.88 (t, J=6.6 Hz, 2H), 6.79 and 7 P.Kaszynski and D. Lipiak, in Materials for Optical Limiting, ed. 6.85 (AB d, J=9.1 Hz, 4H); 13C NMR, d 13.83, 14.10, 22.18, R. Crane, K. Lewis, E. V. Stryland and M. Khoshnevisan, MRS, 22.66, 25.97, 28.98, 29.17, 29.30, 29.34, 29.53 (2C), 31.13, 1995, 374, 341. 31.87, 38.36, 68.37, 74.21, 84.36, 114.91, 121.48, 143.73, 157.16, 8 P.Kaszynski, J. Huang, G. S. Jenkins, K. A. Bairamov and D. Lipiak, Mol. Cryst. Liq. Cryst., 1995, 260, 315. 161.62; 11B NMR, d -14.2 (d, JBH=166 Hz); EIMS, m/z 9 K. Czuprynski, A. G. Douglass, P. Kaszynski and W. Drzewinski, 493–487 (max. at 490, 33, M), 244–237 (max. at 241, 100%); Liq. Cryst., submitted. IR 2954, 2926, 2855, 2614, 1761, 1504, 1242, 1188 cm-1. Anal. 10 P. Kaszynski and K.Czuprynski, Chem. Commun., submitted. Calc. for C24H46B10O3: C, 58.74; H, 9.45. Found: C, 58.85; 11 A. G. Douglass, K. Czuprynski, M. Mierzwa and P. Kaszynski, H, 9.44%. Chem. Mater., in press. 12 R. B. King, Russ. Chem. Bull., 1993, 42, 1283. 13 V. I. Bregadze, Chem. Rev., 1992, 92, 209 and references therein. 4-Pentylbicyclo[2.2.2]octane-1-carboxylic acid 4-pentyloxy- 14 R.N. Grimes, Carboranes, Academic Press, New York, 1970, 15 G. W. Gray and S. M. Kelly, Mol. Cryst. Liq. Cryst., 1981, 75, 95. phenyl ester (5BO5).15 Prepared and purified by the method 16 R. Dabrowski, J. Dziaduszek, T. Szczucinski and Z. Raszewski, used for 5BO10 giving 472 mg (66% yield): mp 49.5 °C; Mol. Cryst. Liq. Cryst., 1984, 107, 411. 1H NMR, d 0.87 (t, J=7.2 Hz, 3H), 0.91 (t, J=7.1 Hz, 3H), 17 V.V. Korshak, N. I. Bekasova, A. I. Solomatina, T. M. Frunze, 1.09-1.44 (m, 18H), 1.70-1.77 (m, 2H), 1.86-1.91 (m, J= A. A. Sakharova and O. A. Mel’nik, Izv. Akad. Nauk. SSSR, Ser. 7.8 Hz, 6H), 3.90 (t, J=6.4 Hz, 2H), 6.83 (d, J=9.1 Hz, 2H), Khim., 1982, 31, 1904. 6.89 (d, J=9.1 Hz, 2H); 13C NMR, d 14.00, 14.07, 22.43, 18 P. Adomenas, A. Nenishkis and D.Girdzhyunaite, J. Org. Chem. USSR, 1982, 18, 1100. 22.66, 23.33, 28.15, 28.59, 28.93, 30.36, 30.46, 32.78, 39.21, 19 R. Dabrowski, J. Szulc and B. Sosnowska, Mol. Cryst. Liq. Cryst., 41.31, 68.32, 114.90, 122.15, 144.31, 156.57, 177.11; EIMS, 1992, 215, 13. m/z 386 (13), 180 (18), 179 (100), 123 (20), 109 (29%); IR 20 J. Przedmojski, personal communication. 2954, 2922, 2870, 2856, 1746, 1504, 1458, 1227, 1198 cm-1; 21 The more commonly used reduced temperature scale (T/TNI in K) UV, lmax/nm ( log emax) 225 (4.01), 278 (3.30), 285 (3.21).gives similar results. Anal. Calc. for C25H38O3: C, 77.68; H, 9.91. Found: C, 77.76; 22 I. H. Ibrahim and W. Haase, J. Phys. (Paris), 1979, 40, 191. Data not available for 5PH5. H, 9.84%. J. Mater. Chem., 1998, 8(11), 2391–2398 239723 M. Takahashi, S. Mita and S. Kondo, Mol. Cryst. Liq. Cryst., 30 R. Martin, Monatsh. Chem., 1981, 112, 1155. 1986, 132, 53. 31 V. Vill, Liq. Cryst. 3.0, Hamburg, 1997, compound #22714. 24 M. F. Vuks, Opt. Spectrosc. (Engl. Transl.), 1966, 20, 361. 32 Experimental value for B12H122- has been measured as 22.0 A° 3; 25 The molar volume contribution for the carborane cage was esti- A. Kaczmarczyk and G. B. Kolski, Inorg. Chem., 1965, 4, 665; mated to be 126±9 cm3 mol-1 by statistical analysis of eight calculated (MNDO) value is 18.8 A° 3. Similarly for p-carborane disubstituted o- and m-carborane compounds. To our knowledge the calculated value is 18.7 A° 3 and for benzene 10.2 A° 3 (exptl. no density data have been reported for disubstituted p-carboranes. 10.4 A° 3). 26 The temperature dependence of the calculated molar volume for 33 A. Saupe and W. Maier, Z. Naturforsch., Teil A, 1961, 16, 816. 5BO5 was assumed to be equal to that measured experimentally 34 K. Toriyama, K. Suzuki, T. Nakagomi, T. Ishibashi and for 5CH5. K. Odawara, in The Physics and Chemistry of Liquid Crystal 27 R. F. Fedors, Polym. Eng. Sci., 1974, 14, 147. Devices, ed. G. Sprokel, Plenum, New York, 1976, pp. 153–171. 28 P. Kromm, H. Allouchi, J.-P. Bideau, M. Cotrait and H. T. Nguyen, Acta. Crystallogr., Sect. C, 1995, 51, 1229. 29 U. Baumeister, W. Brandt, H. Hartung, W. Wedler, H.-J. Deutscher, R. Frach and M. Jaskolski, Mol. Cryst. Liq. Cryst., Paper 8/04322A 1985, 130, 321. 2398 J. Mater. Chem., 1998, 8(11), 2391–2398