Molecular Design of Thermotropic Liquid Crystalline Polyhydroxy

It is now well established that single-chain amphiphiles of this type usually ... it would be possible to design molecules which are able to form ther...
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Langmuir 2000, 16, 6701-6708

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Molecular Design of Thermotropic Liquid Crystalline Polyhydroxy Amphiphiles Forming Type 1 Columnar and Cubic Mesophases Konstanze Borisch,† Carsten Tschierske,*,† Petra Go¨ring,‡ and Siegmar Diele‡ Institute of Organic Chemistry, University Halle, Kurt-Mothes-Strasse 2, D-06120 Halle, Germany, and Institute of Physical Chemistry, University Halle, Mu¨ hlpforte 1, D-06108 Halle, Germany Received February 23, 2000. In Final Form: May 17, 2000 Novel amphiphilic molecules, derivatives of gallic acid combining three hydrophilic 3,4-dihydroxypropyloxy (1-glyceryl) groups and one alkyl chain via central aromatic linking units (N-alkyl-3,4,5-tris(3,4dihydroxypropyloxy)benzamides and n-alkyl 3,4,5-tris(3,4-dihydroxypropyloxy)benzoates) have been synthesized. Their thermotropic and lyotropic phase behavior was investigated by hot stage polarized light optical microscopy, by differential scanning calorimetry, and in some cases by X-ray scattering. Lamellar (SA), type-1 bicontinuous cubic (CubV1), and type-1 hexagonal columnar (Colh1) mesophases have been found for the solvent-free compounds solely in dependence on the length of their alkyl chains. Hence, with the exception of the micellar cubic phase (CubI1) all main mesophase types found in normal (type-1) lyotropic systems have been successfully realized as their thermotropic analogues in the absence of any solvent. Additionally, binary systems of different amphiphiles have been investigated. In these systems phase sequences were obtained, which resemble those of lyotropic systems. The analysis of the induced phases is proposed to be also a useful tool to assign the phase structure of thermotropic cubic mesophases. On addition of protic solvents Colh1 phases were introduced or stabilized, but no micellar cubic phases were found.

Introduction Many amphiphilic molecules can form not only micelles and lyotropic phases in aqueous systems but also thermotropic mesophases as pure materials.1,2 Especially the amphiphilic polyhydroxy compounds and carbohydrate derivatives can have a wide variety of different thermotropic mesophases.3-10 The formation of large dynamic hydrogen bonding networks between the hydroxy groups and also the microsegregation11 of the incompatible hydrophilic and lipophilic parts of the individual molecules into separate regions are important for their selforganization. It is now well established that single-chain amphiphiles of this type usually organize to lamellar * To whom correspondence may be addressed. Fax: (+49 345) 5527030. E-mail: [email protected]. † Institute of Organic Chemistry. ‡ Institute of Physical Chemistry. (1) Tschierske, C. Prog. Polym. Sci. 1996, 21, 775. (2) Paleos, C. M.; Tsiourvasaleos, D. Angew. Chem. 1995, 107, 1839. (3) Praefcke, K.; Kohne, B.; Eckert A.; Hempel, J. Z. Naturforsch. 1990, 45b, 1084. (4) Lattermann G.; Staufer, G. Liq. Cryst. 1989, 4, 347. (b) Lattermannn, G.; Staufer, G. Mol. Cryst. Liq. Cryst. 1990, 191, 199. (c) Staufer, G.; Schellhorn M.; Lattermann, G. Liq. Cryst. 1995, 18, 519. (d) Schellhorn M.; Lattermann, G. Liq. Cryst. 1994, 17, 529. (e) Schellhorn M.; Lattermann, G. Macromol. Chem. Phys. 1995, 196, 211. (5) Eckert, A.; Kohne B.; Praefcke, K. Z. Naturforsch. 1988, 43b, 878. (b) Praefcke, K.; Kohne, B.; Stephan W.; Marquardt, P. Chimia 1989, 43, 380. (c) Praefcke, K.; Marquardt, P.; Kohne B.; Stephan, W. J. Carbohydr. Chem. 1991, 10, 539. (6) Borisch, K.; Diele, S.; Go¨ring, P.; Tschierske, C. J. Chem. Soc., Chem. Commun. 1996, 237. (b) Borisch, K.; Diele, S.; Go¨ring, P.; Mu¨ller H.; Tschierske, C. Liq. Cryst. 1997, 22, 427. (c) Borisch, K.; Diele, S.; Go¨ring, P.; Kresse, H.; Tschierske, C. Angew. Chem., Int. Ed. Engl. 1997, 36, 2087. (d) Borisch, K.; Diele, S.; Go¨ring, P.; Kresse, H.; Tschierske, C. J. Mater. Chem. 1998, 8, 529. (7) Veber, M.; Cheylan, E.; Czernecki, S.; Xie, J. Liq. Cryst. 1996, 21, 197. (b) Beginn, U.; Keinath, S.; Mo¨ller, M. Liq. Cryst. 1997, 23, 35. (8) Reviews: (a) Goodby, J. W. Mol. Cryst. Liq. Cryst. 1984, 110, 205. (b) Jeffrey, G. A. Acc. Chem. Res. 1986, 12, 179. (c) Jeffrey G. A.; Wingert, L. M. Liq. Cryst. 1992, 12, 179. (d) Prade, H.; Miethchen R.; Vill, V. J. Prakt. Chem. 1995, 337, 427. (e) Goodby, J. W. Liq. Cryst. 1998, 24, 25.

phases (SA). Double-chain compounds mostly form columnar mesophases (Colh2), but in some cases also bicontinuous cubic mesophases (CubV2, e.g., compound 16d in Figure 1), and smectic phases have been found. Amphiphiles with three aliphatic chains, such as compound 26a (Figure 1), can organize to columnar phases, and elongation of the chains could lead to micellar cubic mesophases built up by discrete inverted micelles (CubI2).6,12 Thus, the phase sequence found in dependence on the molecular structure resembles that one of lyotropic systems. As in all the above-mentioned thermomesophases with curved polar/apolar interfaces the more polar molecular parts and also the stronger cohesive forces (hydrogen bonding) are located inside the aggregates surrounded by the continuum of the molten lipophilic alkyl chains, the curvature of their polar-apolar interfaces is negative, and the columnar and cubic thermomesophases of these amphiphiles can be regarded as reversed (type 2) mesophases. This organization of amphiphilic molecules is now a well-established concept for the design of columnar (9) Selected more recent studies: (a) Goodby, J. W.; Watson, M. J.; Mackenzie, G.; Kelley, S. M.; Bachir, S.; Bault, P.; Gode, P.; Goethals, G.; Martin, P.; Ronco, G.; Villa, P. Liq. Cryst. 1998, 25, 1390. (b) Gode, P.; Goethals, G.; Goodby, J. W.; Haley, J. A.; Kelley, S. M.; Mehl, G. H.; Ronco, G.; Villa, P. Liq. Cryst. 1998, 25, 31. (c) Smits, E.; Engberts, J. B. F. N.; Kellogg, R. M.; Van Doren, H. A. Liq. Cryst. 1997, 23, 481. (d) Miethchen, R.; Faltin, F. J. Prakt. Chem. 1998, 340, 544. (e) Dahlhoff, W. V.; Radkowski, K.; Zugenmaier, P.; Dierking, I. Z. Naturforsch 1995, 50b, 405. (f) van Doren, H. A.; Terpstra, K. R. J. Mater. Chem. 1995, 5, 2153. (10) Selected reports on lyotropic properties of carbohydrate mesogens: (a) van Doren, H. A.; Wingert, L. M. Recl. Trav. Chim. PaysBas 1994, 113, 260. (b) Sakya, P.; Seddon, J. M.; Templer, R. H. J. Phys. II 1994, 4, 1311. (c) Sakya, P.; Seddon, J. M.; Vill, V. Liq. Cryst. 1997, 23, 409. (d) Bonicelli, M. G.; Ceccaroni, G. F.; La Mesa, C. Colloid Polym. Sci, 1998, 276, 109. (e) Ha¨tzschel, D.; Schulte, J.; Enders, S.; Quitzsch, K. Phys. Chem. Chem. Phys. 1999, 1, 895. (11) Tschierske, C. J. Mater. Chem. 1998, 8, 1485. (12) Columnar and cubic mesophases of dendritic molecules: Balagurusamy, V. S. K.; Ungar, G.; Percec, V.; Johansson, G. J. Am. Chem. Soc. 1997, 119, 1539.

10.1021/la000259v CCC: $19.00 © 2000 American Chemical Society Published on Web 07/11/2000

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Figure 1. Molecular structures and transition temperatures of two representative polyhydroxy amphiphiles forming inverted cubic mesophases and proposed models of their cubic phases.6 (a) Inverted bicontinuous cubic phase (CubV2) with Ia3d lattice; the microsegregated polar regions are located inside the branched cylinders. (b) Inverted micellar cubic phase (CubI2) with Pm3n lattice; the positions of the closed spheroidic micelles in the Pm3n lattice of the CubI2 phase are shown as dots; the size of the dots is not related to the micelle size, additionally, these micelles could have a nonspherical shape; the microsegregated polar regions are located inside the aggregates. Footnote a: The SA phase can only be observed on cooling from the isotropic liquid state, on heating a direct transition from CubV2 to the isotropic liquid state is found.

and cubic supermolecular thermotropic liquid crystals.6 However, in lyotropic systems the situation is quite different. Here, normal type (type 1) lyotropic phases are very common, in which the lipophilic parts are located inside the aggregates, surrounded by a polar continuum formed by the polar headgroups and the solvent molecules.13 The question arises, if it would be possible to design molecules which are able to form thermotropic type 1 columnar and cubic mesophases in the absence of any solvent. Indeed, some occasional reports on columnar and cubic mesophases of molecules with large polar groups have recently occurred, but no systematic studies have been carried out.14-16 Therefore, we decided to design novel amphiphilic polyhydroxy compounds with large polar groups which should organize to thermotropic mesophases representing analogues of normal lyotropic systems and to study their fundamental structure-property relationships. Results and Discussion Synthesis. Compounds 3 and 4 have been obtained from ethyl 3,4,5-trihydroxybenzoate (ethyl gallate) as shown in Scheme 1. Etherification with excess allyl bromide gave 3,4,5-triallyloxybenzoic acid17 which was transformed into the corresponding benzoyl chloride by treatment with SOCl2. The crude acid chloride was aminolyzed or alcoholyzed with appropriate n-alkylamines or n-alkanoles in the presence of DMAP to give the benzamides 6 and the benzoates 7, respectively. After (13) Seddon J. M.; Templer R. H. In Handbook of Biological Physics; Lipowsky, R., Sackmann, E., Eds.; Elsevier: Amsterdam, 1995; Vol. 1, p 97. (14) Vill, V.; Bo¨cker, T.; Thiem, J.; Fischer, F. Liq. Cryst. 1989, 6, 349. (15) Vill, V. Habilitationsschrift, Hamburg, 1997, p 35. (16) Borisch, K.; Tschierske, C.; Go¨ring, P.; Diele, S. J. Chem. Soc., Chem. Commun. 1998, 2711. (17) Kasztreiner, E.; Borsy, J.; Vargha, L. Biochem. Pharmacol. 1962, 11, 651.

purification, these 3,4,5-triallyloxybenzoyl compounds were completely dihydroxylated using catalytic quantities of OsO4 and N-methylmorpholine-N-oxide as reoxidant.18 The purification of the final compounds was done by means of preparative thin-layer chromatography followed by repeated crystallization. Purity and the structure of the final product were confirmed by thin-layer chromatography, elemental analysis, mass spectrometry, 1H NMR spectroscopy, and 13C NMR spectroscopy. Compounds 3 and 4 incorporate three stereogenic centers which have been prepared in their racemic forms. Therefore, all synthesized materials represent racemic multicomponent mixtures of diastereomers. No attempts to obtain pure stereoisomers have been done, because, according to our experiences, the mesomorphic properties are only marginally influenced, whereas the melting points and the crystallization tendency of the pure stereoisomers can be rather high. Because this may inhibit the observation of the liquid crystalline phases, the diversity of the materials is in this case a desired feature. Thermotropic Properties of the Amides 3. The mesomorphic properties of the amides 3 are summarized (18) Van Rheenen, V.; Cha, D. Y.; Hartley, W. M. Org. Synth. 1979, 58, 43.

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Scheme 1. Synthesis of the Compounds 3-5

Figure 2. Arrangement of the molecules in the intercalated SA phase of 3/14. Reagents and Conditions: (i) CH2dCH-CH2Br, K2CO3, CH3CN, reflux; (ii) NaOH, H2O, EtOH, reflux, then HCl/H2O; (iii) SOCl2, reflux, (iv) RXH, DMAP (X ) NH, DMF, X ) O, toluene), reflux; (v) Catalyst OsO4, NMMNO, acetone, H2O, 20 °C. Table 1. Phase Transition Temperatures T/°C and Corresponding Enthalpy Values ∆H/kJ mol-1 (lower lines in italics) of the N-Alkyl-3,4,5-tris(3,4-dihydroxypropyl-1-oxy)benzamides 3/na

compound

n

3/16

16

3/14

14

3/12

12

3/10

10

3/8

8

phase transitions cr1 93 4.9 cr 140 30.5 cr1 93 3.4 cr 141 29.2 cr 142 29.7

cr2 133 30.7 SA 212 is 1.1 cr2 135 32.0 (CubV1 119 0.5 (Colh1 106) is 0.6

SA 248 is 7.2 CubV1 182 0.02 Colh1 135) is 0.3

SA 184 is 0.9

a Abbreviations: cr ) crystalline solid; S ) smectic A phase; A Colh1 ) normal hexagonal columnar phase; CubV1 ) normal bicontinuous cubic phase; is ) isotropic liquid state.

in Table 1. All synthesized compounds 3 show liquid crystalline properties, whereby the stability of the liquid crystalline phases strongly decreases on reducing the chain length. Hence, the mesophases of the long-chain compounds are enantiotropic, whereas those of the short-chain derivatives are only monotropic (metastable). Much more interestingly, three completely different mesophase typess SA, columnar, and cubicscan be found exclusively depending on the chain length. Only lamellar phases (SA) were found for compounds 3/14 and 3/16 with long alkyl chains. By observation between crossed polarizers they appear with fan-shaped textures which rapidly align hometropically and show typical oily streak textures on shearing. The diffraction pattern of 3/14 is characterized by a diffuse scattering in the wide angle region and one reflection in the small angle

region corresponding to a layer thickness of d ) 3.75 nm at 165 °C. The length of the molecules L, assuming a most extended shape with all-trans conformation of the alkyl chains (CPK models), amounts to only L ) 3.1 nm. Thus, the layer thickness is larger than the molecular length (d/L ) 1.2), but significantly smaller than twice the length, indicating a bilayer structure with an unusual large degree of intercalation. Assuming a complete intercalation of the chains including the amide groups (see Figure 2) would give a layer thickness of 4.0 nm. This value is still larger than the experimentally observed d values. We must therefore additionally assume a strong disorder of the alkyl chains and a partial overlap of the polar groups as required by the hydrogen-bonding interactions. The phase behavior of the next shorter homologue 3/12 is even more interesting. On cooling its SA phase the formation of nearly circular, optically completely isotropic domains can be observed at 182 °C, which rapidly coalesce on further cooling. This isotropic phase is not fluid, but it can be plastically deformed, which points to a cubic 3D structure. X-ray investigations indicate a diffuse scattering in the wide-angle region which is a characteristic feature for a liquidlike disorder of the alkyl chains. Additionally, two independent sharp reflexes were found in the smallangle region at θ ) 1.16° and θ ) 1.37°. In comparison with the values of the cubic phase of compound 4/12 (Table 4) and together with the other observations they support the existence of a cubic mesophase. No smectic phase can be detected for the next shorter homologue 3/10. Cooling this compound from the isotropic liquid state gives a spherulitic texture as typical for columnar phases. On further cooling a highly viscous and optically isotropic mesophase is formed again. It is reasonable to assume that also this phase is a cubic mesophase. Compound 3/8 with the shortest chain has exclusively a columnar phase. Because the mesophases of 3/8 and 3/10 are monotropic, more detailed investigations were not possible. Thermotropic Properties of the Esters 4. To get materials with lower melting points, we have synthesized the compounds (4) in which the amide groups were replaced by ester groups (see Table 2). Indeed, all transition temperatures are reduced in comparison to the amides 3, but the mesophase type obviously does not depend on the structure of this linking unit. Hence, for the amides and the esters with the same chain length the same mesophases were observed. The only exception is compound 4/10 for which a direct transition between the

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Table 2. Phase Transition Temperatures T/°C and Corresponding Enthalpy Values ∆H/kJ mol-1 (lower lines in italics) of the Alkyl 3,4,5-Tris(3,4-dihydroxypropyl-1-oxy)benzoates 4/n

compound

n

4/14

14

4/12

12

4/10

10

4/9

9

4/8

8

4/6

6

a

phase transitions cr 95 56.0 cr 89 37.8 cr 75 30.7 cr1 64 22.6 cr 63 24.7 cr 74 is 26.6

SA 172 is 0.9 CubV1 133 0.7 CubV1 108 is 0.6 cr2 78 0.4 Colh1 66 is 0.4

SA 152 is 0.1 (CubV1 58)a is 0.3

Figure 3. Small-angle X-ray diffraction pattern of an aligned sample of the SA phase of 4/12 at 143 °C.

On cooling a Colh1 phase is observed (see text). Table 3. Bragg Reflections of the SA Phase of 4/12 at 135 °C θ/deg

d/nm

hkl

1.253 2.504 3.760

3.52 1.76 1.17

001 002 003

Table 4. Bragg Reflections of the Cubic Mesophase of 4/12 at 100 °C θexp/deg 1.19 1.37 1.81 1.95

θcalc/deg

hkl

1.37 1.81 1.94

211 220 321 400

cubic mesophase and the isotropic liquid state is found instead of the Cub-SA-dimorphism of the related compound 3/10. Compound 4/12 which shows an enantiotropic SA-Cub dimorphism was investigated in more detail. The SA phase of 4/12 is characterized by three sharp reflections in the small-angle region with a ratio 1:2:3, confirming the layer structure. The layer thickness (d) is larger than the molecular length (L) and indicates again a deeply intercalated bilayer structure (see Table 3, d ) 3.52 nm at 135 °C, L ) 2.8 nm, d/L ) 1.25). In addition to the sharp 00l reflections a diffuse scattering is found in the small-angle region. By investigation of a well-oriented monodomain (see Figure 3) a scattering is observed which is positioned out of the meridian with qz ∼ 0.5q001 (z is the component parallel to the meridian), indicating a 2D short-range order with a periodicity of 2d in the direction of the layer normal and a periodicity perpendicular to it. This could be due to a partial collapse of the smectic layers giving rise to ribbons which are arranged in a two-dimensional lattice with short-range order. This points to the occurrence of a certain degree of frustration in this SA phase which could be the first step toward its complete collapse which takes place on further cooling and leads to the formation of a cubic phase. The diffraction pattern of this cubic mesophase is characterized by a diffuse scattering in the wide-angle region and four reflections in the small-angle region, which allows us to index this phase to an Ia3d cubic space group

(see Table 4).19 The lattice parameter can be calculated to acub ) 9.1 nm at T ) 100 °C. Because this cubic mesophase occurs in the homologous series 4 at the transition from a smectic layer structure (4/14 and 4/12) to columnar mesomorphism (4/8), it should represent a bicontinuous cubic phase (CubV), related to the bicontinuous cubic phases occurring between lamellar and hexagonal phases of lyotropic systems. Also in these lyotropic systems Ia3d phases have often been found.13 Their structure can be described by Gyroid IPMS (infinite periodic minimal surfaces) which separate the two interwoven bilayers. These IPMS give rise to a labyrinth of two distinct systems of cylinders, branched three-by-three as shown in Figure 1a. As the polar regions of these molecules are significantly larger than the lipophilic ones, the lipophilic chains should fill the space inside these cylinders whereas the hydrophilic groups are surrounding them; i.e., this mesophase should be a bicontinuous cubic phase of the normal type (CubV1). Compound 4/10 forms exclusively a cubic phase, whereas the next shorter homologue 4/9 shows a cubic/columnar dimorphism: On cooling, the columnar phase occurs first (at 58 °C), but it is rapidly replaced by the cubic phase. In the heating scan only a direct transition from the cubic phase to the isotropic liquid state occurs at 58 °C. This means that the columnar phase is metastable and can only be observed by supercooling the cubic 3D structure. It is remarkable that in the series of compounds 3 and 4 the cubic phases can sometimes occur as a single mesophase (4/10), but more often accompanied by a smectic or a columnar phase. Compound 4/8 has exclusively a columnar phase which shows the same characteristic spherulitic texture (see Figure 4) as those of the related amides 3/8 and 3/10. The X-ray diffraction pattern of this phase is characterized by three sharp reflexes in the small-angle region and a diffuse scattering in the wide-angle region. The ratio of the (19) It seems that the cubic phase of 3/12 exhibits essentially the same diffraction pattern, but only the two most intensive reflections in the small angle region (211 and 220) can be observed. On the basis of the similarity in relative position and intensity of these two reflexes with those of the 211 and 220 reflections in the diffraction pattern of the cubic phase 3/12, we can deduce on a Ia3d space group also for this cubic phase.

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Figure 4. Polarized optical photomicrograph of the Colh1 phase of compound 4/8 at 60 °C.

positions of the small angle reflections is 1:x3:2 proofing a hexagonal 2D lattice with a hexagonal lattice parameter of ahex ) 3.6 nm at T ) 60 °C. The number of molecules which should be arranged in average in the cross section of a 0.45 nm thick slice of the columns amounts about n ) 5 molecules as calculated by the equation n ) (ahex2/2)‚ x3h‚(NA/M)‚F, with F ) 1 g/cm3, NA ) Avogadro constant, M ) molecular mass, and h ) 0.45 nm. Because this columnar phase occurs in a phase sequence SA-CubV1Col on reduction of the chain length, this phase should also be a normal-type phase with the alkyl chains assembled in the centers of the columns (Colh1). Discussion of the Thermotropic Phase Sequences. Three different mesophases SA, CubV1, and Colh1 have been realized by the single-chain amphiphiles 3 and 4 solely by changing their chain length. Remarkably, the long-chain compounds of both series show exclusively smectic layer structures. This is surprising, because the cross-sectional areas of the polar region of these molecules (ca. 0.55-0.60 nm2 as determined using CPK models) are significantly larger than the cross-sectional area of the alkyl chains (0.21 nm2 for fluid chains20). Simple geometric packing considerations suggest that all synthesized compounds should be able to form cylindrical aggregates.20 The missing of columnar mesomorphism in the case of longchain compounds can be explained taking into account the results of the X-ray scattering investigations of the SA phases. They indicate that an unusual strong intercalation of the alkyl chains occurs. This intercalation doubles the effective cross-sectional area of the lipophilic region whereas that one of the polar groups remains nearly the same. Thus, the difference in the space filling of polar and lipophilic molecular parts is partially compensated by intercalation. It seems that these systems try to escape the formation of curved aggregates as far as there are other possibilities for efficient space filling. Surprisingly, the reduction of the alkyl chain length by only four CH2 units from C12 to C8 leads to the transition from the smectic phase via the CubV1 phase to the Colh1 phase. This dramatic

influence of the chain length is rather surprising. To understand it, one should recall the strongly intercalated structure of the SA phases. The comparison of the crosssectional areas (see above) indicate, that intercalation alone cannot efficiently fill the space between the alkyl chains. Free space remains between them, which additionally requires a significant conformational disorder of these chains.21 This strong conformational disorder is only possible with long chains. For short alkyl chains and/ or at reduced temperatures a more dense packing of these rigid chains is required, which reduces the effective diameter of the lipophilic regions. The short-range 2D order in the SA phase of 4/12 could be due to ribbonlike aggregates (Figure 5b) which may be the first step of the complete collapse of the layer structure. It ends up with the nearly circular cylinder structure of the aggregates in the hexagonal columnar phase of 4/8 (see Figure 5c). So, a main conclusion is that the polar molecular parts must have a diameter which is at least twice as large as the one of the lipophilic parts and, even more importantly, the aliphatic chains have to be short to obtain thermotropic type 1 mesophases. Binary Systems of Different Amphiphiles. As mentioned above, the bicontinuous cubic phases of compounds 3 and 4 can occur beside or below smectic or columnar phases. The same phase sequences have been found for the bicontinuous cubic phases of the inverted type.6d Therefore, the microstructure of cubic mesophases (CubV1 vs CubV2) cannot be concluded from their position in the thermotropic phase sequence. To additionally confirm the proposed type 1 structure of the investigated cubic and columnar phases, binary systems consisting of different amphiphiles have been studied. In the contact regions between the cubic phases of the homologous compounds 3 and 4 and also in the contact regions between their columnar phases, an uninterrupted miscibility was found and no other mesophase was induced. However in the contact regions between the cubic phases of compounds 3 or 4 and the reversed bicontinuous cubic phase of com-

(20) Isrelachvili, J. Intermolecular and surface forces, 2nd ed.; Academic Press: London, 1992; p 370.

(21) Indeed, the unusual small layer thickness, found in the double layer SA phase of 3/14 is in line with the required conformational disorder.

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Figure 6. Phase diagram of the binary system 1 + 4/10. The transition temperatures observed in the heating scans (polarizing microscopy) were recorded; crystalline phases are not shown.

Figure 5. Possible arrangements of five molecules of 4/8 (a) intercalated bilayer, (b) intercalated bilayer with closely packed alkyl chains, here the polar groups interrupt the layer structure with formation of ribbons, (c) cross section through a cylindrical aggregate built up by partially intercalated alkyl chains which are surrounded by a close shell of the polar groups as proposed for the Colh1 phase of 4/8.

pound 1 (see Figure 1a) extremely broad regions of SA phases were induced, reaching from X1 ) 0.1 to X1 ) 0.98 for the system 1 + 4/10 (see Figure 6). Remarkably, the stability of this induced phase is significantly higher than the mesophase stability of the cubic phases of both pure compounds. The occurrence of an SA phase with zero interface curvature indicates that the polar-apolar interface curvature should change its sign in the contact region. This means that both cubic phases must have opposite signs of their polar-apolar interface curvatures. Because an inverted bicontinuous structure was proven for compound 1,6b,d the type I structure (CubV1) of the cubic phases occurring in the homologous series 3 and 4 is confirmed. These experiments show that the combination of X-ray scattering and investigation of binary systems allows the

precise determination of the microstructure of thermotropic cubic mesophases. X-ray scattering alone can only provide the type of cubic 3D latice and the lattice parameter, whereas the shape of the aggregates (bicontinuous vs micellar) and also the sign of polar-apolar interface curvature can be provided by investigations of appropriate binary systems. Thus, both methods are fully complementary to one another. Interestingly, such investigations can be further extended and can include mesophases with very different structures. As an example, the phase diagram of the binary system consisting of compound 4/10 and the triple chain amphiphile 2 is shown in Figure 7. In the contact regions between the type 1 bicontinuous cubic mesophase (CubV1) of 4/10 and the reversed (type 2) micellar cubic phase (CubI2) of compound 2 the CubV1 phase of 4/10 is at first destabilized and completely replaced by an SA phase which is found in a concentration range between X2 ) 0.03 and X2 ) 0.55. With increasing concentration of compound 2 a reversed columnar mesophase (Colh2) is induced (X2 ) 0.55 and X2 ) 0.85), which on further increase of X2 is destabilized again and finally replaced by the CubI2 phase of 2. This phase sequence is in accordance with the phase sequence of lyotropic systems on changing the solvent concentration. Only the CubV2 phase which could be expected between the SA phase and the Colh2 phase was not detected. In the contact region between the columnar phase of 4/8 and the CubI2 phase of 2 even five different mesophases can be observed: Colh1-CubV1-SA-Colh2-CubI2.22 This phase sequence represents a major part of the hypothetical lyotropic phase sequence23 of detergent solvent systems. (22) This phase sequence was observed by polarized-light microscopy of the contact region. Due to the high crystallization tendency in the contact region, it was not possible to construct a binary phase diagram. (23) The complete phase sequence has only recently been obtained in a ternary system consisting of an amphiphilic block copolymer, a lipophilic, and a polar solvent: Alexandridis, P.; Olsson, U.; Lindmann, B. Langmuir 1998, 14, 2627.

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follows the general rules developed for lyotropic systems. Therefore, in accordance with the well-known phase sequence of lyotropic systems (1) novel mesophases can be induced in solvent-free binary systems of amphiphiles having structurally different mesophases.

CubI1-Colh1-CubV1-SA-CubV2-Colh2-CubI2

(1)

Major parts of this phase sequence have been realized with such binary systems. Furthermore, the analysis of the induced phases is an important tool to assign the phase structure of thermotropic cubic mesophases. Finally, these new amphiphiles represent novel amphotropic materials, i.e., compounds which can form thermotropic as well as lyotropic liquid crystalline phases. This additionally shows that thermotropic and lyotropic mesomorphism are strongly related in these polyhydroxy amphiphiles. Experimental Section

Figure 7. Phase diagram of the binary system 2 + 4/10. The transition temperatures observed in the heating scans (polarizing microscopy) were recorded; crystalline phases are not shown. Table 5. Clearing Temperatures T/°C of the Columnar Mesophases (Colh1) of the Solvent-Saturated Samples of Selected Compounds 3 and 4 compound

solvent

T/°C

3/12 3/14 4/8 4/10 4/12 4/16

formamide formamide water ethylene glycol formamide formamide

178 186 109 97 135 132

Lyotropic Properties of Compounds 3 and 4. Compounds 3 and 4 were additionally investigated with respect to their lyotropic properties with protic solvents such as water, formamide, and glycerol. Though these investigations were only preliminary, they show that the thermotropic mesophases of all compounds 3 and 4 can be influenced by these protic solvents. The results are summarized in Table 5. The solvent-saturated samples of all investigated compounds show exclusively columnar mesophases. In the case of compound 4/8 the columnar phase is continuously stabilized on addition of water. For compounds with smectic and/or bicontinuous cubic phases the columnar phases are induced by the solvents. Because the solvent molecules are built between the headgroups, the curvature of the polar-apolar interfaces increases and the transition to more strongly curved aggregates can be found. It was however not possible to induce a type 1 micellar cubic mesophase (CubI1). Summary Three different mesophases SA, CubV1, and Colh1 have been realized by the single-chain amphiphiles 3 and 4 solely by changing their chain length. Hence, with exception of normal cubic mesophases consisting of closed spheroidic micelles (CubI1) all main mesophase types found in lyotropic systems have now been successfully realized as thermotropic analogues.24 This shows that indeed the thermotropic polymorphism of polyhydroxy amphiphiles (24) Polyhydroxyamphiphiles forming the inverted thermotropic phases CubV2, Colh2, and CubI2 have recently been described; see ref 6.

Techniques. Transition temperatures (given in °C throughout) were measured using a Mettler FP 82 HT hot stage and control unit in conjunction with a Nikon Optiphot 2 polarizing microscope, and they were confirmed using differential scanning calorimetry (Perkin-Elmer DSC-7, heating and cooling rate 10 K min-1). The accuracy of the transition is about (0.5 K. If not otherwise stated the transition enthalpies were obtained from the first heating scan. Materials. Ethyl 3,4,5-trihydroxybenzoate (Fluka), 3-aminopropane-1,2-diol (Aldrich), 1-alkanoles (Merck), 1-alkylamines (Aldrich), 1-tetradecyl bromide (Merck), allyl bromide (Merck), N-methylmorpholine N-oxide (60% solution in water, Aldrich), and osmium tetroxide (0.01 M solution in tert-butanol) were used as obtained. Confirmation of the structures of the products was obtained by 1H and 13C NMR spectroscopy (Varian Gemini 200, Varian Unity 400, and Varian Unity 500) and mass spectrometry (Intectra GmbH, AMD 402, electron impact, 70 eV). Microanalyses were performed using a CHNF932 (LECO Co.) elemental analyzer. The purity of all compounds was additionally checked by thin-layer chromatography (Merck, silica gel 60F 254). The general synthetic procedures are described below, together with the analytical data of the compounds 4/12, 5/12, 6/12, and 7/12 as representative examples. The analytical data of all other compounds are given in the Supporting Information. 3,4,5-Triallyloxybenzoic Acid (5). Ethyl 3,4,5-trihydroxybenzoate (5.9 g, 003 mol), allyl bromide (8.6 mL, 0.1 mol), potassium carbonate (37.2 g, 0.27 mol), and KI (1.0 g) were added to dry acetonitrile (200 mL). The mixture was refluxed with stirring for 5 h. The potassium carbonate was filtered off and washed twice with CH2Cl2 (100 mL), and the solvent was evaporated. The residue was dissolved in 100 mL of ethanol. A solution of KOH (2.6 g, 0.046 mol) in 40 mL of water was added, and the mixture was refluxed for 1 h. After cooling, the mixture was diluted with water (250 mL), acidified with concentrated hydrochloric acid (6 mL), and extracted three times with diethyl ether (200 mL). The solution was washed with water (50 mL) and brine (50 mL) and dried over Na2SO4. The solvent was distilled off and the residue was purified by preparative thin-layer chromatography (Chromatotron, Harrison Research). Yield 3.9 g (45%). Mp 74 °C (ref 17: 78-79 °C). δH (200 MHz; CDCl3; 27 °C; TMS; J/Hz): 4.50-4.58 (6H, m, OCH2), 5.10-5.40 (6H, m, CH2d), 5.91-6.14 (3H, m, CHd), 7.28 (2H, s, H-ar). δC (100 MHz; CDCl3; 27 °C; TMS): 171.5 (CO), 152.4, 142.8, 132.9, 109.4 (C-ar), 134.2

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Langmuir, Vol. 16, No. 16, 2000

(CH2CH), 118.0, 117.8 (CH2CH), 69.9 (OCH2). MS (70 eV, EI): m/z 290 ([M]+ 89.3%). N-Alkyl-3,4,5-triallyloxybenzamides (6). 5 (1.0 g, 3.4 mmol) and thionyl chloride (10 mL) were refluxed for 3 h. The excess thionyl chloride was completely distilled off under reduced pressure, and the residue was dissolved in dry methylene chloride (15 mL). The obtained solution was added to a solution of the appropriate n-alkylamine (16.2 mmol) and DMAP (10 mg) in dry DMF (20 mL) under inert conditions with stirring at 80 °C. The resulting mixture was heated at this temperature for 4 h and was stirred for additional 24 h at room temperature. Afterward, water (100 mL) and diethyl ether (150 mL) were added. The organic phase was separated, and the aqueous solution was extracted twice with diethyl ether (50 mL). The combined organic phases were washed with 5% hydrochloric acid (50 mL), water (50 mL), saturated NaHCO3 solution (50 mL), and brine (50 mL). The organic layer was dried with Na2SO4, and the solvent was evaporated in vacuo. The residue was purified twice by preparative thin-layer chromatography (Chromatotron, petroleum ether eluent). N-Dodecyl-3,4,5-triallyloxybenzamide (6/12). Synthesized from 5 and 1-dodecylamine (3.0 g). Yield: 0.87 g (59%). Mp 55°C. δH (400 MHz; CDCl3; 27 °C; TMS; J/Hz): 0.86 (3H, t, J 7, CH3), 1.24-1.32 (18H, m, CH2), 1.551.62 (2H, m, CH2), 3.37-3.45 (2H, m, NCH2), 4.55-4.61 (6H, m, OCH2), 5.15-5.43 (6H, m, CH2d), 6.00-6.12 (3H, m, CHd), 6.95 (2H, s, H-ar). δC (100 MHz; CDCl3; 27 °C; TMS): 167.3 (CO), 152.7, 140.9, 130.3, 106.7 (C-ar), 134.4, 133.1 (CH2CH), 117.9, 117.7 (CH2CH), 70.2 (OCH2), 40.1 (NCH2), 31.8, 29.5, 29.4, 29.2, 26.9, 22.6 (CH2), 14.0 (CH3); MS (70 eV, EI): m/z: 457 ([M]+ 49.3%). n-Alkyl Triallyloxybenzoates (7). 3,4,5-Triallyloxybenzoyl chloride was prepared as described above from 6 (1.0 g, 3.4 mmol) and thionyl chloride (10 mL) and dissolved in toluene (10 mL). This solution was added to a solution of the appropriate primary alcohol (16.2 mmol) and DMAP (10 mg) in dry toluene (25 mL) under inert conditions with stirring. The resulting mixture was refluxed for 4 h. Afterward, water (100 mL) and diethyl ether (100 mL) were added. The organic phase was separated, and the aqueous solution was extracted twice with diethyl ether (50 mL). The combined organic phases were washed water (50 mL), saturated NaHCO3 solution (50 mL), and brine (50 mL). The organic layer was dried with Na2SO4, and the solvent was distilled off. Then, the solvent was evaporated in vacuo and the residue was purified twice by preparative thin-layer chromatography (Chromatotron, petroleum ether eluent). Dodecyl 3,4,5-Triallyloxybenzoate (7/12). Synthesized from 5 and 1-dodecanol (3.0 g). Yield: 1.3 g (94%). Mp 40 °C. δH (200 MHz; CDCl3; 27 °C; TMS; J/Hz): 0.86 (3H, t, J 6, CH3), 1.27-1.37 (18H, m, CH2), 1.75-1.76 (2H, m, CH2), 4.29 (2H, t, J 6, COOCH2), 4.61-4.65 (6H, m, OCH2), 5.21-5.49 (6H, m, CH2d), 6.01-6.12 (3H, m, CHd), 7.30 (2H, s, H-ar). δC (100 MHz; CDCl3; 27 °C; TMS): 167.4 (CO), 153.4, 143.0, 125.5, 108.9 (C-ar), 134.3, 133.2 (CH2CH), 117.9, 117.8 (CH2CH), 70.0 (OCH2), 65.2

Borisch et al.

(COOCH2), 31.8, 29.6, 29.5, 29.4, 29.2, 26.6, 25.9, 22.6 (CH2), 14.0 (CH3). MS (70 eV, EI): m/z 458 ([M]+ 100%). N-Alkyl-3,4,5-tris(3,4-dihydroxypropyloxy)benzamides (3) and n-Alkyl 3,4,5-Tris(3,4-dihydroxypropyloxy)benzoates (4). 6/n or 7/n (1.0 mmol) and Nmethylmorpholine N-oxide (6.0 mmol, 0.6 mL of 60% solution in water) were dissolved in acetone (20 mL). Osmium tetroxide (0.7 mL of a 0.01M solution in tertbutanol) was added, and the solution was stirred for 24 h at 20 °C. Afterward 5 mL of sodium sulfite solution was added and the mixture was stirred for 2 h at 20 °C. The mixture was filtered over a silica bed. The residue was carefully washed twice with acetone (50 mL), and the solutions were evaporated in vacuo. The residue was purified by column chromatography (chloroform eluent) followed by repeated thin-layer chromatography (chromatotron; the polarity of the eluent was gradually changed from pure chloroform to chloroform/methanol containing 25 vol % methanol) and finally by crystallization from acetone. N-Dodecyl-3,4,5-tris(2,3-dihydroxypropoxy)benzamide (3/12). Synthesized from 6/12 (0.5 g). Yield: 0.19 g (34%). cr1 93 cr2 135 CubV1 182 SA 184 is. Anal. Found: C, 59.84; H, 8.60; N, 2.66. Calcd for C28H49O10N: C, 60. 09; H, 8.82; N, 2.50. δH (400 MHz; [2H6]DMSO; 27 °C; TMS; J/Hz): 0.84 (3H, t, J 7, CH3), 1.23-1.27 (18H, m, CH2), 1.491.51 (2H, m, CH2), 3.19-3.22 (2H, m, NCH2), 3.33-3.54 (6H, m, CH2OH), 3.67-3.71 (1H, m, CH), 3.80-3.84, 3.863.96 (6H, m, OCH2), 4.00-4.04 (2H, m, CH), 4.51-4.55 (1H, m, OH), 4.63-4.66 (3H, m, OH), 4.94 (2H, d, J 5, OH), 7.15 (2H, s, H-ar), 8.36 (1H, t, J 6, NH). δC (100 MHz; [2H6]DMSO; 27 °C; TMS): 165.6 (CO), 152.2, 139.9, 129.7, 105.9 (C-ar), 70.7 (OCH2), 70.5, 70.1 (CH), 62.8, 62.7 (CH2OH), 40.1 (NCH2), 31.2, 29.1, 29.0, 28.9, 28.7, 28.6, 26.5, 22.0 (CH2), 13.8 (CH3); MS (ESI): m/z: 560 ([M + 1]+ 100%). Dodecyl 3,4,5-Tris(2,3-dihydroxypropoxy)benzoate (4/ 12). Synthesized from 7/12 (0.5 g). Yield: 0.10 g (16%). cr 89 CubV1 133 SA 152 is. Anal. Found: C, 59.51; H, 8.52. Calcd for C28H48O11: C, 59.98; H, 8.69. δH (400 MHz; [2H6]DMSO; 27 °C; TMS; J/Hz): 0.84 (3H, t, J 7, CH3), 1.221.34 (18H, m, CH2), 1.67-1.70 (2H, m, CH2), 3.42-3.51 (6H, m, CH2OH), 3.70-3.71, 3.79-3.80 (3H, m, CH), 3.823.92, 3.97-4.05 (6H, m, OCH2), 4.23 (2H, t, J 6, COOCH2), 4.52-4.55 (1H, m, OH), 4.63-4.68 (3H, m, OH), 4.95 (2H, d, J 5, OH), 7.22 (2H, s, H-ar). δC (125 MHz; [2H6]DMSO; 30 °C; TMS;): 165.3 (CO), 152.2, 141.9, 124.6, 107.7 (Car), 70.7 (OCH2), 70.5, 69.9 (CH), 64.6 (COOCH2), 64.7, 62.5 (CH2OH), 31.2, 28.9, 28.8, 28.0, 25.3, 22.0 (CH2), 14.8 (CH3); MS (ESI): m/z 561 ([M + 1]+ 71.8%). Acknowledgment. This work was supported by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie. Supporting Information Available: Analytical data (NMR, MS, elemental analysis) of compounds 3-7. This material is available free of charge via the Internet at http://pubs.acs.org. LA000259V