Novel dibenzo[fg,op]naphthacene discotic liquid crystals: A versatile rational synthesis

Article abstract of DOI:10.1039/b111067p

A rational synthesis for dibenzo[fg,op]naphthacene derivatives (also named as dibenzopyrene) is described. The methodology involves the preparation of a quaterphenyl using a palladium-catalysed cross-coupling of arylboronic acids followed by chemical or photochemical cyclization. The versatility of the process is shown by the formation of various dibenzo[fg,op]naphthacene derivatives having identical or non-identical peripheral chains. All the new compounds exhibit a single mesophase, which has been identified by X-ray diffractometry and optical microscopy as hexagonal colunmar (Col_h). X-Ray results confirm that hexasubstituted dibenzonaphthacenes are more ordered than octasubstituted derivatives.

Full text of DOI:10.1039/b111067p

View Article Online / Journal Homepage / Table of Contents for this issue
Novel dibenzo[fg,op]naphthacene discotic liquid crystals: a versatile
rational synthesis
Sandeep Kumar,* Jaishri J. Naidu and D. S. Shankar Rao
Centre for Liquid Crystal Research, P.O. Box 1329, Jalahalli, Bangalore, 560 013, India.
E-mail: clcr@vsnl.com
Received 4th December 2001, Accepted 6th February 2002
First published as an Advance Article on the web 27th March 2002
A rational synthesis for dibenzo[fg,op]naphthacene derivatives (also named as dibenzopyrene) is described. The
methodology involves the preparation of a quaterphenyl using a palladium-catalysed cross-coupling of
arylboronic acids followed by chemical or photochemical cyclization. The versatility of the process is shown by
the formation of various dibenzo[fg,op]naphthacene derivatives having identical or non-identical peripheral
chains. All the new compounds exhibit a single mesophase, which has been identified by X-ray diffractometry
and optical microscopy as hexagonal columnar (Colh). X-Ray results confirm that hexasubstituted
dibenzonaphthacenes are more ordered than octasubstituted derivatives.
many recent publications. Bock and Helfrich have demon-
Introduction
strated for the first time the existence of ferroelectricity in the
columnar liquid crystals of 1,2,5,6,8,9,12,13-octakis[(S)-2-
heptyloxypropanoyloxy)dibenzopyrene.8,9 Synthesis of various
octaalkoxy-substituted DBP and their photophysical proper-
ties have been reported by the Ringsdorf and Markovitsi
groups.10,11 Mesomorphic properties of five homologues of
the 1,2,5,6,8,9,12,13-octaalkoxy-DBP (Fig. 1) and their charge
transfer complexes with 2,4,7-trinitrofluoren-9-one are
reported by Zamir et al.12 The electro-optical and electro-
mechanical effects in these materials were also reported very
recently.13,14
Because of the larger core having more delocalized p
electrons than triphenylene, a greater degree of p–p interaction
and hence higher charge carrier mobility was expected in
these derivatives.10 However, the charge carrier mobility in the
octasubstituted DBP derivatives was found to be one order of
magnitude lower than in the columnar phase of hexaalkoxy-
triphenylenes.15 This could be because of less ordered columnar
packing due to the steric hindrance caused by the ‘bay region’
alkoxy chains and this results in lower charge carrier mobility.
It is anticipated that the removal of these alkoxy chains from
the 1 and 8 positions will give a better core–core interaction
and, therefore, high charge carrier mobility. Energy minimized
structures of an octasubstituted DBP (Fig. 2a) and hexasub-
stituted DBP (Fig. 2b) clearly suggest that hexasubstituted
DBP derivatives should form more ordered columnar meso-
phase due to less steric hindrance of the bay region side chains.
All the dibenzo[fg,op]naphthacene discotic liquid crystals
prepared so far are octasubstituted. These derivatives were
synthesized following the method of Musgrave and Webster16
which involves the oxidation of 3,3’,4,4’-tetramethoxybiphenyl
The discovery of discotic liquid crystals by Chandrasekhar in
1977 has opened up a whole new field of liquid crystal
research.1 In general, these mesogens have flat or nearly flat
aromatic cores surrounded by more than one long aliphatic
chain. The mesophases of these compounds usually have a
columnar structure in which the disks are stacked one on top of
the other to form columns. Attractive intermolecular interac-
tions between the aromatic cores are primarily responsible for
the stacking of cores. One may expect that, up to some extent,
increasing the core–core attractive interactions (for example
by large polycyclic aromatic cores) while maintaining some
minimal constraints on the number, size, and nature of the
side chains, would enhance molecular stacking and lead to
the formation of novel liquid crystalline mesophases. This
prompted researchers around the world to explore several
polycondensed aromatics for the formation of discotic liquid
crystals. A number of liquid crystals based on naphthalene,
anthracene, phenanthrene, triphenylene, perylene, dibenzo-
pyrene, etc., have been prepared and extensively studied.2
These materials are of increasing interest as self-organizing
molecular wires through which charge or excitons can migrate
rapidly.3 Conductivity along the molecular columns was
reported to be several orders of magnitude higher than that
observed for organic polymers.4 Very high charge carrier
mobility has been observed in the highly ordered columnar
phases of various discotic liquid crystals.4,5 Because of these
properties, their potential applications in conducting, photo-
conducting systems, optical data storage, light emitting diodes,
photovoltaic solar cells, gas sensors, and other devices have
been envisaged.3,4 It has recently been demonstrated that the
degree of order determines charge mobility in columnar liquid
crystalline materials.5b,c It is expected that an increase in the
orbital overlap area will lead to higher charge mobility. Van de
Craats has very recently observed a positive effect of increasing
the macrocyclic core size on the intracolumnar charge carrier
mobility. Hexabenzocoronenes show remarkably high charge
carrier mobility (up to 0.38 cm2 V21 s21) in the liquid cry-
stalline phase.6 Schmidt-Mende et al. recently used hexaben-
zocoronene based discotic liquid crystals as a hole transporting
layer to construct an efficient organic photovoltaic solar cell.7
Discotic liquid crystals based on dibenzo[fg,op]naphthacene,
also named as dibenzopyrene (DBP), have been the subject of
Fig. 1 Structure of octaalkoxydibenzonaphthacene derivatives.
DOI: 10.1039/b111067p
J. Mater. Chem., 2002, 12, 1335–1341
This journal is # The Royal Society of Chemistry 2002
1335

View Article Online
oxide. Thin-layer chromatography (TLC) was performed on
aluminium sheets precoated with silica gel (Merck, Kieselgel
60, F254). HPLC analysis was performed on a Merck-Hitachi
LaChrom 2000 machine using an UV detector and Merck
LiChrospher Si 60 (5mm) column. Mass spectra were recorded
on a JEOL J600H spectrometer in FAB1 mode using
m-nitrobenzyl alcohol (NBA) matrix. NMR spectra were
recorded on a 200 or 400 MHz machine (Bruker). All chemical
shifts are reported in d units downfield from Me4Si, and J
values are given in Hz. Transition temperatures were measured
using a Mettler FP82HT hot stage and central processor in
conjunction with a Leitz DMRXP polarizing microscope as
well as by differential scanning calorimetry (DSC7 Perkin-
Elmer). X-Ray studies were performed using an image plate
detector (MAC Science DIP1030). Unoriented samples con-
tained in sealed Lindemann glass capillaries were irradiated
with Cu Ka rays obtained from a sealed-tube generator (Enraf-
Nonius FR 590) in conjunction with double mirror focusing
optics. The synthesis of different hexaalkoxy-DBN derivatives
is outlined in Scheme 1. The 2,2’-dibromo-4,4’-dihydroxybi-
phenyl 3 was prepared as reported.19
Fig. 2 MM2 Energy minimised structures of (a) octaalkoxydibenzo-
naphthacene and (b) hexaalkoxydibenzonaphthacene derivatives.
by chloranil in 70% sulfuric acid to 2,5,6,9,12,13-hexamethoxy-
dibenzo[fg,op]naphthacene-1,8-quinone and its 1,10-quinone
isomer. Reductive acetylation followed by alkylation resulted
in the synthesis of octasubstituted-DBP liquid crystals. Direct
oxidation of tetrakis(pentyloxy)biphenyl by chloranil to
hexakis(pentyloxy)dibenzo[fg,op]naphthacene-1,8-quinone and
its conversion to octakis(pentyloxy)-DBP by the above
mentioned procedure has also been reported by the Ringsdorf
group.10 We have recently reported that VOCl3 can be
efficiently utilized under very mild reaction conditions for the
preparation of two quinones.17
Hitherto, the synthesis of any low degree substituted
DBP discotic liquid crystal is not known. In a preliminary
communication, we have recently reported a novel, regiospe-
cific synthesis of variable degree substituted dibenzo[fg,op]-
naphthacene derivatives.18 The versatility of the process is now
examined by applying it to the synthesis of various dibenzo-
[fg,op]naphthacene derivatives having identical and non-
identical peripheral chains.
Experimental
General information
Chemicals and solvents (AR quality) were used without any
purification. Column chromatographic separations were per-
formed on silica gel (230–400 mesh) and neutral aluminium
Scheme 1 Synthesis of hexaalkoxydibenzonaphthacene derivatives.
1336
J. Mater. Chem., 2002, 12, 1335–1341

View Article Online
Synthesis of quaterphenyls 8a–f; general procedure I
Synthesis of 2,2’-dibromo-4,4’-dialkoxybiphenyls 4a–f; general
procedure:
A mixture of 3,4-dialkoxyphenylboronic acid 6a–f (4 mmol) in
ethanol–ether (2 : 1, y5 ml) was added to a stirred mixture of
2,2’-dibromo-4,4’-dialkoxybiphenyl 4a–f (1 mmol), tetrakis-
(triphenylphosphine)palladium(0) (0.04 mmol) and 2 M aqu-
eous sodium carbonate (10 ml) in degassed THF (15 ml). The
reaction mixture was refluxed for 20 h under nitrogen. After
cooling to room temperature, it was poured over water and
extracted with diethyl ether (4 6 30 ml). The combined extracts
were washed with water followed by brine and dried over
anhydrous sodium sulfate. Solvent was removed under vacuum
and the crude product was purified by column chromatography
over silica gel. Elution of the column with hexane–dichloro-
methane yielded quaterphenyls 8a–f in 25–30% yield.
Powdered KOH (448 mg, 8 mmol) was mixed with DMSO
(2 ml) at room temperature and stirred for 10 min. 2,2’-
Dibromo-4,4’-dihydroxybiphenyl (344 mg, 1 mmol) followed
by appropriate alkyl bromide (4 mmol) was added and the
reaction mixture was stirred at 60 uC for 12 h and then worked
up by adding ice–water and extracting with hexane. The
combined extracts were washed with water followed by brine
and dried over anhydrous sodium sulfate. Solvent was removed
under vacuum and the crude product was purified by column
chromatography over silica gel. Elution of the column with
hexane yielded 2,2’-dibromo-4,4’-dialkoxybiphenyls 4a–f in
60–80% yield.
General procedure II
4a. dH (200 MHz, CDCl3): 7.19 (2H, d, J 2.5), 7.12 (2H, d, J
8.5), 6.88 (2H, dd, J 2.5, 8.5), 3.98 (4H, t, J 6.4), 1.8 (4H, m),
1.52 (4H, m), 0.99 (6H, t, J 7.2).
A mixture of diboronic acid 5b or 5d (1 mmol), 3,4-
dialkoxyiodobenzene 7 (2.2 mmol), tetrakis(triphenylphos-
phine)palladium(0) (0.03 mmol) and 2 M aqueous sodium
carbonate (10 ml) in degassed DME (50 ml) was refluxed for
15 h under nitrogen. Work-up and purification as mentioned
above yielded quaterphenyls 8b or 8d in about 20% yield.
4b. dH (200 MHz, CDCl3): 7.19 (2H, d, J 2.5), 7.12 (2H, d, J
8.5), 6.88 (2H, dd, J 2.5, 8.5), 3.97 (4H, t, J 6.5), 1.8 (4H, m),
1.52 (8H, m), 0.94 (6H, t, J 7.2); m/z 484 (M1, 100%).
8a–f. All the quaterphenyls give similar 1H NMR spectra
differing in only the number of alkyl chain CH2 protons. dH
(200 MHz, CDCl3): 7.27 (2H, d, J 8.4), 6.84 (2H, dd, J 2.6, 8.4),
6.69 (2H, d, J 2.6), 6.52 (2H, d, J 8.1), 6.17 (2H, br s), 6.17 (2H,
br s), 3.94 (8H, m), 3.54 (4H, m), 1.8–1.3 (alkyl chain CH2, m),
0.86 (18H, t, J 6.7).
4c. dH (200 MHz, CDCl3): 7.19 (2H, d, J 2.5), 7.12 (2H, d, J
8.5), 6.88 (2H, dd, J 2.5, 8.5), 3.98 (4H, t, J 6.4), 1.8–1.3 (16H,
m), 0.92 (6H, m); m/z 512.3 (M1, 100%).
4d. dH (200 MHz, CDCl3): 7.19 (2H, d, J 2.5), 7.12 (2H, d, J
8.5), 6.88 (2H, dd, J 2.5, 8.5), 3.97 (4H, t, J 6.4), 1.8–1.3 (20H,
m), 0.88 (6H, m); m/z 540.4 (M1, 100%).
Mass spectra of all the compounds give correct molecular ion
peaks.
4e. dH (200 MHz, CDCl3): 7.19 (2H, d, J 2.5), 7.12 (2H, d, J
8.5), 6.88 (2H, dd, J 2.5, 8.5), 3.97 (4H, t, J 6.4), 1.8–1.3 (24H,
m), 0.88 (6H, m); m/z 568.4 (M1, 30%).
Synthesis of hexaalkoxydibenzo[fg,op]naphthacenes 9a–f;
general procedure
Photocyclization method. An Ace Glass Incorporated photo-
chemical reaction assembly (catalog no. 7840) was used for the
photochemical cyclization reactions. An equimolar mixture of
quaterphenyl 8a–f and iodine in dry benzene (200 ml) was
irradiated for 48 h in argon atmosphere. Aqueous sodium
thiosulfate solution (20%, 100 ml) was added in the reaction
mixture and it was extracted with dichloromethane. The
combined extracts were washed with water and dried over
anhydrous sodium sulfate. Solvent was removed under vacuum
and the crude product was purified by repeated column
chromatography over silica gel and neutral alumina. Elution of
the column with hexane–dichloromethane yielded hexaalkoxy-
dibenzo[fg,op]naphthacene 9a–f in 10–15% yield.
4f. dH (200 MHz, CDCl3): 7.19 (2H, d, J 2.5), 7.12 (2H, d, J
8.5), 6.88 (2H, dd, J 2.5, 8.5), 4.0 (4H, t, J 6.9), 1.8–1.3 (20H,
m), 0.95 (6H, d, J 6.3), 0.88 (12H, d, J 6.5); m/z 624.2 (M1,
25%).
Synthesis of diboronic acids 5b and 5d; general procedure:
To a solution of 2,2’-dibromo-4,4’-dialkoxybiphenyl (1 mmol)
in dry THF (20 ml), a 2.5 M solution of n-butyllithium in
hexane (2.2 mmol) was added dropwise at 278 uC. The
reaction mixture was stirred at this temperature for 1 h under
argon. A solution of trimethyl borate (4.4 mmol) in dry THF
was added dropwise at the same temperature. The reaction
mixture was allowed to warm to room temperature overnight
and then stirred with 10% aqueous HCl (20 ml) for 2 h. The
product was extracted with ethyl acetate. Combined extracts
were dried over anhydrous sodium sulfate. Solvent was
removed under vacuum and the crude product was purified
by column chromatography over silica gel. Elution of the
column with ethyl acetate yielded the diboronic acid in 20%
yield.
Chemical cyclization method. To a solution of quaterphenyl
8b (1 mol) in dry dichloromethane (10 ml), anhydrous FeCl3
(4 mmol) was added. The reaction mixture was stirred at room
temperature for 30 min under N2 atmosphere. The reaction was
quenched with methanol (5 ml) and after that it was poured
over ice–water (20 ml) and extracted with CH2Cl2 (4 6 20 ml).
Purification of the crude product as mentioned above furnished
the hexakis(pentyloxy)-DBN 9b in 10% yield.
5b. dH (200 MHz, CDCl3): 7.51 (2H, d, J 8.7), 7.35 (2H, d, J
2.8), 7.05 (2H, dd, J 8.7, 2.8), 4.82 (2H, s), 4.01 (4H, t, J 6.5), 1.8
(4H, m), 1.6–1.3 (8H, m), 0.93 (6H, t, J 6.8).
9a. 1H NMR (400 MHz, CDCl3): d 8.10 (4H, s), 8.02 (4H, s),
4.31 (4H, t, J 6.4), 4.24 (8H, t, J 6.5), 1.9 (12H, m), 1.5 (12H,
m), 1.0 (18H, m); 13C NMR (50 MHz, CDCl3): d 157.0, 150.0,
130.2, 124.8, 118.0, 107.7, 106.6, 69.3, 68.3, 31.7, 31.5, 19.4,
14.0; HPLC: 99.0% (silica, hexane–dichloromethane: 7 : 3,
flow 1.0 ml min21, retention time 14.36 min); MS: m/z 734.3
(100%); elemental anal: calcd for C48H62O6, C 78.44, H 8.50:
found, C 78.26, H 8.65%.
5d. dH (200 MHz, CDCl3): 7.51 (2H, d, J 8.7), 7.35 (2H, d, J
2.8), 7.04 (2H, dd, J 8.7, 2.8), 4.78 (2H, s), 4.01 (4H, t, J 6.5), 1.8
(4H, m), 1.6–1.3 (16H, m), 0.93 (6H, t, J 6.8).
Synthesis of 3,4-dialkoxyphenylboronic acids 6a–f
IR data: the same spectrum is observed for all compounds
9a–f: nmax/cm21 2934, 1604, 1532, 1408, 1381, 1263, 1177, 1075.
UV–vis data: all samples were measured in dichloromethane
3,4-Dialkoxyphenylboronic acids 6a–f were prepared following
literature methods.20,21
J. Mater. Chem., 2002, 12, 1335–1341
1337

View Article Online
and similar spectrum is observed for all the compounds.
lmax/nm 268, 286, 307, 337, 354, 377, 398.
Thermal behavior
All the DBN derivatives 9a–f were found to be liquid crystalline
with a very broad mesophase temperature range. The nature of
these new liquid crystals was studied by differential scanning
calorimetry (DSC) and by optical microscopy with polarized
light. Transition temperatures and transition enthalpies were
determined by DSC. Data obtained from the heating and
cooling cycles of DSC are collected in Table 1. The peak
temperatures are given in uC and the numbers in parentheses
9b. 1H NMR (400 MHz, CDCl3): d 8.18 (4H, s), 8.09 (4H, s),
4.37 (4H, t, J 6.4), 4.30 (8H, t, J 6.5), 2.0 (12H, m), 1.57 (24H,
m), 1.0 (18H, m); 13C NMR (50 MHz, CDCl3): d 157.0, 149.9,
130.1, 124.7, 107.5, 106.5, 69.5, 68.6, 29.1, 28.4, 22.6, 14.1;
HPLC: 99.9% (silica, hexane–dichloromethane: 7 : 3, flow
1.0 ml min21, retention time 8.31 min); MS: m/z 819.4 (40%);
elemental anal: calcd for C54H74O6, C 79.18, H 9.11: found, C
79.30, H 9.32%.
indicate the transition enthalpy (DH) in kJ mol21
.
As may be seen from the data, all compounds exhibit a
very similar behaviour, viz. they form a single enantiotropic
mesophase with a broad mesophase range. Isotropic tempera-
ture decreases on increasing number of carbon atoms in the
peripheral chains. For the crystalline to mesophase transition,
DH increases with increasing number of carbon atoms in side
chains. This may reflect selective melting of the side chains.
Conversely, the enthalpy difference associated with the
transition from mesophase to isotropic liquid is approximately
constant, reflecting unstacking of the aromatic cores. This
behavior is typical of discotic mesophases. The degree of
supercooling observed for columnar mesophase to isotropic
temperature is not significant compared with that found for
crystalline to columnar phase temperature. Similar behaviour
has been shown for many triphenylene based discotic liquid
crystals.20 It is interesting to compare the thermal behaviour of
these hexaalkoxy-DBP derivatives with octaalkoxy-DBP
derivatives.12 All the hexasubstituted derivatives are thermally
more stable (having higher isotropic temperature) than the
octasubstituted dibenzopyrenes. The lower melting and clear-
ing temperatures of octaalkoxy-DBP could be due to the
presence of two extra alkoxy chains and the steric hindrance
caused by these chains. Triphenylene based discotic liquid
crystals having an extra alkoxy chain in the bay region also
exhibit similar effects. 1,2,3,6,7,10,11-Heptaalkoxytripheny-
lenes have much lower melting and isotropic temperatures
compared to symmetrically substituted 2,3,6,7,10,11-hexaalko-
xytriphenylenes.23 Seven-tail triphenylene discotic liquid crys-
tals in which one of the peripheral chains is attached to the core
via an amide linkage show very high clearing temperatures.24
This is due to the H-bonding of the amide group and, therefore,
can not be compared with the normal alkoxy triphenylene
derivatives. The use of branched chains often reduces melting
and isotropic temperatures.25 Some effects of the introduction
of branched-chains into mesogens on the mesomorphism have
been summarised by Ohta and co-workers.25b On replacing two
n-butyloxy tails in compound 9a by branched chains, both
melting and clearing temperatures were lowered. The resulting
compound 9f exhibits an isotropic temperature about 71 uC
lower than 9a. The results are in accordance with the
9c. 1H NMR (400 MHz, CDCl3): d 8.18 (4H, s), 8.02 (4H, s),
4.29 (4H, t, J 6.5), 4.23 (8H, t, J 6.5), 1.91 (12H, m), 1.5 (36H,
m), 1.0 (18H, m); HPLC: 98% (silica, hexane–dichloromethane:
8 : 2, flow 1.0 ml min21, retention time 12.63 min); other
spectral data could not be obtained because of the paucity of
the material.
9d. 1H NMR (400 MHz, CDCl3): d 8.10 (4H, s), 8.01 (4H, s),
4.30 (4H, t, J 6.3), 4.23 (8H, t, J 6.3), 1.92 (12H, m), 1.5 (48H,
m), 0.85 (18H, brs); 13C NMR (100 MHz, CDCl3): d 157.2,
150.1, 130.4, 124.9, 118.2, 107.8, 106.7, 69.8, 68.8, 32.1, 29.9,
29.7, 29.4, 26.5, 22.9, 14.3; HPLC: 99.6% (silica, hexane–
dichloromethane: 7 : 3, flow 1.0 ml min21, retention time
11.50 min); elemental anal: calcd for C66H98O6, C 80.28, H
10.00: found, C 80.34, H 10.28%.
9e. 1H NMR (400 MHz, CDCl3): d 8.18 (4H, s), 8.09 (4H, s),
4.37 (4H, t, J 6.6), 4.30 (8H, t, J 6.6), 1.99 (12H, m), 1.5 (60H,
m), 0.90 (18H, t, J 6.6). ); HPLC: 98.5% (silica, hexane–
dichloromethane: 8.5 : 1.5, flow 1.0 ml min21, retention time
11.50 min). Other spectral data could not be obtained because
of the paucity of the material.
9f. 1H NMR (400 MHz, CDCl3): d 8.18 (s, 4H), 8.09 (s, 4H),
4.41 (t, J 6.6, 4H), 4.31 (t, J 6.6, 8H), 2.0–1.3 (m, 42H), 1.07 (m,
18H), 0.89 (d, J 6.5, 6H); 13C NMR (100 MHz, CDCl3): d
156.9, 150.0, 130.1, 124.8, 118.2, 107.8, 106.5, 69.3, 66.9, 39.3,
37.4, 36.6, 31.5, 30.0, 27.9, 24.7, 22.7, 19.8, 19.3, 13.9; HPLC:
99.9% (silica, hexane–dichloromethane: 7 : 3, flow 1.0 ml min21
,
retention time 7.58 min); MS: m/z 902.2 (100%); elemental anal:
calcd for C60H86O6, C 79.78, H 9.60: found, C 79.69, H 9.82%.
Results and discussion
Synthesis
For the synthesis of various dibenzo[fg,op]naphthacene (DBN)
derivatives 9a–f the strategy outlined in Scheme 1 was
envisaged. The synthesis is based on the preparation of key
intermediate quaterphenyl 8 starting from 3-nitrobromoben-
zene 1. The 2,2’-dibromo-4,4’-dihydroxybiphenyl 3 was pre-
pared as reported by Cornforth.19 Alkylation of the diphenol
with the appropriate 1-bromoalkane yielded 2,2’-dibromo-
4,4’-dialkoxybiphenyls 4. The quaterphenyls 8 were prepared
either by coupling 4 with dialkoxyboronic acids 6 or by first
converting the dibromobiphenyls 4 to diboronic acids 5 and
then reacting with dialkoxyiodobenzenes 7. Photocyclodehy-
drogenation of the quaterphenyls 8 in the presence of an excess
of iodine furnished the desired 2,5,6,9,12,13-hexaalkoxy-DBN
derivatives 9a–f. The cyclization can also be achieved by
oxidative coupling using FeCl3 as oxidizing agent. The use of
FeCl3 for the synthesis of various triphenylene derivatives is
well documented.20,22 Final compounds were characterized
from their 1H NMR, 13C NMR, IR, UV–vis, mass and
elemental analysis.
Table 1 Phase transition temperatures (peak temp.) and enthalpies of
hexasubstituted dibenzopyrene derivatives. Cr ~ crystal, Colh
hexagonal columnar liquid crystalline phase, I ~ isotropic
~
Thermal transitions/uC and enthalpy
changes/kJ mol21 in parentheses
Compound
Heating scan
Cooling scan
9a
9b
Cr 147.7 (27.7) Colh
236.9 (2.9) I
Cr 120.5 (52.1) Colh
223.1 (4.1) I
Cr 143 Colh 188 I
Cr 143.4 (86.2) Colh
179.7 (2.8) I
Cr 134 Colh 161 I
Cr 118.7 (63.8) Colh
165.5 (2.8) I
I 234.4 (2.6) Colh
117.0 (29.4) Cr
I 219.9 (3.7) Colh
96.3 (54.0) Cr
I 187 Colh 102 Cr
I 178.2 (2.6) Colh
103.4 (85.8) Cr
I 159 Colh 96 Cr
I 161.4 (4.3) Colh
87.8 (63.6) Cr
9ca
9d
9ea
9f
aBased on polarizing microscopy.
1338
J. Mater. Chem., 2002, 12, 1335–1341

View Article Online
Fig. 4 (a) X-Ray diffraction pattern obtained for compound O5DBP at
27 uC; (b) x-averaged one-dimensional intensity vs. 2h profile. The
sharp peak at low angle (2h # 5u) corresponds to the disc diameter.
Whereas the first diffuse peak at 2h # 18u is from the aliphatic chains,
the fairly sharp peak at higher angles (y25u) is due to the stacking of
˚
the rigid cores within a column giving a core–core separation of 3.63 A.
at fixed intervals from the isotropic phase, viz., y10 uC, 30 uC
and 70 uC. Fig. 4a and 4b show a representative diffraction
pattern and the derived one-dimensional intensity vs. 2h profile
obtained at 27 uC for O5DBP; Fig. 5a and 5b show the same
for the compound 9b at 120 uC. In the low-angle region, four
sharp peaks—one strong and three weak reflections—are seen
whose d-spacings are in the ratio of 1 : 1/d3 : 1/d4 : 1/d7.
Identifying the first peak with the Miller index 100, the ratios
conform to the expected values from a two-dimensional
hexagonal lattice. In the wide-angle region two diffuse
Fig. 3 (a) Optical textures of 9a obtained on cooling from the isotropic
liquid at 226 uC (crossed polarizer, magnification 6 200). (b) Optical
textures of 9d obtained on cooling from the isotropic liquid at 150 uC
(crossed polarizer, magnification 6 200). These textures resemble the
usual texture of columnar phase.
observations reported by Cross et al. for various unsymme-
trical triphenylene derivatives. In a number of unsymmetrical
triphenylene derivatives a maximum in the columnar phase–
isotropic clearing point is found for the most symmetrical
triphenylene, i.e. when all the six alkoxy chains are of equal
length.20 The lower melting and clearing temperatures of the
compound 9f could be due to the unsymmetrical nature (by
virtue of non-identical peripheral chains) and the presence of
two branched chains in the molecule. The decrease in the
transition temperature due to the disorder and stereohetero-
geneity caused by branched chains is well documented.25,26
Microscopy with polarized light suggested that all com-
pounds 9a–f form hexagonal columnar mesophases. Classical
textures of columnar mesophases appeared upon cooling
from the isotropic liquid. These textures are very similar to
the known textures for the Colh phases shown by several
well characterized discotic liquid crystals.27 Fig. 3 shows two
representative examples of such photomicroscopic pictures of
compound 9a and 9d obtained on cooling from the isotropic
liquid.
X-Ray diffraction studies
To verify our hypothesis that the hexaalkoxydibenzo[fg,op]-
naphthacene derivatives should form more ordered columnar
phases than octasubstituted derivatives, we prepared octakis-
(pentyloxy)dibenzo[fg,op]naphthacene (O5DBP, Fig. 1, R ~
C5H11)10 and performed X-ray investigations on both the
octakis(pentyloxy) (O5DBP) and hexakis(pentyloxy)dibenzo-
[fg,op]naphthacene (9b) derivatives under similar conditions.
Diffraction patterns for compound O5DBP with eight alkoxy
substituents and 9b with six alkoxy substituents were recorded
Fig. 5 (a) X-Ray diffraction pattern obtained for compound 9b at
120 uC; (b) x-averaged one-dimensional intensity vs. 2h profile. The
peaks at 2h # 5u, 19u and 25u are from the disc diameter, aliphatic
chains and due to the packing of rigid cores within a column.
J. Mater. Chem., 2002, 12, 1335–1341
1339

View Article Online
Table 2 X-Ray diffraction data for octa- and hexa-substituted
dibenzonaphthacene derivatives giving the disc diameter d100, the
core–core separation (dcore–core), and the correlation length for the
intracolumnar order
˚
d-Spacing numbers/A
dalkyl
Correlation
˚
length/A
Compound Temp./uC d100
dcore–core
chain
O5DBP
O5DBP
O5DBP
9b
9b
9b
87
67
27
210
190
150
120
21.19
21.20
21.87
21.17
21.17
21.08
20.98
3.89
3.81
3.63
3.60
3.59
3.55
3.53
4.51
4.43
4.23
4.84
4.80
4.75
4.70
37.84
41.83
82.12
93.78
113.86
123.32
143.82
9b
Fig. 7 Plot of correlation length vs. Tiso 2 T for O5DBP (open circle)
and for 9b (solid circle). Higher values for the correlation length in the
case of compound 9b than for the compound O5DBP indicate that the
phase is more ordered in compound 9b.
˚
reflections are seen. The broad one centered at y4.6 A
corresponds to the liquid-like order of the aliphatic chains. The
relatively sharper one seen at higher 2h-values is due to the
stacking of the molecular cores one on top of another. The
diffuse nature of the peak suggests that the stacking of discs
within each column is correlated over short distances only. All
these features are characteristics of the Colh phase.
a great deal of control in the modification of the dibenzo-
[fg,op]naphthacene discotic core. Whereas the synthesis of
octasubstituted dibenzo[fg,op]naphthacene to give low molar
mass discotic liquid crystals is fairly easy, the synthesis of any
low degree substituted dibenzo[fg,op]naphthacene has so far
not been achieved. A variety of dibenzo[fg,op]naphthacene
discotics can now be designed and created using the above
methodology. A comparison of the X-ray results for octasub-
stituted and hexasubstituted-dibenzo[fg,op]naphthacene clearly
indicate, as predicted, that hexasubstituted dibenzo[fg,op]-
naphthacene derivatives are more ordered than octasubstituted
derivatives and, therefore, are better candidates for charge
migration studies.
Table 2 summarizes the different spacings extracted by
fitting each of the peaks in the one-dimensional profile to a
Lorentzian form. Also shown is the correlation length for the
intracolumnar packing arising out of the core–core interaction.
The correlation length is calculated using the FWHM of the
sharper peak at higher angles and the Scherrer expression.28 As
can be noted from the table the disc diameter remains the same
˚
at y21 A for both O5DBP and 9b irrespective of the number of
alkoxy chain substituents, i.e., whether it is 8 or 6. Also the disc
diameter remains constant with varying temperature for both
the compounds.
Acknowledgement
Fig. 6 and 7 show plots of the average stacking distance
(or core–core separation) and core–core correlation length as a
function of Tiso 2 T, Tiso being the isotropic temperature. The
core–core distance shows a strong temperature dependence for
O5DBP but has a smaller variation for 9b. It should be noted
here that the value of the core–core distance is smaller for a
compound with six alkoxy substituents to the core (9b) than
that for one with eight alkoxy substituents (O5DBP). The core–
core correlation length is higher for the compound with six
alkoxy substituents (9b) than for the compound O5DBP with
eight alkoxy substituents. A probable reason is that the density
of the core–core packing increases as the number of alkoxy
substituents on the central core decreases. We have observed a
similar behaviour in a triphenylene based system also.23
In conclusion, the methodology outlined above has given us
We are very grateful to Professor S. Chandrasekhar and
Professor H. Ringsdorf for many helpful discussions. We thank
Professor A. Srikrishna for providing photolysis apparatus and
helpful discussions. Mr Sanjay K. Varshney is thanked for his
technical assistance.
References
1
S. Chandrasekhar, B. K. Sadashiva and K. A. Suresh, Pramana.,
1977, 9, 471.
2
A. N. Cammidge and R. J. Bushby, in Handbook of Liquid Crystals
ed. D. Demus, J. Goodby, G. W. Gray, H.-W. Spiess and V. Vill,
Wiley-VCH, 1998, Vol. 2B, Ch. VII.
3
4
N. Boden, R. Bissell, J. Clements and B. Movaghar, Liq. Cryst.
Today, 1996, 6, 1.
(a) S. Chandrasekhar, in Handbook of Liquid Crystals, ed.
D. Demus, J. Goodby, G. W. Gray, H.-W. Spiess and V. Vill,
Wiley-VCH, 1998, Vol. 2B, Ch. VIII; (b) N. Boden and
B. Movaghar, in Handbook of Liquid Crystals, ed. D. Demus,
J. Goodby, G. W. Gray, H.-W. Spiess and V. Vill, Wiley-VCH,
1998, Vol. 2B, Ch. IX.
5
(a) P. G. Schouten, J. M. Warman, M. P. deHaas, M. A. Fox and
H. L. Pan, Nature, 1991, 353, 736; (b) D. Adam, P. Schuhmacher,
J. Simmerer, L. Ha¨ussling, K. Siemensmeyer, K. H. Etzbach,
H. Ringsdorf and D. Haarer, Nature, 1994, 371, 141; (c) A. M. van
de Craats, J. M. Warman, M. P. deHaas, D. Adam, J. Simmerer,
D. Haarer and P. Schuhmacher, Adv. Mater., 1996, 8, 823;
(d) P. G. Schouten, J. M. Warman, M. P. deHaas, C. F. van
Nostrum, G. H. Gelinck, R. J. M. Nolte, M. J. Copyn,
J. W. Zwikker, M. K. Engel, M. Hanack, Y. H. Chang and
W. T. Ford, J. Am. Chem. Soc., 1994, 116, 6880.
6
7
(a) A. M. van de Craats, J. M. Warman, A. Fechtenkotter,
J. D. Brand, M. A. Harbison and K. Mullen, Adv. Mater., 1999,
11, 1469; (b) A. M. van de Craats and J. M. Warman, Adv. Mater.,
2001, 13, 130.
L. Schmidt-Mende, A. Fechtenkotter, K. Mullen, E. Moons,
R. H. Friend and J. D. Mackenzie, Science, 2001, 293, 1119.
H. Bock and W. Helfrich, Liq. Cryst., 1992, 12, 697.
H. Bock and W. Helfrich, Liq. Cryst., 1995, 18, 387.
Fig. 6 Plot of core–core spacing as a function Tiso 2 T for O5DBP
(open circle) and for the compound 9b (solid circle). Tiso being the
clearing point temperature. Notice the smaller core–core spacing for
the compound 9b than for the compound O5DBP indicating better
core–core packing in the compound 9b.
8
9
1340
J. Mater. Chem., 2002, 12, 1335–1341

View Article Online
10 P. Henderson, H. Ringsdorf and P. Schuhmacher, Liq. Cryst.,
1995, 18, 191.
11 P. Uznanski, S. Marguet, D. Markovitsi, P. Schuhmacher and
H. Ringsdorf, Mol. Cryst. Liq. Cryst., 1997, 293, 123.
12 S. Zamir, D. Singer, N. Spielberg, E. J. Wachtel, H. Zimmermann,
R. Poupko and Z. Luz, Liq. Cryst., 1996, 21, 39.
R. C. Borner, R. J. Bushby, A. N. Cammidge and M. V. Jesudason,
Liq. Cryst., 1993, 15, 851; (c) H. Naarmann, M. Hanack and
R. Mattmer, Synthesis, 1994, 477; (d) N. Boden, R. J. Bushby,
A. N. Cammidge and G. Headdock, Synthesis, 1995, 31.
23 S. Kumar, M. Manickam, S. K. Varshney, D. S. Shankar Rao and
S. K. Prasad, J. Mater. Chem., 2000, 10, 2483.
13 H. Kitzerow and H. Bock, Mol. Cryst. Liq. Cryst., 1997, 299, 117.
14 A. Jakli, M. Muller, D. Kruerke and G. Heppke, Liq. Cryst., 1998,
24, 467.
24 N. Boden, R. J. Bushby, A. N. Cammidge and G. Headdock,
J. Mater. Chem., 1995, 5, 2275.
25 (a) P. G. Schouten, J. F. van der Pol, J. W. Zwikker, W. Drenth
and S. J. Picken, Mol. Cryst. Liq. Cryst., 1991, 195, 291;
(b) K. Ohta, Y. Morizumi, H. Ema, T. Fujimoto and I.
Yamamoto, Mol. Cryst. Liq. Cryst., 1991, 208, 55; (c) D. M.
Collard and C. P. Lillya, J. Am. Chem. Soc., 1991, 113, 8577;
(d) S. Kumar, D. S. Shankar Rao and S. K. Prasad, J. Mater.
Chem., 1999, 9, 2751.
26 S. Kumar and S. K. Varshney, Angew. Chem., Int. Ed., 2000, 39,
3140.
27 (a) J. Billard, J. C. Dubois, N. H. Tinh and A. Zann, Nouv.
J. Chim., 1978, 2, 535; (b) S. Kumar, E. J. Wachtel and E. Keinan,
J. Org. Chem., 1993, 58, 3821.
15 Unpublished results of Ringsdorf group.
16 O. C. Musgrave and C. J. Webster, Chem. Commun., 1969, 712;
O. C. Musgrave and C. J. Webster, J. Chem. Soc. (C), 1971, 1393.
17 S. Kumar and S. K. Varshney, Synthesis, 2001, 305.
18 S. Kumar and J. J. Naidu, Liq. Cryst., 2001, 28, 1435.
19 J. Cornforth, R. H. Cornforth and R. T. Gray, J. Chem. Soc.,
Perkin Trans. 1, 1982, 2289.
20 S. J. Cross, J. W. Goodby, A. W. Hall, M. Hird, S. M. Kelly,
K. J. Toyne and C. Wu, Liq. Cryst., 1998, 25, 1.
21 T. Yatabe, M. A. Harbison, J. D. Brand, M. Wagner, K. Mullen,
P. Samori and J. P. Rabe, J. Mater. Chem., 2000, 10, 1519.
22 (a) H. Bengs, O. Karthaus, H. Ringsdorf, C. Baehr, M. Ebert and
J. M. Wendorff, Liq. Cryst., 1991, 10, 161; (b) N. Boden,
28 See e.g. X-ray Diffraction Procedures, ed. H. P. Klug and
L. E. Alexander, John Wiley & Sons, New York, 1954, ch. 9.
J. Mater. Chem., 2002, 12, 1335–1341
1341