Polyhedral Liquid Crystal Silsesquioxanes-

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Polyhedral Liquid Crystal SilsesquioxanesGeorg H. Mehl* and Isabel M. SaezDepartment of Chemistry, University of Hull, Hull HU6 7RX, UK

Synthetic routes towards monodisperse poly-hedral liquid-crystalline silsesquioxanes aredescribed. The liquid-crystalline phase beha-viour of these structurally uniform materials,with various silsesquioxane core and mesogenicside-chain structures, were analysed and com-pared with those of related oligomeric andpolymeric materials. Common features of thesolid-state behaviour of these organic/inorganichybrids, which form self-organized structures in

their soft-matter states, were identified. Copy-right # 1999 John Wiley & Sons, Ltd.

Keywords: silsesquioxane; liquid crystal; poly-hedral; metallomesogen; monodisperse

INTRODUCTION

Although polyhedral silsesquioxanes have beenknown for a considerable time13 and are thesubject of several reviews,412 only fairly recentlyhas their use as components of liquid-crystallinesystems become the focus of attention1320.

The materials to which polyhedral LC silses-quioxanes currently show the closest resemblancein chemical structure are rod-like organic mesogensterminally substituted with alkylsiloxane groups orsimilarly substituted siloxane rings, stars and linearpolysiloxanes.

For low-molar-mass materials the chemical

structure defines the occurrence and the stabilityrange of liquid-crystal (LC) phases. For linear side-chain polymers it is well established that for a givenpendant side-chain the LC properties are stronglydependent on size (degree of polymerization),

polydispersity and the tacticity of the backbone;this highlights the importance of well characterizedmaterials to the understanding of their phasebehaviour21,22.

The use of silsesquioxanes as molecular scaf-folding for liquid-crystalline systems rests on theirdiscrete molecular structure. It allows for mono-disperse backbone entities, which have a definedmolecular geometry in which the core provides acompact, fixed directionality to the resulting

molecule. This eliminates a major problem in theinvestigation of the side-chain liquid-crystal poly-mer systems, namely the dependency of liquid-crystal properties on polydispersity and tacticity,while combining the main advantages of both side-chain LC polymers, i.e. their robustness andprocessability, with the advantages of low-molar-mass systems, i.e. their fast response to externalstimuli.

METHODS OF SYNTHESISA liquid-crystalline silsesquioxane is constituted ofthe following components: the silsesquioxane corea; an attachment linking the hydrocarbon spacer tothe inorganic core, possibly via a silicon-organicspacer b; the hydrocarbon spacer c separating themesogen and the polyhedral centre; and finally themesogenic group d, as shown in Fig. 1, a spacefilled structure is shown in Fig. 2.

Although it is conceptually not necessary, allpolyhedral silsesquioxane liquid crystals reportedso far have been synthesized by a convergent route,

in such a manner that the silsesquioxane core andthe liquid-crystalline side-chain are synthesizedseparately and fused in the last step.

Cores

The core structures discussed in this contributionare depicted in Fig. 4. The functionalization of thesilsesquioxane cores is relatively difficult and thevariety of coupling reactions that can be toleratedby the polyhedral core is rather limited2330. There

APPLIED ORGANOMETALLIC CHEMISTRYAppl. Organometal. Chem. 13, 261272 (1999)

Copyright # 1999 John Wiley & Sons, Ltd. CCC 02682605/99/04026112 $17.50

* Correspondence to: G. H. Mehl, Department of Chemistry,University of Hull, Hull HU6 7RX, UK.E-mail: [email protected]/grant sponsor: EPSRC.Contract/grant sponsor: DERA (Malvern).


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are two common pathways to silsesquioxane cores,either via controlled hydrolysis of appropriatelyfunctionalized trichlorosilanes yielding, a variety ofstructures depending on the reaction conditions andthe work-up, or via controlled condensation ofsilicic acid and subsequent functionalization.

The most frequently used cores are the cubicoctasilsesquioxanes. Octa(hydridosilsesquioxane),Si8O12H8 (T8H8) (core B) was first synthesized in195931. The use of this material in the past has beenhampered by the lack of an efficient synthesis andthe difficulty in separating the oligomers producedin the hydrolysis of Cl3SiH. However T8H8 is nowavailable reproducibly in gram quantities27,32. Thehigher polyhedral oligomers T10, T12 and T14 haverecently been isolated in gram quantities33 and theseparation of T8T18 by size-exclusion chromato-

graphy (SEC) has been reported.34 Though thereare at present no reports of the use of these cores asscaffolding for liquid-crystal materials, their struc-ture makes them of interest as substrates for novelLC systems.

Octa(hydridodimethylsiloxy)octasilsesquioxane

(core A) is accessible in gram quantities through asynthetic route via the reaction of [NMe4]2[SiO4]with ClHSiMe2

5,913,35.Extensive earlier work912 investigating the

reaction conditions which direct the condensationof silicic acid to a particular polyhedral structurehighlighted the influence of the size of the alkylchains in the tetra-alkylammonium hydroxide onthe condensation reaction. It was found that thesynthesis of trigonal prismatic systems (core E) isbest achieved using tetraethylammonium hy-

Figure 1 Structure of the elements building up a liquid-crystal silsesquioxane.

Figure 2 Spindle-type structure of liquid-crystalline silsesquioxanes.

Copyright # 1999 John Wiley & Sons, Ltd. Appl. Organometal. Chem. 13, 261272 (1999)

262 G. H. MEHL AND I. M. SAEZ


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droxide, whereas the octa(dimethylsiloxy)silsesqui-oxane core A is favourably obtained using tetra-methylammonium hydroxide as co-reagent anddeca(dimethylsiloxy)silsesquioxane (core D) isyielded using tetrabutylammonium hydroxide912.

Silylation

The strategy used for the synthesis of liquid-crystalsilsesquioxanes has so far proceeded exclusivelyvia a modification of the silsesquioxane coresthrough hydrosilylation of a terminal olefin,undoubtedly because the similar route to the relatedliquid-crystal side-chain polyorganosiloxanes iswell documented21,22. This synthetic route can befollowed through two pathways, either by ahydrosilylation reaction of a mesogen containinga terminal alkene group with a hydridosilsesquiox-

ane, or by a hydrosilylation of an alkenylsilses-quioxane with a mesogen terminated with ahydridosilicon functionality.

The systems described here follow the first routeand make use of the addition of the SiH bond ofthe appropriate hydridosilsesquioxane to the term-inal carboncarbon double bond on the flexiblespacer of the mesogen that is to constitute theappended side-chain3642

Hydrosilylation studies using T8H8 were firstdisclosed by Herren and Calzaferri, reporting thesynthesis of monosubstituted alkyl and ferrocenylheptahydridosilsesquioxanes2326 using hexa-chloroplatinic acid as catalyst. They found that inthe case of unsubstituted linear alkenes the reactionproceeded exclusively to the a-isomer; for vinyl-ferrocene as reactant a similar result was observed(the assignment ofa and b is shown in Fig. 3). Thisresult was confirmed for unsubstituted alkenes27,but for allyloxy derivatives significant amounts ofthe branched b-isomer were found15,29. For vinylsiloxanes ca 21% b-addition occurs and simulta-neously a vinyl/H exchange takes place onsilicon27. However, in the case of allylsiloxanesthe regioselectivity of the reaction was found to be

higher as it produced only the linear a-isomer, butthe allyl/H exchange process was still present43.

Similarly, the hydrosilylation of unsubstitutedterminal alkenes with (Si8O12)(OSiMe2H)8 (A)yields exclusively the linear a-isomer44, whereas a

conjugated olefin, e.g. vinylferrocene, yields bothisomers in an a/b ratio of ca 9:135.

If their LC properties are to be considered, the aand b isomers are expected to show very differentproperties. If different isomers are present in amaterial it is a liquid-crystal mixture. Unless theratio of a/b isomers is known and strictlyreproducible, it is very difficult to draw a correla-tion between the chemical structure and the ensuingphysical properties1620,45.

We observed that for silsesquioxane cores theaddition to the terminal alkene group of the flexiblespacer of the mesogen is regiospecific within the

limits of detection used, affording only the lineara-isomer, using Karstedts catalyst at roomtemperature. The b-addition product is usuallyidentified by a signal in the vicinity of 2.0 ppm inthe 1H NMR spectrum due to RSiCH(R')CH3and by the signal of the SiCinternal upfield fromthe SiCterminal in the

29Si NMR spectrum of thesematerials.

The structural principles of organic low-molar-mass rigid rod-like mesogens have been extensivelyreviewed and discussed (most recently, in Refs 36and 37), and will not be addressed here. Metal-containing liquid crystals (metallomesogens) are amuch younger addition to the field of liquidcrystals, and the role of the metal centre in theformation of stable mesophases is not yet fullyexplored38.

A somewhat delicate synthetic task is posed bymetallomesogens. As the side-chain contains apotentially reactive metal centre, the method offunctionalization of the cage not only needs to betolerant of the cage and the functional groups of themesogen, but also designed to avoid any possibleinterference of the metal in the process. This isparticularly relevant for the hydrosilylation reac-

tion: in a metal-catalysed reaction in the presence ofanother metal, the choice of catalyst is of crucialimportance, and the mildest experimental condi-tions should be attained in order to avoid sidereactions involving the metal centre.

There are very few studies involving theinfluence of the type of catalyst on the conversionof polyhedral hydridosiloxane cores in the presenceof liquid-crystalline groups. Recently, it appearsthat the very robust Speiers catalyst (H2PtCl6 inisopropanol) has been employed less often thanFigure 3 Assignment ofa and b addition.


POLYHEDRAL LIQUID CRYSTAL SILSESQUIOXANES 263


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during the initial period of the synthesis of siloxanegroups containing liquid-crystalline side-chains.The use of various cyclopentadienylplatinumderivatives or Karstedts catalyst (platinum(divi-nyltetramethyldisiloxane), 33.5% platirum con-

centration in xylene) allow for considerably lowerreaction temperatures, but an increased sensitivityof the catalyst to hydrolysis has to be taken intoaccount. However, under most circumstances this iscompensated by the good conversion results atroom temperature.

In our experiments we observed that Karstedtscatalyst was very effective at room temperature,requiring reaction times of 13 h, without any signof interference with the metal complexes or otherLC side-chains used by us.

To ensure complete reaction of the hydrido-silsesquioxane with the mesomorphic alkene, a

small excess of the alkene is required. We haveused typically a 10% molar excess of the alkene forlinear terminal olefins and cyanobiphenyl deriva-tives; however, for derivatives of lower solubility,e.g. metallomesogens, a larger excess is advan-tageous in order to ensure complete conversion.Incomplete conversion leaves unreacted Si-Hbonds which, when heated in the solid state, canreact with other functional groups causing cross-linking and degradation of the sample.

Upon completion of the reaction the very activeKarstedts catalyst is deactivated by addition of asmall amount of triphenylphosphine. The deactiva-tion of the catalyst avoids contamination by thedeposition of platinum metal. It has been shownthat the presence of platinum affects the transitiontemperatures and the electro-optical behaviour21,22.

PURIFICATION

As shown for side-chain mesogenic poly-siloxanes21,22, one of the difficulties encounteredin the purification of the LC silsesquioxanes is that,

even in the case of complete conversion of theSiH moieties, effective removal of the excessalkene used is required in order to obtain repro-ducible transition temperatures in materials fromdifferent batches.

The presence of small amounts of isomericalkenes making up the hydrocarbon spacer in asilsesquioxane oligomer might bring about adecrease in transition temperatures and/or a modi-fication of the type of mesophase exhibited.

We have found column chromatography unsuit-

able for purification so far, as silica gel does notseem to provide the desired separation, whereas theuse of flash-grade silica gel tends to causeirreversible adsorption of the samples onto thesupport.

The usual work-up procedure is the precipitationof a solution of the silsesquioxane in dichloro-methane into a large volume of methanol (volume/volume ratio 91:10); this procedure is repeateduntil no monomer is detected in the silsesquioxaneby thin-layer chromatography. The merits of TLCdetection in the course of removal of alkenes havebeen described in detail39,40.

Gel permeation chromatography is particularlyuseful to ascertain the monodispersity of thematerials, and it provides a first-hand indication.However, due to the absence of calibrationstandards of similar structures, the absolute values

measured have to be treated with some caution.

LIQUID CRYSTAL PROPERTIES

Polyhedral LC silsesquioxanes can be viewed as anelement in the series of materials ascending fromlow-molar-mass liquid-crystal to linear side-chainpolymers. In order to achieve such a structuralarrangement the mesogens of one polyhedralmolecule are aligned antiparallel along one axis,as depicted in Fig. 1 and Fig. 2, giving a spindletype of structure. This leads to a structure in whichthe polyhedral cores are situated in the centres ofthe layers, separated from the mesogenic cores bythe mainly hydrocarbon layer of the spacer. Thisidealized concept of a fluid-like layered phase leadsto a picture of a sequence of layers of silsesquiox-anes (A), methylene groups (B) and mesogenic,usually aromatic, cores (C) in an ABCCBAsequence (Refs 16 and 17 and references therein,and Ref. 44; see also Fig. 1). For the description andunderstanding of the liquid-crystalline phase beha-viour of these materials, their essentially trichoto-

mous molecular structure has to be taken intoconsideration. The type and stability of the liquid-crystalline phase will be determined by thestructure of the mesogenic side-chains and thestructure of the organo-silicon core.

In order to evaluate the solid-state properties ofthese materials they have to be compared withstructurally related oligomeric and polymeric liquidcrystals. The chemical structures of the side-chainsdiscussed are shown in Fig. 5.

For side-chain LC polymers it has been estab-




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lished that for a given side-chain the phasetransition temperatures increase with increasingnumbers of repeat units, until a plateau is reachedabove which the transition temperatures are con-stant21,22. Depending on the polydispersity of thematerial and the structure of the backbone, this

value is reported to be obtained between 12 and 30repeat units. Thus in the region of degrees ofpolymerization between 2 and 12 the changes inproperties are the largest.

For the attachment of a cyanobiphenyl group asmesogenic core, separated from core A by methy-

Figure 4 Structures of the silicon-containing cores.




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lene spacers containing more than four methylenegroups (side-chains 1, 2 and 3) calorimetric studiesshow that the materials 10, 11 and 12 exhibit liquid-crystal phases16,17. The values for the transitiontemperatures, and the associated enthalpies andentropies for the second heating cycles, of each

compound are listed in Table 1. Thermal polarizedlight transmission microscopy reveals the forma-tion of a focal-conic defect texture, with hyperbolicand elliptical lines of optical discontinuity, as eachmaterial is cooled from the amorphous liquid intothe liquid-crystal state. These observations classifythe high-temperature liquid-crystal phase of eachmaterial as smectic A. Mechanical shearing of thespecimens in the microscope shows that the high-temperature mesophase flows easily and has arelatively low viscosity. In addition, the short time

required to form focal-conic defect textures oncooling from the isotropic liquid indicates that thecompounds have properties more in keeping withlow-molar-mass materials than with polymers.

Increasing the number of methylene units in the

spacer chain between the cyanobiphenyl andsilsesquioxane core units from four to 11 carbonatoms results in the isotropization temperaturebeing increased from 93.9 (10) to 128.5 C (12),the LC phase preceding the isotropization being asmectic A phase.

For the structurally related materials 13, 14 and15 with a cross-shaped tetrakis(dimethylsiloxane)core C, a similar trend in the development of thephase behaviour could be observed; the isotropiza-tion temperature is increased from 88.7 (13) to129.7 C (15) as the spacer increases from four toeleven methylene units16,17.

Comparison with cyclic materials based ontetramethylsiloxane rings F reveals that the transi-tion temperatures are roughly similar for compound1644, which has a spacer of six methylene groups. It

shows the transition from the smectic A phase to theisotropic phase at 118 C. The material 17 with thespacer of 11 methylene units becomes isotropic at141.3 C.

Comparison with linear polymers with a siloxanebackbone which exhibit isotropization temperaturesof 132 C for the polymer 18 (side-chain 1), 166 Cfor 19 (side-chain 2) and 157 C for 20 (side-chain3) showed that they follow a similar pattern to thatfound for the oligomers.

For the oligomeric materials the increase fromfour to eight side-chains does not affect thetransition temperatures significantly, but the silses-quioxane based materials tend to have higher glasstransition temperatures. The decrease in the valueof the glass transition temperature with increasingspacer length can be attributed to the plastifyingeffect of the alkyl groups. Additionally thesilsesquioxane-based materials with the longerspacers exhibit a rich, though not yet fully explored,crystalline polymorphism at lower temperatures.

The values for the isotropization enthalpy,reduced molar entropy (DSmol/R) and the reducedmolar entropy per side-chain (DSmol/nR) for thesilsesquioxane systems shown in Table 2 indicatethat an increase of the size of the enthalpy peakoccurs with increasing spacer length, suggestingthat there is an increase in the ordering of themolecules within the liquid-crystal phase. Thisconjecture is supported by the fact that there is alsoan increase in the calculated reduced molar entropy(DSmol/R), from 4.35 (10) to 9.68 (12), as the series

Figure 5 Structures of the liquid-crystalline side-chains.




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is ascended. For the related tetrahedral systems thevalues of the reduced molar entropy increase from2.14 for 13 to 6.33 for 15. Dividing these values bythe number of side-chains present in the molecule(DSmol/nR) reveals that for the materials with theshortest spacer similar values of 0.54 occur for the

tetrahedral and for the silsesquioxane-based core.For all the materials, investigated an increase inspacer length leads to an increase in the values forthe reduced molar entropy per side-chain. The risefor the silsesquioxane-based compounds is howeverless than that of the smaller tetrahedral materials.

X-ray diffration can be used to elucidate the LCphase structure further. The high-temperatureliquid-crystal phase of the materials 1012 showedan X-ray diffraction pattern which was obtainedfrom fibre samples recorded in Lindemann tubesand found to be typical of the smectic A phase8. Thevalues detected are listed in Table 3. The diffusewide-angle halo at 4.44.5 A is typical for thelateral distance between mesogenic units. Diffusereflections at 8.2 A for the tetrahedral materialsand of 12.013.2 A were attributed to the periodi-city of the arrangement of the silicon-containingcores13,5,8.

Table 1. Transition temperatures (C) and enthalpy data [DH (J g1) DCp (J g1 K1)]

Compound CoreSide-chain

Tg (C)[DCp (J g

1 K1)] Transitions a(C) [DH (J g1)]

10 A 1 11.0 [0.22] SA 93.9 [4.31] Iso

11 A 2 3.0 [0.32] S1 22.5 [0.19] SA 116.5 [5.40] Iso12 A 3 7.5 [0.28] cr 34.4 [10.2] S2 54.6 [0.77] S3 64.5 [1.20] SA 128.5 [8.66] Iso13 C 1 9.6 [0.34] SA 88.7 [4.85] Iso14 C 2 14.7 [0.36] SA 118.7 [7.83] Iso15 C 3 6.3 [0.31] cr 38.7 [18.29] (SC 42 [ ]) SA 129.7 [5.40] Iso16b H 2 57 SA 118 [5.6] Iso17 H 3 cr 59.9 [25.7] SA 141.3 [11.1] Iso18c I 1 28 SA 132 Iso19c I 2 14 SA 166 Iso20c I 3 1 SC 48 SA 157 Iso

a S1, S2, S3 are smectic or soft-crystal phases.b From Ref. 44.c From Refs 21 and 22.

Table 2. Reduced entropies (DSmol/nR) and re-duced entropies per side-chain (DSmol/nR)

Compound Core Side-chain DS/R DS/nR

10 A 1 4.35 0.5411 A 2 5.50 0.6912 A 3 9.68 1.2113 C 1 2.14 0.5414 C 2 3.46 0.8715 C 3 6.33 1.58

Table 3. The d-spacings (A ) determined by X-ray diffraction for compounds1012

Compound Side-chain T (C) d-spacings (A)

10 1 25.0 4.5 11.3 32.211 2 25.0 4.5 12.0 17.6a 35.4a

12 3 75.0 4.5 13.2 21.6 43.112 3 85.0 4.5 13.2 21.4 42.812 3 100.0 4.5 13.2 21.3 42.812 3 110.0 4.5 13.2 21.0 42.112 3 120.0 4.5 13.2 21.0 42.112 3 125.0 4.5 13.2 21.0 42.1

a Reflections are very weak.




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The values calculated from the small-anglereflections observed for the d-spacings of thesmectic layers of the silsesquioxane materials listedin Table 3 of 32.2 for 10 to 43.1 A for 12 at 25 Care consistent with a structure where the silicon-containing cores are located in the central regionsof the layers, and there is some overlap of theterminal cyanobiphenyl groups8,16,17. This classi-fies the high-temperature smectic phase as asmectic Ad (SAd) phase.

The situation is more complex, however, for thetetrahedral material 15, for which on cooling theoccurrence of a smectic C phase preceding crystal-lization was observed. X-ray diffraction data of this

compound showed that at 70

C in the smectic Aphase an arrangement requiring two sublattices isrequired. After 24 h of annealing reflections ofroughly similar intensities relating to periodicitiesof 38.7 and 46.7 A could be observed. This can beaccommodated with a phase structure in whichsome of the mesogens overlap completely and someonly partly17.

The influence of the stiffness of the mesogeniccore on the transition temperatures has beendemonstrated by Kreuzer et al.14 for materials

based on the core A, as has been shown formesogens 4 and 5 terminated with aromatic cyanogroups leading to compounds 21 and 22. Thetransition temperatures are listed in Table 4.

The variation in the core structure, employingcores E, A and D and thus resulting in materials 23,24 and 25, is accompanied by an increase in theisotropization temperature from 142 to 164 C14.An interesting feature is the occurrence of crystal-line phases as the number of branches rises from sixto ten while the structure of the mesogens isretained. It can be assumed that the crowding of themesogens in the vicinity of the silsesquioxane coresleads to additional ordering in the alkyl chains. A

comparison of the core structures A and B revealedfor the mesogen 6 that the isotropization tempera-tures are relatively unaffected by the presence ofthe dimethylsiloxane group in A. The isotropizationtemperature from the smectic A phase is reported14

to be 157 C for 24 and 160 C for 26. The pressureof the dimethylsiloxane depresses the crystalliza-tion by 20 C, from 128 C in 26 to 108 C in 24.

The structure of the materials derived from theside-chain 7 which promotes the formation of tiltedphases and the silicon-containing cores; the results

Table 4. Transition temperatures (C) (Ref. 14) and (Ref. 15)

Compound Core Side-chain Tg (C) Transitions (C)

21 B 4 cr 119 SA 146 Iso22 B 5 102 SX 127 SA b300 Iso

23 A 6 46 cr 128 SA 157 Iso24 B 6 108 cr 108 SA 160 Iso25 D 6 51 cr 122 SA 164 Iso26 E 6 30 SX 37 SA 142 Iso36 B 8 cr 90 SX 184 N Iso

Table 5. Transition and glass transition temperatures (C), specic heats DCp (J g1 K1) and transition

enthalpies DH (J g1) of materials derived from side-chain 7, determined by DSC measurements(10 Cmin1)

Compound Core Tg (C) [DCp (J g1K1)] Phase transitions a(C) [DH (J g1)]

8 Monomer cr 78.5 SC 106.1 [0.68] SA 157.2 [2.93] N 168.9 [1.51] Iso27 C 8.4 [0.47] SCa 125.0 [0.13] SC 154.4 [0. 1] SA 164.4 []

b Iso28 H 10.3 [0.54] SXb 69.5 [1.03] SC 184.2 []

b N 193.3 [8.0] Iso29 F 11.2 [0.37] SC 182.5 [ ] SA 188.7 [6.82] Iso30 E 11.5 [0.14] SC 191.5 [2.1] Iso31 B cr 143.0 [21.1] SC 208.0 [0. 2]SA 214.0 [6.4] Iso32 I 19.0 [0.15] S4C 223.5 [0. 1] SA 234.3 [5.33] Iso

a SX is an unresolved phase.b Unresolvable by DSC.




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are listed in Table 520. The type of core employedinfluences the phase behaviour considerablythrough its geometry, the concentration of stereo-isomers, its tendency to phase-separate and thenumber of branches present in the molecules.

To fit cyclic liquid-crystalline materials made ofsmall rings consisting of different isomers intosmectic phases, they have to be inscribed in acylindrical (spindle-shaped) structure, with thesmallest radius in the core (the area of thesiloxanes) and the largest at the terminal groupsof the molecule. Due to the limited number ofmesogens per core, one molecule cannot fill theenveloping cylinder. It has to be filled by side-chains of other molecules.

Low glass transition temperatures (Tg) and lowviscosites can be explained in this model as a resultof the flexibility of the core and gaps in the

cylinder. Increasing the diameter of the core fromfour siloxane groups (H) in 28 to five (F) in 29 leadsto an increase in the number of gaps. The glasstransition temperature is not much affected by thisincrease in disorder, but the transition temperaturesof the mesophases decrease and a highly orderedphase present (smectic X) is lost altogether.

The use of hexasilsesquioxane E or octasilses-quioxane cores B, leading to materials 30 and 31,where the gaps in the cylinder are filled, results inan increase in the mesophase stability, which is notmuch lower than in material 32 where the mesogen7 is attached to a much larger linear polymer I.

The compound 27 with the tetrahedral core C hasa markedly different phase behaviour from theother materials; this can be rationalized in thefollowing manner. There is an odd number of atomsseparating the two pairs of anti-parallel orientedmesogens. For the material in a layered phase thisfavours a minimum-energy conformation of themolecules where two pairs of mesogens of onemolecule are not aligned parallel but lie at an angle.This distinguishes this type of materials fromcuboid systems (cores A, B) where an even numberof atoms separates anti-parallel mesogens. The

effects of the oddeven effect in liquid crystals arediscussed extensively elsewhere (see Refs 21, 22,36 and 37, and references therein).

Due to its crowded central core (C) with its eightmethyl groups, the glass transition and clearingtemperature are much lower than in all the othermaterials. Due to the possibility of intramolecularinteractions facilitated through the close proximityof the side-chains, that are liquid-crystalline phasescan be realized similar to those found for the othermaterials. Additionally, an alternating smectic C

(SCa) phase occurs at temperatures below 125 Cand can be identified by its transition enthalpy at thetransition from the smectic C phase (DH= 0.13J g1), and by two and four brush defects in itsSchlieren texture which are associated with the SCa

phase. This texture could not be observed for theother materials.

The use of a metallomesogen as a side-chainopens the possibility of generating materials whichare predominantly inorganic in the structure of theirscaffolding. The inclusion of metals in mesogensmakes it possible to combine properties associatedwith metallic materials, e.g. magnetism and/orconductivity, with liquid-crystalline phase beha-viour.

The addition of a pentamethylsiloxane unit G toside-chain 9 results in compound 33, where thestability range of the smectic A phase is enhanced

to 240 C45. The results are listed in Table 6. Fusing9 to a tetrameric cyclic siloxane ring system resultsin material 34, consisting of several stereoisomers,and leads to a further enhancement of theisotropization temperature to 296 C. This is thefirst example of a polyhedral silsesquioxane withappended metallomesogens18. The nickel-modifiedcompound 35 has been prepared by hydrosilylationof the non-symmetrical, diamagnetic, square-planarnickel complex 9 with (Si8O12)(OSiMe2H)8 (A).Investigation of its solid-state properties revealedthat the crystalline state is enhanced up to 219 C,when decomposition sets in.

Systems where more than one type of mesogen isattached to a silsesquioxane core or not all of thereactive SiH groups are converted are attractivebecause they promise to offer tailoring of the LCproperties by the use of appropriately selected side-chains. Preparatively, the problem of controllingthe uniformity of the products can be envisaged tobe difficult.

The few reports available14 indicate that ifsuitable side-chains of the type of 8 are included,crystalline phases are suppressed and glass transi-tions appear. Additionally, for such a system 36

Table 6. Transition temperatures (C) determinedby DSC measurements (10 C min1) for the 9 and3335

Compound Core Side-chain Phase transitions (C)

9 9 cr 181 SA 190 Iso33 G 9 cr 184 SA 240 Iso34 H 9 cr 196 SA 296 dec35 A 9 cr 219 dec




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consisting of core B and side-chain 8 where four ofthe eight SiH groups were reacted, the firstnematic phase in LC silsesquioxane materials couldbe observed,15 with a stability range of the nematicphase up to 184 C (see Table 4). Altogether the

phase behaviour of these systems seems to becomeclose to that of cyclic siloxanes46.

For materials where two rigid side-chains areemployed, the phase transition temperatures seemto depend strongly on the core structure, with thesystems containing more aromatic cores in the side-chain having higher transition temperatures.

EXPERIMENTAL

Synthesis of octakis[4-cyano-4'-(6-dimethylsiloxy)hexyloxy-

biphenyl]octasilsesquioxane (11)

A solution of toluene (20 ml), 4-cyano-4'-(5-hexenyloxy)biphenyl) (0.55 g, 1.58 mmol) and10 ml of a 3.0 3.5 % solution of Karstedtscatalyst dissolved in xylene was gently aerated for20 s, then a solution of octakis (hydrodimethyl-siloxy)octasilsesquioxane (0.1 g, 0.098 mmol) indry toluene (10 ml) was added dropwise at roomtemperature over a period of 1 h. A few minutesafter the addition was complete no SiH band(2142 cm1) could be detected by IR spectroscopy.A spatula-tipful of triphenylphosphine was added inorder to convert the catalyst to the less reactivetriphenylphosphine complex and the solution wasconcentrated under reduced pressure. The oligo-meric material was isolated by precipitation inmethanol. The product was separated by filtration,and purified by repetitive precipitation frommethanol until no monomer could be detected bychromatographic methods. Recrystallization frompentane yielded 0.31 g of 2 (85%).

Spectral data: IR (KBr): n = 3020 (ArH), 2940(CH), 2220 (C=N), 1600 (Ar), 860 cm1

(CH);1

H NMR (270 MHz, CDCl3, TMS):= 7.65 (m, 4H, ArH), 7.52 (m, 2H, ArH),6.95 (m, 2H, ArH), 3.93 (tr, 2H, ArOCH2),1.75 (m, 2H, OCH2CH2), 1.45 (m, 6H,CH2CH2CH2), 0.62 (tr, 2H, CH2Si), 0.14(s, 6H, (CH3Si);

13C NMR (67.8 MHz, CDCl3):= 0.01 (CH3Si), 18.14 (CH2Si), 23.49, 26.31,24.78, 33.75, (CH2CH2CH2), 67.44(CH2O), 110.82 (Ar C4', CCN), 115.19 (ArC3, C5), 126.89, 128.48 (Ar C2, C6, C2', C6'),131.56 (Ar C1), 132.49 (Ar C3', C5'), 144.71 (Ar

C1'), 160.11 (Ar C4, OC), 118.87 (CN);29Si

NMR (53.5 MHz, CDCl3, TMS):= 12.34(SiCH3), 108.8 (Si(O)4).

Octakis(([8-(3-(10-decyloxydi-methylsiloxy)-6-hydroxyphenyl)-2-phenyl-4,7-diazocta-1,3,7-tri-enato](2-)nickel(II)))octasilses-quioxane (35)

Nickel(II) complex 9 (0.5105 g, 1.01 mmol) wasadded to a toluene solution (50 ml) containing 10 "lof a 33.5% solution of bis(divinyltetramethyldi-siloxane)platinum(0) in xylene, and the whole wasstirred (1 h) at room temperature. A gentle streamof air was blown through the red solution for a few

seconds. A solution of octa(hydrodimethylsiloxy)-octasilsexquioxane (0.1017 g, 0.10 mmol) intoluene (20 ml) was added dropwise over a periodof 2 h. A brownred solid separated as the additionprogressed. After a further 1 h, the IR spectrum ofthe reaction mixture showed complete conversion.At this point a few crystals of triphenylphosphinewere added to deactivate the catalyst. The ochre-redsolid was filtered off and dried in vacuo. The solidwas washed with tetrahydrofuran to remove anyunreacted monomer, dissolved in dichloromethane(5 ml) and filtered through a Pasteur pipette loadedwith Hyflo (filter aid) to remove any platinum(0)residues. The product was precipitated out ofsolution by addition to rapidly stirred methanol(750 ml). This procedure was repeated until thesolution showed no TLC-detectable amount ofmonomer. The ochre-red solid was filtered off anddried in vacuo. [yield 0.2717 g (53%).]

Analysis: Calcd for C249H328O44N16Si16Ni16: C,56.97; H, 6.52; N, 4.42. Found: C, 55.39; H, 6.38;N, 4.42%. MS (FAB): m/z, no M detected; NiSi-containing clusters of lower fragments. GPC(CHCl3, polystyrene standard): 5775 (Mw) and asecond band at 2Mw. Mn = 5075; Mw/Mn = 1.13. IR

(KBr disc; n cm1

): 1607 (vs, sharp; C=

N, C=

Oand C=CN coordinated); 1085 (vs, broad;SiOSi). 1H NMR (CD2Cl2): d 7.11 (m, 8H;aromatic C6H5, HC=N, HC=O and NCH=C);6.83 (m, 1H, Hb); 6.68 (m, 1H, Ha); 6.38 (m, 1H,Hc); 3.74 [t (J= 7 Hz) 2H, CH2O); 3.30 (S,4H,NCH2); 1.68 (m, 2H, CH2CH2O); 1.30[m, 14H, (CH2)7]; 0.60 (t (J= 7 Hz) 2H,CH2Si); 0.13(, 6H, SiCH3).

29Si{1H} NMR(CDCl3; Cr(acac)3): d 12.65 (s, Si(CH3)2CH2, Msilicon); 108.8 (s, SiO3; Q silicon).




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SUMMARY

Studies of liquid-crystal polyhedral silsesquioxanematerials have been described, relating to theirsynthesis, specific problems connected with the

nature of the silsesquioxane cage, and the specialproperties that their geometry imparts to theirmesogenic behaviour. The synthesis of thesematerials is based on a silsesquioxane cagemodification process, starting with a suitablyfunctionalized cage, to which the mesogens areattached. All the systems described so far are basedon the hydrosilylation reaction of hydrido-silses-quioxanes and mesogenic alkenes.

The liquid-crystalline phase behaviour of thesestructurally uniform materials is related to that ofcomparable oligomeric and polymer systems anddepends strongly on the structure of the mesogenic

side-chains. This allows for the tailoring of glasstransition temperatures, the type of liquid-crystal-line phases and phase transition temperatures over awide temperature range. The possibility of includ-ing metals in the LC side-chains highlights theversatility in this field of research, which isconcerned with the soft-matter state of inorganic/organic hybrids.

Acknowledgements We thank Professor J. W. Goodby formany helpful discussions and the Engineering and PhysicalSciences Research Council (EPSRC) and the Defence Evalua-tion and Research Agency (DERA, Malvern) for the financial

support of this research.

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Polyhedral Liquid Crystal Silsesquioxanes-

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