J. Chem. SOC., Faraday Trans. I, 1982, 78, 713-724 Classification of the Mesophase of Di-isobutylsilanediol BY JOHN D. BUNNING AND JOHN E. LYDON Astbury Department of Biophysics, University of Leeds, Leeds LS2 9JT AND COLIN EABORN AND PATRICIA M. JACKSON School of Molecular Sciences, University of Sussex, Brighton BN1 8QJ AND JOHN W. GOODBY AND GEORGE W. GRAY* Department of Chemistry, University of Hull, Hull HU6 7RX Received 25th February, 198 1 The thermotropic liquid crystal mesophase formed by di-isobutylsilanediol has been known for over twenty-five years, but its nature and structure have remained uncertain and the physical characteristics of the phase appeared unique and quite different from those of any other recognised type of liquid crystal. In a previous preliminary publication, it was suggested that the mesophase of di-isobutylsilanediol is in fact of the discotic type only very recently characterised in certain organic systems.Here we describe more fully our grounds for this suggestion. A model is proposed for the structure of this mesophase; this offers an explanation as to why other dialkylsilanediols do not behave similarly. A reinterpretation of earlier X-ray crystallographic data led us to suggest that the mesophase consists of hydrogen-bonded dimers. The model is supported by data from optical microscopy, thermal analysis and miscibility studies. The optical textures shown by the mesophase and those observed at the crystal mesophase boundary are discussed and compared with those of the discotic phases of the benzene hexa-n-alkanoates.The structure of the thermotropic mesophase of di-isobutylsilanediol, (i-Bu),Si(OH), (1) must present one of the longest-standing problems in the field of liquid crystal research. This compound was first prepared by Eabornl in 1952. He noticed that it had a ‘double melting point ’, suspected liquid crystal behaviour, and passed a sample to Hartshorne for optical examination. Hartshorne found that the mesophase was puzzling : microscopic textures unlike those of any previously encountered mesophase type were formed and the situation was further complicated because the molecular structure (plate 1) offered no clue as to which type of mesophase was likely. A preliminary X-ray diffraction study of the crystalline solid was carried out by Bernal et aL2 but this did not give any clear implications concerning the molecular ordering in the mesophase.Eaborn and Hartshorne reported3 the optical observations in detail and made a tentative assignment of the phase as smectic, but they could offer no convincing explanation of the peculiar features of the microscopic texture. Their model incor- porated the extended hydrogen-bonded chain shown in fig. 1 which at that time was thought to exist in the crystal structure of another silanediol derivative. Although many thousands of liquid crystal phases must have been examined in the meantime, none has been reported with textures identical to those of (I) and no closely related compounds have been reported with comparable mesogenic properties. For twenty-five years, the problem of the structure of this mesophase has lain dormant.We have attempted a comprehensive structural study of the mesophase of (I) using a range ofphysical techniques. In addition to repeating Hartshorne’s optical microscopy, FAR 1 713 24714 ME SO PHASE OF D 1-1 SOB U TY L S I L ANED I 0 L we have extended the X-ray investigation to a study of the mesophase itself and we have undertaken differential thermal analysis and miscibility tests. On the basis of these investigations, we have come to the conclusion that this mesophase is discotic4 and it must therefore be regarded, in retrospect, as the first example of this type of phase to be reported. We have already made a preliminary report5 suggesting that the phase is discotic, but now describe fully the grounds for the conclusion that this is indeed the case.i-Bu \ Y-? I 1 I t I 1 ?-Y \ 1 8 ' : A-0 \ / ii i-Bu i-Bu FIG. 1.-Extended system of hydrogen bonding suggested by Eaborn and Hartshorne3 for both the crystalline solid and the mesophase of di-isobutylsilanediol. A scheme of association of hydroxyl groups in this manner was proposed by Kakudo and Watase (Technul. Rep. Osaka Univ., 1952, 2, 247) for the crystalline solid of diethylsilanediol and by Kakudo, Kasai and Watase (J. Chem. Phys., 1953, 21, 1894) for diallylsilanediol. In both cases their models were based on a combination of preliminary X-ray studies and infrared absorption spectra and cannot be regarded as unequivocal. A full crystal structure determination of diphenylsilanediol has recently been reported by Parkanyi and Bocelli (Cryst.Struct. Cumrnun., 1978, 7, 335) and in this case all of the hydrogen bonds are of the usual -OH. .O- type, and there is no evidence of OH groups lying as opposed dipoles. RESULTS MESOPHASE FORMATION I N DIALKYLSILANEDIOLS As reported by Eaborn and Hartshorne3 the behaviour of di-isobutylsilanediol in forming a mesophase is apparently unique and in their paper they give a substantial list of dialkylsilanediols which either they or others had observed to give normal melting behaviour. In the course of this work dicyclohexylsilanediol has been reprepared and found to give no mesophase and this is also the case for the following silanediols prepared at the University of Sussex (m.p./"C in parenthesis) : di-n-hexyl- (86-87) ; methyl-n-pentyl-( 57) ; methyl-n-octyl-( 57) ; methyl-n-decyl-( 59-60) ; methyl- n-dodecyl-(67-68); methyl-n-octadecyl-(87-88); methylcyclohexyl-(97-98); di-o-tolyl- (138-139).In some cases, decomposition of the sample is a problem and this was particularly true with methyl-n-pentylsilanediol and methyl-n-octylsilanediol, but even in these instances it seems clear that no mesophases are formed. Perhaps dialkylsilanediols having a secondary carbon p to the silicon would be closer analogues to di-isobutylsilanediol and these may be worthy of examination in the future. OPTICAL MICROSCOPY The transition temperatures observed for di-isobutylsilanediol were : crystal - mesophase - isotropic liquid crystal - mesophase - isotropic liquid. 88.4 O C 98.7 O C 77 O C 95.6 O C Eaborn and Hartshorne3 have given a detailed description of the changes observed at the isotropic liquid -+ mesophase transition and at the crystal + mesophase andBUNNING, LYDON, EABORN, JACKSON, GOODBY A N D GRAY 715 mesophase + crystal transitions.We have repeated these observations and we concur with their descriptions. A brief summary of these is given below. There are two major additions to their description which we shall make. First, we shall distinguish between the effects of rapid cooling and of slow cooling of the isotropic liquid, since the cooling rate dramatically affects the texture of the mesophase formed. Secondly, we shall describe the appearance of homeotropic* dendrites of mesophase not reported previously. ISOTROPIC LIQUID -+ MESOPHASE TRANSITION Some degree of supercooling always occurs at this transition.When the mesophase is formed by rapid cooling the mosaic fan texture shown in plate 2 arises. When viewed with crossed polars, this always appeared grey or pale yellow in colour, indicating that the degree of birefringence was very low. By contrast, slow cooling of the isotropic liquid produces a star-like or fern-like dendritic growth of areas of mesophase as shown in plate 3(a)-(c).t Most of these growths are homeotropic and are difficult to see against the isotropic background (and can be seen most clearly with the analyser removed, by virtue of the Becke line effect at their edges), but there are occasional birefringent regions. We suggest that the homeotropic areas have grown on the glass surfaces (and are therefore parallel to the stage) and are being viewed down the axis of a uniaxial indicatrix, whereas the birefringent areas are growing in the bulk of the sample and are randomly aligned. Although the dendrites have rounded corners they are very similar in appearance to dendrites of crystalline solid phases, having branching at specific angles and showing a high degree of mirror symmetry along the dendrite arms.The strong suggestion of six-fold symmetry at the centres of these stars coupled with the prevalence of branching angles of 60’ implies that each entire growth is a single domain of mesophase and that it is being viewed down a six-fold axis. (In such a situation, one should perhaps refer to the optical property as pseudo-homeotropic, since the viewing direction is a six-fold rotation axis rather than one of infinite order.) On further cooling, when all of the sample has been converted to mesophase, the rod texture shown in plate 4(a) or (b) develops. At first sight, the appearance of this texture of bright rods running through a dark background suggests an open lattice framework of some kind.However, the dark areas are homeotropic (not isotropic) and because the intensity of the light regions gradually fades into the background it would appear that the ‘rods’ are actually curved regions at the edges of extended homeotropic domains. This hypothesis is reinforced by the observation that when viewed with polarised light, in the absence of an analyser, the contrast between these ‘rods’ and the surrounding mesophase regions is lost once in every 180’ of rotation of the stage.When viewed between crossed polars, they extinguish every 90’. Using a specially designed thin hot stage,g Eaborn and Hartshorne made a conoscopic examination of this texture. The homeotropic regions were found to be optically negative and the uniaxial indicatrix was found to tilt at an increasing angle as the axis of a rod was approached. SOLID -+ MESOPHASE TRANSITIONS Rapid cooling of the mesophase produces the solid in the form of spherulites of radial, acicular crystals (plate 5). When the temperature of a sample in this state is raised, the broad pattern of the spherulites is maintained by the mesophase, but the * Optically extinct because the dendritic area is being viewed along the uniaxial optical axis.? Note that although Eaborn and Hartshorne used the word ‘dendritic’ in their descriptions of optical textures of (I), they were not describing this texture, but rather the branching pattern of the rod texture mentioned below. 24-2716 MES 0 PHASE OF D 1-1 SOB U TY L S I L ANE D I 0 L fine detail changes dramatically. The radial pattern of individual crystals gives place to a tangential pattern of bands (plate 6). This was termed the striated band texture by Hartshorne. If a single crystal (grown from solvent) is heated, it also gives a striated band and on cooling reverts to more or less its original form. The polarisation colours of the mesophase are noticeably lower than those of the corresponding single crystal. COMPARISON OF THE MESOPHASES OF BENZENE HEXA-n-HEPTANOATE AND DI-ISOBUTY LSI L ANEDIOL The textures which we have observed for the mesophase of (I) were totally dissimilar to textures shown by any known smectic, nematic or cholesteric phases.There are how- ever, some points of resemblance with those described by Chandrasekhar et aL4 for the new discotic phases of certain esters of hexahydroxybenzene. Moreover, like the discotic phases, and unlike smectic and nematic phases, the phase exhibited by the silanediol was shown to be optically negative uniaxial. Therefore, a comparison was made between these textures and those of the discotic phase exhibited by benzene- hexa-n-heptanoate (11). For comparison, plates 7-9 show the typical textures exhibited by the discotic phase of benzene hexa-n-heptanoate (11).Plate 7 shows the mosaic-fan texture of (11). This texture is clearly very similar to plate 2 for the silanediol and this texture appears to be typical of both the ester and the diol. Plate 8 shows the pseudo-fan texture of the discotic phase. This texture was coloured pale yellow and grey as was that shown in plate 2 for the silanediol. The striated pattern in plate 8 is also very similar to the striated band structure shown in fig. 2 of the paper by Eaborn and Hart~horne.~ Plate 9 shows the feather texture of the discotic phase of (11). This texture was not exhibited by (I) and therefore may be typical only of (11). Thus, it can be seen that there are some points of similarity in the optical properties of the mesophases of (I) and (11).Moreover, it is now clear that other discogenic systems form discotic phases with the rod texture' and also the homeotropic dendritic texture. DIFFERENTIAL THERMAL ANALYSIS Differential thermal analysis was used to confirm the transition temperatures observed with hot-stage optical microscopy and to measure the enthalpy changes for the solid + mesophase and mesophase -+ isotropic liquid transitions. The differential thermal analysis trace for a heating cycle is shown in fig. 2. The trace for cooling is considerably broader than that for heating, and we found that for a second and subsequent heating cycles the traces continue to broaden. It appears therefore that the compound is prone to thermal decomposition and that even the first heating of a sample to observe the mesophase is accompanied by an appreciable degree of decomposition.The enthalpy values obtained are: AH(crystal1ine solid -+ mesophase) = 7.6 kJ mol-l, AH(mesophase -+ isotropic liquid) = 7.2 kJ molt'. The magnitude of AH(mesophase -+ isotropic liquid) is unusually large, as is the ratio of the enthalpy change at the mesophase-+isotropic transition to that at the crystal -+ mesophase transition (ca. 0.94).* We take these facts to indicate that a considerable fraction of the intermolecular bonding of the crystal is retained in this mesophase. * In systems involving smectic and nematic phases, this ratio is very low (0.2 or less), whereas for the discotic system benzene hexaheptanoate it is 0.67.J. Chem. SOC., Faraday Trans. 1, Vol. 78, part 3 PLATE 1 .-Model of di-isobutylsilanediol (I).PLATE 2.-Mosaic fan texture of (I); crossed polars ( x 100). BUNNING, LYDON, EABORN, JACKSON, GOODBY AND GRAY Plates 1 and 2 (Facing p . 716)J . Chem. SOC., Faraday Trans. 1, Vol. 78, part 3 Plate 3 PLATE 3.-Star-like or fern-like dendritic growth areas of the mesophase of (I), obtained on slow cooling. Most areas are homeotropic, as shown by the dotted white lines in (b), which have been drawn to make the boundaries clear [ x 75 for (b) and (c); x 50 for (a)]; crossed polarisers ( x 150). BUNNING, LYDON, EABORN, JACKSON, GOODBY AND GRAYJ . Chem. SOC., Faraday Trans. 1, Vol. 78, part 3 Plate 4 PLATE 4.-Rod texture of the mesophase of (I); crossed polars [ x 100 for (a) and (b)]. The structure around the sharp boundaries where the rods intersect (such as the arrowed regions) is discussed under fig.10. BUNNING, LYDON, EABORN, JACKSON, GOODBY AND GRAYJ . Chem. SOC., Faraday Trans. 1, Vol. 78, part 3 Plates 5 and 6 PLATE 5.-Crystalline texture of (I) obtained by rapid cooling of the mesophase; crossed polars ( x 100). PLATE 6.-Mesophase texture of (I) obtained by heating the crystalline texture shown in plate 5 ; this is the striated band texture of Hartshorne; crossed polars ( x 100). BUNNING, LYDON, EABORN, JACKSON, GOODBY AND GRAYJ. Chem. SOC., Faraday Trans. 1, Vol. 78, part 3 Plates 7 and 8 PLATE 7.-Discotic mesophase of benzene hexa-n-heptanoate (11) ; mosaic fan texture; crossed polars ( x 73, compare with plate 2. PLATE 8.-Striated texture of the discotic mesophase of (11); crossed polars ( x 75); similar to the striated band texture of plate 6.BUNNING, LYDON, EABORN, JACKSON, GOODBY AND GRAYJ . Chem. Soc., Faraday Trans. 1, Vol. 78,part 3 Plates 9 and 10 PLATE 9.-Feather texture of the discotic mesophase of (11); crossed polars ( x 75). PLATE 10.-(a) Model of the hydrogen-bonded dimer of di-isobutylsilanediol. (b) Model showing the stacking of two hydrogen-bonded dimers of di-isobutylsilanediol. BUNNING, LYDON, EABORN, JACKSON, GOODBY AND GRAYBUNNING, LYDON, EABORN, JACKSON, GOODBY A N D GRAY t I M E S O P H A S E *-- I I I I CRYSTALLINE I I ISOTROPIC 717 increasing temperature FIG. 2.-Differential thermal analysis trace for di-isobutylsilanediol obtained on heating a sample of the material, initially as the crystalline solid, at a heating rate of 10 O C min-I.X-RAY DIFFRACTION The X-ray diffraction pattern of the mesophase of (I) is very simple. Only two factors are apparent: an outer diffuse ring corresponding to a repeat distance of 4.7 A and an inner ring corresponding to a repeat distance of ca. 11-12 A. This inner ring is of an intermediate type. It is by no means as sharp as those found for smectic mesophases, but it is not as diffuse as the inner rings given by isotropic samples or by nematic mesophases. The only type of mesophase which has been observed to give a diffraction pattern of this type is the recently characterised discotic phase. MISCIBILITY STUDIES To test our suspicion that the mesophase of (I) was of the discotic type, a miscibility study was carried out with benzene hexa-n-heptanoate as the standard reference discotic material.The diagram of state for binary mixtures of these two materials is shown in fig. 3 : it can be seen that there is indeed a region of continuous miscibility connecting components (I) and (11). This is shown by the darker shaded region and means that a single, continuous phase consisting of varying proportions of two molecular species [ranging from 100% of (I) to 100% of (II)] exists across the diagram of state. Thus, for every composition from left to right across the diagram of state, there is a temperature range, admittedly narrow in places, over which the texture of the liquid crystal phase is typically of the discotic type. This temperature range is that between the temperature at which the last trace of crystalline solid disappears and that at which the first sign of isotropic liquid appears, i.e. the commencement of the two-phase region consisting of isotropic liquid and discotic phase denoted by the lighter shading in fig.3. On the basis of Sackmann’s miscibility principle, the phases of (I) and (11) are therefore of the same type. It is apparent that the mesophase -, isotropic liquid transition temperatures are considerably depressed and there may be a number of factors responsible for this. First, since it is necessary to preheat mixtures to ensure thorough mixing of the isotropic liquids, the thermal instability of (I) must give rise to some contaminants.718 ME SO PHASE OF D 1-1 SOBU TY L SI L AN ED I 0 L I" Q) * 80 .. . . . . . 100 % I 100 % II composition - FIG. 3.-Phase diagram for binary mixtures of di-isobutylsilanediol (I) and benzene hexa-n-heptanoate. (11). The range over which the one-phase discotic region exists is shown by the darker tone. The paler area indicates the two-phase region where the discotic phase and the isotropic liquid co-exist. At high concentrations of (11) the occurrence of the crystalline solid + mesophase transition is difficult to detect and the dotted line represents where the eutectic might be expected to lie. Numerous compositions in the narrow one-phase region [25-70% of (I)] were in fact examined; only a few points have been indicated in the figure. The continuity of phase was confirmed by contact preparations (G. W.Gray and D. G. Mc- Donnell, Mol. Cryst. Liq. Cryst. Lett., 1977,34,211). On cooling such a preparation to 90 OC, two discotic regions were separated by the isotropic liquid and two-phase regions, but below 86OC, through supercooling effects, a single discotic phase of uniform texture extended across the preparation. Secondly, the two molecular species have very different structures. As will be discussed later, we suggest that the molecular units in (I) have approximate 4-fold rotational symmetry whereas the molecules of (11) have 6-fold symmetry. This difference may however not be significant in the context of discotic mesophase formation since discogenic molecules have now been discovered with 2, 3, 4 and 6-fold rotational symmetry. See, for example, the recent review of discotic phases by Billard.' Nevertheless, in spite of the departure from ideality, we consider that the miscibility continuum observed affords strong support for the hypothesis that the mesophase of (I) is discotic.DISCUSSION MOLECULAR ORDERING I N THE MESOPHASE AND THE CRYSTALLINE SOLID We suggest that the basic structural unit of the mesophase is the hydrogen-bonded dimer shown in fig. 4 and plate lO(a), rather than the extended chain shown in fig. 1. We picture these dimers as stacking on top of one another [plate lO(b)] giving columns which pack together in a hexagonal array. We propose a model for the crystalline solid where further hydrogen bonding links the dimers together, as shown in fig. 5, and causes them to lie tilted with respect to the stack axis.Crystallographic studies to test this hypothesis are at present being undertaken. The preliminary X-ray investigation of the crystalline solid carried out by Bernal et aL2 was reported by Eaborn and Hart~horne.~ The unit cell is triclinic with a = 14.8 A, b = 5.06 A, c = 28.8 A, a = 90°, #I = 121°, y = 96O. At first sight, this does not suggest any obvious model for the molecular arrangement in the mesophase. However, we note that two of the angles are close to 90' and the third is near to 120O. This suggests that it might be possible to redraw the latticeas a distorted hexagonal array:BUNNING, LYDON, EABORN, JACKSON, GOODBY A N D GRAY 719 i-Bu i-Bu \ / 0 -H 0 7\ I I H Y I I I I I I I H - 0 \ /O ii i-Bu i-Bu FIG. 4.-Hydrogen-bonded dimer of di-isobutylsilanediol, which we suggest is the basic structural unit of both the mesophase and the crystalline solid.1 -- HO,si,O--- H HO,,i 'I ,O H - - - HO,si,O 'i H - - - L -I-- - - w-w- FIG. 5.-Extended scheme of hydrogen bonding which we suggest occurs in the crystalline solid of di-isobutylsilanediol. Upper: perspective sketch showing the way the molecules stack in the crystalline solid. Lower: A view perpendicular to the stack axis. Note that symmetry considerations do not require that the dimers should lie normal to the stack axis. this can be done if additional lattice points are included half way along the 28 A axis, as shown in fig. 6. The predominant spacing of such a lattice would be 28/2 x 2/3/2 = 12 A, and the molecules are pictured as lying in the array shown in fig.7. This arrangement appears to be compatible with the partial crystallographic information at present available. This transition from solid to mesophase is pictured as involving untilting the molecular dimers to bring them perpendicular to the stack720 MESOPHASE OF DI-ISOBUTYLSILANEDIOL 14 FIG. 6. FIG. 7. FIG. 6.-Unit cell of the crystalline state of di-isobutylsilanediol. The shaded parallelogram represents the ac face of the unit cell. If additional lattice points are added half-way along the c axis as shown, a distorted hexagonal arrangement is produced. FIG. 7.-Stacking of columns in the pseudo-hexagonal lattice suggested for the crystalline solid. FIG. 8.-Alternative orientations which we suggest each molecule can adopt in the mesophase by virtue of its approximate 4-fold symmetry. axis; presumably this will allow an increase in their rotational motion about this axis.The dimer has approximate 4-fold symmetry and if we add the possibility that each assembly can take up either of two orientations (as shown in fig. 8), this would apparently introduce a satisfactory degree of randomness to account for the partial diffuseness of the 11-12 A ring, If the isobutyl group were to be replaced by a different alkyl residue, say a straight chain, the dimers could no longer have a compact disc-like shape unless the alkyl chain adopted a very non-extended conformation, unusual for a thermotropic mesophase. We can see, therefore, why mesophase formation is not a general property of the alkyl- silanediols and may be restricted to the isobutyl compound.As discussed below, a number of observations of Eaborn and Hartshorne3 are explicable if we assume that the dimers are inclined at an angle to the needle axis in the crystal and if they lie normally (or at an angle more close to 90°) to the stack axis in the mesophase. The crystal + mesophase transition would therefore involve a realignment of the molecules and the breaking of the inter-dimer (but not the intra-dimer) hydrogen bonds. By analogy with other hydrogen bonding for molecules of comparable size (such as alkanols and carboxylic acids), we would have expected an appreciable amount of intra-dimer hydrogen bonding to persist into the isotropic liquid. However, since the mesophase -, isotropic liquid transition enthalpy is only slightly less than that of the crystal mesophase transition, we conclude that the majority of the intra-dimer hydrogen bonds are broken at the mesophase -+ isotropic liquid transition.BUNNING, LYDON, EABORN, JACKSON, GOODBY AND GRAY 72 1 FIG.9.-Form of disclination which may occur in the rod texture of the mesophase. The curved surfaces in this figure have been drawn to indicate the alignment of the dimer molecules. It is not intended to imply that the mesophase is divided into layers. In some instances the disclinations lie on the surfaces of the slide and cover slip (rather than in the bulk of the phase). In these cases, the pattern of the molecular alignments corresponds to the upper or lower half of this figure [cf. fig. 12(B) and the discussion of conoscopic figures near to the disclinations in ref.(3)]. FIG. 10.-Boundary where two disclination rods intersect. Examples of intersections of this type can be seen in plates 4(u) and (b). Since we are dealing with an area of interface rather than a line of intersection, this boundary remains in focus when the microscope is raised and lowered. OPTICAL TEXTURES Eaborn and Hartshorne3 devoted a considerable portion of their 1955 paper to a description of the optical textures of the mesophase of (I). In the absence of any satisfactory model for the molecular ordering in the mesophase, they were, however, at a loss for an explanation of these textures. In the discussion below, we shall attempt to show that the discotic model offers a satisfactory explanation for the textures of the mesophase itself and, coupled with the model for the crystalline solid given above, it appears to offer a convincing explanation of the changes observed on both heating and cooling the crystalline solid/mesophase boundary.As mentioned above, the appearance of the dendritic islands of mesophase indicates an underlying 6-fold symmetry and this is clearly compatible with the mesophase model suggested. The ‘rods’ in the rod texture are disclination lines where the planes of the dimers lie radially, as shown in fig. 9. Wherever these disclinations intersect, a clear line of intersection can be seen [see plate 4(a) and (b)]. This line remains in focus as the microscope is raised and lowered and, as indicated in fig. 10, represents a plane of interface.As described by Eaborn and Hart~horne,~ the light lines of the rod texture often have a notched appearance and the extinction directions near to these are often oblique (rather than strictly parallel and perpendicular to the disclination). We suggest that these two phenomena are related and have their explanation in terms of the pattern of molecular orientation shown in fig. 11, where the planes of the dimeric molecules describe conical surfaces which approach the disclination line obliquely. We suggest that the reason why these textures are so unlike those of conventional smectic phases must lie in the different relative values of the elastic constants of this722 MESOPHASE OF DI-ISOBUTYLSILANEDIOL FIG. I 1 .-Conical pattern of molecular orientation which we suggest explains the notched appearance of some areas of the rod texture and the oblique extinction in regions adjacent to these disclinations. FIG.12.-Two alternative models proposed for the rod texture by Eaborn and Hartshorne3 (redrawn from their paper). A, The mesophase is visualized as being nematic in type. The vertical and tilted lines represent Si-OH chains of the type shown in fig. 1. B, Here the mesophase is smectic. The curved layers shown are composed of Si-OH chains lying side-by-side. There are objections to both models. Eaborn and Hartshorne preferred B, although, as they stated, it is not easy to see how a structure of this type could give large perfectly homeotropic regions. mesophase (which may not be shared by all discotic phases).The phase has the same mechanical characteristics as a sheet of flexible card: it can be easily bent in one direction, but once it is bent it is much more difficult to bend in a direction at right angles. For comparison, the original postulates (A and B) for the rod texture made by Eaborn and Hartshorne3 are in fig. 12. Model B is essentially a smectic structure with layers of Si-OH chains (of the type shown in fig. 1) running parallel to the layers. A major problem of the model, as discussed by Eaborn and Hart~horne,~ is the difficulty in explaining the perfectly homeotropic nature of the bulk of the sample. Eaborn and Hartshorne3 referred to the areas of the rod texture which appear light under crossed polars as biitonnets. We would however prefer to avoid the use of this term completely because of possible confusion.We suggest that the term should be used solely to describe islands of smectic phase separating out of the isotropic liquid. CHANGES OBSERVED AT THE CRYSTAL/MESOPHASE BOUNDARY It appears that the texture changes observed at the solid + mesophase transition are explicable in terms of a realignment of the dimers from a tilted to a more normal arrangement. At the transition therefore, the structure contracts along the stack axis and expands along a perpendicular direction. For a single isolated crystal, this causesBUNNING, LYDON, EABORN, JACKSON, GOODBY A N D GRAY 723 transverse breaks to occur giving the striated band appearance as described by Eaborn and Hartshome. For spherulites the effect is slightly different.The contraction along the stack axis (i.e. along the radii of the spherulite) causes tangential cracks to occur and the structure changes from a radial array of needles to a tangential array of mesophase domains. The tangential expansion of the phase cannot be accommodated by the spherulite and the mesophase domains are forced to tilt out of a strictly tangential alignment giving patterns as shown in plate 6 . Eaborn and Hartshorne have described a further phenomenon at the solid + meso- phase transition. This was observed when a single crystal was laid across a hole in a cover slip and heated. The parts of the crystal which were in contact with the glass adopted a convoluted scalloped structure and an explanation was offered in terms of a specific interaction between the mesophase and the glass surface.We suggest that the differential expansion of the sample at the solid + mesophase transition offers an alternative explanation. The parts of the crystal in contact with the glass surface will be hotter than the region over the hole and the transition will occur there first. Thus the outer regions of the sample are attempting to contract along one direction and expand along another before any dimension changes occur in the central region; this strain results in a scalloped undulation at the edges. EXPERIMENTAL MATERIALS Di-isobutylsilanediol was made as described by E a b o d and the other dialkylsilanediols mentioned were prepared and purified by the general procedures described by Harris.8 The dichlorodiorganosilanes used in the preparations were provided by Petrarch Systems.OPTICAL MICROSCOPY A N D MISCIBILITY STUDIES Microscopic studies were carried out using a Nikon LKe polarising microscope equipped with a Mettler FP52 heating stage and FP5 control unit. DIFFERENTIAL THERMAL ANALYSIS D.t.a. was carried out using a Stanton Redcroft low-temperature differential thermal analyser. The observed transition temperatures agreed with those obtained by optical microscopy. The system was calibrated using indium metal as standard and enthalpy values are believed to be accurate to f 10%. X-RAY D I FFR A c T ION The X-ray diffraction patterns were obtained with a camera specially constructed for liquid crystal studies in the Astbury Department of Biophysics, Leeds University.The sample was contained in a 0.3 mm diameter Lindemann glass tube and Cu Ka radiation was used. The diffraction pattern was recorded on a flat photographic film. CONCLUSIONS We may summarise our findings and hypotheses in the following form: (1) We concur with the experimental observations of Eaborn and Hartshorne3 but we draw radically different conclusions about the nature of the mesophase. (2) We suggest that the mesophase is discotic and that the basic unit is a dimer in which two molecules are held together by perfectly conventional hydrogen bonds. (3) We suggest that in the crystalline solid the dimer units are hydrogen-bonded together in tilted stacks in an approximately hexagonal array and that at the crystal to mesophase transition the inter-dimer hydrogen bonds break and the dimers take up an untilted orientation. (4) We suggest that the elastic constants of this mesophase are radically different from those of smectic or nematic phases and this gives rise to the distinctive optical textures.724 ME SO PHASE OF D I-ISOB U TY L S I LA NED IOL The authors gratefully acknowledge financial support of their research work by the S.R.C. We also thank Dr N. H. Hartshorne for a critical reading of the manuscript. C. Eaborn, J. Chem. SOC., 1952, 2840. Results obtained by J. D. Bernal, C. H. Carlisle and A. de Rahim and reported in ref. (3). C. Eaborn and N. H. Hartshorne, J. Chem. SOC., 1955, 549. * S. Chandrasekhar, B. K. Sadashiva and K. A. Suresh, Pramana, 1977, 9, 471; a comparison with photomicrographs of discotic textures in the article by J. Billard, J. C. Dubois, N. H. Tinh and A. Zann in Nouv. J. Chim., 1978, 2, 535 is also of interest. J. D. Bunning, J. W. Goodby, G. W. Gray and J. E. Lydon, Springer Series in Chemical Physics 11, Liquid Crystals of One- and Two-Dimensional Order, ed. W. Helfrich and G. Heppke (Springer-Verlag, Berlin, 1980), p. 397. N. H. Hartshorne and A. Stuart, Crystals and the Polarising Microscope (Edward Arnold, London, 4th edn, 1970), p. 447 et seq. 'I J. Billard, Springer Series in Chemical Physics 11, Liquid Crystals of One- and Two-Dimensional Order, ed. W. Helfrich and G. Heppke (Springer-Verlag, Berlin, 1980), p. 383. G. I. Harris, J. Chem. SOC., 1963, 5978; J. Chem. SOC. B, 1970, 488 and 492. (PAPER 1/327)