Probing the structural factors influencing columnar mesophase formation and stability in triphenylene discotics

Article abstract of DOI:10.1039/b913678a

Series of structurally related substituted triphenylene derivatives were designed and synthesised to interrogate key features which determine mesophase formation and stability, and to challenge the general conclusions previously proposed by us and others.

Full text of DOI:10.1039/b913678a

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Probing the structural factors influencing columnar mesophase formation
and stability in triphenylene discoticsw
´
Andrew N. Cammidge,* Celine Chausson, Hemant Gopee, Juanjuan Li and David L. Hughes
Received (in Cambridge, UK) 8th July 2009, Accepted 21st October 2009
First published as an Advance Article on the web 4th November 2009
DOI: 10.1039/b913678a
Series of structurally related substituted triphenylene derivatives
were designed and synthesised to interrogate key features which
determine mesophase formation and stability, and to challenge the
general conclusions previously proposed by us and others. It is
apparent that no single, simple principle can be universally applied.
Discotic and columnar liquid crystals1 are an intensively
investigated class of organic materials. Their discovery in
19772 sparked curiosity-driven research with diverse molecular
cores demonstrated as scaffolds supporting liquid crystal
behaviour3 and a rich polymesomorphism was discovered.3–5
Three main classes of discotic cores have received particular
attention, namely phthalocyanines,6 coronenes7 and
triphenylenes8 (Fig. 1). Applications are diverse and include
light-emitting diodes,9 field-effect transistors10 and photo-
voltaic devices11 as well as optical materials such as compensating
films for displays.12 Electronic applications exploit particular
properties of columnar phases which can give rise to easily
aligned self-healing (fluid) materials with high charge carrier
mobilities.13
Fig. 2 Selected examples illustrating the sensitivity of mesophase
stability to modest structural variation.
mesophase stability was observed. The analysis was extended
to triphenylenes themselves and a weaker correlation was
described. Symmetrical hexaalkoxy triphenylenes such as 2
(Fig. 2) are the archetypal discotic liquid crystals, typically
showing columnar hexagonal mesophases over reasonably
wide temperature ranges.4b Removal of one alkoxy chain,
however, destroys mesophase formation17 but it is restored
when a conjugating substituent is incorporated.18 The dependence
on extension of the core p-system is made clear when similar
chain-length substituents are compared (4 and 5; 6–9). It is
clear that cyanide 7 results in the most dramatic mesophase
stabilisation, but a significant enhancement is found for apolar
acetylene 6.18a Non-conjugating methyl substitution destroys
mesophase behaviour whereas methoxy supports it.18b,19 Our
assessment is that a number of factors combine to govern
mesophase formation and stability in triphenylene discotics.
Core extension by substitution with conjugating groups appears
to be a crucial factor. It is recognised that non-conjugating
systems pose different stereochemical requirements and are
likely to destabilise columnar mesophase formation for steric
reasons also. Attachment of electron withdrawing substituents
to the p-electron-rich alkoxytriphenylene core leads to the
greatest enhancement of mesophase stability probably
through polarisation and strengthening of p–p interactions
between the aromatic cores.16 Indeed, combination of
electron-rich discotic materials with electron-accepting
aromatics such as TNF is well known to lead to stabilised
columnar mesophases.20
We and others have focused on the triphenylene core which
provides synthetic and structural versatility. Structural
features controlling mesophase type and stability14,15 were
found to be extremely subtle but we concluded that only
conjugating substituents that were able to extend the
triphenylene’s p-system supported mesophase formation and
that only in very rare cases would mesophase formation be
observed when non-conjugating substituents were present.
Williams and co-workers16 recently argued that mesophase
behaviour can be rationalised by considering the electron
donating/withdrawing character of the substituents. Within a
series of substituted azatriphenylene derivatives (1) a reasonable
correlation between substituent Hammet parameters and
Fig. 1 The most extensively studied discotic liquid crystal cores.
In this communication we report the synthesis and
properties of substituted triphenylene discotics designed to
probe the relationship between molecular structure and
mesophase behaviour and to provide derivatives with device
potential. For this study we kept the parent triphenylene intact
and investigated the introduction of aryl substituents with
varying electronic characteristics. In order to preserve the
School of Chemistry, University of East Anglia, Norwich, UK NR4 7TJ.
E-mail: a.cammidge@uea.ac.uk; Fax: +44 (0)1603592011;
Tel: +44 (0)1603592011
w Electronic supplementary information (ESI) available: For full
experimental details and X-ray data. CCDC 725154. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/b913678a
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Scheme 1 Synthesis and mesophase properties of new 2,3- and 3,6-substituted triphenylenes.
basic molecular shape as disc (or oval), the study focused on
introduction of new substituents at the triphenylene 2,3- or
3,6-positions.15,21
The key intermediates were dibromides 10 and 12, synthesised
by bromination of the corresponding tetraalkoxytriphenylenes.
Suzuki–Miyaura coupling22 of 10 and 12 with appropriate aryl
boronic acids afforded the target triphenylene discotics
(Scheme 1). The thermal behaviour (determined by polarised
optical microscopy and DSC) of the first set of new triphenylenes
is also shown in Scheme 1.
Within this series of triphenylenes, the diphenyl derivatives
can be considered as parent structures. The 2,3-diphenyl-
triphenylene 14 does not show a mesophase whereas the
corresponding derivative bearing the same substituents at
positions 3- and 6- shows a columnar hexagonal mesophase.
This general trend of behaviour is mirrored through most of
the series, with 2,3-diaryl triphenylenes 16, 18, 20 and 22
melting directly to isotropic liquids and their 3,6-substituted
counterparts showing hexagonal mesophases. Within the
series of 3,6-diaryl derivatives it can be seen that introduction
of electron donating groups (compound 17) has little effect on
the mesophase behaviour compared to parent triphenylene 15.
However, this comparison is perhaps not ideal because extra
substituents have been introduced in 17. Within the two series
we minimised additional substituent effects by choosing
groups with similar (2 atom) lengths but different electronic
properties. Diethyl derivative 19, which can be considered
essentially neutral in terms of electronic effects, exhibits a
significantly more stable mesophase than 17 and has properties
intermediate between 21 and 23 which bear electron with-
drawing aldehyde and nitrile groups, respectively. The trend is
therefore not strong, although the most powerful electron
withdrawing substituents (nitriles on 23) lead to the most
stable mesophase. Interestingly, this derivative shows particularly
strong tendency for homeotropic alignment between glass
slides such that it is formed instantly and spontaneously as
the material is cooled into the mesophase. Crystals suitable for
X-ray diffraction were obtained for 23 (Fig. 3).23 Within
Fig. 3 X-Ray crystal structure of 23.
slipped columns it can be seen that the flat triphenylene cores
stack in an antiparallel sense and it seems reasonable to
assume that this relationship is preferred as the molecules
assemble into symmetrical columns within the mesophase.16
The above observations suggested modifications to parent
systems 14 and 15 to investigate the importance of the
electronic character of the substituent further and determine
whether its electronic character is a general controlling factor
governing mesophase stability. The phenyl substituents of 14
and 15 were therefore swapped, respectively, for electron
deficient pyridines and electron-rich thiophenes and furans,
again using straightforward Suzuki–Miyaura coupling
(Scheme 1). The thermal behaviour of the new materials is
shown in Fig. 4. It is immediately clear from the results in Fig. 4
that mesophase behaviour in these compounds does not follow
a
trend where electron withdrawing substituents favour
columnar phases. In fact, the opposite trend is apparent with
electron poor pyridine substituents destroying mesophase
behaviour completely. p-Excessive thiophenes however result
in columnar mesophase formation through all compounds
studied. Once again introduction of the new aryl groups at
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¨
the 3,6-positions results in much more stable columnar
mesophases compared to 2,3-substitution. Comparison of isomers
27/28 and 29/30 reveals a weak link between mesophase stability
and the position of thiophene substitution (a versus b).
The observation that introduction of thiophene substituents
supports columnar mesophase formation could prove
particularly important because such materials can be considered
as through-conjugated discotic analogues of the extensively
investigated oligothiophenes.24 The furan substituted derivatives
also show formation of mesophases and their behaviour, which is
more closely related to electron deficient derivatives 21/23,
further contradicts a simple correlation between mesophase
stability and electronic nature of substituent.
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In conclusion we have designed and synthesised series of
structurally related substituted triphenylene derivatives in order
to interrogate key features which determine mesophase formation
and stability. It is apparent that no single, simple principle can be
widely applied. Structural features, proposed by us and others,
such as the importance of conjugating substituents or the electronic
nature of the substituents, provide useful guidelines but are
insufficient to reliably predict and explain all the observed trends
in isolation. Perturbation of molecular structure through variation
of substituent (nature, position, number etc.) has wider impact on
bulk phase properties and it is impossible to change, for
example, electronic properties without concomitant steric and
conformational change. These effects must therefore be considered
in concert to achieve the fine balance of properties which lead to
mesophase formation at the expense of crystallinity or disorder.
We gratefully acknowledge support from UEA, CVCP
(ORS award) and Dstl for funding and the EPSRC Mass
Spectrometry Service Centre at Swansea.
21 A. N. Cammidge and H. Gopee, Chem. Commun., 2002, 966.
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23 Crystal data: C56H66N2O4, ca. 0.7(H2O), M = 843.7. Triclinic,
ꢀ
space groupP1 (no. 2), a = 11.215(1), b = 12.998(2), c = 17.424(4) A,
a = 77.29(3)1, b = 80.52(1)1, g = 80.53(1)1, V = 2422.0(7) A3.
Z = 2, Dc = 1.157 g cmÀ3, F(000) = 910, T = 140(1) K,
m(Mo-Ka) = 0.7 cmÀ1, l(Mo-Ka) = 0.71069 A. Total no. of
reflections recorded, to ymax = 25.41, was 13 816 of which 8194
were unique (Rint = 0.063); 5686 were ‘observed’ with I > 2sI. At
the conclusion of the refinement, wR2 = 0.165 and R1 = 0.082
(B2) for all 8194 reflections weighted w = [s2(Fo2) + (0.0989P)2]À1
with P = (Fo2 + 2Fc2)/3; for the ‘observed’ data only, R1 = 0.058.
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Notes and references
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2 S. Chandrasekhar, B. K. Sadashiva and K. A. Suresh, Pramana,
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3 A. N. Cammidge and R. J. Bushby, in Handbook of
Liquid Crystals, ed. D. Demus, J. W. Goodby, G. W. Gray,
H.–W. Spiess and V. Vill, WILEY-VCH, Weinheim, vol. II, p. 693.
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Chem. Commun., 2009, 7375–7377 | 7377