Antiaromatic twinned triphenylene discotics showing nematic phases and 2-dimensional π-overlap in the solid state

Article abstract of DOI:10.1039/c0cc00311e

Construction of formally antiaromatic discotic twins based on dehydroannulene cores linked through non-adjacent positions on triphenylenes gives stable molecules with intriguing molecular and supramolecular properties and, unlike the majority of discotic

Full text of DOI:10.1039/c0cc00311e

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Antiaromatic twinned triphenylene discotics showing nematic phases and
2-dimensional p-overlap in the solid statew
Liang Zhang, Hemant Gopee, David L. Hughes and Andrew N. Cammidge*
Received 10th March 2010, Accepted 10th May 2010
First published as an Advance Article on the web 20th May 2010
DOI: 10.1039/c0cc00311e
Construction of formally antiaromatic discotic twins based on
dehydroannulene cores linked through non-adjacent positions
on triphenylenes gives stable molecules with intriguing molecular
and supramolecular properties and, unlike the majority of
discotic triphenylenes and similar materials, the twins form
nematic mesophases on heating.
Scheme
1
Attempted synthesis of dehydroannulene-twinned
Triphenylene derivatives are among the most widely studied
discotic liquid crystals.1–3 Many derivatives form stable
columnar mesophases that demonstrate anisotropic conduction
and lend themselves to diverse applications in molecular
electronics.4 Discotic nematic systems are much rarer but
currently find application as compensating films for displays.5
Simple twinned triphenylenes have received recent attention
and a general observation is that linking two discotic
triphenylene units via an appropriately lengthed, rigid spacer
can yield nematic material.6 Flexible spacers can preserve
columnar mesophase formation or destroy mesophase
behaviour completely depending on length and nature of the
tether.7–9 In all such simple twins, communication between
the triphenylenes is minimal or non-existent so the basic
properties of the discotic system are not modified or enhanced.
We were intrigued to interrogate the properties of rigidly
fused discotic twins and reasoned that such structures would
present a flattened, board-shaped architecture and that
the resulting conjugated systems would show significant
electronic perturbation. Our initial design focused on twinned
dehydroannulene structure 2. It can be seen that the structure
is severely constrained to be planar at the core, yet presents
a formally antiaromatic system. This antiaromatic twin,
however, is expected to have poor thermal stability due
to facile decomposition pathways made possible by the
1,2-substitution pattern in the annulene (e.g. pericyclic
processes). This did indeed prove to be the case and preliminary
experiments on the dimerization of 1 showed MALDI-MS
evidence for traces of the product but dominant
decomposition (Scheme 1).
triphenylenes linked conventionally through ortho-sites.
Fig.
1
Dehydroannulene-twinned triphenylenes linked through
3,6-positions; a conjugated (a 36 p-electron, formally antiaromatic)
pathway is shown in red.
maintained yet decomposition via pericyclic reactions is
essentially impossible (Fig. 1). Synthesis of twin 3a was
achieved following the sequence shown in Scheme 2.
Dibromide 4 was coupled with protected acetylene under
Sonogashira coupling conditions.13 Deprotection (NaH)
yielded triphenylene diacetylene 6. Formation of dehydro-
annulene twin 3a was achieved in low yield by treatment of
6 with Pd/Cu(I)/PPh3 in refluxing diisopropyl amine. Reaction
conditions were optimised14 to give twin 3a in 27% yield along
with higher cyclic dehydroannulene oligomers (Scheme 3).
Twins 3 are particularly interesting from both a molecular
and supramolecular sense and their behaviour and properties
are distinctly different from typical triphenylene discotics,
their non-cyclic counterparts 7 (Scheme 2, synthesized in a
stepwise manner—see ESIw) and the higher oligomers.
By design the molecular conformation of 3 is forced to be
planar and fully conjugated at the core and the molecule
Synthetic advances in our group and in others, however,
provide access to triphenylene derivatives with almost
any substitution pattern.3,10–12 Dehydroannulene formation
through the triphenylene 3,6-positions, 3, appeared particularly
attractive because the key conjugated antiaromatic core is
presents
a board-like profile. Chemical characterisation
supports this structure in solution. For example, 1H NMR
spectroscopy shows a characteristic signal at low field for the
proton located within the formally antiaromatic p-system
(9.23 ppm compared to 8.64 for the corresponding proton of
7). Outer protons are shifted upfield as expected.
School of Chemistry, University of East Anglia, Norwich NR4 7TJ,
UK. E-mail: a.cammidge@uea.ac.uk; Fax: +44 (0)1603592011;
Tel: +44 (0)1603592011
w Electronic supplementary information (ESI) available: Full experi-
mental, characterisation details and spectra of 3a. CCDC 769122. For
ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c0cc00311e
Electronic absorption spectra show further evidence of
conjugation with 3a giving a complex absorption spectrum
with the longest wavelength band red-shifted by around 25 nm
to 425 nm when compared to model compounds 6 and 7.
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Fig. 2 Absorption (7.4 Â 10À6 M) and fluorescence (7.4 Â 10À8 M)
spectra of 3a and 7 in dichloromethane.
Scheme 2 Synthesis of dehydroannulene-twinned triphenylenes 3
and nematic phase of 3a as observed by polarising optical microscopy
(216 1C, 200Â).
Scheme 3 Optimised synthesis of twin 3a and higher cyclic oligomers.
The emission spectrum of twin 3a is particularly distinctive
and shows a very large Stokes shift of ca. 70 nm and
broadened peaks, whereas the emission of the model 7 is sharp
and shifted by only ca. 10 nm (Fig. 2). The peak shape and
shift suggest significant reorganisation of 3 in its excited state.
Unlike the majority of discotic triphenylenes and similar
materials, twin 3a forms a nematic mesophase on heating and
displays characteristic textures when observed by polarising
optical microscopy (Scheme 2). It is interesting to note that
open twin 7 and higher cyclic oligomers 8 and 9, each able
to adopt non-planar conformations, are non-mesogenic. The
corresponding longer chain derivative, 3b, was synthesized
Fig. 3 X-Ray crystal structure of 3a and cartoon showing inter- and
intracolumnar communication in the solid.
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in order to lower the transition temperatures and is found to
form a stable nematic phase from 110–131 1C.
J. D. MacKenzie, Science, 2001, 293, 1119; (d) W. Pisula,
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684.
¨
Crystals of 3a suitable for X-ray diffraction15 were obtained
by slow evaporation from hexane/THF. The crystal structure,
Fig. 3, reveals the anticipated near-planar molecular
conformation. Most interesting, however, is the molecular
packing in the crystal. It can be seen that the two triphenylene
units from each twin stack in adjacent columns with
an intracolumnar triphenylene–triphenylene separation of
3.48 A, leading to a unique supramolecular arrangement
where there is now electronic communication both along and
across the stacks. Face-to-face columnar organisation of the
whole molecules is prevented because it would leave a void
within the core and this no doubt accounts for the observed
exclusive nematic behaviour of twins 3.
5 (a) H. Mori, Y. Itoh, Y. Nishuira, Y. Nakamura and
Y. Shinagawa, Jpn. J. Appl. Phys., 1997, 36, 143; (b) K. Kawata,
Chem. Rec., 2002, 2, 59.
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P. S. Martin and Z. Lu, J. Mater. Chem., 1999, 9, 1391.
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472.
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M. V. Jesudason, Liq. Cryst., 1993, 15, 851; (b) N. Boden,
R. J. Bushby and A. N. Cammidge, J. Chem. Soc., Chem. Commun.,
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2773.
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2697.
13 K. Sonogashira, J. Organomet. Chem., 2002, 653, 46.
14 W. B. Wan, S. C. Brand, J. J. Pak and M. M. Haley, Chem.–Eur.
J., 2000, 6, 2044.
15 Crystal data for 3a (CCDC 769122): C92H116O8, M = 1349.8.
Monoclinic, space group P21/a (equiv. to no. 14), a = 8.8608(6),
In conclusion we have prepared a new class of formally
antiaromatic discotic twins based on dehydroannulene cores
linked through non-adjacent positions on triphenylenes.
Construction in this fashion gives stable, planar, board-like
molecules with intriguing molecular and supramolecular
properties. The X-ray crystal structure of twin 3a shows
the molecule to exist in the expected near-planar conformation
and its packing is particularly intriguing, giving an
arrangement where there is now electronic communication
both along and across the stacks.
b
=
27.2688(16),
V = 3924.6(5) A3, Z = 2, Dc = 1.142 g cmÀ3, F(000) = 1464,
120(2) K, m(Mo-Ka) l(Mo-Ka)
0.7 cmÀ1
0.71073 A. Crystals are yellow plates. Intensity data measured by
thin-slice o- and j-scans on Bruker-Nonius KappaCCD
c = 16.3205(12) A, b = 95.592(2)1,
T
=
=
,
=
a
We thank the EPSRC National Mass Spectrometry
Service Centre, Swansea, and the EPSRC National X-ray
Crystallography Centre, Southampton for support.
diffractometer (at EPSRC National Crystallography Service) with
Mo-Ka radiation and 10 cm confocal mirrors monochromator.
Total no. of reflections recorded, to ymax = 22.51, was 21 077 of
which 5106 unique (Rint = 0.070); 3735 ‘observed’ with I > 2s(I).
Data processed with DENZO/SCALEPACK,16 with absorption
corrections in SADABS.17 Structure determined by direct methods
in SHELXS,18 and refined in SHELXL.18 Final wR2 = 0.141 and
R1 = 0.10018 for all 5106 reflections weighted w = [s2(Fo2) +
Notes and references
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2
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C. Hagele, G. Scalia, R. Judele, E. Kapatsina, S. Sauer,
A. Schreivogel and M. Tosoni, Angew. Chem., Int. Ed., 2007, 46,
4832.
data only, R1 = 0.066. In final difference map, highest peak
ca. 0.22 eAÀ3
16 Z. Otwinowski and W. Minor, Processing of X-ray diffraction data
collected in oscillation mode, Methods Enzymol., 1997, 276,
307–326.
.
3 (a) S. Kumar, Liq. Cryst., 2004, 31, 1037–1059; (b) S. Kumar, Liq.
Cryst., 2005, 32, 1089.
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Synth. Met., 2000, 111, 15; (c) L. Schmidt-Mende,
17 G. M. Sheldrick, SADABS—Program for calculation of absorption
corrections for area-detector systems, version 2007/2, Bruker AXS Inc.,
Madison, Wisconsin, USA, 2007.
18 G. M. Sheldrick, SHELX-97—Programs for crystal structure
determination (SHELXS) and refinement (SHELXL), Acta
Crystallogr., Sect. A: Found. Crystallogr., 2008, 64, 112–122.
A. Fechtenkotter, K. Mullen, E. Moons, R. H. Friend and
¨
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Chem. Commun., 2010, 46, 4255–4257 | 4257