Chem. Mater. 2008, 20, 1909–1915
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Guanidinium Chlorides with Triphenylene Moieties Displaying Columnar Mesophases Sven Sauer,† Nelli Steinke,† Angelika Baro,† Sabine Laschat,*,† Frank Giesselmann,‡ and Willi Kantlehner§ Institut für Organische Chemie, UniVersität Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany; Institut für Physikalische Chemie, UniVersität Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany; and Fakultät Chemie/Organische Chemie, Hochschule Technik and Wirtschaft Aalen, BeethoVenstr. 1, D-73430 Aalen, Germany ReceiVed October 15, 2007. ReVised Manuscript ReceiVed December 20, 2007
Novel guanidinium chlorides 6a-f which are tethered to a pentaalkyloxytriphenylene unit have been prepared by a five-step sequence from hexaalkyloxytriphenylenes 1a-f. The mesomorphic properties of 6a-f were studied by DSC, POM, and X-ray diffraction. Columnar rectangular mesophases were found for derivatives 6b-f with long alkyl chains. Detailed SAXS experiments revealed a change of the common Colr (C2/m) phase to Colr (P2m) phase with increasing chain length. Derivative 6a with pentyl side chain displayed only unidentified crystalline or plastic crystalline properties.
Introduction Ionic liquids with thermotropic liquid crystalline properties have received increasing interest since the pioneering work by Seddon,1 Levelut,2 Haramoto,3 Kresse,4 Kato,5 and others.6,7 Particularly, mesomorphic ionic liquids derived from imidazolium ions have been extensively investigated due to their high ionic conductivity and good thermal stability.8 In contrast to imidazolium and pyridinium9 ionic liquid crystals, the corresponding guanidinium salts have only been rarely exploited with regard to both their ionic liquid
* Corresponding author: Tel +49-711-685 64565; Fax +49-711-685 64285; e-mail
[email protected]. † Institut für Organische Chemie, Universität Stuttgart. ‡ Institut für Physikalische Chemie, Universität Stuttgart. § Hochschule Technik and Wirtschaft Aalen.
(1) (a) Holbrey, J. D.; Seddon, K. R J. Chem. Soc., Dalton Trans. 1999, 2133. (b) Gordon, C. M.; Holbrey, J. D.; Kennedy, A. R.; Seddon, K. R. J. Mater. Chem. 1998, 8, 2627–2636. (c) Bowlas, C. J.; Bruce, D. W.; Seddon, K. R. Chem. Commun. 1996, 1625. (2) Artzner, F.; Veber, M.; Clerc, M.; Levelut, A.-M. Liq. Cryst. 1997, 23, 27. (3) (a) Haramoto, Y.; Nanasawa, M.; Ujiie, S. Liq. Cryst. 2001, 28, 557– 560. (b) Haramoto, Y.; Nansawa, M.; Ujiie, S.; Holmes, A. B. Mol. Cryst. Liq. Cryst. 2000, 348, 129–136. (c) Haramoto, Y.; Kusakabe, Y.; Nansawa, M.; Ujiie, S.; Mang, S.; Moratti, S. C.; Holmes, A. B. Liq. Cryst. 2000, 27, 263. (d) Haramoto, Y.; Akiyama, Y.; Segawa, R.; Nanasawa, M.; Ujiie, S.; Holmes, A. B. Liq. Cryst. 1999, 26, 1425– 1428. (e) Haramoto, Y.; Akiyama, Y.; Segawa, R.; Nanasawa, M.; Ujiie, S.; Holmes, A. B. Bull. Chem. Soc. Jpn. 1999, 72, 875–878. (f) Haramoto, Y.; Akiyama, Y.; Segawa, R.; Ujiie, S.; Nanasawa, M. Liq. Cryst. 1998, 24, 877–880. (g) Haramoto, Y.; Akiyama, Y.; Segawa, R.; Ujiie, S.; Nanasawa, M. J. Mater. Chem. 1998, 8, 275–276. (h) Haramoto, Y.; Ujiie, S.; Nansawa, M. Liq. Cryst. 1996, 21, 923–925. (i) Haramoto, Y.; Yin, M.; Matukawa, Y.; Ujiie, S.; Nanasawa, M. Liq. Cryst. 1995, 19, 319. (4) (a) Bernhardt, H.; Weissflog, W.; Kresse, H. Mol. Cryst. Liq. Cryst. A 1999, 330, 1451–1455. (b) Kresse, H. Liq. Cryst. 1998, 25, 437– 439. (c) Bernhardt, H.; Weissflog, W.; Kresse, H. Liq. Cryst. 1998, 24, 895–897. (5) Yoshio, M.; Mukai, T.; Kanie, K.; Yoshizawa, M.; Ohno, H.; Kato, T. Chem. Lett. 2002, 320.
behavior10 and their liquid crystalline properties.11 This is somewhat surprising because guanidines are valuable reagents, auxiliaries, and catalysts in organic synthesis.12 We were therefore interested to explore tetramethylguanidinium salts which are tethered to a pentaalkyloxytriphenylene moiety with regard to synthetic access and mesomorphic properties. The results are described below.
(6) (a) Marcos, M.; Martin-Rapun, R.; Omenat, A.; Barbera, J.; Serrano, J. L. Chem. Mater. 2006, 18, 1206–1212. (b) Chen, H.; Kwait, D. C.; Gönen, Z. S.; Weslowski, B. T.; Abdallah, D. J.; Weiss, R. G. Chem. Mater. 2002, 14, 4063. (c) Yoshio, M.; Mukai, T.; Kanie, K.; Yoshizawa, M.; Ohno, H.; Kato, T. AdV. Mater. 2002, 14, 351. (d) Rocaboy, C.; Hampel, F.; Gladysz, J. A. J. Org. Chem. 2002, 67, 6863–6870. (e) Yoshio, M.; Mukai, T.; Kanie, K.; Yoshizawa, M.; Ohno, H.; Kato, T. Chem. Lett. 2002, 320. (f) Neve, F.; Francescangeli, O.; Crispini, A.; Charmant, J. Chem. Mater. 2001, 13, 2032–2041. (g) Dzyuba, S. V.; Bartsch, R. A. Chem. Commun. 2001, 1466–1467. (h) Veber, M.; Berruyer, G. Liq. Cryst. 2000, 27, 671. (i) Lee, K. M.; Lee, C. K.; Lin, J. B. Chem. Commun. 1997, 899. (j) Chachaty, C.; Bredel, T.; Tistchenko, A. M.; Caniparoli, J. P.; Gallot, B. Liq. Cryst. 1988, 3, 815–824. (7) (a) Reviews on nonconventional liquid crystals: Kato, T.; Mizoshita, N.; Kishimoto, K. Angew. Chem. 2006, 118, 44-74; Angew. Chem., Int. Ed. 2006, 45, 38-68. (b) Tschierske, C. Ann. Rep. Prog. Chem., Sect. C 2001, 97, 191–297. (c) Tschierske, C. J. Mater. Chem. 1998, 8, 1485–1508 Review on columnar liquid crystals: (d) Laschat, S.; Baro, A.; Steinke, N.; Giesselmann, F.; Hägele, C.; Scalia, G.; Judele, R.; Kapatsina, E.; Sauer, S.; Schreivogel, A.; Tosoni, M. Angew. Chem. 2007, 119, 49164973; Angew. Chem., Int. Ed. 2007, 46, 4832-4887. (8) (a) For recent examples see: Chiou, J. Y. Z.; Chen, J. N.; Lei, J. S.; Lin, I. J. B. Mater. Chem. 2006, 16, 2972–2977. (b) Kumar Pal, S.; Kumar, S. Tetrahedron Lett. 2006, 47, 8993–8997. (c) Cui, L.; Zhu, L. Liq. Cryst. 2006, 33, 811–818. (d) Dobbs, W.; Suisse, J.-M.; Douce, L.; Welter, R. Angew. Chem. 2006, 118, 4285-4288; Angew. Chem., Int. Ed. 2006, 45, 4179-4182. (e) Motoyanagi, J.; Fukushima, T.; Aida, T. Chem. Commun. 2005, 10, 1–103. (f) Suisse, J. M.; BelleminLaponnaz, S.; Douce, L.; Maisse-Francois, A.; Welter, R. Tetrahedron Lett. 2005, 46, 4303–4305. (g) Kumar, S.; Kumar-Pal, S. Tetrahedron Lett. 2005, 46, 2607–2610. (h) Yoshizawa, H.; Mihara, T.; Koide, N. Liq. Cryst. 2005, 32, 143–149. (i) Yoshio, M.; Mukai, T.; Ohno, H.; Kato, T. J. Am. Chem. Soc. 2004, 126, 994–995. (j) Tosoni, M.; Laschat, S.; Baro, A. HelV. Chim. Acta 2004, 87, 2742–2749. (k) Yoshio, M.; Mukai, T.; Kanie, K.; Yoshizawa, M.; Ohno, H.; Kato, T. AdV. Mater. 2002, 14, 351–354.
10.1021/cm702967c CCC: $40.75 2008 American Chemical Society Published on Web 01/31/2008
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Chem. Mater., Vol. 20, No. 5, 2008 Scheme 1
Sauer et al. Table 1. Phase Transition Temperatures [°C] and Enthalpies [kJ mol-1] of Guanidinium Chlorides 6a phase transitions (onset [°C]) and transition enthalpies (given in parentheses) [kJ/mol]
compound 6a 6b 6c 6d 6e 6f
1. 2. 1. 2. 1. 2. 1. 2. 1. 2. 1. 2.
cooling heating cooling heating cooling heating cooling heating cooling heating cooling heating
I 68/(-2.3) × 24/(-13.5) Cr Cr 53/(16.2) × 58/(2.1) I I 110/(-3.4) Colr1 81/(-1) Colr2 34/(-10.0) Cr Cr 46/(12.2) Colr2 81/(1) Colr1 106/ (3.6) I I 104/(-3.8) Colr 40/(-29.4) Cr Cr1 47/(17.5) Cr2 55/(11.1) Colr 95/ (4.0) I I 111/(-6.0) Colr 30/(-35.5) Cr Cr 44/(34) Colr 103/(5.9) I I 118/(-6.6) Colr 30/(-39.1) Cr Cr1 41/(9.8) Cr2 49/(29.5) Colr 115/ (6.7) I I 94/(-5.5) Colr 20/(-26.9) Cr Cr 45/(38.3) Colr 91/(5.8) I
a
The following phases were observed: crystalline phase Cr, columnar rectangular phase Colr, isotropic liquid I, crystalline or plastic crystalline phases with unknown phase structure X.
Results and Discussion Synthesis of Guanidinium Chlorides. As shown in Scheme 1, the synthesis of guanidinium salts 6 commenced with the known hexaalkoxytriphenylenes 1,18 which were (9) (a) Taubert, A.; Steiner, P.; Mantion, A. J. Phys. Chem. B 2005, 109, 15542. (b) Taubert, A. Acta Chim. SloV. 2005, 52, 183. (c) Badoux, J.; Judeinstein, P.; Cahard, D.; Plaquevent, J.-C. Tetrahedron Lett. 2005, 46, 1137–1140. (d) Kumar, S.; Kumar Pal, S. Tetrahedron Lett. 2005, 46, 4127–4130. (e) Taubert, A. Angew. Chem. 2004, 116, 5494; Angew. Chem., Int. Ed. 2004, 43, 5380. (f) Haristoy, D.; Tsiourvas, D. Liq. Cryst. 2004, 31, 697–703. (g) Haristoy, D.; Tsiourvas, D. Chem. Mater. 2003, 15, 2079–2083. (h) Haramoto, Y.; Miyashita, T.; Nanasawa, M.; Aoki, Y.; Nohira, H. Liq. Cryst. 2002, 29, 87–90. (i) Binnemans, K.; Bex, C.; Van Deun, R. J. Inclusion Phenom. Macrocycl. Chem. 1999, 35, 63–73. (j) Hessel, V.; Ringsdorf, H.; Festag, R.; Wendorff, J. H. Makromol. Chem., Rapid Commun. 1993, 14, 707–718. (10) Mateus, N. M. M.; Branco, L. C.; Lourenco, N. M. T.; Afonso, C. A. M. Green Chem. 2003, 5, 347–352. (11) (a) Mathevet, F.; Masson, P.; Nicoud, J.-F.; Skoulios, A. J. Am. Chem. Soc. 2005, 127, 9053–9061. (b) Kim, D.; Jon, S.; Lee, H.-K.; Baek, K.; Oh, N.-K.; Zin, W.-C.; Kim, K. Chem. Commun. 2005, 5509– 5511. (12) (a) Ishikawa, T.; Kumamoto, T. Synthesis 2006, 73, 7–752. (b) Yamamoto, Y.; Kojima, S. The Chemistry of Amidines and Imidates; Patai, S., Rappoport, Z., Eds.; John Wiley & Sons: New York, 1991; Vol. 2, pp 485–526.
treated with bromocatecholborane in CH2Cl2 at room temperature according to the procedure by Kumar13 to give the pentaalkyloxymonohydroxytriphenylenes 2 in moderate yields (see Table 1 of the Supporting Information) together with unreacted starting material 1, which could be recovered. Subsequent Williamson etherification of hydroxytriphenylenes 2 with 1,4-dibromobutane in the presence of K2CO3 in 2-butanone under reflux yielded the corresponding bromides 3 in 85–92%. Displacement of the bromide by an amino function was achieved by treatment of bromides 3 with NaN3 in DMF at 100 °C followed by reduction with LiAlH4 in THF at room temperature, and the amines 5 were isolated in good yields. Finally, the guanidinium moiety was introduced by heating amines 5 together with N,N,N′,N′tetramethylchloroformadinium chloride19,20 in the presence of NEt3 in CH2Cl2 and subsequent treatment with NaOH. The desired guanidinium chlorides 6 could be isolated in moderate to good yields. Mesomorphic Properties of Guanidinium Chlorides. All derivatives 6 were investigated by differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). The phase transition temperatures and enthalpies for the triphenylene guanidinium salts are given in Table 1. A typical DSC experiment is shown in Figures 1 and 2. Whereas compound 6a displayed only a crystal-to-crystal (or crystal-to-soft crystal) transition, the other derivatives 6b-f displayed broad enantiotropic mesophases from about 45 up to 115 °C. A more detailed view on the DSC results (13) Kumar, S.; Manickam, M. Synthesis 1998, 1119–1122. (14) Gerrard, W.; Lappert, M. F.; Mountfield, B. A. Chem. Soc. 1959, 1529– 1535. (15) Kumar, S.; Naidu, J. J.; Varshney, S. K. Mol. Cryst. Liq. Cryst. 2004, 411, 355–362. (16) Paraschiv, I.; Giesbers, M.; van Lagen, B.; Grozema, F. C.; Abellon, R. D.; Siebbeles, L. D. A.; Marcelis, A. T. M.; Zuilhof, H.; Sudhölter, E. J. R. Chem. Mater. 2006, 18, 968–974. (17) Kantlehner, W.; Haug, E.; Mergen, W. W.; Speh, P.; Maier, T.; Kapassakalidis, J.; Bräuner, H.-J.; Hagen, H. Liebigs Ann. Chem. 1984, 108–126. (18) (a) Kumar, S. Chem. Soc. ReV. 2006, 35, 83. (b) Kumar, S. Liq. Cryst. 2004, 31, 1037–1059. (19) Kantlehner, W.; Greiner, K. Liebigs Ann. Chem. 1990, 96, 5–973. (20) (a) Kremzow, D.; Seidel, G.; Lehmann, C. W.; Fürstner, A. Chem.sEur. J. 2005, 11, 1833–1853. (b) Ohno, K.; Ishida, W.; Kameta, K.; Oda, K.; Machida, M. Heterocycles 2003, 317–322. (c) Wittmann, H.; Raab, V.; Schorm, A.; Plackmeyer, J.; Sundermeyer, J. Eur. J. Inorg. Chem. 2001, 1937–1948. (d) Knorr, R.; Trzeciak, A.; Bannwarth, W.; Gillessen, D. Tetrahedron Lett. 1989, 30, 1927– 1930.
Guanidinium Chlorides with Triphenylene Moieties
Chem. Mater., Vol. 20, No. 5, 2008 1911 Table 2. X-ray Diffraction Data for Triphenylene Guanidinium Chlorides 6b-f compound mesophase 6b
Colr (65 °C) P2/a
6b
Colr (85 °C) C2/m
6c
Colr (80 °C) C2/m
6d
Colr (80 °C) C2/m
6d
Colr (100 °C) P2m
6e
Colr (80 °C) P2m
6f
Colr (85 °C) P2m
Figure 1. DSC thermogram for 6f during (a) first cooling and (b) second heating (heating/cooling rate 10 K min-1).
Figure 2. DSC thermogram for 6b during (a) first cooling and (b) second heating (heating/cooling rate 10 K min-1).
Figure 3. Fan-shaped texture of 6d as seen between crossed polarizers at 76 °C upon slow cooling from the isotropic phase (magnification ×200).
(Figure 2) revealed that the triphenylene guanidinium salt 6b exhibited two different columnar mesophases with a first melting transition at 46 °C, a second phase transition at 81 °C, and a clearing point at 106 °C. For the other compounds 6c-f only one mesophase was visible in the DSC curves. Under the optical polarizing microscope the guanidinium salts 6b-f exhibited fan-shaped textures typical for columnar mesophases (Figure 3).
Hkl 200 110 210 300 400 220 410 320 420 600
dexp dcalcd lattice Fcalcd [Å] [Å] parameter [Å] [g cm-3] 41.4 39.6 30.7 27.1 20.9 19.6 18.7 17.5 15.2 13.5
41.4 39.6 30.5 27.6 20.7 19.8 18.8 17.5 15.2 13.8
∼4.4 diffuse halo 110 38.9 38.9 200 33.6 33.6 020 24.0 23.8 310 20.4 20.3 220 19.5 19.4 400 16.8 16.8 130 15.5 15.5 ∼ 4.2 diffuse halo 110 40.9 40.9 200 35.8 35.8 020 25.1 24.9 310 21.5 21.5 220 20.4 20.4 400 17.8 17.9 130 16.1 16.2 ∼ 4.2 diffuse halo 110 42.5 42.5 200 36.9 36.9 020 26.5 26.0 310 22.4 22.2 220 21.2 21.3 400 18.5 18.4 130 17.0 16.9 ∼ 4.4 diffuse halo 100a,b 41.7 42.0 010a,b,c 36.4 36.7 110 27.6 27.6 200 21.0 21.0 020 18.3 18.3 a 120 16.7 16.8 220 13.1 13.8 030a,,c 11.9 12.2 ∼ 4.6 diffuse halo 100a,b 43.1 43.3 010a,b,c 37.4 37.6 110 28.4 28.4 200 21.6 21.6 a 17.2 17.2 120 a,b,c 030 12.4 12.5 ∼ 4.6 diffuse halo 100a,b 44.8 44.9 010a,b,c 38.7 39.3 110 29.6 29.6 200 22.5 22.5 120a 17.7 18.0 310 13.8 14.0 030a,b,c 12.8 13.1
a ) 82.8 b ) 45.1
0.83
a ) 67.2 b ) 47.7
1.01
a ) 71.6 b ) 49.8
0.91
a ) 73.7 b ) 52.1
0.86
a ) 42.0 b ) 36.7
1.03
a ) 43.3 b ) 37.6
1.04
a ) 44.9 b ) 39.3
1.03
∼ 4.5 diffuse halo a
The liquid crystalline phases were studied in more detail by X-ray diffraction, which was performed on extruded samples, tempered into the mesophase. Guanidinium salts 6b-f gave typical diffraction patterns for columnar mesophases. The wide-angle scattering diffractions of the mesophase only showed a broad halo corresponding to the liquidlike aliphatic chains. The reflections in the small-angle region are summarized in Table 2.
This indexation is not consistent with the extinction rule of a two-dimensional rectangular C2/m lattice: hk: h + k ) 2n + 1. b This indexation is not consistent with the extinction rule of a two-dimensional rectangular P21/a lattice: h0: h ) 2n + 1. 0k ) 2n + 1. c This indexation is not consistent with the extinction rule of a two-dimensional rectangular P2/a lattce: 0k ) 2n + 1.
Typical SAXS diffraction patterns are shown in Figure 4. For the mesophases of 6b-d at 85 °C (6b), 80 °C (6c), and
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Figure 5. CPK model of triphenylene guanidinium salt molecule 6c with all-trans conformation of the alkyl chains generated by Chem3D.
Figure 4. X-ray (SAXS) diffraction patterns of compound 6f at 85 °C (top) and compound 6c at 80 °C (bottom) after extusion from the columnar rectangular phase.
80 °C (6d), respectively, an indexation was found which is characteristic for a columnar rectangular lattice with C2/m symmetry. All observed reflections follow the reflection condition for Miller indices h + k ) 2n. The lattice parameters a and b are given in Table 2. Upon elongation of the chain length the lattice parameters are also increasing. The number of molecules forming a columnar unit can be calculated from the X-ray density given by21
consists of eight molecules. With this information in hand, it is possible to recalculate the densities of the investigated compounds 6. The results are shown in Table 2. The recalculated densities (assuming z ) 8) are in the range from 1.01 g/cm3 for 6b at 85 °C and 0.86 g/cm3 for 6e at 80 °C, which is typical value for organic compounds. That leads to two different requirements for the unit cell: first, one unit cell must consist of two columns; second, eight molecules must be positioned in one unit cell. Upon considering the shape of one triphenylene guanidinium chloride molecule (Figure 5) and the ionic interactions between the guanidinium groups and the chlorine ions, presumably four molecules aggregate to a disk. On the basis of the above results, we thus propose a unit cell shown in Figure 6, in which two disks lead to two columns. The C2/m space group implies that the disks are tilted. We were able to obtain a well-orientated sample from 6c at 80 °C. By WAXS experiments, it was possible to determine the tilt angle from the angular dispersion of the π-π reflection at 3.5 Å, which represents the disk-disk interaction. The tilt angle was calculated from this experiment to be 29°. The angular dispersion is shown in Figure 7. The theoretical tilt angle for this symmetry group while taking the planarity of the molecules into account is given by
( )
where F ) density, z ) number of molecules in the columnar unit, M ) molecular weight, NA ) Avogadro’s constant, A ) columnar cross section, and h ) height of the columnar unit. Using the cell parameters a and b, A could be calculated. Together with the height of one columnar unit given by the WAXS reflections at around 4.5 Å, it is possible to estimate the number of molecules in the unit cell. Assuming F ) 1 g/cm3, z is calculated to be 7.9, 8.8, and 9.0 molecules for 6b,c, d (at 85, 80, and 80 °C, respectively). Therefore, we assume that the unit cell of the rectangular C2/m lattice
a (2) b√3 where Φ ) tilt angle, a ) smaller lattice parameter, and b ) larger lattice parameter.22 The theoretical tilt angle for 6c was calculated to 34°. This is in good agreement with the experimental value, thus supporting the symmetry group assignment. This model is further supported by the lattice parameters. The diameter of a disk of four molecules together with the alkyl chains was estimated by CPK models using Chem3D. According to modeling data the diameter of one disk with guanidinium head group but without alkyl chains is ∼36 Å. This is a constant value for every investigated derivative of 6. For a typical heptyl chain (6c) the whole diameter of
(21) Lehmann, M.; Köhn, C.; Meier, H.; Renker, S.; Oehlhof, A. J. Mater. Chem. 2006, 16, 441–451.
(22) Gearba, R. I.; Bondar, A. I.; Goderis, B.; Bras, W.; Ivanov, D. A. Chem. Mater. 2005, 17, 2825–2832.
zM F ) NAAh
(1)
Φ ) arccos
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Figure 8. Schematic drawing of the proposed unit cell with P2m symmetry.
Figure 6. Schematic drawing of the disk formed by four guanidinium chloride molecules (top) and proposed unit cell with C2/m symmetry (bottom), formed by two columns per cell.
Figure 7. Angular dispersion of 6c (π-π interaction reflex, Φ ) tilt angle).
central disk and peripheral side chains is ∼46 Å. By comparing this value with the calculated lattice parameters a (71.6 Å) and b (49.8 Å), it is obvious that two disks are fitting well into the rectangular unit cell with C2/m symmetry. For the samples 6d-f (100, 80, and 85 °C, respectively) the best indexation leads to a rectangular P2m lattice for the columnar mesophase. A typical diffraction pattern of this lattice is shown in Figure 4 (top). The corresponding unit cell is shown in Figure 8. Ohta first discovered this rectangular two-dimensional lattice for discotic liquid crystals.23 The resulting Miller indices for compounds 6d-f are similar to those reported by Ohta. The common lattices for columnar rectangular (23) Komatsu, T.; Ohta, K.; Watanabe, T.; Ikemoto, H.; Fujimoto, T.; Yamamoto, I. J. Mater. Chem. 1994, 4, 537–540.
mesophases (P21/a, P2/a, and C2/m) could be excluded since the Miller indices for the observed mesophase could not be applied to these extinction rules as shown in Table 2. We therefore propose a plain lattice with P2m symmetry, which has no extinction rules for these columnar rectangular mesophases. The lattice parameters for the compounds 6d-f (100, 80, and 85 °C, respectively) are given in Table 2. With growing chain length the cell parameters are increasing as well. Assuming a density of ∼1 g/cm3, it was possible to estimate the number of molecules in the unit cell as described above. The rectangular P2m lattice requires four single triphenylene guanidinium chloride molecules. With this estimation we calculated the densities around 1.03 g/cm3. Similarly to the C2/m unit cell, four molecules form one single disk, which itself aggregates to the columnar superstructure. One of these columns builds up the unit cell. Since only one column is needed to build up the lattice, the parameters a and b should be approximately half of the parameters as in the case of a C2/m symmetry. This is the case with lengths around a ) 43 Å and b ) 37 Å compared to a ) 72 Å and b ) 50 Å. Upon comparison of the different SAXS diffraction pattern of compounds 6b-f (Figure 9), it is obvious that there is a change of symmetry of the columnar rectangular mesophases depending on both chain length and temperature. The molecules with shorter chain length (C6, C7) show the typical C2/m symmetry. The molecules with longer chains (C9, C10) show P2m symmetry. For compound 6d with octyl chains it is obvious that there is a change in symmetry with increasing temperature. At lower temperatures (80 °C) the common C2/m symmetry was found. Upon rising the temperature to 100 °C, a symmetry change to P2m was induced. Unfortunately, this symmetry change could only be observed by SAXS experiments. The DSC curve of 6d revealed only one mesophase. This observation is not unusual for columnar mesophases, since transition enthalpies could be very small. There are some examples where changes from columnar rectangular mesophases to columnar hexagonal
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Figure 9. SAXS diffraction patterns of 6b-f. Symmetry change between C8 at 80 °C and C8 at 100 °C. Indexation is given for the different cell types.
Figure 10. Possible mechanisms for symmetry change from C2/m to P2m.
mesophases or even to isotropic phase occur and those changes could not be observed by DSC.24–26 We propose the following model in order to explain the C2/m to P2m transition in the C8 derivative 6d (Figure 10). With increasing temperature and chain length the tilt angle of the disks decreases; i.e., the ellipsoid cross section of the columns transforms into a more circular cross section, (24) Tinh, N. H.; Foucher, P.; Destrade, C.; Levelut, A. M.; Malthete, J. Mol. Cryst. Liq. Cryst. 1984, 111, 277–292. (25) Deschenaux, R.; Schweissguth, M.; Vilches, M.-T.; Levelut, A. M.; Hautot, D.; Long, G. J.; Luneau, D. Organometallics 1999, 18, 5553– 5559. (26) Kang, S. H.; Kim, M.; Lee, H.-K.; Kang, Y.-S.; Zin, W.-C.; Kim, K. Chem. Commun. 1999, 93.
resulting in an increased lateral distance between two columns (equivalent to the lattice parameter b in the C2/m lattice and the diagonal b′ of the P2m lattice) due to the increased steric requirements of the alkyl chains. Simultaneously, the lattice angle γ increases up to 90°, which corresponds to the plain rectangular cell (P2m). Thus, while keeping the intercolumnar distance constant, the plain rectangular lattice can accommodate the sterically more bulky high-temperature phase of 6d more easily than the C2/m lattice. Similarly, with increasing chain lengths the C2/m unit cell cannot accommodate the two columns due to increasing steric hindrance, and thus the P2m lattice is preferred which contains only one column per unit cell. Ohta discussed a
Guanidinium Chlorides with Triphenylene Moieties
similar model for symmetry transitions of phthalocyanine complexes.23 It should be noted that Janietz observed a C2/m to P2m transition depending on the stoichiometry of binary mixtures of triazines and benzoic acids.27 Furthermore, Tschierske described the C2/m to P2m transition of bolaamphiphilic tetraols with respect to lateral semiperfluoroalkyl chain lengths.28 Compound 6b shows a third columnar rectangular mesophase at 65 °C. The Miller indices could fit to a columnar rectangular P2/a symmetry, since the other symmetries are not in accordance with the extinction rules. Using again the model of disk forming eight molecules should build up the unit cell. That leads to a density of 0.83 g/cm3. Also, compound 6a shows a mesophase due to the observed textures of the molecule. The X-ray investigations of the salts showed a higher ordered mesophase or plastic crystal. However, further studies are required to determine details of this particular phase. (27) Komatsu, T.; Ohta, K.; Watanabe, T.; Ikemoto, H.; Fujimoto, T.; Yamamoto, I. J. Mater. Chem. 1994, 4, 537–540. (28) Cheng, X.; Kumar Das, M.; Baumeister, U.; Diele, S.; Tschierske, C. J. Am. Chem. Soc. 2004, 126, 12930–12940.
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Conclusion It has been shown that ionic liquids containing a guanidinium moiety tethered to a pentaalkyloxytriphenylene unit form disklike aggregates consisting of four guanidinium chloride molecules leading to different columnar rectangular mesophases with a temperature- and chain-length-dependent transition between the common Colr (C2/m) phase and the Colr (P2m) phase. Implications of the different unit cell symmetries on macroscopic properties such as ionic conductivity are currently under investigation. Acknowledgment.GenerousfinancialsupportbytheBundesministerium für Bildung and Forschung (grant 01 RI 05177), the Ministerium für Wissenschaft, Forschung and Kunst des Landes Baden-Württemberg, and the Fonds der Chemischen Industrie is gratefully acknowledged. Supporting Information Available: Full details of the synthetic methods together with the spectroscopic and analytical data for the prepared compounds. This material is available free of charge via the Internet at http://pubs.acs.org. CM702967C
