Synthesis of unsymmetrical dibenzoquinoxaline discotic mesogens

Article abstract of DOI:10.1039/b400998c

Unsymmetrical dibenzophenazine discotic mesogens were prepared via the corresponding tetraalkoxybenzfl derivatives. These dibenzophenazines exhibit Col_ho phases over extremely broad temperature ranges.

Full text of DOI:10.1039/b400998c

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Synthesis of unsymmetrical dibenzoquinoxaline discotic mesogens{
E. Johan Foster, Jarret Babuin, Natalie Nguyen and Vance E. Williams*
Department of Chemistry, Simon Fraser University, 8888 University Dr., Burnaby, British Columbia,
Canada V5A 1S6. E-mail: vancew@sfu.ca; Fax: 11 604 291 3765; Tel: 11 604 291 3352
Received (in Columbia, MO, USA) 20th January 2004, Accepted 20th July 2004
First published as an Advance Article on the web 13th August 2004
Unsymmetrical dibenzophenazine discotic mesogens were
A number of literature methods for oxidizing diphenylacetylene
to benzil were examined in an effort to convert alkynes 4a–c to the
corresponding 1,2-diones 1a–c. Attempts to carry out this
prepared via the corresponding tetraalkoxybenzil derivatives.
These dibenzophenazines exhibit Colho phases over extremely
broad temperature ranges.
17
transformation using KMnO4 were only nominally successful,
yielding only trace quantities of the desired products. Considerably
better results were obtained by employing DMSO as the oxidant in
the presence of 5 mol% PdCl2.18 These electron-rich alkynes are
more readily oxidized than diphenylacetylene, and could be
converted to the corresponding benzil derivatives at lower
temperatures and catalyst loadings. Equally satisfactory results
were obtained from the analogous I2-mediated DMSO oxidation,19
which affords the diones in near quantitative yields. Since both I2
and PdCl2 yield almost identical results, the much less expensive I2
was employed for these conversions. The overall yields obtained for
the preparation of the unsymmetrical benzil derivatives from the
iodinated precursors using this route ranged from 45–53%; this
compares favourably with the yields obtained for the symmetrical
derivatives by the benzoin condensation method (vide supra).
The successful assembly of the unsymmetrical benzils 1a–c
affords access to a broad array of potential mesogens. As part of
this preliminary investigation, we examined a series of dicyanodi-
benzoquinoxaline derivatives, 6a–c. The symmetrical analog, 6f,
has been shown to possess a remarkably broad mesophase range;10
this is a common feature of polar discotic mesogens20,21 that
makes these molecules attractive candidates for device applica-
tions. Oxidative cyclization of the benzil derivatives 1a–c to the
phenanthrene-9,10-diones, 5a–c, followed by condensation with
diaminomaleonitrile yields the desired products 6a–c (Scheme 2).
For comparison, the symmetrical analogues 6d and 6e were also
prepared.
The tendency of many disc-shaped molecules to self-assemble into
columnar liquid crystal phases was first recognized in the 1970’s.
Recently, such discotic mesogens have been targeted as candidates
for a variety of device applications including photovoltaics,1,2
LEDs,3 xerography4 and chemical sensing.5 Understanding the
relationship between molecular structure and macroscopic pro-
perties is crucial for realizing the full potential of these materials.
Molecular symmetry is one factor that is expected to play a decisive
role in determining both the stability and the ordering of a
columnar mesophase. Although a broad range of discotic
mesogens have been examined to date, the vast majority of these
have been highly symmetrical molecules. The relatively few
reported examples of unsymmetrical discogens have almost
invariably been based on the triphenylene core.6–8 The limited
attention received by lower symmetry molecules can be traced to
the greater challenges associated with their synthesis.
3,3’,4,4’-Tetraalkoxybenzil derivatives (1) have been widely
exploited as precursors for a variety of mesogens, including
triphenylenes,9 diazatriphenylenes,10,11 phthalocyanines,12 metallo-
mesogens13 and macrodiscotic species.14 Given their remarkable
versatility in this regard, these diones present an inviting entry point
into the synthesis of low-symmetry discotic molecules. Symmetrical
benzils (R1 ~ R2) can be prepared using a variety of methods,
including the benzoin condensation route reported by Wenz.9 In
this manner, we were able to assemble compounds 1d–1f in four
steps from veratraldehyde in average overall yields of 29%.
Unfortunately, while the benzoin condensation has been used to
cross-couple electronically disparate aldehydes,15,16 it is not a viable
method for the formation of benzil derivatives such as 1a–c, that
have nearly identical functional groups on both rings.
The phase behavior for each of these compounds was examined
using polarized optical microscopy (POM), X-ray diffraction
(XRD) and differential scanning calorimetry (DSC); these results
are summarized in Table 1. With the exception of the non-
mesogenic dimethoxy derivative, 6a, all members of this series are
liquid crystalline over broad temperature ranges. These liquid
crystals exhibit the characteristic dendritic textures of columnar
For this reason, we examined a different approach whereby these
unsymmetrical benzil derivatives were synthesized in five steps from
1,2-dialkoxybenzenes (Scheme 1). Treatment of 1,2-dialkoxyben-
zenes with I2 and HIO3 in acetic acid affords the iodinated deriva-
tives 2a–d. Compound 2d was then converted to the corresponding
terminal alkyne 3 by treatment with trimethylsilylacetylene
under standard Hagihara–Sonagashira conditions and subsequent
removal of the TMS group. Palladium-catalyzed coupling of this
alkyne with the appropriate iodobenzene derivatives, 2a–c affords
the unsymmetrical products 4a–c in excellent overall yields.
Scheme 1 Reagents and conditions: (a) TMS–acetylene, PdCl2(PPh3)2, CuI,
(i-Pr)2NH, THF; (b) K2CO3, MeOH–THF; c) Pd(PPh3)4, CuI, (i-Pr)2NH,
THF; (d) I2, DMSO, 145 uC.
{ Electronic supplementary information (ESI) available: full synthetic and
analytical details, POM images. See http://www.rsc.org/suppdata/cc/b4/
b400998c/
2 0 5 2
C h e m . C o m m u n . , 2 0 0 4 , 2 0 5 2 – 2 0 5 3
T h i s j o u r n a l i s ß T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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The intercolumnar distances obtained from the XRD data also
point to a lower density of packing in the case of low symmetry
analogues. While the a spacing for the symmetrical derivatives 6d–f
increases uniformly with increasing chain length, the unsymme-
trical derivatives show larger intercolumnar distances than would
be expected based on this trend. In particular, we note that the
intercolumnar distance of 6c is somewhat larger than that of 6f,
despite the former being the smaller molecule. Likewise, while the
average chain lengths of 6b and 6e are the same, the former exhibits
an appreciably larger intercolumnar spacing.
In conclusion, we have developed a convenient synthesis of
unsymmetrical discotic mesogens. Our preliminary studies indicate
that reducing the symmetry of the mesogen provides a practical
method for shifting the phase transitions to lower temperatures,
and as such is complementary to other strategies22 developed for
creating room temperature columnar phases. Although these early
studies have focused on derivatives that contain only two different
functional groups, this approach could be adapted to the assembly
of analogues in which all four alkyl chains differ. Further investiga-
tions are under way to examine such compounds and other
mesogenic derivatives of compounds 1a–f.
Scheme 2 Reagents and conditions: (a) VOF3, BF3?Et2O, CH2Cl2;
(b) diaminomaleonitrile, AcOH, reflux.
Table 1 Phase behaviour of compounds 6a–f. Transition temperatures
and enthalpies were determined by DSC (scan rate ~ 5 uC min21).
Intercolumnar spacings, a, were determined by XRD
The authors gratefully acknowledge the financial assistance of
Simon Fraser University and the Natural Sciences and Engineering
Research Council (NSERC).
Notes and references
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hexagonal phases when viewed by POM. The XRD patterns of
these mesophases exhibit peaks that index to the (100) and (110)
spacings of a two-dimensional hexagonal lattice, and a broad peak
˚
at approximately 3.5 A, which corresponds to the p–p stacking
distance. On the basis of these observations, the liquid crystals were
identified as ordered columnar hexagonal phases (Colho).
Breaking the symmetry in this class of mesogens has a dramatic
effect on their observed phase ranges. While the symmetrical
derivatives 6d–f exhibit columnar phases over similar temperature
ranges, both the Tm and Tc of the unsymmetrical mesogens 6b and
6c are appreciably lowered. This results in phase ranges that are of
similar breadth to those of the parent compounds, but that are
shifted downwards by 20–40 uC. This effect becomes more pro-
nounced as the disparity between the chain lengths, R1 and R2,
increases. This depression of the phase transitions is consistent with
a lower efficiency of packing of the columnar structures formed by
unsymmetrical derivatives. A similar effect has been observed with
triphenylene derivatives.6–8
C h e m . C o m m u n . , 2 0 0 4 , 2 0 5 2 – 2 0 5 3
2 0 5 3