A columnar liquid crystal based on triphenylphosphine oxide - Its structural changes upon interaction with alkaline metal cations

Article abstract of DOI:10.1039/b514520a

A triphenylphosphine oxide (TPPO) compound bearing 3,4,5- tridodecyloxybenzyloxy moieties exhibits a columnar liquid crystalline phase, and by changing its self-assembled structure, is responsive to alkaline metal cations due to cation-phosphine oxide int

Full text of DOI:10.1039/b514520a

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
A columnar liquid crystal based on triphenylphosphine oxide—its
structural changes upon interaction with alkaline metal cations{
Tsukasa Hatano and Takashi Kato*
Received (in Cambridge, UK) 13th October 2005, Accepted 4th January 2006
First published as an Advance Article on the web 27th January 2006
DOI: 10.1039/b514520a
8
A triphenylphosphine oxide (TPPO) compound bearing 3,4,5-
tridodecyloxybenzyloxy moieties exhibits a columnar liquid
crystalline phase, and by changing its self-assembled structure,
is responsive to alkaline metal cations due to cation–phosphine
oxide interactions.
interactions. Thus, we expected a new class of ion-active self-
assembled columnar materials to be developed by employing
triphenylphosphine oxide (TPPO) groups as the building blocks of
liquid crystals. TPPO has a bowl-like shape that can self-assemble
9
into 1D arrays. The PLO group can be used as a polar moiety for
1
0
self-assembly. We have designed liquid crystalline molecule 1,
with a TPPO group in the centre of the bowl and pyrogallol
moieties with long alkyl chains at the periphery of the bowl,
expecting that the binding of TPPO to metals will change its liquid
crystalline behaviour. To the best of our knowledge, TPPO-based
liquid crystals and their metal complexes have not yet been
reported.
The fabrication of ordered nanostructures by using molecular self-
organisation processes has attracted attention due to their
potential as functional materials such as ion- and electron-active
1
2–7
materials. Columnar liquid crystals can be used for the one-
2,4–7
dimensional (1D) transportation of charge, ions and energy.
4
For 1D ion-active liquid crystals, an imidazolium group, crown
5
6
ethers and oligo(ethylene oxide)s are incorporated into meso-
genic molecules to form columnar structures. Recently, ion-
responsive liquid crystals derived from folic acid have been
Compound 1 was synthesised by nucleophilic substitution of the
1
1
hydroxyl groups of tris(4-hydroxyphenyl)phosphine oxide with a
4,12
benzyl chloride derivative.
Compound 1 shows endothermic
7
prepared. The change in their hydrogen-bonded structures,
peaks at 23 and 56 uC in differential scanning calorimetry (DSC)
thermograms upon heating. These peaks correspond to the
crystalline–columnar and columnar–disordered isotropic transi-
tions, respectively. The enthalpy change of the columnar–
induced by ion dipolar interactions between the pterin ring and
metal cation, is key to the transition processes. Our intention in the
present study is to prepare new ion-responsive liquid crystals to
develop novel dynamically-functioned materials. Here we report
on a thermotropic liquid crystalline compound consisting of a
phosphine oxide group, as shown in Fig. 1. Phosphine oxide
groups can bind various metal cations such as alkaline, alkaline
earth, transition and lanthanide metal ions through ion dipolar
2
1
disordered isotropic transition is 4.0 kJ mol . The focal conic
texture typical of columnar phases (Col) is observed under a
polarising optical microscope (Fig. 2). The X-ray diffraction
(XRD) pattern of 1 presents two sharp peaks at 34.0 and 16.9 s,
and a broad peak at around 8 s (Fig. 3). These sharp peaks
correspond to the (100) and (200) reflections. The broad peak at 8
s can be assigned to the (001) reflection, based on the finding that
a packing distance of a TPPO derivative has been estimated to be
13
ca. 6 s from a single crystal structure.
The oriented sample was prepared by the mechanical shearing
of polydomain samples in sandwiched KBr crystals or glass plates.
The polarising optical microscope image and small angle X-ray
Fig. 1 Structure of phosphine oxide liquid crystal 1.
Department of Chemistry and Biotechnology, School of Engineering,
The University of Tokyo, Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan.
E-mail: kato@chiral.t.u-tokyo.ac.jp; Fax: (+81) 3-5841-8661
{
Electronic Supplementary Information (ESI) available: The preparation
of 1, experimental details and phase identification. See DOI: 10.1039/
b514520a
Fig. 2 Polarising optical microscopic image of 1 at 48 uC.
Chem. Commun., 2006, 1277–1279 | 1277
This journal is ß The Royal Society of Chemistry 2006

View Article Online
Fig. 3 X-Ray diffraction pattern of 1 at 30 uC.
Fig. 5 Phase diagrams of 1 with (a) lithium and (b) sodium ions. M:
mesophase, Cub: cubic phase, Col: columnar phase and Iso: disordered
isotropic phase.
changed the assembled structure of 1. A cubic phase (Cub) is
observed for the mixture of LiOTf and 1 in the ratio of 0.1 : 1.0
from 51 to 69 uC.{ Focal conic textures typical of columnar phases
are observed for mixtures in the ratio of LiOTf to 1 from 0.2 to
0.6.{ The transition temperatures from columnar to disordered
isotropic phases are increased more than 30 uC by the addition of
LiOTf. The stacking distance of the complexes increase from 8.0 to
9.5 s upon addition of LiOTf.{ These results suggest that LiOTf
interacts with the TPPO groups and is maintained by a cation–
phosphine oxide interaction.{ In addition, the lithium cation has a
1
4
coordination number of 2 and is in a linear arrangement. The
columnar phase obtained from the mixture of LiOTf and 1 in the
ratio of 0.5 : 1.0 shows the highest thermal stability, as shown in
Fig. 5a. It is likely that this strong enhancement of the columnar
phase is caused by the formation of a sandwich-like complex
involving ion dipolar interactions, as schematically shown in Fig. 6
left.
Fig. 4 Schematic illustration of an assembled structure of 1 in the
columnar phase.
diffraction (SAXD) pattern support the alignment of 1.{ Polarised
infrared (IR) spectra of this sample at 45 uC were obtained for two
directions: parallel and perpendicular to the shear direction.{ The
absorbance of the PLO stretching in the spectrum parallel to the
shear direction is larger in intensity than in the spectrum
perpendicular to this same shear direction, indicating that 1D
alignment of the PLO group parallel to the shear direction has
been achieved.{ These results suggest that the columnar phase
formed by 1 is composed of stacks of TPPO, in which the
phosphine oxide groups are pointing in the same direction in a
single column (Fig. 4).
The cubic phase is also observed for mixtures of NaOTf and 1 in
the ratios 0.1 to 0.3 (Fig. 5b). The SAXD profile for the cubic
phases at 60 uC exhibits reflections of 31.8 (100), 22.6 (110), 18.4
K
(
111) and 15.9 s (200) (Fig. 7).§ The reciprocal d-spacing of 1 : 2 :
K K
3
: 4 is characteristic of primitive cubic lattice structures
Pm3m). The cubic phase obtained from a mixture of NaOTf and
in approximately the ratio 0.15 : 1.0 is stable at the highest
(
1
temperature, as shown in Fig. 5b. This strong enhancement of the
cubic phase may be caused by the formation of a polyhedral-like
complex involving ion dipolar interactions, as schematically shown
The phase transition behaviour of mixtures of 1 and lithium or
sodium ions were examined. These mixtures were prepared by the
slow evaporation of a THF solution of 1, containing the requisite
amount of lithium triflate (LiOTf) or sodium triflate (NaOTf),
followed by 6 h of drying under vacuum. Fig. 5 shows the phase
diagrams of 1 with LiOTf and NaOTf.{ The addition of LiOTf
1
5
in Fig. 6 (right). " It was previously reported that sphere-like
complexes with polyhedral arrangements were yielded from
alkaline and alkaline earth metal ions by cation–phosphine oxide
1
5
interactions. These results suggest that the primitive cubic lattice
Fig. 6 Schematic illustration of the assembled structural changes of 1 initiated by the addition of alkaline metal ions.
1
278 | Chem. Commun., 2006, 1277–1279
This journal is ß The Royal Society of Chemistry 2006

View Article Online
I. M. Saez, R. P. Tuffin, G. Mackenzie, R. Auz e´ ly-Velty, T. Benvegnu
and D. Plusquellec, Chem. Commun., 1998, 2057; M. L. Bushey,
A. Hwang, P. W. Stephens and C. Nuckolls, Angew. Chem., Int. Ed.,
2002, 41, 2828; F. J. M. Hoeben, P. Jonkheijm, E. W. Meijer and
A. P. H. J. Schenning, Chem. Rev., 2005, 105, 1491; T. Kato and
M. Yoshio, in Electrochemical Aspects of Ionic Liquids, ed. H. Ohno,
Wiley-Interscience, New York, 2005, ch. 25, pp. 307–320.
2
R. J. Bushby and O. R. Lozman, Curr. Opin. Solid State Mater. Sci.,
2
002, 6, 569; A. F. Th u¨ nemann, D. Ruppelt, C. Burger and K. M u¨ llen,
J. Mater. Chem., 2000, 10, 1325; D. Guillon, Struct. Bonding, 1999, 95,
1; T. Kato and N. Mizoshita, Curr. Opin. Solid State Mater. Sci., 2002,
, 579; D. Adam, P. Schuhmacher, J. Simmerer, L. Haussling,
4
6
K. Siemensmeyer, K. H. Etzbach, H. Ringsdorf and D. Haarer,
Nature, 1994, 371, 141; R. C. Smith, W. M. Fischer and D. L. Gin,
J. Am. Chem. Soc., 1997, 119, 4092; V. Percec, M. Glodde, T. K. Bera,
Y. Miura, I. Shiyanovskaya, K. D. Singer, V. S. K. Balagurusamy,
P. A. Heiney, I. Schnell, A. Rapp, H.-W. Spiess, S. D. Hudsonk and
H. Duank, Nature, 2002, 419, 384.
3 M. C. Artal, K. J. Toyne, J. W. Goodby, J. Barber a´ and D. J. Photinos,
J. Mater. Chem., 2001, 11, 2801; P. Hindmarsh, M. J. Watson, M. Hird
and J. W. Goodby, J. Mater. Chem., 1995, 5, 2111.
Fig. 7 Small angle X-ray diffraction pattern of the cubic phase obtained
at 60 uC for the mixture of NaOTf and 1 in the ratio of 0.1 : 1.0. The inset
is a magnified XRD pattern.§
4
M. Yoshio, T. Mukai, H. Ohno and T. Kato, J. Am. Chem. Soc., 2004,
126, 994.
structure of the octahedral complex can be formed in the cubic
phase.
5
U. Beginn, G. Zipp, A. Mourran, P. Walther and M. M o¨ ller, Adv.
Mater., 2000, 12, 513; C. F. van Nostrum, S. J. Picken, A.-J. Schouten
and R. J. M. Nolte, J. Am. Chem. Soc., 1995, 117, 9957.
V. Percec, J. A. Heck, D. Tomazos and G. Ungar, J. Chem. Soc., Perkin
Trans. 2, 1993, 2381.
K. Kanie, T. Yasuda, S. Ujiie and T. Kato, Chem. Commun., 2000,
1899; K. Kanie, M. Nishii, T. Yasuda, T. Taki, S. Ujiie and T. Kato,
J. Mater. Chem., 2001, 11, 2875; T. Kato, T. Matsuoka, M. Nishii,
Y. Kamikawa, K. Kanie, T. Nishimura, E. Yashima and S. Ujiie,
Angew. Chem., Int. Ed., 2004, 43, 1969; Y. Kamikawa, M. Nishii and
T. Kato, Chem.–Eur. J., 2004, 10, 5942.
L. J. Charbonni e` re, R. Ziessel, M. Montalti, L. Prodi, N. Zaccheroni,
C. Boehme and G. Wipff, J. Am. Chem. Soc., 2002, 124, 7779;
R. Selvaraju, K. Panchanatheswaran and V. Parthasarathi, Acta
Crystallogr., Sect. C: Cryst. Struct. Commun., 1998, 54, 905;
M. R. Caira and B. J. Gellatly, Acta Crystallogr., Sect. B: Struct.
Crystallogr. Cryst. Chem., 1980, 36, 1198; M. Baaden, G. Wipff,
M. R. Yaftian, M. Burgard and D. Matt, J. Chem. Soc., Perkin Trans.
2, 2000, 1315; F. Arnaud-Neu, J. K. Browne, D. Byrne, D. J. Marrs,
M. A. McKervey, P. O’Hagan, M. J. Schwing-Weill and A. Walker,
Chem.–Eur. J., 1999, 5, 175; P. Delangle, J.-P. Dutasta, J.-P. Declercq
and B. Tinant, Chem.–Eur. J., 1998, 4, 100; H. Boerrigter, W. Verboom
and D. N. Reinhoudt, J. Org. Chem., 1997, 62, 7148.
B. Xu and T. M. Swager, J. Am. Chem. Soc., 1993, 115, 1159; T. Komori
and S. Shinkai, Chem. Lett., 1993, 8, 1455; S. Sawamura, K. Kawai,
Y. Matsuo, K. Kanie, T. Kato and E. Nakamura, Nature, 2002, 419,
702; Y. Matsuo, A. Muramatsu, R. Hamasaki, N. Mizoshita, T. Kato
and E. Nakamura, J. Am. Chem. Soc., 2004, 126, 432.
In conclusion, a columnar liquid crystal based on a TPPO
moiety has been reported for the first time; the TPPO liquid crystal
having metal binding ability. Structural changes are observed for
the TPPO complex due to the interaction between TPPO and
alkaline metal cations. The octahedral arrangement of 1 with
sodium ions results in the formation of a cubic phase, while the
linear arrangement of 1 with lithium ions induces a columnar
phase. This liquid crystal material may be useful for the
transportation, sensing and extraction of alkaline metal ions.
Partial financial support by a Grant-in-Aid for Creative
Scientific Research in the ‘‘Invention of Conjugated Electric
Structures and Novel Functions’’ (no. 16GS0209) (T. K.) from the
Japan Society for the Promotion of Science (JSPS), Scientific
Research B (no. 17350065) (T. K.) and from the 21st Century
COE program of The Frontier of Fundamental Chemistry
Focusing on Molecular Dynamism’’ (T. K.) from the Ministry
of Education, Culture, Sports, Science and Technology is grate-
fully acknowledged. We also thank Drs. M. Yoshio, M.
Moriyama, K. Kishimoto and M. Nishii at the University of
Tokyo for their kind assistance. T. H. thanks the Research
Fellowship of the Japan Society for the Promotion of Science.
6
7
8
9
`
10 N. Malek, T. Maris, M.-E. Perron and J. D. Wuest, Angew. Chem., Int.
Ed., 2005, 44, 4021.
11 B. P. Friedrichsen, D. R. Powell and H. W. Whitlock, J. Am. Chem.
Soc., 1990, 112, 8931.
Notes and references
{
Macro-phase separation was observed for 1 with LiOTf and NaOTf in
the ratio 1.0 : 0.7 and 1.0 : 0.5, respectively.
The halo at 4.5u originates from the X-ray scattering of the poly(imide)
Kapton) film used as the sample holder.
The lattice constant of a single crystal (ref. 15) of a mixture of TPPO and
1
2 V. S. K. Balagurusamy, G. Ungar, V. Percec and G. Johansson,
J. Am. Chem. Soc., 1997, 119, 1539.
3 A. L. Spek, Acta Crystallogr., Sect. C: Cryst. Struct. Commun., 1987, 43,
233; O. B. Shawkataly, K. Ramalingam, S. Selvakumar, H.-K. Fun
and A. R. Ibrahim, Acta Crystallogr., Sect. C: Cryst. Struct. Commun.,
997, 53, 1451.
4 X. Tian, R. Fr o¨ hlich and N. W. Mitzel, Dalton Trans., 2005, 380;
§
(
"
1
1
Li cation in the ratio of 4 : 1 is between 14 and 17 s. The size of the
tridodecyloxybenzyloxy group close to 8 s by interdigitation and/or
bending of alkyl chains. The lattice constant (31.8 s) is appropriate for the
size of the micellar structure.
1
1
L. Bourget-Merle, P. B. Hitchcock and M. F. Lappert, J. Organomet.
Chem., 2004, 689, 4357.
1
5 J. P. Fackler Jr, C. A. L o´ pez and R. E. P. Winpenny, Acta Crystallogr.,
Sect. C: Cryst. Struct. Commun., 1992, 48, 2218; M. B. Hursthouse,
W. Levason, R. Ratnani, G. Reid, H. Stainer and M. Webster,
Polyhedron, 2005, 24, 121.
1
For example: J. Alper, Science, 2002, 295, 2397; R. F. Service, Science,
002, 295, 2399; T. Kato, Science, 2002, 295, 2414; O. Ikkala and G. ten
Brinke, Chem. Commun., 2004, 2131; J. W. Goodby, G. H. Mehl,
2
This journal is ß The Royal Society of Chemistry 2006
Chem. Commun., 2006, 1277–1279 | 1279