Soluble and liquid-crystalline ovalenes

Article abstract of DOI:10.1002/anie.200503601

The extremely persistent self-assembly of ovalene tetraesters (see picture) into liquid-crystalline columns from below room temperature to over 375°C gives these first soluble derivatives of "circumnaphthalene", one of the most stable mesophases known. The close contact of the large π systems within the columnar stacks and their good solubility makes these flexible disks promising tools for organic electronics. (Figure Presented)

Full text of DOI:10.1002/anie.200503601

Angewandte
Chemie
Self-Assembly
DOI: 10.1002/anie.200503601
Soluble and Liquid-Crystalline Ovalenes**
Salima Saïdi-Besbes, Éric Grelet, and Harald Bock*
Dedicated to Dr. Nguyen Huu Tinh
on the occasion ofhis 65th birthday
Large polycyclic organic p systems of high symmetry, such as
[
1]
[2]
[3]
acenes, rylenes, and fullerenes, are not only exciting
objects of synthesis and of great fundamental interest, but are
also gaining more and more importance as electronically and
optically active components in devices such as field-ffect
[
4]
[5]
transistors and solar cells.
In 1948, Clar reported the synthesis of the orange
compound 1, which he called ovalene, which he obtained by
treating either 1,14-benzobisanthene (2) or bisanthene (3)
with maleic anhydride under oxidative conditions to obtain
the corresponding carboxylic anhydrides 4 and 5, respectively,
[
6]
followed by decarboxylation (Scheme 1). Geometrically,
ovalene may be regarded as a two-dimensional analogue of
fullerene C , since their cross-sections are approximately
7
0
equivalent, just as coronene corresponds in shape to a cross-
section of C .
6
0
There have been no reports of soluble ovalene-di- or
-
tetracarboxylic acid derivatives derived from 4 or 5 in the
literature since Clarꢀs first synthesis of the insoluble parent
hydrocarbon and the carboxylic anhydrides.Clar reported
that 5 was dark brown, while 4 was dark red.Their large
aromatic systems, which promise good orbital overlap
between neighboring molecules in the solid state, makes
ovalene derivatives interesting candidates for organic opto-
electronics, provided that they can be processed, namely are
[*] Dr. S. Saïdi-Besbes, Dr. É. Grelet, Dr. H. Bock
Centre de Recherche Paul Pascal
CNRS/UniversitØ Bordeaux I
115, Av. Schweitzer, 33600 Pessac (France)
Fax: (+33)5-5684-5600
E-mail: bock@crpp-bordeaux.cnrs.fr
[**] We are grateful to the RØgion Aquitaine and to the Association
Universitaire de la Francophonie for financial support.
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1783

Communications
in pyridine gave a mixture of dihydro-
and higher hydrogenated bisan-
[
11,12]
thenes,
which we used directly in
the benzogenic oxidative Diels–Alder
reaction to give 6 and 5, without
attempting to prepare 3 by oxidation
[
11]
with chloranil. Blue, fully aromat-
ized bisanthene (3) is regenerated
in situ during heating in nitrobenzene
and adds maleic anhydride upon
reaching reflux temperature.
To obtain esters 8 and 9, we then
faced the synthetic challenge of ester-
ifying efficiently the quite unreactive
insoluble anhydrides with long and
branched alcohols, in particular 2-
ethylhexanol, which we had used on
other aromatic cores to induce high
solubility as well as columnar liquid-
crystalline self-assembly at room tem-
[
13,14]
perature.
We had found previously that
large aromatic dianhydrides such as
perylene-3,4,9,10-tetracarboxylic di-
anhydride (PTCDA), reported by
other research groups as being diffi-
Scheme 1. Synthesis of 1 and 2 and of their carboxylic ester derivatives 8 and 9: a) 1. Zn, pyridine, acetic
acid; 2. chloranil; b) maleic anhydride, nitrobenzene; c) soda lime, 4008C; d) RBr, ROH, DBU.
[
15]
cult to esterify, react with a mixture
soluble and film-forming.The elliptical shape of the aromatic
core means that the introduction of solubilizing substituents
may lead to columnar liquid-crystalline (LC) self-assembled
systems with an associated efficient transport of excitons and
of propanol and bromopropane in the presence of potassium
carbonate to give the tetrapropylester 10a (R = n-propyl) in
[
13]
good yield.
However, this method is less efficient with
longer, less-polar alcohols and bromoalkanes such as 2-
ethylhexanol and 1-bromo-2-ethylhexane and gave only low
yields after exceedingly long reaction times (on the order of a
week).By modifying a procedure reported for the esterifica-
tion of phthalocyanine-2,3,6,7,10,11,14,15-octacarboxylic
[
7,8]
charge along the columns.
We therefore set out to prepare a soluble and columnar
liquid-crystalline form of 5, which is completely insoluble in
organic solvents and hard to purify.The synthetic approach
starting from 3 should also lead to soluble derivatives of the
violet benzobisanthene dicarboxylic anhydride (6), which is
obtained, instead of 5, from the reaction of 3 and maleic
anhydride in refluxing nitrobenzene if the reaction time is
[
16]
acid
we succeeded in obtaining high yields of the 2-
ethylhexyl esters 10b and 8b (Scheme 2, R = 2-ethylhexyl)
overnight by esterification with 2-ethylhexanol and 1-bromo-
2-ethylhexane in acetonitrile in the presence of the strong
soluble base 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU).
As expected, X-ray diffraction studies on 8b showed the
presence of a hexagonal columnar liquid-crystalline phase at
room temperature, with a column-to-column distance of 22 Š
and a disk-to-disk spacing of 3.5 Š (Figure 1). The mesophase
is stable over a very large range of temperatures, and no
[
9]
reduced to minutes instead of hours.
We synthesized 5 starting from naphthadianthrone (7,
which is obtained by photocyclization of commercial bian-
[
10]
throne).
To avoid isolation of the very unstable 3, we
followed the procedure developed by Brockmann and
Randebrock.Treatment of 7 with zinc dust and acetic acid
Scheme 2. Room-temperature liquid-crystalline arene tetracarboxylic tetra-2-ethylhexyl esters (R=2-ethylhexyl). The transition temperatures from
the columnar mesophase to the isotropic liquid are given in parentheses. 11 and 13 show subambient melting points of À288C and +128C,
respectively. No transition to a crystalline phase was detected in 12, 10b, and 8b by differential scanning calorimetry down to À608C.
1
784
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Angewandte
Chemie
Figure 2. Growth of homeotropically aligned hexagonal columnar
Figure 1. Room-temperature powder X-ray diffractogram of 8b.
domains of 8c upon cooling through the liquid-to-mesophase transi-
tion. The image was taken by differential interference contrast (DIC)
microscopy in the reflection mode (350 mm”450 mm) of a thin film
between a silicon wafer and a glass slide.
transition to an isotropic liquid phase could be observed up to
our experimental limit of 3758C.
This very wide temperature range of the columnar phase
is in agreement with the evolution of liquid crystallinity as the
core size increases in arene tetracarboxylates: the clarifica-
tion temperature of the room-temperature liquid-crystalline
pyrene, triphenylene, and coronene tetra(2-ethylhexyl) esters
1
1, 12, and 13 increases with core size from the pyrene
[
13]
[17]
(
(
928C) to the triphenylene (1258C) to the coronene ester
1538C). The perylene derivative 10b is an exception to
[
17]
[13]
the rule, as it has a higher clearing temperature (2608C) than
the coronene ester although it has four carbon atoms less,
because stacking of rotated perylene moieties allows excel-
lent space filling without interference from the out-of-plane
[
18]
carboxylate groups.
In order that the LC-to-liquid transition occurred at an
accessible temperature, we replaced half of the 2-ethylhexyl
groups by using 2-decyltetracan-1-ol as well as 2-ethylhexyl
bromide in the esterification.The resulting mixed ester ( 8c
with one R group on each side being 2-ethylhexyl and the
other 2-decyltetradecyl) shows a mesophase-to-liquid tran-
sition at 2208C, and cooling through this temperature yields
the typical six-petaled flowerlike domains of a hexagonal
columnar phase growing in homeotropic alignment (that is,
columns perpendicular to substrates; Figure 2).
To obtain a soluble ovalene derivative with only small
non-absorbing alkyl substituents, we treated 5 with propanol
and 1-bromopropane to yield the tetrapropyl ester 8a (R = n-
propyl).Ester 8a melts above 3758C and is highly soluble in
common organic solvents (> 5% in chloroform).Ovalene
tetraesters 8 are dark brown in the solid state, and their
orange solutions in chloroform show a main absorption
maximum at 359 nm, which is shifted by 15 nm with respect to
unsubstituted ovalene (1: maximum at 344 nm; Figure 3).
In the same manner as ovalene tetraesters 8 are formed
from 5, soluble benzobisanthene diesters 9 are obtained
efficiently from 6 by reaction with 1-bromoalkane, 1-alkanol,
and DBU.They are dark violet solids that give pinkish red
solutions in chloroform with a main absorption maximum at
the same wavelength as ovalene (359 nm) and strong long-
Figure 3. Normalized absorption spectra in chloroform of 8 and 9 and,
for comparison, of their shortened homologues 13 and 14.
wavelength absorptions at 539 and 501 nm.The identical
wavelength of the main absorption peaks of 8 and 9 illustrate
that the two chromophores are electronically related.No
[
19]
corresponding peak is found for solutions of bisanthene (3),
whereas coronene tetraester 13 and the corresponding benzo-
[ghi]perylene diester 14 show the same relationship (main
absorption peak at 313 nm, shifted by 8 nm with respect to
unsubstituted coronene; inset in Figure 3).
In summary, soluble ovalene and benzobisanthene dyes
are readily accessible by an optimized approach from
naphthadianthrone (7) and an efficient esterification proce-
dure.Furthermore, the appropriate choice of the alkyl
substituents leads to ovalene esters that undergo columnar
self-assembly at room temperature over extremely large
temperature ranges.Very few other mesogens, in particular,
[
8,20]
hexasubstituted hexabenzocoronenes
phthalocyanines,
and octasubstituted
are known to self-assemble into col-
umns over temperature ranges of over 3508C.
[
16,21]
Angew. Chem. Int. Ed. 2006, 45, 1783 –1786
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
1785

Communications
Besides improving considerably their synthetic accessi-
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versatility of the ovalene and bisanthene chromophores by
rendering them soluble and strongly self-organizing.
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8
1
7
b: Four portions of zinc powder (4 ” 6 g) and 80% acetic acid (4 ”
0 mL) were added in 30-minute intervals to a refluxing suspension of
(2 g) in pyridine (200 mL).The yellow suspension became green,
then brown.After 5 h, the solution was cooled to room temperature,
the zinc powder filtered off, and water (1 L) added.The precipitate
thus formed was filtered off and dried under vacuum to yield 1.6 g of a
yellow insoluble solid (crude, partially hydrogenated 3).This solid
was heated at reflux for 12 h with maleic anhydride (5 g) in
nitrobenzene (60 mL).A color change from greenish yellow via
blue and violet to brown was observed during warming and the first
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3
0 minutes at reflux.Chloroform was added to the cooled solution,
and the insoluble solid filtered off, washed with chloroform, and dried
under vacuum to yield crude dianhydride 5 (2.9 g). The dianhydride
was suspended in acetonitrile (150 mL), then 2-ethylhexyl bromide
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(
(
0.1 mol, 19.3 g), 2-ethylhexanol (0.1 mol, 13.0 g), and DBU
80 mmol, 12.2 g) added, and the mixture refluxed for 16 h. The
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product, which was purified by column chromatography (silica gel,
dichloromethane) and reprecipitation from butanol.Yield: 2 .7 9 g of a
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1
dianthrone). H NMR (400 MHz, CDCl ): d = 8.59 (d, J = 9 Hz, 4H,
3
ArH), 8.01 (d, J = 9 Hz, 4H, ArH), 7.98 (s, 2H, ArH), 4.90–4.77 (m,
8
H, CH O), 2.22–2.13 (m, 4H, CH), 1.91–1.48 (m, 32H, CH ), 1.25 (t,
2
2
1
3
J = 7 Hz, 12H, CH ), 1.07 ppm (t, J = 7 Hz, 12H, CH ); C NMR
3
3
(
400 MHz, APT, CDCl
3
): d = 169.2 (CO
2
), 127.8 (CArH), 126.8 (CAr),
1
26.1 (CAr), 123.8 (CArH), 123.5 (CAr), 123.0 (CArH), 118.8 (CAr), 116.3
(
CAr), 115.6 (CAr), 69.2 (CH O), 39.3 (CH), 30.8 (CH ), 29.3 (CH ),
2
2
2
2
4.2 (CH ), 23.4 (CH ), 14.4 (CH ), 11.3 ppm (CH ).Elemental
2 2 3 3
analysis calcd for C H O : C 79.81, H 7.68%; found: C 79.99, H
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Tetrahedron
6
8
78
8
7
.82%
a (R = n-propyl): In the same way as described above, crude
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9
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monoanhydride 6 (2.5 g) was obtained if the reflux with maleic
anhydride in nitrobenzene was stopped after 15 min.The anhydride
was esterified with propyl bromide, propanol, and DBU as above and
purified by column chromatography (silica gel, dichloromethane) and
recrystallization from butanol.Yield: 1 .2 9 g of dark red/violet crystals
1
(45% with respect to naphthodianthrone).Mp. 3. 15
8C. H NMR
(400 MHz, CDCl ): d = 7.94 (d, J = 8 Hz, 2H, ArH), 7.82 (d, J = 9 Hz,
3
2
8
H, ArH), 7.51 (d, J = 8 Hz, 2H, ArH), 7.45 (s, 2H, ArH), 7.40 (t, J =
Hz, 2H, ArH), 7.36 (d, J = 9 Hz, 2H, ArH), 4.59 (t, J = 7 Hz, 4H,
CH O), 2.01 (sext, J = 7 Hz, 4H, CH ), 1.19 ppm (t, J = 7 Hz, 6H,
CH ); C NMR (400 MHz, APT, CDCl ): d = 168.8 (CO ), 128.8
2
2
1
3
3
3
2
(CAr), 128.0 (CArH), 127.8 (C ), 127.3 (C ), 125.4 (CArH), 124.9
Ar Ar
(CAr), 124.8 (CAr), 124.4 (CArH), 123.5 (C H), 122.8 (CArH), 121.0
Ar
(CAr), 120.7 (CAr), 119.8 (CAr), 118.3 (CArH), 118.0 (C ), 67.6 (OCH ),
Ar
2
2
8
2.3 (CH ), 10.9 ppm (CH ).Elemental analysis calcd for C H O : C
3.50, H 4.79%; found: C 83.88, H 5.06%.
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Adv. Mater.
2
3
38 26
4
1
[
Received: October 12, 2005
Published online: February 9, 2006
1
Keywords: cycloadditions · electron transfer · liquid crystals
.
pi interactions · self-assembly
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