Liquid-crystalline terpyridines

Article abstract of DOI:10.1039/b709730a

5,5″-Disubstitution of the terpyridine core leads to the first inherently liquid-crystalline terpyridines. Mesophases characteristic of bent-core and calamitic systems may be obtained depending on the core structure employed. The Royal Society of Chemistr

Full text of DOI:10.1039/b709730a

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Liquid-crystalline terpyridines{
Valery N. Kozhevnikov,* Adrian C. Whitwood and Duncan W. Bruce*
Received (in Cambridge, UK) 27th June 2007, Accepted 17th July 2007
First published as an Advance Article on the web 13th August 2007
DOI: 10.1039/b709730a
Mesomorphism in bent-core mesogens is a delicate function of
5,50-Disubstitution of the terpyridine core leads to the first
inherently liquid-crystalline terpyridines. Mesophases charac-
teristic of bent-core and calamitic systems may be obtained
depending on the core structure employed.
the number of rings, the central bend angle and the functional
groups that link the various rings.6 The minimum number of rings
required for mesomorphism in such systems appears to be five and
so the first target compounds were 3-n (8 ¡ n ¡ 16). None of
these derivatives was mesomorphic and all melted directly (ESI{)
to the isotropic liquid between 232 uC (n = 6) to 184 uC (n = 18).
Unlike compounds 3, esters 4-n were liquid crystalline and
mesomorphism was investigated using polarised optical micro-
scopy and DSC; thermal data are collected in the Table. It seems
that the octyloxy homologue (4-8) is not mesomorphic and the
texture formed on cooling, while reminiscent of a Colr (B1) phase,
showed very sharp edges (ESI{). Preliminary X-ray analysis12 also
showed several peaks in the wide-angle region.
2,29:69,20-Terpyridines (terpy) are ubiquitous in coordination
chemistry as ligands in families of metal complexes with real or
potential applications.1 The idea of designing and synthesising
liquid-crystalline terpys and then combining their excellent physical
properties with the benefits of self organisation, is very attractive
and attempts have been made to obtain such materials. Certain
compounds containing the terpy fragment are known to be
mesomorphic, mainly after complexation to metal centres. For
example, 49-alkylterpyridines lead to lyotropic RuII and RhIII
complexes,2 while extended 6,60-terpyridines formed intriguing
motifs when coordinated to CuI.3 As free ligands, however,
terpyridines have not been found to be useful in this regard. To
best of our knowledge the only example of mesomorphic terpy is
reported by Ziessel et al. where the terpy is attached in the
49-position to a hexacatenar diamide, which is clearly responsible
for the mesomorphism as the free ligand4 and in metal complexes.5
The literature shows that, in the design of mesogenic
terpyridines, it is the 49-position or the 6- and 60-positions that
have been functionalised. However, in considering how terpy-
based liquid crystals might be realised, it is important to recall its
bent nature, which suggests that any mesogens that do result will
have mesomorphism characteristic of bent-core systems.6 Bent-
core liquid crystals tend to form lamellar or columnar phases and
are of particular interest as they can show spontaneous symmetry
breaking and are implicated as possible biaxial nematic materials.7
Thus, the 5,50-disubstituted pattern is the most attractive, yet save
for one report8 of terpy functionalised with cholesteryl groups at
the 5,50-positions (which was not mesomorphic), there are no other
examples in the literature. This was also noted by Piguet and co-
workers when they initially described mesomorphism in structu-
rally analogous bis(benzimidazolyl)pyridines.9
The longer-chain homologues, however, showed a different, but
common behaviour. Thus, heating the materials led to a transition
Previously, some of us have accessed 5,50-difunctionalised
terpyridines using established and flexible chemistry in which a
hydrazone is reacted with an aldehyde to give a 1,2,4-triazine,
which is then reacted with norbornadiene to give the product
(Scheme 1).10 Using an anisyl-based hydrazone (1) leads to
compound 2 whose methyl group is readily removed to allow for
further substitution.
Department of Chemistry, University of York, Heslington, York, UK
YO10 5DD. E-mail: db519@york.ac.uk; vk503@york.ac.uk;
Tel: +44 1904 434085
{ Electronic supplementary information (ESI) available: experimental. See
DOI: 10.1039/b709730a
Scheme 1 Synthesis of the mesomorphic terpyridines.
3826 | Chem. Commun., 2007, 3826–3828
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Table 1 Mesomorphism of the new compounds
Compound
Transition
T/uC
DH/kJ mol21
4-8
4-10
Cr–I
285
199
252
256
192
245
253
186
242
249
182
238
244
190
256
163
204
(93)
139
175
(111)
143
150
161
35.7
5.4
8.9
18.2
6.2
10.4
18.9
6.5
Cr–Cr9
Cr9–B2
B2–I
Cr–Cr9
Cr9–B2
B2–I
Cr–Cr9
Cr9–B2
B2–I
Cr–Cr9
Cr9–B2
B2–I
Cr–N
N–I
Cr–N
N–I
(SmC–N)
Cr–N
N–I
(SmC–N)
Cr–SmC
SmC–N
N–I
4-12
4-14
4-16
9.1
17.8
5.9
9.2
17.7
56.5
2.4
42.4
1.8
(24.2)
27.7
1.8
(22.8)
29.5
2.4
6-4
6-8
6-12
6-16
1.4
to an ordered phase with a wide temperature range, after which a
more fluid phase was seen followed by the isotropic liquid. Slow
cooling from isotropic gave a texture that grew dendritically and
then coalesced gradually and was characteristic of a B2 mesophase.
The chiral nature of the phase11 was evident using polarised optical
microscopy where domains of opposite handedness were observed
when the polarisers were uncrossed (Fig. 1). The high temperatures
have so far prevented us carrying out a switching study to
determine the exact symmetry of the B2 phase. Cooling from the
B2 phase gave rise to a texture that appeared to have defects
consistent with a columnar phase yet, as with compound 4-8, the
edges were rather sharp. Despite the relatively low enthalpy of
transition, this phase is assigned as crystalline. Note that X-ray has
so far been unable to ‘catch’ the B2 phase in these compounds due
to its narrow range of existence.12
Scheme 2 Synthesis of the annelated terpyridines 6-n.
rod-like materials. This result was checked carefully and micro-
scopy showed an absence of chiral domains in the N phase and
both broken-fan and schlieren textures in the SmC phase, with the
classic striated texture at the transition (Fig. 3). Further, the
nematic phase of 6-8 was found to be continuously miscible with
that of 4-pentyl-40-cyanoterphenyl at 190 uC. An X-ray single
crystal structure14 of 5 helped in understanding this change in
mesomorphic nature. Thus, the two pentalene rings occupy the
central space (Fig. 2) and, in this way, begin to transform the shape
the core from bent to rod-like, preventing a chevron-like
arrangement in a layered phase. In addition, the angle that the
two peripheral oxygen atoms make with the centroid of the central
pyridine ring is 135u, which is approaching the upper limit for
behaviour characteristic of bent-core mesogens that do not possess
substantial dipoles. Subsequent addition of the two ester groups is,
on the basis of previous studies,16 predicted to open out the central
angle further. The combination of these three factors contribute to
the observation of nematic and smectic mesophases.
The synthetic route employed in realising these terpyridines
allows the central five-ring unit to be varied with a good degree of
flexibility. In order to widen the range of materials available for
study, the dieneophile 1-morpholinocyclopentene13 was employed
which, in reaction with 1, led to annelated terpyridines 5 in 70%
yield (Scheme 2). Demethylation of 5 followed by esterification
using 4-alkoxybenzoic acids led to the new liquid crystals, 6-n.
Thus, in total contrast to 4-n, these annelated terpyridines, 6-n,
showed nematic and smectic C phases, phases characteristic of
Fig. 1 Optical texture of the B2 phase of 4-12 at 250 uC on cooling.
Polarisers are uncrossed as indicated on the figures.
Fig. 2 Two views of the molecular structure of 5.
Chem. Commun., 2007, 3826–3828 | 3827
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Notes and references
1 U. S. Schubert, H. Hofmeier and G. R. Newkome, Modern Terpyridine
Chemistry, Wiley-VCH, Weinheim, 2006; E. C. Constable, Chem. Soc.
Rev., 2007, 36, 246.
2 J. D. Holbrey, D. W. Bruce and G. J. T. Tiddy, J. Chem. Soc., Dalton
Trans., 1995, 1769.
3 R. Ziessel, L. Douce, A. El-Ghayoury, A. Harriman and A. Skoulios,
Angew. Chem., Int. Ed., 2000, 39, 1489.
4 F. Camerel, B. Donnio, C. Bourgogne, M. Schmutz, D. Guillon,
P. Davidson and R. Ziessel, Chem.–Eur. J., 2006, 12, 4261.
5 F. Camerel, R. Ziessel, B. Donnio and D. Guillon, New J. Chem., 2006,
30, 135.
6 R. A. Reddy and C. J. Tschierske, J. Mater. Chem., 2006, 16,
907.
7 L. A. Madsen, T. J. Dingemans, M. Nakata and E. T. Samulski, Phys.
Rev. Lett., 2004, 92, 145505; B. R. Acharya, A. Primak and S. Kumar,
Phys. Rev. Lett., 2004, 92, 145506.
8 E. C. Constable and C. E. Housecroft, Chimia, 1998, 52, 533.
9 H. Nozary, C. Piguet, P. Tissot, G. Bernardinelli, J. C. G. Bu¨nzli,
R. Deschenaux and D. Guillon, J. Am. Chem. Soc., 1998, 120, 12274.
10 V. N. Kozhevnikov, D. N. Kozhevnikov, O. V. Shabunina,
V. L. Rusinov and O. N. Chupakhin, Tetrahedron Lett., 2005, 46,
1521.
11 The chirality of the phases from these non-chiral materials is described
in more detail in: D. R. Link, G. Natale, R. Shao, J. E. Maclenan,
N. A. Clark, E. Korblova and D. M. Walba, Science, 1997, 278,
1924.
12 Data from Dr Bertrand Donnio, IPCMS, Strasbourg, France.
13 V. N. Kozhevnikov, D. N. Kozhevnikov, T. V. Nikitina, V. L. Rusinov,
O. N. Chupakhin, M. Zabel and B. Ko¨nig, J. Org. Chem., 2003, 68,
2882.
14 Summary crystal data for 5: C35H31N3O2; formula weight M =
525.63 g mol21; T = 110(2) K; MoKa radiation l = 0.71073 A; size
˚
Fig. 3 Photomicrographs, taken over ,0.5 uC, of the N–SmC transition
of 6-16: (a) nematic, (b) transition, (c) SmC phase.
0.27 6 0.11 6 0.08 mm3; monoclinic; space group C2/c; a =
˚
˚
˚
16.1072(12) A; b = 15.3466(12) A; c = 11.0231(8) A; a = c = 90u, b =
98.079(2)u; V = 2697.8(4) A ; Z = 4; Dc = 1.294 Mg m23
;
3
˚
m = 0.081 mm21; F000 = 1112; Crystal size; 1.84 , h , 28.36u; 13663
reflections collected = 3362 unique (Rint = 0.0352); 221 ¡ h ¡ 21,
220 ¡ k ¡ 20, 214 ¡ l ¡ 14; Completeness to theta = 28.36u 99.5%;
absorption correction – none; Solved using SHELXS-97 and refined
with SHELXL-9715 with full-matrix least squares on F2 193 parameters;
GoF = 1.052, Ri[I . 2s(I0)] = 0.0456; wR2[I . 2s(I0)] = 0.1221; R1(all
reflections) = 0.0600; wR2(all reflections) = 0.1311; 0.398 . Dr .
The extra steric demands of the pentalene ring also caused
noticeable destabilisation of the crystal phases and mesophases.
For example, 6-12 melts to nematic at 139 uC and clears at 175 uC,
while 4-12 melts to the B2 phase at 245 uC and clears at 253 uC.
These new mesogens open up a wide and exciting prospect for
functional metal complexes, particularly when coupling their
photophysical properties with liquid-crystalline assembly.
20.221 e. A23. CCDC 647485. For crystallographic data in CIF or
˚
other electronic format see DOI: 10.1039/b709730a.
ˇ ´
15 A. Lesac, H. L. Nguyen, S. Narancic, U. Baumeister, S. Diele and D.
W. Bruce, Liq. Cryst., 2006, 33, 167.
The award of an EU Marie Curie Incoming International
Fellowship to VNK. Helpful discussions with John Goodby and
Steve Cowling (York), are acknowledged gratefully.
16 G. M. Sheldrick, SHELX97, Program for the Refinement of Crystal
Structures, University of Go¨ttingen, Germany, 1997.
3828 | Chem. Commun., 2007, 3826–3828
This journal is ß The Royal Society of Chemistry 2007