Ionic Liquid Crystal Dendrimers with Mono-, Di- and Trisubstituted

1206

Chem. Mater. 2006, 18, 1206-1212

Ionic Liquid Crystal Dendrimers with Mono-, Di- and Trisubstituted Benzoic Acids Mercedes Marcos, Rafael Martıń-Rapuń, Ana Omenat, Joaquıń Barbera´, and Jose´ L. Serrano* Departamento de Quı´mica Orga´ nica, Instituto de Ciencia de Materiales de Arago´ n, UniVersidad de Zaragoza-CSIC, 50009 Zaragoza, Spain ReceiVed July 29, 2005. ReVised Manuscript ReceiVed January 9, 2006

The synthesis and characterization of a new family of ionic liquid crystal dendrimers consisting of the ammonium salts of the commercially available poly(propylene imine) (G ) 1-5) dendrimers and 4-decyloxy-, 3,4-didecyloxy-, and 3,4,5-tridecyloxybenzoic acids are reported. The liquid crystalline behavior was investigated by means of differential scanning calorimetry, polarizing-light optical microscopy, and X-ray diffractometry. Dendrimers with mono- and didecyloxybenzoate moieties show lamellar mesomorphism, namely, smectic A, whereas those with tridecyloxybenzoate units show columnar mesomorphism: rectangular for G ) 1 and hexagonal for G ) 2-5. On the basis of the experimental results, we propose models both at the molecular level and in the mesophase for all the materials.

Introduction

Scheme 1. Synthetic Route to Ionic Dendrimers

In the last years, the investigation of liquid crystal (LC) polymers has been directed to a new line of progress which is referred to the so-called hyperbranched polymers and dendrimers.1,2 Two general synthetic approaches have been followed for the preparation of LC dendrimers: (a) the formation of macrostructures with a regular structural growth3-9 by successive introduction of the mesogenic units within the branches in each dendrimer generation5,10-12 or (b) the functionalization of a preexisting dendrimer by the covalent linkage of the mesogenic units to its peripheral functional groups (amino, carboxylate, silane, etc.). In this way, it is possible to obtain dendritic architectures that display LC properties by the introduction of rodlike1,13-22 * To whom correspondence should be addressed. E-mail: [email protected].

(1) Ponomarenko, S. A.; Boiko, N. I.; Shibaev, V. P. Polym. Sci. Ser. C 2001, 43, 1. (2) Saez, I. M.; Goodby, J. W. J. Mater. Chem. 2005, 15, 26. (3) Kim, Y. H. J. Am. Chem. Soc. 1992, 114, 4947. (4) Percec, V.; Kawasumi, M. Polym. Prepr. 1992, 33, 221. (5) Percec, V.; Chu, P. V.; Kawasumi, M.; Toward, M. Macromolecules 1994, 27, 4441. (6) Percec, V. Pure Appl. Chem. 1995, 67, 2031. (7) Plesnivy, T.; Ringsdorf, H.; Schuhmacher, P.; Nuütz, U.; Diele, S. Liq. Cryst. 1995, 18, 185. (8) Gehringer, L.; Guillon, D.; Donnio, B. Macromolecules 2003, 36, 5593. (9) Gehringer, L.; Bourgogne, C.; Guillon, D.; Donnio, B. J. Am. Chem. Soc. 2004, 126, 3856. (10) Percec, V.; Kawasumi, M. Macromolecules 1992, 25, 3843. (11) Bauer, S.; Fischer, H.; Ringsdorf, H. Angew. Chem., Int. Ed. Engl. 1993, 32, 1589 (12) Hudson, S. D.; Jung, H. T.; Percec, V.; Cho, D. W.; Johansson, G.; Ungar, G.; Balagurusamy, U. S. K. Science 1997, 278, 449. (13) Ponomarenko, S. A.; Rebrov, E. A.; Brobonsky, A. Y.; Boiko, N. I.; Muzafarov, A. M.; Shibaev, V. P. Liq. Cryst. 1996, 21, 1. (14) Ponomarenko, S. A.; Boiko, N. I.; Shibaev, V. P.; Richardson, R. M.; Whitehouse, I. J.; Rebrov, E. A.; Muzafarov, A. M. Macromolecules 2000, 33, 5549. (15) Lorenz, K.; Ho¨lter, D.; Mu¨hlhaupt, R.; Frey, H. AdV. Mater. 1996, 8, 414. (16) Baars, M. W. P. L.; So¨ntjens, S. H.; Fischer, H.; Peerlings, H. W. I.; Meijer, E. W. Chem.sEur. J. 1998, 4, 2456. (17) Barbera´, J.; Marcos, M.; Serrano, J. L. Chem.sEur. J. 1999, 5, 1834.

or disklike1,23 mesogenic units at the periphery of the original dendrimer, thus forming a LC shell around the central dendritic nucleus. The formation of a LC phase as a result of the molecular arrangement at the supramolecular level is determined by the microsegregation of the three molecular regions with very diverse characteristics which are present in this type of LC dendrimers (central dendrimeric core, mesogenic units, and terminal flexible chains), in such a way that the molecule tends to adopt the thermodynamically most (18) Marcos, M.; Gimeńez, R.; Serrano, J. L.; Donnio, B.; Heinrich, B.; Guillon, D. Chem.sEur. J. 2001, 7, 1006. (19) Donnio, B.; Barbera´, J.; Gimeńez, R.; Guillon, D.; Marcos, M.; Serrano, J. L. Macromolecules 2002, 35, 370. (20) Barbera´, J.; Gimeńez, R.; Marcos, M.; Serrano, J. L. Liq. Cryst. 2002, 29, 309. (21) Serrano, J. L.; Marcos, M.; Martıń, R.; Gonza´lez, M.; Barbera´, J. Chem. Mater. 2003, 15, 3866. (22) Rueff, J.-M.; Barbera´, J.; Donnio, B.; Guillon, D.; Marcos, M.; Serrano, J. L. Macromolecules 2003, 36, 8368. (23) McKenna, M. D.; Barbera´, J.; Marcos, M.; Serrano, J. L. J. Am. Chem. Soc. 2005, 127, 619.

10.1021/cm051677u CCC: $33.50 © 2006 American Chemical Society Published on Web 02/04/2006

Ionic LC Dendrimers with Substituted Benzoic Acids

Chem. Mater., Vol. 18, No. 5, 2006 1207

Figure 1. FT-IR spectra of the PPI-(NH2)32 dendrimer (black), the substituted benzoic acid (red), and the PPI-benzoate dendrimer (green).

stable conformation.13,16,24,25 In the case of lamellar mesophases, the proposed molecular model leads to consideration of the molecules as super-rods that would be ordered parallel to each other promoting a smectic mesophase. For the columnar mesophases, however, a model in which the molecule adopts a radial disklike conformation has been postulated.25 The mesophase is formed as a result of the packing of these molecules within columns. In all these cases, the LC materials are constituted by covalent neutral molecules. Recently, we have reported on the thermotropic mesomorphic properties of a number of ionic LC dendrimers, which are formed by the spontaneous assembly of long-chain carboxylic acids (HOOC(CH2)mCH3, with m ) 8, 12 and 16) onto the surface of amino-terminated poly(propylene imine) (PPI-(NH2)n) and poly(amidoamine) (PAMAM(NH2)n).26 Therefore, the presence of anisotropic mesogenic units in the structure of a molecule is not anymore necessary to obtain a material with LC properties. This dendrimer system is achieved by converting the dendrimer surface from hydrophilic (-NH2) to hydrophobic (alkyl chains). Ion pairs between n-alkanoic acids and the terminal amine groups of the PPI and PAMAM dendrimers are formed, which means that the ionic interaction plays a key role in the formation of a thermotropic mesophase. The ease in the construction of this type of ionic dendrimers allows the preparation of a great variety of dendrimer systems, which can be compared to their covalent homologues. This is the case of the dendrimers reported by Lattermann et al.,27 prepared by the condensation of the amino-terminated PPI dendrimers (G ) 1-5) with 3,4-didecyloxybenzoyl chloride to afford the corresponding covalent dendrimers with amide bonds. The authors report a crystalline polymorphism as well as a hexagonal columnar mesophase for the first four dendrimer generations, and no mesomorphism for the dendrimer of the fifth generation. In this work, we describe the synthesis and structural characterization of a family of ionic LC dendrimers consisting of the ammonium salts of the commercially available PPI dendrimers (generations 1-5) and 4-decyloxy-, 3,4-didecyloxy-, and 3,4,5-tridecyloxybenzoic acids. Dendrimers with mono- and didecyloxybenzoate units show lamellar mesomorphism, namely, smectic A, and dendrimers (24) Barbera´, J.; Donnio, B.; Gimeńez, R.; Guillon, D.; Marcos, M.; Omenat, A.; Serrano, J. L. J. Mater. Chem. 2001, 11, 2808. (25) Marcos, M.; Omenat, A.; Serrano, J. L. C. R. Chim. 2003, 6, 947. (26) Martıń-Rapuń, R.; Marcos, M.; Omenat, A.; Barbera´, J.; Romero, P.; Serrano, J. L. J. Am. Chem. Soc. 2005, 127, 7397. (27) Cameron, J. H.; Facher, A.; Lattermann, G.; Diele, S. AdV. Mater. 1997, 9, 398.

Table 1. Transition Temperatures and Enthalpies As Determined by DSCa of PPI Ionic Dendrimers (PPI-(4-Ar-OC10)n (First Scan) and PPI-(3,4-Ar-(OC10)2)n and PPI-(3,4,5-Ar-(OC10)3)n (Second Scan)) dendrimer

transitions [°C], (∆H [J g-1]), heating process

PPI-(4-Ar-OC10)4 PPI-(4-Ar-OC10)8 PPI-(4-Ar-OC10)16 PPI-(4-Ar-OC10)32 PPI-(4-Ar-OC10)64 PPI-(3,4-Ar-(OC10)2)4 PPI-(3,4-Ar-(OC10)2)8 PPI-(3,4-Ar-(OC10)2)16 PPI-(3,4-Ar-(OC10)2)32 PPI-(3,4-Ar-(OC10)2)64 PPI-(3,4,5-Ar-(OC10)3)4 PPI-(3,4,5-Ar-(OC10)3)8 PPI-(3,4,5-Ar-(OC10)3)16 PPI-(3,4,5-Ar-(OC10)3)32 PPI-(3,4,5-Ar-(OC10)3)64

C1 46 (7.8) C2 100 (40.6) SmA 192 (10.0) I C1 64 (57.3) C2 87 (1.5) SmA 195 (7.7) I SmA 194 (3.9) I SmA 188 (4.3) I g 38 SmA 157 (1.2) I C 45 (31.2) SmA 125b I C 31 (4.5) SmA 135 (2.8) I C 22 (5.7) SmA 139 (3.4) I C 38 (7.9) SmA 139 (3.0) I C 31 (11.9) SmA 134 (2.0) I Colr 90[b] I Colh 93 (1.1) I Colh 91 (0.8) I Colh 86b I Colh 94b I

a C ) crystal, SmA ) smectic A mesophase, Col ) columnar hexagonal h mesophase, Colr ) columnar rectangular mesophase, I ) isotropic liquid. b Temperatures determined by POM.

containing tridecyloxybenzoate units exhibit columnar mesomorphism, rectangular or hexagonal, depending on the generation number. It is noteworthy that the introduction of 3,4-didecyloxybenzoic acid onto the periphery of PPI dendrimers via ionic interactions affords materials with a smectic A mesophase, at temperatures close to room temperature,26 whereas the corresponding amide-bonded covalent homologues show columnar mesomorphism at temperatures higher than 60 °C. Thus, it can be deduced that the ionic interaction between the dendrimer peripheral quaternary ammonium salts and the benzoate groups with alkyloxy chains is decisive to the generation of a certain type of mesomorphism and to the thermal stability of the mesophase. Results and Discussion Synthesis and Characterization. The synthesis of the ionic dendrimes is outlined in Scheme 1. Amine-terminated PPI dendrimer was added to a solution of the corresponding benzoic acid in dry tetrahydrofuran (THF), in approximately 1:1 (primary amine groups: carboxylic acid groups) stoichiometry, following the method described by Crooks et al.28 The purity of the final dendrimers and the protonation of the amino groups was checked out by Fourier transform infrared (FT-IR) and 1H NMR spectroscopies. (28) Chechik, V.; Zhao, M.; Crooks, R. M. J. Am. Chem. Soc. 1999, 121, 4910.

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Marcos et al.

Figure 2. 1H NMR spectra of the ionic dendrimers: (a) PPI-(NH2)32 dendrimer (black), 4-decyloxybenzoic acid (red), and PPI-(4-Ar-OC10)32 dendrimer (green); (b) PPI-(NH2)32 dendrimer (black), 3,4-didecyloxybenzoic acid (red), and PPI-(3,4-Ar-(OC10)2)32 dendrimer (green); and (c) PPI-(NH2)32 dendrimer (black), 3,4,5-tridecyloxybenozic acid (red), and PPI-(3,4,5-Ar-(OC10)3)32 dendrimer (green).

FT-IR Study. The formation of the ammonium benzoate salts was confirmed by FT-IR measurements. In every case, the carbonyl stretching absorption band at 1674-1689 cm-1 disappeared, while a band at 1543-1558 cm-1 corresponding to the asymmetric stretching mode of the carboxylate groups appeared (Figure 1). Thermal Study. The phase behavior of the ionic dendrimers was investigated by polarizing optical microscopy (POM) and differential scanning calorimetry (DSC). The transition temperatures and enthalpies obtained from the DSC thermograms are gathered in Table 1. In general, the transition temperatures are dendrimer generation independent but significantly influenced by the substituted benzoic acid considered. Ionic dendrimers with mono- and didecyloxybenzoate moieties show a smectic A mesophase for all the generations under study. Dendrimers containing tridecyloxybenzoate units exhibit columnar mesomorphism, rectangular for the first generation and hexagonal for generations 2-5. Molecular models for the different mesophases observed will be proposed according to the structural characterization performed by X-ray diffraction (XRD) experiments. With reference to the thermal stability of these materials, the successive heating scans of the ionic dendrimers PPI(4-Ar-OC10)n promote a progressive decrease in the clearing point of the sample under study as it was already found for the ionic dendrimers formed by reaction of PAMAM or PPI with alkanoic acids.26 As it was demonstrated previously,26 the variation of the clearing points for the ionic dendrimers when subject to successive heating and cooling cycles is due to the partial formation of amide bonds between the terminal amino and the carboxylate groups. Those dendrimers with lower transition temperatures, namely, PPI-(3,4-Ar(OC10)2)n and PPI-(3,4,5-Ar-(OC10)3)n afford reproducible DSC consecutive heating scans, because the temperature reached during the measurements is not high enough to facilitate the reaction between the primary amino groups of the dendrimers and the carboxylate groups of the benzoic acids. For this reason, we give the thermal data of the

Table 2. X-ray Data for the Smectic A Ionic Dendrimers dendrimer PPI-(4-Ar-OC10)4 PPI-(4-Ar-OC10)8 PPI-(4-Ar-OC10)16 PPI-(4-Ar-OC10)32 PPI-(4-Ar-OC10)64 PPI-(3,4-Ar-(OC10)2)4 PPI-(3,4-Ar-(OC10)2)8 PPI-(3,4-Ar-(OC10)2)16 PPI-(3,4-Ar-(OC10)2)32 PPI-(3,4-Ar-(OC10)2)64

mesophase

T [°C]

d [Å]

φa [Å]

hmb [Å]

SmA SmA SmA SmA SmA SmA SmA SmA SmA SmA

RT RT RT RT RT 50 50 50 50 50

31.4 30.8 32.4 31.5 31.4 34.3 34.8 34.8 35.0 35.3

9.8 14.4 20.0 28.8 41.0 11.3 16.1 22.9 32.4 45.8

7.0 7.9 8.9 8.9 8.9 5.2 6.3 6.8 7.1 7.2

a Calculated diameter of the ideal molecular cylinder according to the model proposed in Figure 4, assuming a density ) 1 g/cm3 [r ) φ/2 ) [10 × M/(6.023 × π × d)]1/2, M ) molecular weight of the dendrimer]. b Calculated height of the central cylinder slab occupied by the dendrimer matrix (PPI) [hm ) 10 × Mm/(6.023 × π × r2), Mm ) molecular weight of the PPI dendrimer matrix].

dendrimers for the first heating scan in the case of PPI-(4Ar-OC10)n dendrimers and the second heating scan for the series PPI-(3,4-Ar-(OC10)2)n and PPI-(3,4,5-Ar-(OC10)3)n in Table 1. NMR Study. Ionic dendrimers were studied by 1H NMR spectroscopy. Samples consisting of the corresponding dendrimer were dissolved in CDCl3. The results obtained confirmed the formation of the salts and provided further structural information. The spectra recorded for PPI-(3,4-Ar-(OC10)2)n and PPI-(3,4,5-Ar-(OC10)3)n dendrimers reveal just the protonation of the primary amino groups (signal at 2.90 ppm [-CH2-NH3+]) and no protonation of the tertiary amino groups (absence of the signal at 2.78 ppm [-CH2-CH2)3NH+].29,30 The signals corresponding to the aromatic and oxymethylene protons of the di- and tridecyloxybenzoate units are significantly shifted to higher fields with respect to the pure di- or tridecyloxybenzoic acids (Figure 2). (29) Baars, M. W. P. L.; Karlsson, A. J.; Sorokin, V.; de Waal, B. F. W.; Meijer, E. W. Angew. Chem., Int. Ed. 2000, 39, 4262. (30) Tsiourvas, D.; Felekis, T.; Sideratou, Z.; Paleos, C. M. Liq. Cryst. 2004, 31, 739.


Chem. Mater., Vol. 18, No. 5, 2006 1209

Figure 3. XRD patterns of (a) PPI-(3,4,5-Ar-(OC10)3)64 in the hexagonal columnar mesophase at room temperature (b) PPI-(3,4,5-Ar-(OC10)3)4 in the rectangular columnar mesophase at room temperature (small angle region).

For PPI-(4-Ar-OC10)n dendrimers, however, in addition to the protonation of the primary amino groups, a notquantified protonation of the inner tertiary amino groups of the PPI dendritic scaffold occurs, as indicated by the presence of a signal at 2.75 ppm (Hc) [-CH2-CH2c)3-NH+], which is correlated by heteronuclear single quantum coherence with a signal at 51.4 ppm (Cc).29,30 XRD Study. The lamellar (SmA) mesomorphism of most of the dendrimers was confirmed by powder XRD. The diffraction patterns of these materials consist of a diffuse halo in the wide angle region, corresponding to the distance between the conformationally disordered alkyl chains, and only one maximum in the low angle region for dendrimers of series PPI-(4-Ar-OC10)n, which evidences a long range order. This pattern is consistent with a smectic A mesophase, as observed by POM. The diffraction patterns obtained for PPI-(3,4-Ar-(OC10)2)n dendrimers show two maxima in the low angle region in the reciprocal spacing ratio 1:2, which is indicative of a lamellar order. A smectic A mesophase was assigned according to this result along with the textures observed by POM. Table 2 gathers the values of the layer thickness in the SmA mesophase for all the dendrimers. The XRD patterns obtained from samples of PPI-(3,4,5-Ar(OC10)3)n were not consistent with a lamellar mesomorphism but were consistent with a columnar one. They exhibit a diffuse maximum corresponding to a distance of about 4.5 Å between the conformationally disordered aliphatic chains. When n > 4, the hexagonal columnar phase was assigned by the presence of a set of three sharp reflections in the reciprocal ratio 1:x3:x4 in the low angle region of the patterns (Figure 3a). When n ) 4, a more complex pattern was obtained, which corresponds to a rectangular columnar mesophase (Figure 3b). The cell parameters measured in all cases are shown in Table 3. For each family of PPI-derived ionic dendrimers, the layer thickness is independent of the generation number considered and increases with the number of alkyloxy chains in the benzoate ring. On the basis of previous works on LC dendrimers,16,17 we propose an ideal cylindrical model for the lamellar mesophases, in which the dendrimer matrix occupies the central

Table 3. X-ray Data for the Columnar Ionic Dendrimers

dendrimer

hk

dobs [Å]

PPI-(3,4,5-Ar-(OC10)3)4

20 11 21 31 40 22 10 11 20 10 11 20 10 11 20 10 11 20

38.5 31.8 25.4 20.9 19.5 15.95 32.45 19.0 16.5 32.2 18.5 15.8 33.6 19.4 16.5 32.2 18.5 16.1

PPI-(3,4,5-Ar-(OC10)3)8 PPI-(3,4,5-Ar-(OC10)3)16 PPI-(3,4,5-Ar-(OC10)3)32 PPI-(3,4,5-Ar-(OC10)3)64

dcalc [Å] 38.5 31.9 25.9 20.7 19.2 15.9 32.7 18.9 16.3 31.9 18.4 16.0 33.4 19.3 16.7 32.2 18.6 16.1

mesophase type and lattice constants [Å]

hda [Å]

Colr a ) 77 b ) 35

3.3

Colh a ) 37.8

7.4

Colh a ) 36.9

15.7

Colh a ) 38.6

28.9

Colh a ) 37.1

62.7

a Calculated thickness of the molecular disk, assuming a density F ) 1 g cm-3 [hd ) 10 × M/[6.023 × a × (b/2)], for the Colr phase; hd ) 10 × M/[6.023 × (x3/2) × a2], for the Colh phase; M ) molecular weight].

section whereas the alkoxybenzoate units extend up and down as shown in Figure 4. As a consequence of this molecular model, we propose a molecular organization in the SmA mesophase as shown in Figure 4. The dimensions of these molecular cylinders for each dendrimer can be estimated from the experimental data obtained for d (layer spacing of the SmA mesophase, which is, in fact, the height of the molecular cylinder), assuming that the density is 1 g/cm3. The values calculated for each dendrimer are gathered in Table 2. Several conclusions can be drawn when analyzing these values: first, as it was previously observed when studying the mesomorphic behavior of covalent LC dendrimers,17 the height of the molecular cylinder remains practically constant for all the dendrimer generations considered, whereas the cylinder diameter increases from one generation to the next one. For the first four generations, the height of the cylinder is larger than its diameter, and for the fifth generation, however, this trend is inverted (Figure 5). Second, the surface occupied by the aliphatic chains of the PPI-(4-Ar-OC10)n series is much larger than that occupied by the aliphatic chains, which are double in number,

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Figure 4. Molecular model for the ionic dendrimers with a lamellar mesomorphism and schematic representation of the molecular organization in the SmA mesophase.

Figure 5. Graphical representation of the evolution of the cylinder diameter (φ) and height (d) with the generation number for dendrimers (a) PPI-(4Ar-OC10)n and (b) PPI-(3,4-Ar-(OC10)2)n.

of the PPI-(3,4-Ar-(OC10)2)n series for each generation. This may indicate that the aliphatic chains in the molecules of the first series adopt a curly conformation, whereas the chains of the second series are conformationally extended. And third, the PPI dendrimer matrix slab is more extended in the case of the PPI-(4-Ar-OC10)n series, which means that the PPI matrix occupies a subcylinder higher and narrower than that corresponding to PPI-(3,4-Ar-(OC10)2)n for each generation. Moreover, from the third generation onward, the height of the subcylinder occupied by the central dendrimer slab is almost constant for both series (Figure 4). In a previous work,26 we reported on the structural characterization of the smectic A exhibited by the aliphatic ionic dendrimers PPI-(C10)n (n ) 4, 8, and 16). Taking into account that the only structural difference between the two dendrimer series is the presence of a phenoxy moiety (C6H5O), whose length is 5.3 Å, an increase of about 10 Å would be expected for the d values measured in the aromatic series. The smectic layer spacings found for the aliphatic dendrimers are about 23.5 Å, which are about 8 Å shorter than the values

measured for PPI-(4-Ar-OC10)n, which in our opinion supports the validity of the model proposed. The increase of the number of terminal alkoxy chains as substituents of the aromatic ring and, consequently, the volume of the aliphatic/aromatic region of the dendrimer prevents the accommodation of all these chains on the base surface of this ideal cylinder. The flexibility of the dendrimer matrix (PPI) allows such a molecular conformation in which the aliphatic/aromatic region extends radially from the central dendrimer nucleus, as shown in Figures 6 and 7. The supramolecular organization of these disklike molecules in columns gives rise to the columnar mesomorphism observed. For the hexagonal columnar mesophase, the molecular model consists of a disk with the dendrimer matrix occupying a central subcylinder and the carboxylate chains radially arranged, as represented in Figure 5. The calculated values of the thickness of the disks (Table 3) indicate that, in the case of the second generation, one single molecule occupies a disk 7.4 Å thick, whereas for the generations 3-5, one single dendrimer molecule occupies 2, 4, and 8 slices of an


Chem. Mater., Vol. 18, No. 5, 2006 1211

Figure 6. Molecular model for the ionic dendrimers with a hexagonal columnar mesomorphism and schematic representation of the molecular organization in the mesophase.

Figure 7. Molecular model for the ionic dendrimer PPI-(3,4,5-Ar-(OC10)3)4 with rectangular columnar mesomorphism and schematic representation of the molecular organization in the mesophase.

approximate thickness of 7.4 Å, which means they form cylinders with heights of 15.7, 28.9, and 62.7 Å, respectively. Figure 7 shows the schematic representation of a molecule of dendrimer PPI-(3,4,5-Ar-(OC10)3)4 and the molecular organization within a rectangular lattice. The analogue covalent series of dendrimers PPI-(NHCO-3,4-Ar-(OC10)2)n was synthesized and studied by Lattermann et al.27 Interestingly, these dendrimers show a hexagonal columnar mesophase (n ) 4, 8, 16, and 32). However, when the two constituent moieties (PPI-(NH2)n dendrimer and 3,4-didecyloxybenzoic acid) are forming ionic pairs, as described in this work, all the dendrimers (n ) 4, 8, 16, 32, and 64) present a smectic A mesophase. This drastic modification of the mesomorphic behavior may be due to the formation of H bonds between neighbor molecules in the case of the amide bonds, which would stabilize the disklike or radial conformation (giving rise to columnar

supramolecular organizations, Figure 6) compared to the rodlike or cylindrical one (proposed for the smectic mesophases, Figure 4). Moreover, the crystalline phase is destabilized in the case of the ionic dendrimers, as it was already reported for the structurally related poly(ethylene imine) dendrimers,31 and simultaneously the clearing points are higher. Both facts result in a stabilization of the mesophase when salts are formed. When comparing the lattice parameters measured for the hexagonal columnar mesophase shown by the ionic dendrimers PPI-(3,4,5-Ar-(OC10)3)n and those reported for the covalent dendrimers PPI-(NH-CO-3,4-Ar-(OC10)2)n,27 the most significant result is that the hexagonal lattice of the covalent dendrimers is larger than that of the ionic ones (parameter a is ca. 8 Å smaller for the ionic dendrimers). (31) Cameron, J. H.; Facher, A.; Stebani, G.; Lattermann, G. AdV. Mater. 1995, 7, 578.

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Two reasons may justify this difference: The first is the temperature at which the XRD experiments were performed (70-100 °C for the covalent series and room temperature in the present work). Second, the increase of the number of alkyloxy chains that have to be accommodated around the central dendrimer nucleus may force it to adopt a more elongated conformation (along the column axis) so that the effective diameter of the molecular disk decreases.

Marcos et al.

4-decyloxy- and 3,4-didecyloxybenzoic acids are used. However, ionic dendrimers containing 3,4,5-tridecyloxybenzoate units afford columnar mesophases. The comparison of the results obtained with those reported for the analogue series of covalent dendrimers allows us to conclude that the intra- and intermolecular interactions within these systems play a key role in the mesomorphism of the materials which just differ in the type of bond (ionic or covalent) between the constituent parts of the dendrimers.

Conclusions We have described a simple noncovalent dendrimer system which exhibits thermotropic LC behavior. This system is achieved by converting the dendrimer surface from hydrophilic (-NH2) to hydrophobic (alkyl/aromatic chains). The method is based on the formation of ion pairs between substituted benzoic acids and the terminal amine groups of PPI dendrimers. The results reported herein indicate that these systems tend to assemble in smectic LC phases, when

Acknowledgment. This work has been supported by the CICYT of Spain and the FEDER funds (EU) under Project Nos. MAT2002-04118-C02-01 and MAT2003-07806-C01, by the European Union under the RTN Project “Supermolecular Liquid Crystal Dendrimers (LCDD)” (No. HPRN-CT2000-00016), and by the Diputacioń General de Aragoń. R.M.-R. acknowledges a fellowship from the Ministerio de Educacioń y Ciencia (Spain). CM051677U

Ionic Liquid Crystal Dendrimers with Mono-, Di- and Trisubstituted

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