MALDI-TOF mass spectrometry, and elemental analysis (see
the Supporting Information).
The LC properties of viologens 1-3 are summarized in
Table 1 (see also the Supporting Information). Viologen 1
Table 1. Liquid-Crystalline Properties of Viologens 1-3
compd
n
phase transitiona
lattice parameterb (Å)
c
1
2
3
8
12
16
Cr -15 (5.1) Colh
a ) 33.7
a ) 69.5, b ) 34.4
a ) 74.9, b ) 39.1
c
Cr 47 (43.2) Colr
c
Cr 64 (98.2) Colr
a Transition temperatures (°C) and enthalpies (kJ mol-1, in parentheses)
determined by DSC on the second heating at 10 °C min-1. Cr: crystalline.
Colh: hexagonal columnar. Colr: rectangular columnar. b Determined by
XRD data in the Colh phase for 1 and in the Colr phases for 2 and 3 at
100 °C. c The isotropization temperatures could not be determined because
thermal degradation occurred above 160 °C before reaching the isotropic
state.
Figure 1. Molecular structures of liquid-crystalline viologens 1-3
and their model compound 4.
materials that show interesting properties such as electro-
chromism and electrical conductivity,6 and are also employed
as components of supramolecular architectures.7-9 For further
applications of viologens, the introduction of ordered self-
organized structures as well as the control of intermolecular
interactions in the condensed state should be of great
importance.
Herein we report on a new series of redox-active liquid
crystals 1-3 based on viologens that form columnar as-
semblies. To date, a few thermotropic LC viologens exhibit-
ing smectic and/or nematic phases have been reported.10 To
our knowledge, however, there is no precedent of viologens
which are capable of forming columnar LC phases.
New viologens 1-4 (Figure 1) were prepared via the
quaternization reaction of 4,4′-bipyridyl with 2 equiv of
the corresponding trialkoxy-substituted benzyl bromide,
followed by the counterion exchange. All these compounds
were characterized by 1H and 13C NMR spectroscopy,
bearing six octyloxy chains shows a hexagonal columnar
(Colh) phase, whereas viologens 2 and 3 form rectangular
columnar (Colr) phases. The crystalline (Cr)-Colh phase
transition for 1 is observed at -15 °C and the Colh phase is
stable up to 160 °C. At 160 °C, the compound starts to
decompose gradually. The transition temperatures of crystal-
to-liquid crystal increase with the increase of the alkoxy chain
length.
X-ray diffraction (XRD) measurements were conducted
for viologens 1-3. The XRD pattern of 1 taken at 100 °C
(Figure 2a) clearly shows three reflection peaks in the
small-angle region with the reciprocal d-spacing ratio of
1:x3:2, characteristic of a Colh structure. In addition, a
diffuse halo around 4.5 Å arising from the conformationally
disordered alkoxy chains is observed, whereas no distinct
peaks corresponding to the stacking periodicity could be
detected in the wide-angle region. The intercolumnar distance
(a) of 1 in the Colh phase is calculated to be 33.7 Å from
the XRD data (Table 1). This value agrees with the fully
extended molecular length of ca. 36 Å for 1 (Supporting
Information).
(5) (a) Allard, E.; Oswald, F.; Donnio, B.; Guillon, D.; Delgado, J. L.;
Langa, F.; Deschenaux, R. Org. Lett. 2005, 7, 383. (b) Aprahamian, I.;
Yasuda, T.; Ikeda, T.; Saha, S.; Dichtel, W. R.; Isoda, K.; Kato, T.; Stoddart,
J. F. Angew. Chem., Int. Ed. 2007, 46, 4675. (c) Matsuo, Y.; Muramatsu,
A.; Kamikawa, Y.; Kato, T.; Nakamura, E. J. Am. Chem. Soc. 2006, 128,
9586. (d) Sawamura, M.; Kawai, K.; Matsuo, Y.; Kanie, K.; Kato, T.;
Nakamura, E. Nature 2002, 419, 702. (e) Deschenaux, R.; Schweissguth,
M.; Levelut, A.-M. Chem. Commun. 1996, 1275. (f) Deschenaux, R.;
Schweissguth, M.; Vilches, M.-T.; Levelut, A.-M.; Hautot, D.; Long, G.
J.; Luneau, D. Organometallics 1999, 18, 5553. (g) Turpin, F.; Guillon,
D.; Deschenaux, R. Mol. Cryst. Liq. Cryst. 2001, 362, 171.
(6) (a) Monk, P. M. S. The Viologens: Physicochemical Properties,
Synthesis and Applications of the Salts of 4,4′-Bipyridine; Wiley: Chichester,
UK, 1998. (b) Bird, C. L.; Kuhn, A. T. Chem. Soc. ReV. 1981, 10, 49. (c)
Sliwa, W.; Bachowska, B.; Zelichowicz, N. Heterocycles 1991, 32,
2241.
(7) (a) Anelli, P. L.; Ashton, P. R.; Ballardini, R.; Balzani, V.; Delgado,
M.; Gandolfi, M. T.; Goodnow, T. T.; Kaifer, A. E.; Philp, D.; Pietraszk-
iewicz, M.; Prodi, L.; Reddington, M. V.; Slawin, A. M. Z.; Spencer, N.;
Stoddart, J. F.; Vicent, C.; Williams, D. J. J. Am. Chem. Soc. 1992, 114,
193. (b) Amabilino, D. B.; Stoddart, J. F.; Williams, D. J. Chem. Mater.
1994, 6, 1159. (c) Balzani, V.; Credi, A.; Raymo, F. M.; Stoddart, J. F.
Angew. Chem., Int. Ed. 2000, 39, 3348. (d) Flood, A. H.; Ramirez, R. J.
A.; Deng, W.-Q.; Muller, R. P.; Goddard, W. A.; Stoddart, J. F. Aust. J.
Chem. 2004, 57, 301.
(8) (a) Balzani, V.; Bandmann, H.; Ceroni, P.; Giansante, C.; Hahn, U.;
Kla¨rner, F.-G.; Mu¨ller, U.; Mu¨ller, W. M.; Verhaelen, C.; Vicinelli, V.;
Vo¨gtle, F. J. Am. Chem. Soc. 2006, 128, 637. (b) Jeon, W. S.; Kim, H.-J.;
Lee, C.; Kim, K. Chem. Commun. 2002, 1828. (c) Ong, W.; Grindstaff, J.;
Sobransingh, D.; Toba, R.; Quintela, J. M.; Peinador, C.; Kaifer, A. E. J.
Am. Chem. Soc. 2005, 127, 3353. (d) Loeb, S. J. Chem. Commun. 2005,
1511. (e) Kawaguchi, Y.; Harada, A. Org. Lett. 2000, 2, 1353.
Meanwhile, there are obvious differences in the XRD
patterns as well as the columnar structures as for 2 and 3.
As shown in Figure 2b, the XRD pattern of 2 at 100 °C
gives two intense peaks at 34.0 and 30.4 Å with a set of
smaller peaks in the small-angle region. This pattern is
consistent with a Colr phase with c2mm symmetry.11 This
structural difference in the Colh and Colr phases can be
explained in terms of the aspect ratio for respective viologen
(9) Recently, viologen moieties have been used for the preparation of
liquid-crystalline [2]rotaxanes: Suhan, N.; Loeb, S. J.; Eichhorn, S. H. Abstr.
90th Can. Chem. Conf. 2007, May, No. 363.
(10) (a) Tabushi, I.; Yamamura, K.; Kominami, K. J. Am. Chem. Soc.
1986, 108, 6409. (b) Yamamura, K.; Okada, Y.; Ono, S.; Kominami, K.;
Tabushi, I. Tetrahedron Lett. 1987, 28, 6475. (c) Haramoto, Y.; Yin, M.;
Matukawa, Y.; Ujiie, S.; Nanasawa, M. Liq. Cryst. 1995, 19, 319. (d)
Bhowmik, P. K.; Akhter, S.; Han, H. J. Polym. Sci., Part A: Polym. Chem.
1995, 33, 1927. (e) Bhowmik, P. K.; Han, H.; Cebe, J. J.; Burchett, R. A.;
Acharya, B.; Kumar, S. Liq. Cryst. 2003, 30, 1433. (f) Bhowmik, P. K.;
Han, H.; Nedeltchev, I. K.; Cebe, J. J. Mol. Cryst. Liq. Cryst. 2004, 419,
27.
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