J. Phys. Chem. 1996, 100, 3131-3136
3131
Tetrahedrally Substituted Tribenzosilatranes: Mesomorphic Properties and Structures† Pierre Bassoul,* Jacques Simon, and Corinne Soulie´ ESPCI-CNRS, 10 rue Vauquelin, 75231 Paris Cedex 05, France ReceiVed: July 11, 1995; In Final Form: October 5, 1995X
The tribenzosilatrane moiety has been substituted with four long paraffinic chains. The physicochemical properties (optical microscopy, differential scanning calorimetry) of the condensed phases have been determined as a function of the chain length. Several crystalline or mesomorphic phases have been characterized by X-ray diffraction. In some cases, original layered structures have been found with a hexagonal order within the lamellae. However, none of the mesophases can be attributed to the class of plastic crystals. A globularity criterion has been defined which relates the shape of the molecular unit with the type of arrangement in the mesophases.
Introduction Computer calculations are now routinely used in molecular engineering for predicting the physicochemical properties of real or hypothetical molecular units. However, for obtaining a material, one must also master the organization of the molecular units in the condensed phases. This can be called a “supramolecular engineering” approach. Supramolecular engineering is difficult to use for designing three-dimensional periodic crystals.1 The structure of mesophases is more easily predicted from the characteristics of the molecular units: rodlike molecules often yield smectic or nematic liquid crystals, discoı¨d molecules can afford columnar mesophases, and globular compounds may lead to plastic crystals. Molecules able to form plastic crystals are rather rare.2 Postel and Riess3 defined a criterion related to the probability of obtaining a plastically crystalline phase. A parameter R was taken such as
R ) dm/Dmax where dm is the minimum distance between the centers of the molecules in the condensed phase and Dmax is the maximum diameter of the molecule. The parameter R measures the degree of interpenetration of the molecular units in the condensed phase and is therefore correlated with their rotational freedom. R must be larger than 0.81 in order to get plastically crystalline phase.3 Long-chain tetrahedrally substituted tribenzosilatranes have been synthesized by our group.4,5 The parameter R cannot be used for designing such molecular units since dm must be deduced from the structure of the condensed phase. We then decided to find a more general parameter able to offer a guideline for synthesizing new mesogens. Supramolecular Engineering of Plastic Crystals A globularity criterion has been established which does not necessitate the knowledge of the structure of the condensed phase. The parameter G is defined as the ratio of the van der Waals volume of the molecular unit (Vmol) over the volume of the sphere (Vmax) in which the molecule can be inscribed (Figure 1). It has been experimentally shown that all compounds from cyclohexane to triethylenediamine (Table 1) form plastically †
In memory of Lucien Simon. X Abstract published in AdVance ACS Abstracts, December 15, 1995.
0022-3654/96/20100-3131$12.00/0
Figure 1. Spheres in which can be inscribed adamantane and (Me3Si)4S6. Left: adamantane, G ) 0.64 (plastic crystal). Right: (Me3Si)4S6, G ) 0.46 (crystal).
TABLE 1: Globularity Criterion As Defined by the Parameters R (from Ref 3) or G (See Text) compound cyclohexane cyclohexanol cyclopentane adamantane quinuclidine (CH3)3CCl cyclobutane dl-camphor P4S10 TEDa HMTAb UF6 (CCH2Cl)4S6 P4O10 (CH3Si)4S6 Te(OH)6 a
G) R (Postel-Riess) Vmol (Å3) Dmax (Å) Vmax (Å3) Vmol/Vmax 0.94 0.89 0.88 0.88 0.88 0.85 0.84 0.84 0.82 0.82 0.77 0.75 0.74 0.65 0.65 0.62
100.50 107.44 83.75 144.54 110.31 93.12 74.06 179.14 268.39 115.44 132.37 113.51 287.33 142.68 261.92 124.19
6.57 7.01 6.61 7.55 7.22 7.00 6.22 8.45 9.34 7.10 7.29 7.82 11.77 9.31 10.28 9.19
148.49 180.37 151.22 225.34 197.07 179.59 126.00 315.91 426.62 187.40 202.85 259.39 853.74 422.52 568.82 406.39
0.68 0.60 0.55 0.64 0.56 0.52 0.59 0.57 0.63 0.62 0.65 0.45 0.34 0.34 0.46 0.31
Triethylenediamine. b Hexamethylenetetramine.
crystalline phases (0.82 e R e 0.94). On the contrary, solid crystals are obtained from hexamethylenetetramine (HMTA) to Te(OH)6 (0.62 e R e 0.77). The two different families of compounds can alternatively be distinguished by using the parameter based on the volumes (G in Table 1). In this case, the G factor must be superior to approximately 0.5 in order to get plastic crystals. However, one of the compounds (HMTA) does not follow the rule. In the present case, a tribenzosilatrane substituted with four paraffinic chains has been studied4,5 (Figure 2). The use of Postel and Riess’ criterion is strictly impossible for this type of compound since above a given temperature, the long paraffinic chains are in a quasi-liquid state. © 1996 American Chemical Society
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Figure 2. Representation of one of the tetrasubstituted tribenzosilatrane molecules synthesized. Figure 4. Various substituted tribenzosilatranes synthesized.
Figure 3. Geometrical parameters associated with the tribenzosilatrane subunit.
The volume of the rigid core alone is first calculated from the geometrical parameters of the molecular subunit (Figure 3). The volume of the sphere in which is inscribed the rigid core is 4/3π (4.25)3 ) 321 Å3. The real molecular volume which can be deduced from the van der Waals radii is 258 Å3: the difference (63 Å3) corresponds to empty spaces. Four alkyl chains are linked to the rigid core. Their contribution to the radius of the molecular unit must be estimated in their molten state. From density measurements of various hydrocarbon liquids, it has been possible to estimate the volume of (CH2) and (CH3) groups:6
VCH2 ) 25 Å per subunit; VCH3 ) 54 Å per subunit 3
3
These values are given at room temperature. The corresponding volumes of the molten chains may then be readily calculated. In the example given in Figure 2
C12H25-: 11 × 25 + 54 ) 329 Å3 (×3) C8H17-:
7 × 25+ 54 ) 229 Å3 total ) 1216 Å3
It is now necessary to calculate the thickness of the liquidlike medium due to the molten paraffin chains around the rigid core. This can be made since the surface of the (virtual) sphere on which the paraffinic tails are linked is known:
S ) 4π(4.25)2 ) 227 Å2 Consequently, 227/4 ) 56.7 Å2 (∼57 Å2) is the area available for each paraffinic chain. This is more than the values found in mesophases7 (22-28 Å2). The extent of the paraffinic chains in a quasi-liquid state is therefore 1216/(4 × 57) ) 5.3 Å by considering an average volume for the four chains. The volume of the sphere including the side chains is then 4/3π(4.25 + 5.3)3 ) 3646 Å3 (∼3650 Å3). A small correction to this last value must be made since empty spaces are present in the rigid core
Figure 5. Thermogram corresponding to compound 3 (4.4.4.4): (a) first heating; (b) subsequent cyclings.
which must be filled before the surrounding liquidlike medium can form: 3650 - 63 ) 3587 Å3. The globularity factor G can now be calculated:
G ) (258 + 1216)/3587 ) 0.41 It is seen that the tetrasubstituted tribenzosilatrane is situated approximately at the frontier delimiting crystals and plastic crystals. Preparation of the Compounds The synthesis of substituted tribenzosilatranes was the object of previous publications.4,5 The compounds studied and the abbreviations used have been indicated in Figure 4. The products 1-8 could be isolated by recrystallization from pentane at -30 °C. They are obtained as white microcrystalline powders with the exception of compounds 2 (2*.4.4.4) and 3 (4.4.4.4) which yield large single crystals with platelike and hexagonal shapes, respectively. Physicochemical Properties: DSC and Optical Microscopy Differential scanning calorimetry (DSC) and optical microscopy with crossed polarizers have been used to characterize the mesophases. a. 1-Butyl-((13,17,21-tributyl)-3,4,6,7,10,11-tribenzo)silatrane 3 (4.4.4.4). The thermogram of compound 3 shows two peaks during the first heating but only one in the subsequent cyclings (Figure 5). At room temperature the aspect of the compound seems crystalline and highly colored for samples obtained by cooling the isotropic phase. At 130 °C, the presence of colored domains still indicates birefringent phases, but the whole mass does not seem crystalline (Figure 6). The transition to the isotropic liquid occurs at 130.4 °C. The texture is not characteristic of any of those known for conventional liquid crystalline phases. A large supercooling is
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J. Phys. Chem., Vol. 100, No. 8, 1996 3133
Figure 6. Compound 3 (4.4.4.4): observations in optical microscopy under crossed polarizers at 130 °C.
Figure 7. Optical microscopy of compound 6 (1.12.12.12) at 54 °C (under pressure).
observed (transition at 62.9 °C), indicating a transition toward a highly ordered phase. b. 1-Methyl-((13,17,21-tridodecyl)-3,4,6,7,10,11-tribenzo)silatrane 6 (1.12.12.12). DSC measurements indicate transitions at 57 °C (48 J g-1) and 65 °C (3 J g-1) with the reappearance of birefringency at 40 °C by cooling the sample. At room temperature, the texture is constituted of spherulites with fairly thick grain boundaries. These later start to disappear around 53 °C to form a viscous mass. By applying a pressure,
a new texture appears (Figure 7). The isotropic phase is observed at 56.5 °C. The other derivatives present closely related behaviors, and their properties have been gathered in the next section. c. Series (x.4.4.4); x ) CH3, 1; x ) CHdCH2, 2; x ) C4H9, 3; x ) C8H17, 4. One or two transitions are observed for the compounds belonging to the series (x.4.4.4). The clearing point T2 decreases when the length of the hydrocarbon chain linked to the silicon atom increases. No clear correlation can be found
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TABLE 2: Transition Temperatures for Compounds 1-4a 1 (1.4.4.4) 2 (2*.4.4.4) 3 (4.4.4.4) 4 (8.4.4.4)
T1
T2
∆T
(134)b 91.4 130.4 62
158.4 144 133.4 65.9
70 66.9 62.9
∆H1
∆H2
(45.6) 45.6 28.8 50.7 35.8 36
∆S1
∆S2
(0.106) 0.106 0.079 0.122 0.088 0.106
a T ) transition temperature; T ) clearing point; ∆T ) supercool1 2 ing; ∆H and ∆S ) enthalpy and entropy of fusion, respectively (in J g-1 and J g-1 K-1). b The values corresponding to a second heating cycle are in italic.
TABLE 3: Transition Temperatures for Compounds 6-8 6 (1.12.12.12) 7 (8.12.12.12) 8 (12.12.12.12)
T1
T2
∆T
∆H1
∆H2
57 60 44.6
65 65 45.6
25 65 33.3
47.6 3.2 95.2 76.1
∆S1
∆S2
0.14 0.003 0.28 0.24
TABLE 4: Transition Temperatures for Compounds 4, 5, 7, and 9 9 (8.0.0.0) 4 (8.4.4.4) 5 (8.8.8.8) 7 (8.12.12.12)
T1
T2
71.5 62 20 60
78.1 65.9 22 65.1
∆T
∆H1,2
∆S1,2
20 65
29.53 36 15.2 95.2
0.084 0.106 0.052 0.280
for the lowest temperature transitions related to either crystalcrystal or crystal-ordered mesophase transitions (Table 2). In some cases, the thermogram obtained after a heatingcooling cycle is different from the first one. For example, compound 1 (1.4.4.4) at first yields a single peak at 158.4 °C; the subsequent thermograms show a transition at 134 °C. This behavior is closely related to the one already encountered for compound 3 (4.4.4.4). The peaks at 134 and 158.4 °C may be simultaneously observed with various cooling rates: the relative intensities depend on the conditions. The sum of the enthalpies is always of the order of 45 J g-1. Compound 2 (2*.4.4.4) also presents a first transition with a corresponding small enthalpy. The derivative 4 (8.4.4.4) shows two very close transitions; the first one can be detected by a shoulder on the clearing point transition peak. d. Series (x.12.12.12): x ) CH3, 6; x ) C8H17, 7; x ) C12H25, 8. The derivative 6 (1.12.12.12) differs from its homologues because the first transition corresponds to a higher enthalpy (48 J g-1) than the second one (3 J g-1). However, a large supercooling is observed (Table 3). Compounds 7 (8.12.12.12) and 8 (12.12.12.12) have very close thermal behaviors with the transition and isotropic peaks almost in coincidence. The clearing point temperatures are significantly lower in the dodecyl series as compared to the butyl one. e. Series (8.x.x.x): x ) H, 9; x ) C4H9, 4; x ) C8H17, 5; x ) C12H25, 7. See Table 4. Structural Determinations All the previously mentioned compounds have been studied by X-ray diffraction. In no case a single crystal could be obtained. In the further sections only results leading to an unambiguous unit cell with a perfect agreement between experimental and calculated lines will be given (see experimental section). In all cases, the diagrams may be associated with a hexagonal cell. For the compounds 2 and 8 in the low-temperature phases, a systematic extinction 001, 1 ) 3n, is observed. The c parameter is larger than the a one and increases with the length of the side chains on the phenyl rings (Table 5).
Figure 8. Postulated two-dimensional arrangement of the tribenzosilatrane rigid cores in a layer.
TABLE 5: Hexagonal Unit Cells Found for Various Tetrasubstituted Silatranes 2 (2*.4.4.4) 8 (12.12.12.12) 3 (4.4.4.4) 4 (8.4.4.4) 5 (8.8.8.8)
phasea
a (Å)
c (Å)
extinction laws
LT, 25 °C LT, 25 °C HT, 25 °C HT, 25 °C HT, 12 °C
9.25 9.62 12.72 16.96 16.74
32.1 61.6 11.93 11.05 10.95
00l, l ) 3n 00l, l ) 3n none none none
a LT ) low-temperature phase; HT ) quenched high-temperature phase.
TABLE 6: Compactness and Number of Molecules per Unit Cell Corresponding to the Structures Shown in Table 5 2 (2*.4.4.4) 8 (12.12.12.12) 3 (4.4.4.4) 4 (8.4.4.4) 5 (8.8.8.8)
phase
Z
compactness (%)
LT, 25 °C LT, 25 °C HT, 25 °C HT, 25 °C HT, 12 °C
3 3 2 3 2
59.5 61.0 60.5 62.6 56.8
It is known that most molecular crystals have a compactness in the range 60-75%.8 It is therefore possible to calculate the most probable number of molecules per unit cell. The values found for the tetrasubstituted tribenzosilatranes situate them in the range of the expected compactness (Table 6). a. Compounds 2 (2*.4.4.4) and 8 (12.12.12.12). These two derivatives present a hexagonal lattice with a ∼ 9 Å. From previous studies on triptycene mesogens,9 a layered arrangement of the molecular units can be postulated (Figure 8). From CPK models, the hexagonal lattice parameter can be estimated to be of the order of 9 Å; such a value is actually found by X-ray diffraction. It is then necessary to find a structural model for the stacking of the layers. Since a 31 axis has been evidenced, three different layers must be considered. From the diffraction pattern, no information is available concerning the position of one molecular unit in the second layer relatively to one belonging to the first one. The structure which can be consequently proposed is showed in Figure 9. A relative shift of the layers can preserve the P31 space group. It is worth noting that this condensed phase is polar since, for symmetry reasons, all molecular dipole moments are aligned in the same direction. The molecular dipole moment arising from the Si r N interaction has been estimated to be of the order of 5 D.10 b. Compound 3 (4.4.4.4). In this case, the hexagonal lattice parameter is significantly larger than before: 12.72 Å compared to 9.2-9.6 Å. The distance between the molecules being larger, the paraffinic chains must somehow fill the voids between them. A rough estimation of the diameter of the molecular unit with the paraffinic chains completely folded around the rigid core
Tetrahedrally Substituted Tribenzosilatranes
J. Phys. Chem., Vol. 100, No. 8, 1996 3135
Figure 12. Most probable structure of the high-temperature phase of 4 (8.4.4.4) (space group P31m). Figure 9. Model proposed for the structure of the low-temperature phase of 2 (2*.4.4.4) and 8 (12.12.12.12) (space group P31).
Figure 13. An example of macrochiral center. Figure 10. Proposed structure for the high-temperature phase of 3 (4.4.4.4) (space group P3m1).
TABLE 7: Data for Compound 2 (2*.4.4.4) a ) b ) 9.25 Å R ) β ) 90°
c ) 32.1 Å γ ) 120°
space group P31 dhkl
theor
obsd
dhkl
theor
003 100 101 102 103 104 006 105
10.7 8.01 7.77 7.13 6.41 5.67 5.35 5.00
10.7 8.01 7.76 7.23 6.43 5.63 5.32 5.05
110 111 106 112 113 114 200
4.62 4.57 4.45 4.44 4.24 4.00 3.98
obsd
4.43 4.43 4.21 4.01
TABLE 8: Data for Compound 8 (12.12.12.12) a ) b ) 9.62 Å R ) β ) 90°
c ) 61.6 Å γ ) 120°
space group P31
Figure 11. One of the columns found in the structure of the hightemperature phase of 3 (4.4.4.4).
may be given. A value of 12 Å is found which corresponds to the hexagonal lattice parameter actually found. In (4.4.4.4), there are two molecules per unit cell, may be accommodated within a layer; the second molecule of the unit cell is situated above this plane. X-ray data cannot give any indication concerning the distance from the layer plane. No orientational order between the molecular units is expected. The condensed phase may be either polar or apolar (Figure 10). The structure can also be viewed as formed of columns along the c direction (Figure 11). Since the value of the c parameter cannot correspond to two molecules of tribenzosilatrane, a polar head-to-tail longitudinal arrangement must be postulated within the columns. The relative orientations of these columns are not determined by the symmetry of the unit cell. A random distribution of the orientations is also possible. c. Compound 4 (8.4.4.4). The c parameter observed for the high-temperature phase of this compound is approximately
dhkl
theor
obsd
dhkl
theor
obsd
003 006 100 101 102 103 104 105 009 106 107 108 109
20.53 10.26 8.33 8.25 8.04 7.71 7.33 6.90 6.84 6.47 6.05 5.65 5.29
20.53 10.30 8.34
0012 1010 110 111 112 113 1011 114 115 1012 116 117 200
5.13 4.95 4.81 4.79 4.75 4.68 4.65 4.59 4.48 4.37 4.35 4.22 4.16
5.09 4.99
8.04 7.72 7.37 6.96 6.83 6.41 6.15 5.69 5.29
4.72 4.72 4.62 4.45 4.34 4.34 4.26 4.18
the same as for the derivative 2 (4.4.4.4), indicating a similar packing of the hexagonal layers. However, the unit cell contains three molecules against only two in the previous case. On the other hand, the a parameter is significantly larger although the substituents of the phenyl rings are the same; this is an indication of the involvement of the paraffinic chains in the hexagonal layer. The most probable structure is shown in Figure 12.
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TABLE 9: Data for Compound 3 (4.4.4.4) a ) b ) 12.72 Å R ) β ) 90° dhkl 001 100 101 110 002 111 200 102 201 112 120 202
theor
obsd
11.93 11.01 8.09 6.36 5.96 5.61 5.51 5.24 5.00 4.35 4.16 4.04
11.93 11.04 8.11 6.36 5.98 5.62 5.51 5.24 4.99 4.36 4.15 4.05
TABLE 11: Data for Compound 5 (8.8.8.8)
c ) 11.93 Å
a ) b ) 16.74 Å R ) β ) 90°
γ ) 120° dhkl 003 211 103 300 301 122 113 203 220 302 221 130
theor 3.98 3.93 3.74 3.67 3.51 3.41 3.37 3.22 3.18 3.13 3.07 3.05
c ) 10.95 Å γ ) 120°
obsd
dhkl
theor
obsd
dhkl
theor
obsd
3.98 3.93 3.74 3.67 3.51 3.42 3.37 3.21 3.18 3.13 3.07
100 001 101 110 200 111 201 120 002 102 121
14.50 10.95 8.74 8.37 7.25 6.65 6.04 5.45 5.47 5.12 4.90
14.48 11.00
300 112 301 202 220 310 221 122 131 003 400
4.83 4.58 4.42 4.37 4.18 4.02 3.91 3.87 3.77 3.65 3.62
4.72 4.72
8.30 7.25 6.15 5.47 5.47 5.11 5.00
4.37 4.20 4.02 3.95 3.87 3.63 3.63
Conclusion TABLE 10: Data for Compound 4 (8.4.4.4) a ) b ) 16.86 Å R ) β ) 90°
c ) 11.05 Å γ ) 120°
dhkl
theor
obsd
dhkl
theor
obsd
100 001 101 110 200 111 201 002 120 012 121
14.60 11.05 8.81 8.43 7.30 6.70 6.09 5.52 5.52 5.17 4.94
14.60 10.97
300 112 301 202 220 130 221 212 131 003 302
4.87 4.62 4.45 4.40 4.21 4.05 3.94 3.90 3.80 3.68 3.65
4.88 4.61 4.49 4.44 4.25 4.03 3.96 3.86 3.82 3.68 3.62
7.31 6.73 6.10 5.52 5.52 5.18 4.95
The relative positions of c-axis of the three different molecules in the unit cell cannot be determined from the X-ray pattern. From the previous structural data, it can be postulated that one of the molecules is below and the other above the reference hexagonal plane. In the case the three molecules would be situated within the same layer, the distance between the subunits would be 9.73 Å, a value compatible with the size of the rigid core. d. Compound 5 (8.8.8.8). The cell parameters corresponding to the compounds 4 and 5 are very comparable. However, the last cell contains only two molecules compared to three for the compound 4 (8.4.4.4). A model comparable to the one postulated for the high-temperature phase of 3 (4.4.4.4) is the most likely. Experimental Section Calculations of the van der Waals volumes have been made with the software Moldraw.11 Lengths and angles have been determined from the structure of 1-phenyltribenzosilatrane,12 and data are available from the literature. Volumes have been computed with the program Fastvolout.11 See Tables 7-11.
The tetrahedrally substituted tribenzosilatranes presently studied do not form plastic crystals. The molecular parameter defined to predict whether or not such mesophases could be obtained seems nevertheless relevant since the value found indicated that substituted tribenzosilatranes should be at the border between crystals and plastic crystals. In the mesophases characterized, the rigid cores are more or less organized whereas the paraffinic substituents are disorganized. The type of arrangement of the rigid cores is tightly correlated with their shape and their approximate C3V symmetry. These results confirm that the motto which states that “Les chaıˆnes sont plus ordonne´es que les coeurs” is not correct. From the reactional pathway presently established, it is possible to link four different groups on the tribenzosilatrane rigid moiety; this is a route to the fabrication of “macrochiral centers” in which the chirality is brought by long, voluminous chains of different lengths or of different chemical natures. Such studies are in progress. References and Notes (1) Maddox, J. Nature 1988, 335, 201. (2) Sherwood, J. N. The Plastically Crystalline State; J. Wiley & Sons: New York, 1979. (3) Postel, M.; Riess, J. G. J. Phys. Chem. 1977, 81, 2634. (4) Soulie´, C.; Simon, J. New J. Chem. 1993, 17, 267. (5) Soulie´, C.; Bassoul, P.; Simon, J. J. Chem. Soc., Chem. Commun. 1993, 114. (6) Engel, M. K.; Bassoul, P.; Bosio, L.; Lehmann, H.; Hanack, M.; Simon, J. Liq. Cryst. 1993, 15, 709. (7) Weber, P.; Guillon, D.; Skoulios, A. Liq. Cryst. 1991, 9, 369. (8) Kitaı¨gorodsky, A. I. Molecular Crystals and Molecules; Academic Press: New York, 1973. (9) Norvez, S.; Simon, J. Liq. Cryst. 1993, 14, 1389. (10) Voronkov, M. G.; Mazheika, I. B.; Zelchan, G. I. Chem. Heterocycl. Compd. USSR 1965, 58. (11) Softwares Moldraw and Fastvolout from: Cense, J. M. ENSCP: Paris, 1989. (12) Boer, F. P.; Turley, J. W.; Flynn, J. J. J. Am. Chem. Soc. 1968, 90, 5102.
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