Synthesis of crown ether-linked discotic triphenylenes

Article abstract of DOI:10.1021/ol902637z

[Chemical equaction presented] Novel triphenylene dimers linked by a central crown ether core have been synthesized and characterized. The crown ether is most conveniently formed as a final step to permit purification and isolation of ion-free material, and extension of the protocol permits synthesis of triad structures linked though a 27-crown-9 macrocycle. The latter compounds present a new discotic motif that supports mesophase formation.

Full text of DOI:10.1021/ol902637z

ORGANIC
LETTERS
2010
Vol. 12, No. 3
472-475
Synthesis of Crown Ether-Linked
Discotic Triphenylenes
Juanjuan Li,†,‡ Zhiqun He,*,† Hemant Gopee,‡ and Andrew N Cammidge*,‡
Key Laboratory of Luminescence and Optical Information, Ministry of Education,
Institute of Optoelectronic Technology, Beijing Jiaotong UniVersity, Beijing 100044,
People’s Republic of China, and School of Chemistry, UniVersity of East Anglia,
Norwich NR4 7TJ, United Kingdom
a.cammidge@uea.ac.uk; zhqhe@bjtu.edu.cn
Received November 16, 2009
ABSTRACT
Novel triphenylene dimers linked by a central crown ether core have been synthesized and characterized. The crown ether is most conveniently
formed as a final step to permit purification and isolation of ion-free material, and extension of the protocol permits synthesis of triad structures
linked though a 27-crown-9 macrocycle. The latter compounds present a new discotic motif that supports mesophase formation.
Discotic and columnar liquid crystals have been extensively
studied due to their potential application in organic electronic
devices since their discovery in 1977.1,2 Their supramolecular
architecture combined with the π-electron rich molecular
cores make discotic systems an attractive emerging class of
prospective organic optoelectronic materials that could find
use in light emitting3 and photovoltaic4 devices and field
effect transistors.5
To use discotic materials in inexpensive organic optoelec-
tronic device applications, two challenging issues must be
addressed. Reproducible thin films must be prepared from
solution-based processes, and the molecular alignment within
the films must be controlled. Low molecular weight organics
often show poor film processability while polymers can be
too viscous, leading to weakened control of molecular
orientation.
† Beijing Jiaotong University.
‡ University of East Anglia.
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10.1021/ol902637z  2010 American Chemical Society
Published on Web 01/05/2010

Triphenylene has become the most widely studied scaffold
for discotic liquid crystals.6 Its core, like most discotic liquid
crystals, is generally flat, rigid and aromatic and is sur-
rounded by flexible peripheral side chains. Synthetic ad-
vances over recent years have permitted investigation of a
wide range of symmetrical (hexasubstituted) and unsym-
metrically substituted derivatives.6-9 These structures often
form columnar mesophases with beneficial macroscopic
molecular orientation. To improve their processability and
functionality, twinned and oligomeric structures have re-
ceived particular attention. It is found that columnar me-
sophase formation is favored if the individual discotic units
are joined by a flexible spacer of appropriate length and
composition.10 Rigid spacers can lead to formation of
nematic phases.11
In this paper we report the synthesis and properties of
discotic triphenylenes linked through a crown ether. Our first
targets were ditriphenyleno-18-crown-6 derivatives 2 (Figure
1).12 The supramolecular properties of crown ethers are well-
known and we envisaged that incorporation of the crown
structure could conceivably contribute to enhanced columnar
order.13-15 The crown ether linked structure itself presents
an unusual motif for interrogation of structural features
controlling mesophase formation in triphenylene discotics.
In addition, the crown ether linking group can potentially
offer the new twinned materials a further functionality or
property via a combination of electron and ionic conductivity
within one framework if an extended columnar alignment
can be achieved. The corresponding “open” bisterphenyl-
18-crown-6 derivatives 1 are known16 and present a non-
planar (with respect to the cores) molecular structure. These
materials are reported to show columnar phases and are more
accurately described as polycatenar materials17 rather than
discotics. As such, although interesting in their own right,
they are unlikely to offer favorable properties for optoelec-
tronic applications because they lack extended π-systems.
Figure 1. Typical hexaalkoxytriphenylenes (HATn), the hexagonal
columnar mesophase, polycatenar bisterphenyl-18-crown-6 (1) and
target ditriphenyleno-18-crown-6 (2).
The synthesis of unsymmetrically substituted triphenylenes
can be readily achieved through direct oxidative coupling
between suitably substituted biphenyl and benzene deriva-
tives.7 This protocol was developed, and is most suitable,
for synthesis of differentially substituted hexaalkoxytriph-
enylenes, which is the key structural motif present in target
ditriphenyleno-18-crown-6 2. Tetraalkoxybiphenyl 3 (R )
n-hexyl) (Scheme 1) was most conveniently prepared using
(6) (a) Kumar, S. Liq. Cryst. 2004, 31, 1037–1059. (b) Kumar, S. Liq.
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Scheme 1
. Attempted Synthesis using Oxidative
Biphenyl-Benzene Coupling
924
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Suzuki-Miyaura cross-coupling.18 Oxidative coupling was
attempted using iron(III) chloride under the usual conditions
(dichloromethane, room temperature followed by reductive
methanol workup).
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Org. Lett., Vol. 12, No. 3, 2010
473

Analysis of the crude product revealed it to comprise a
complex mixture of components with no evidence for 2b.
Ether hydrolysis is a known side reaction19 in such oxidative
couplings employing FeCl3, and it seems likely that this
pathway accounts for the majority of side products. The
reaction stoichiometry and conditions were varied but in all
cases resulted in the same disappointing outcome.
Scheme 3
.
Synthesis of Target Triphenylene Twins 2 using
Crown Formation as the Final Step
An alternative synthesis was therefore required and we
reasoned that intermediate bisterphenyl 1 could be converted
to target 2 through oxidative or photoinduced20 cyclization
(Scheme 2). Such oxidative cyclizations of terphenyls are
Scheme 2
.
Ring Closure of Bis(terphenyl) Crown Ether 1 to
Give the Target Triphenylene Twin 2b
ditriphenyleno-crown ether 2b. The reaction requires two
cyclizations per molecule and was expected to be slow. This
was indeed the case, and conversion of less than 5% was
achieved after 4 weeks irradiation. It appears that the reaction
rate slows after the initial conversion, probably because the
formed triphenylenes act as better chromophores. The rate
was deemed not to be viable for preparation of the target
compound in the quantities eventually required for charac-
terization and evaluation.
It was decided therefore to construct the crown ether of
the molecule in the final step and employ the route outlined
in scheme 3. 2,3-Dimethoxy-6,7,10,11-tetrakishexyloxyt-
riphenylene 7 was prepared as previously described7 and
demethylated using lithium diphenylphosphide or
thiophenol.7b,21,22
Dihydroxytriphenylene 8 could be converted to the second
required starting material 2,3-bis(chloroethoxy)ethoxy triph-
enylene 9 but we favored synthesis of this material via FeCl3
mediated oxidative coupling between biphenyl 3 and 1,2-
bis(chloroethoxy)ethoxybenzene 10. Crown ether formation
was then achieved using sodium carbonate as base and the
resulting product mixture purified relatively easily by column
chromatography. Pure 2b has a surprisingly high melting
point (175 °C) and disappointingly shows no mesophase
behavior. Complexation with potassium raised the melting
point but did not induce mesophase formation. Further
generally smooth and efficient. Bisterphenyl 1 (R ) n-hexyl)
was easily prepared by Suzuki-Miyaura coupling between
bishexyloxyphenyl boronic acid 5 (Scheme 3) and tetrabro-
modibenzo-18-crown-6 6. Cyclisation with FeCl3 gave a
mixture that appeared to contain significant quantities of the
desired product (mass spectral evidence) alongside many
other components. Separation proved difficult and, although
the compound could be separated and isolated, it was always
1
contaminated with trace impurities. HNMR spectra were
poorly resolved indicating that the product contaminants
included iron salts that could not be easily removed. Clearly
the presence of even trace quantities of ionic impurities
would be unacceptable for any envisaged optoelectronic
applications and characterization so this route, and all others
employing metallic oxidants in the final step, were rejected.
Photochemical cyclization, employing a UV immersion
lamp and iodine, provided a metal-free route to the target
(19) Kumar, S.; Lakshmi, B. Tetrahedron Lett. 2005, 46, 2603.
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1986, 721. (b) Cammidge, A. N.; Gopee, H. Chem. Commun. 2002, 966.
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474
Org. Lett., Vol. 12, No. 3, 2010

hexyl) and found no evidence of the reported mesophase
behavior,16 suggesting that the geometrical constraints are
also present in the open system.
Scheme 4. Synthesis, Mesophase Behavior and Polarized
Optical Microscopy Images of Triphenylene Discotic Triads 12
We therefore turned our attention to more flexible, triad
systems, namely triphenyleno-27-crown-9 derivatives 12.
Their synthesis, shown in scheme 4, was significantly
simplified by our findings in the earlier work. The crown
ether macrocycle was assembled in a stepwise manner,23
initially by slow addition of bis(2chloroethyl)ether to dihy-
droxytriphenylene 8 to give the open twinned triphenylene
inermediate 11. Reaction of dihydroxide 11 with dichloride
9 using cesium carbonate as base afforded the 27-crown-9
macrocyle.
The behavior of these flexible, large macrocyclic triph-
enylene triads significantly contrasts the smaller twinned
crown derivatives 2. Triads 12 present a structural motif that
has no real comparison with any other discotic assembly and
show mesophases up to 176 °C (pentyl) and 186 °C (hexyl).
The mesophase behavior was characterized by DSC (ESI)
and polarized optical microscopy (Scheme 4) where textures
characteristic of columnar hexagonal phases are observed.
The balance between discotic structure and flexibility
therefore clearly supports columnar mesophase formation,
and the liquid crystal phases show a strong tendency to align
(homeotropically) on the glass surfaces.
In summary, we have reported a convenient synthesis of
triphenylene discotics twinned through an 18-crown-6 core.
Formation of the crown ether is performed as a final step to
permit purification and isolation of ion-free material, and
extension of the protocol permits synthesis of triad structures
linked though a 27-crown-9 macrocycle. The latter com-
pounds present a new discotic motif that supports mesophase
formation. The mesophases align easily on surfaces making
them of interest for both fundamental investigation of
structural factors controlling mesophase formation in dis-
cotics and for optoelectronic device applications.
Acknowledgment. J.L. thanks the China Scholarship
Council, Ministry of Education. We thank the EPSRC
Service at Swansea, U.K. for Mass Spectrometry and
Chemical Analysis Centre at Peking for additional analyses.
This work was funded by the NSF China (20674004), the
ISTCP China (2008DFA61420) and the 111 project China
(B08002).
derivatives were therefore prepared using our optimized
synthesis. However, those bearing both pentyl (2a) and decyl
(2c) chains similarly displayed no mesophase behavior. This
appeared to contrast with the reported properties of the related
polycatenar structures 1 and we reasoned that this most likely
arises because of the nonplanar arrangement of appended
aryl groups favored by the 18-crown-6 core. Terphenyl units
present in 1 are able to twist in order to approach a geometry
that can support columnar structure and cyclization to give
the triphenylene systems removes this option and disfavors
close columnar packing. However, we re-examined 1 (R )
Supporting Information Available: Experimental pro-
cedures, and compound characterization data. This material
is available free of charge via the Internet at http://pubs.acs.org.
OL902637Z
(23) Pedersen, C. J. Org. Synth. Coll. 1988, 6, 395.
Org. Lett., Vol. 12, No. 3, 2010
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