Enforced liquid crystalline properties of dibenzo[a,c]phenazine dimer and self assembly

Article abstract of DOI:10.1039/c0jm01636e

Dibenzo[a,c]phenazine dimers with the following features: (i) short spacer, (ii) peripheral chains shorter and longer than the spacers, and (iii) reduced symmetrical structure of the disc were synthesized. The dibenzo[a,c]phenazine disc has enforced the s

Full text of DOI:10.1039/c0jm01636e

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Enforced liquid crystalline properties of dibenzo[a,c]phenazine dimer and self
assembly†
Mei-Chun Tzeng,a Su-Chih Liao,a Tsu-Hsin Chang,a Shih-Chi Yang,a Min-Wei Weng,b Hsiao-Ching Yang,b
a
Michael Y. Chiang, Zhang Kai, Jishan Wu and Chi Wi Ong
c
c
a
*
Received 28th May 2010, Accepted 25th October 2010
DOI: 10.1039/c0jm01636e
Dibenzo[a,c]phenazine dimers with the following features: (i) short spacer, (ii) peripheral chains shorter
and longer than the spacers, and (iii) reduced symmetrical structure of the disc were synthesized. The
dibenzo[a,c]phenazine disc has enforced the self-assembly of discotic dimers connected through six
methylene spacers to form mesophases through a folded conformation in the mesophase. An X-ray
diffraction study pinpoints the preferential intra-molecular bridging conformation of these dimers at
the mesophase. These new discotic dimers showed polymesomorphism and glass formation.
Interestingly, these discotic dimers form organogels with non-polar hydrocarbon solvents and long
chain aliphatic alcohols. A superstructure arising from association of the nanofibers was observed with
transmission electron microscopy.
a glassy state have been widely studied mainly because they are
Introduction
an ideal model for higher oligomers and polymers, and also due
to their ease in producing a pure product. The relationship
between the liquid crystalline behavior and the length and parity
of the flexible spacers in dimeric rod-shaped liquid crystalline
materials have been widely studied and understood.16,17
However, for the dimeric discotic-family of mesogens, the effect
of peripheral substituent and spacer has not been thoroughly
investigated and fully understood. Among the most studied
dimeric DLCs with a flexible spacer are the triphenylenes18–23
which have been recently reviewed24 together with other dimers
based on benzene,25,26 phthalocyanine,27 cyclotetraveratrylene28
and hexabenzocoronenes.29 In all the discotic dimers reported,
the spacer lengths are long and must be equal to or longer than
the peripheral side chains to show mesomorphism, a property
closely related to the ease of reorientation between the discogens
in neighboring columns with intercalated arrangement. The
mesophases for these dimeric discotic liquid crystals have
generally been discussed in the context of the discogens being
arranged in an inter-columnar manner19,24,25 with the two
subunits of the dimer residing in neighboring columns (non-
intercalated or intercalated), although the possibility of an intra-
molecular bridging with the two subunits residing in the same
column cannot be ruled out.20
Discotic liquid crystalline (DLCs) materials consisting of a disc-
like rigid aromatic core with several flexible aliphatic side chains
that form columnar order assemblies in the mesophase have
attracted attention for the past decades due to their promising
one-dimension charge and energy migration along their axes. As
a consequence, DLCs have become increasingly important for
potential applications as functional materials in field effect
transistors, photoconductors, light emitting diodes, photovoltaic
solar cells, and gas sensors.1–7 A high degree of long-range order
of the DLCs, which is important for the undisturbed one-
dimension charge migration along the column, has enabled them
to compete favorably with single-crystal materials.8 The presence
of microscopic local defects at the columnar axis significantly
reduced the charge mobility, and therefore the device perfor-
mance. Hence, a wide range of variation of the core and/or the
side chains of the discotic mesogens have been studied to ensure
long range ordering at the mesophase and a useful transition
temperature for fabrication of devices.5,6,9–11
It has now become apparent that for real applications as active
materials in devices, the ease of processing and mechanical
robustness of the DLCs have to be taken into consideration.
Therefore, recent interest has been centered on the synthesis of
novel functionalized dimers, oligomers and polymers with the
hope of attaining all the combinations of the properties necessary
for DLCs, especially in their ability to form highly ordered
anisotropic glasses on cooling down to below the glass transition
temperature.12–17 Dimeric liquid crystalline compounds with two
mesogenic units linked together by a flexible spacer that can form
The various possible stacking arrangements for the discotic
dimers are shown in Fig. 1. But, unlike the calamatic dimers
whereby the terminal chain can be longer than the spacer to give
a non-intercalated arrangement in the mesophase, such a system
has not been reported for the discotic dimers. For such unprec-
edented discotic dimers to show mesomorphism, we envisage
that this will involve mainly an intramolecular arrangement
between the core–core interactions which is unknown to date.
Substituted dibenzo[a,c]phenazine has been reported to enhance
stabilised columnar structure through an antiparallel packing of
the core and is a candidate of choice for this study.30,31a,b In the
present work, we hope to apply this property of the mesogenic
dibenzo[a,c]phenazine and extend the study to unconventional
discotic dimers containing peripheral chains longer than the
aDepartment of Chemistry, National Sun Yat-Sen University, Kaohsiung,
Taiwan 804. E-mail: cong@mail.nsysu.edu.tw; Tel: +886 7 5252000-3923
bDepartment of Chemistry, Fujen University, Hsing-Chuang, Taipei
County, Taiwan. E-mail: yanghc@gmail.com; Tel: +886 2-29052534
cDepartment of Chemistry, National Singapore University (ACP
Program), Singapore. E-mail: chmwuj@nus.edu.sg; Tel: +65-6516-2677
† Electronic supplementary information (ESI) available: further
experimental details. See DOI: 10.1039/c0jm01636e
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Fig. 1 Possible stacking of discotic dimers into columnar structures at the mesophase.
Scheme 1 The synthesis of dibenzo[a,c]phenazine dimers DiQXs and DiHDBPs.
spacer, and specifically with a much shorter spacer than reported,
with the hope of creating novel liquid-crystalline materials.
Interestingly, when four alkoxy peripheral chains are longer than
the spacer, this will at the same time create reduced symmetry in
each unit, an advantage for the formation of Col glasses. Such
a system has never been synthesized and this report will provide an
insight for the first time. We now report on the synthesis of this
new series of compounds which were found to exhibit meso-
morphism as determined by polarized optical microscopy (POM),
differential scanning calorimetry (DSC) and X-ray diffraction
(XRD). The anticipated advantage of having a reduced symmetry
in each unit of the dimer was found to give Col glass.
Scheme 1. Also, we envisioned that it will be more appropriate to
introduce the flexible methylene spacer of the desired discotic
dimers at an early stage through the bis-alkylation of the mono-
alkylated catechol. The versatile intermediate 2 can be readily
prepared from the reaction of monoalkylated catechol with the
appropriate dibromoalkane in good yield. A two steps nitration of
2 gave the tetra-nitration product 3 in good yield; and is subse-
quently reduced to the tetra-amine 4. The target dimeric discotic
compounds, DiQXs and DiHDBPs, are readily accessible by the
straightforward condensation of 4 with 1,2-diketone derivatives 5
and 6, respectively. The large structural differences between the
reactants and products at every stages of the synthesis had enabled
us to readily obtain DiQXs and DiHDBPs in high purity.
Results and discussion
Synthesis
Liquid crystalline properties (POM, DSC and XRD)
The synthetic strategy of the discotic dimers makes use of the
monoalkylated catechol31 as starting material and is outlined in
The phase-forming properties of bisquinoxaline DiQX(6,6,6) and
DiQX(6,6,12) were found to be non-mesogenic, melting directly
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from the crystalline solid to an isotropic liquid. The non-
planarity of the unfused aromatic ring precluded the mesophase
formation.
discotic system possessing the shortest spacer with mesogenic
properties known to date. Even more intriguing, DiHDBP(6,6,8)
with the peripheral side chains longer than the spacer length was
also found to be mesogenic under POM. The transition
temperatures and enthalpies of these two compounds measured
by DSC are summarized in Table 1. The XRD profile for
The new DiHDBP(6,6,6) with six methylene groups as the
spacer was found to be liquid crystalline under a polarized
optical microscope (POM) (Fig. 2a, c). This constitutes a dimeric
Fig. 2 Left: liquid crystalline properties of DiHDBP(6,6,6) (a) polarized optical micrograph of Colh mesophase at 115 ꢀC, ꢁ200, with cover glass in
cooling process (black region ¼ homeotropic); (b) the corresponding powder X-ray diffractogram of Colh; (c) polarized optical micrograph of the Colr
phase at room temperature, ꢁ200, with cover glass in cooling process; (d) the corresponding powder X-ray diffractogram of Colr. Right: Graphical
representation for the intercolumnar distance and mesophases as the peripheral length increases.
Table 1 The phase behaviors of DiQXs and DiHDBPs. The transition temperatures (ꢀC) and enthalpies (in parentheses/kJ molꢂ1) were determined by
a
DSC at 10 ꢀC minꢂ1
.
Range of columnar
phase/ꢀC
Persistent time of
supercooling Col phase
Compound
DiQX(6,6,6)
DiQX(6,6,12)
Phase transition
DiHDBP(6,6,6)
DiHDBP(6,6,8)
DiHDBP(6,6,10)
DiHDBP(6,6,12)
DiHDBP(6,10,6)
109.6
several hours
79.6
88.0
several hours
several hours
55.0
several hours
128.0
127.4
4 months > T > 1 month
126.0
125.7
DiHDBP(6,12,6)
>4 months
a
K, crystalline phase; Colh, hexagonal columnar mesophase; I, isotropic phase; G, glassy state. Persistent time of supercooling Col phase: the time
needed for supercooled Col phase to return to crystalline at room temperature.
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DiHDBP(6,6,6) in Fig. 2 showed at the small angle region one
very sharp strong reflection indexed as (100) and the second weak
reflection indexed as (110) with a reciprocal spacing ratio of
1:(O3) for a Colh lattice at high temperature; and on cooling the
reflection indexed at (100) splits into two reflections corre-
sponding to the Miller indices of (110) and (200) for a Colr lattice.
The texture from POM at these temperatures also showed the
change from Colh to Colr phase. The number of molecules in an
ab*c unit is generally calculated to be 2 for a rectangular lattice
based on the formula32 Z ¼ (r{(abc)*10ꢂ24}N)/M, whereby r is
the density, a and b are the lattice constants of the rectangular
of fully extended DiHDBP(6,6,6) (extrapolating from the struc-
ꢀ
ture) gave 46.46 A, much larger than that observed in the XRD
ꢀ
with an intercolumnar distance of 31.74 A. This discrepancy
becomes more obvious later with longer spacer length. This
observation, together with the ease of complete reorientation,
indicates the subunits of the dimer are residing in the same
column (intramolecular). The P6mm hexagonal phase trans-
formation to the P2m rectangular phase is illustrated in Fig. 3. At
first, on cooling down, the side chains became more rigid, leading
to the slight expansion of the hexagonal b parameter (Fig. 3b).
The final transformation of the hexagonal phase to the P2m
rectangle phase can be envisioned as the gliding of the central
columns vertically (Fig. 3c), this time causing further expansion
laterally between the columns. This is in line with the a0 and
b0 parameters of the P2m rectangle observed for the XRD
(Table 2). Thus, for DiHDBP(6,6,6) and DiHDBP(6,6,8), the
columns can slide freely and randomly reorientate about the
columnar axes. Our dibenzo[a,c]phenazine is the first dimeric
system with a short spacer to show mesomorphism reported to
date. Although Boden21 had implied that dimeric discotics with
a short spacer can have low strain bridging only in an intra-
molecular manner (intra-1,2 and intra-1,3), no such short spacer
has been shown to be liquid crystalline. Furthermore, since
packing of the dimer can be inter-molecular and intra-molecular
(Fig. 1) one will expect a greater ease of the intra-molecular
packing to undergo reorientation. As such, it appears that the
dimer adopts a folded conformation in the liquid crystalline
state. Furthermore, a simplified Universal Force Field calcu-
lation34a,b of the unsubstituted DiHDBP-dimer shows that the
ꢀ
mesophase (A), c is the mean stacking distance within the column
ꢀ
(A), M is the molecular weight and N is Avogadro’s number.
However, the number of molecules (Z) in the rectangular unit cell
for DiHDBP(6,6,6) and DiHDBP(6,6,8) were calculated to be
around 1 when the density was assumed to be 1 g cmꢂ3. This
implies that their mesophases have a rectangular lattice with
a Colr (P2m) symmetry.32,33 Calculation of the molecular length
Fig. 3 The transformation from hexagonal to rectangular packing.
Table 2 Variable-temparature XRD data for DiHDBPs
Obsd (calcd)
ꢀ
spacing/A
Number of
Molecule in Colr (Z)
ꢀ
Lattice constant/A
ꢀ
Halos obsd/A
Compound
Mesophase
Miller indices
DiHDBP(6,6,6)
Colho at 120 ꢀ
C
a ¼ 21.52
18.64
10.71
19.30
15.87
19.85
11.46
9.96
20.88
16.80
10.55
7.49
21.47
12.37
22.02
8.13
10.76
18.73
10.82
7.09
18.92
16.95
10.55
8.44
19.04
11.00
9.61
7.18
19.16
17.22
(100)
4.64, 3.56
4.46, 3.58
4.56, 3.58
(110)/(003)
(110)
(200)
(100)
(110)
(200)
(110)
(200)
(310)/(003)
(420)
(100)
(110)
(100)
(210)
(003)
(100)
(100)/(003)
(210)/(002)
(110)
Colro at 30 ꢀC
Colho at 85 ꢀ
a ¼ 31.74, b ¼ 24.31
a ¼ 22.92
Z ¼ 0.99
Z ¼ 1.00
DiHDBP(6,6,8)
C
Colro at 30 ꢀC
a ¼ 33.60, b ¼ 26.65
4.42, 3.52
DiHDBP(6,6,10)
DiHDBP(6,6,12)
Colho at 50 ꢀ
Colho at 50 ꢀ
C
C
a ¼ 24.79
a ¼ 25.43
4.50, 3.54
4.54, 3.55
DiHDBP(6,10,6)
Colho at 120 ꢀ
C
a ¼ 21.63
4.54, 3.50
4.42, 3.55
Colro at 30 ꢀC
a ¼ 33.90, b ¼ 22.80
Z ¼ 0.95
(200)
(300)/(003)
(400)
(100)
(110)
(200)
(210)/(002)
(110)
(200)
DiHDBP(6,12,6)
Colho at 120 ꢀ
C
a ¼ 21.99
4.60, 3.50
4.50, 3.53
Colro at 30 ꢀC
a ¼ 34.44, b ¼ 23.06
Z ¼ 0.96
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no crystallization was detected by DSC and XRD, revealing the
formation of Colh glassy phase. Surprisingly, DiHDBP(6,6,12),
whereby the peripheral tetraalkoxy chain is twice the spacer
length was also found to be mesogenic. The increasing peripheral
chain length in DiHDBP(6,6,12) further increases the enthalpies
of the phase transitions (second heating), a property mainly
observed for main-chain discotic polymers, whereby accounting
for their Colh glass phase. It is likely that the intra-molecular
stacked columns can be further interlinked by their side-groups
through enhanced interdigitalization. Thus DiHDBP(6,6,10) and
DiHDBP(6,6,12) mesogenic properties behave rather similar to
that of the polymers.35 The influence of increasing the length of
the side-chains on the intracolumnar distance and mesogenic
temperature is summarized in Fig. 2. It can be clearly seen that as
the side-chain length increases the intercolumnar distance also
increases, and there is a lowering of the melting (TK-Col) and
clearing temperature (TCol-I).
Fig. 4 The possible structures by the force field calculations viewed from
the side (without the methoxy groups for clarity).
extended (all-trans) conformation A is less favourable than the
folded conformation B (calculated to be about 20 Kcal molꢂ1
)
(Fig. 4). The ability of DiHDBP(6,6,6) to reorientate readily
from Colh to Colr can be taken as an indication of the two
subunits of the dimer residing in the same column (intra-
molecular packing of the folded conformation), and in agree-
ment with our modeling results.
Although our data indicates the intra-molecular packing of the
dimeric discotic system at the mesophase, at this point we cannot
totally rule out the possibility of the inter-molecular participa-
tion. We hope to prove the true nature of the intra-molecular
stacking by synthesizing DiHDBP(6,6,10) and DiHDBP(6,6,12)
which have an increased length of the tetraalkoxy peripheral side
chains (R), while maintaining the length of the short spacer
(-O(CH2)6O-) and the internal chain. The extremely long tet-
raalkoxy peripheral chains in the dimeric discotic molecules must
surely forbid the individual discs from aligning on top of each
other within a column in the intermolecular stacking manner. If
these dimeric compounds showed mesomorphism, this must be
derived solely from an intra-molecular stacking into ordered
columns. More importantly, the individual discs in this dimer
also becomes less symmetrical, whereby introducing an extra
degree of disorder that can favor the glassy phase at room
temperature. DiHDBP(6,6,10), with a four methylene group
longer spacer, was also found to be liquid crystalline under POM.
DiHDBP(6,6,10) shows an exothermic transition (second heating
run) typically seen during supercooling for a glass to crystalline
transition before melting into the LC phase, while the second
endothermic transition suggests isotropization as seen in the
DSC (see Fig. S1b, in ESI†). The high enthalpy for isotopization
for longer side chains suggests an increasing van der Waals
interactions of the longer side chains between the subunits for the
dimeric unit within the respective column. On cooling, however,
The most significant proof of intracolumnar stacking of the
dimers in the columnar phase comes from the lengthening of the
spacer in DiHDBP(6,10,6) and DiHDBP(6,12,6). We postulated
that the lengthening of the spacer length will lead to different
observations in their XRD at the mesophase for the different
packing: (i) unchanged intercolumnar distance for intra-molec-
ular stacking, and (ii) lengthening of the intercolumnar distance
for inter-molecular stacking. Analysis of the X-ray diffraction
data for the Colh phase of DiHDBP(6,10,6) and DiHDBP(6,12,6)
clearly showed that their intercolumar distances remain fairly
ꢀ
constant at around 21.63–21.99 A, irrespective of the spacer
length (see Fig. S2†). Their intercolumnar distances are very
similar to that found for DiHDBP(6,6,6). We have thus further
substantiated that the discs adopted solely an intracolumnar
stacking conformation for DiHDBPs. Transition from Colh to
the Colr phase on cooling was again observed for
DiHDBP(6,10,6) and DiHDBP(6,12,6), driven by a reduction in
chain mobility.
The intercolumnar distance a from the XRD is much shorter
than that of a fully extended dimer, again implying intra-
molecular packing. However a slight shortening in the rectan-
gular length parameters as calculated by O3 ꢁ a from the
hexagon, and broadening of the rectangular width was observed,
accounted for the reduction in chain mobility whereby at the
molecular level this could result in a slightly different spatial
profile. At room temperature, these form glassy phases that can
Table 3 Gelation behaviors of dimer and monomer in different organic solvents
Solubility and critical gelation concentration
Solvent
DiHDBP(6,6,8)
DiHDBP(6,6,10)
DiHDBP(6,12,6)
Monomer 6
Monomer 12
Hexane
1-Octanol
Cyclohexane
Ethyl acetate
Ethanol
16 mg mLꢂ1
18 mg mLꢂ1
25 mg mLꢂ1
38 mg mLꢂ1
Insoluble
Insoluble
Insoluble
Insoluble
Soluble
10 mg mLꢂ1
12 mg mLꢂ1
13 mg mLꢂ1
Insoluble
Insoluble
Insoluble
Insoluble
Insoluble
Soluble
Insoluble
Insoluble
Soluble
Insoluble
Insoluble
Insoluble
Insoluble
Insoluble
Soluble
Soluble
38 mg mLꢂ1
Soluble
55 mg mLꢂ1
Insoluble
Insoluble
Insoluble
Insoluble
Soluble
Soluble
10 mg mLꢂ1
Soluble
25 mg mLꢂ1
Insoluble
Insoluble
Insoluble
Insoluble
Soluble
Acetone
Dimthylformamide
Acetonitrile
Dichloromethane
Tetrahydrofuran
Toluene
Soluble
Soluble
Soluble
Soluble
Soluble
Soluble
Soluble
Soluble
Soluble
Soluble
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be stable for months. Another interesting observation is the
increase of isotropic temperatures from DiHDBP(6,6,6) to
DiHDBP(6,10,6). These observations led to the conclusion that
the dimeric discotic molecules form folded conformations
predominantly, resulting in the exclusive formation of intra-
columnar stacking within the column, a result extremely different
from the previously reported dimers.
carried out to investigate the nature of the fiber. The TEM thin
film was prepared from 1 ꢁ 10ꢂ5 M hexane solution aged for 18 h
and drop-cast onto the copper grid. The TEM image clearly
showed one-dimensional association of the nanofibers. The dif-
fractogram shows that there was no repeating units in the asso-
ciated fibers.
Photophysical properties
Gel formation
Fig. 6 shows the UV–vis absorption and fluorescence spectra of
DiHDBP(6,6,6) and DiHDBP(6,10,6) in 10ꢂ5 M CH2Cl2 solu-
tion. The spectra in solution showed similar UV–vis absorption
at similar wavelengths at 284 nm for the discotic dimers
DiHDBP(6,6,6) and DiHDBP(6,10,6). The UV–vis absorption of
Eleven solvents consisting of hydrocarbon solvents, alkyl alco-
hols, and polar solvents were used to test the gelation ability of
the dimers synthesized. The dimer DiHDBP(6,6,8) was found to
form gels with hexane, cyclohexane, 1-octanol and ethyl acetate.
The dominant forces responsible for DiHDBP(6,6,8) to form
a gel include; intra-molecular stacking, p–p interactions, dipole–
dipole interaction, and van der Waals forces. Furthermore, the
lengthening of the peripheral alkyl chains in DiHDBP(6,6,10)
favors gel formation, probably through a better interdigitaliza-
tion between the alkyl groups of the columns which formed
complex 3D intertwining networks. The critical concentrations
required for gel formation are summarized in Table 3. These
organogels formed are thermo-reversible and show a luminescent
property under a UV lamp. The lengthening of the spacer, as in
DiHDBP(6,12,6), did not promote gel formation. We have
previously synthesized the monomeric dibenzo[a,c]phenazine
with hexahexyloxy side-chains (Monomer 6) and hexadocecyloxy
side-chains (Monomer 12).31 It was found that the dibenzo[a,c]-
phenazine monomers only form gels in hydrogen bonding (1-
octanol) and polar solvent (ethyl acetate).
Monomer
6 (hexahexyloxydibenzo-[a,c]phenazine) was also
recorded and shows a similar trend.
The fluorescence spectra for all these compounds upon exci-
tation at 284 nm shows nearly identical emission peaks at around
544 nm. Thus, photophysical data might suggest that they are
not folded in the 10ꢂ5 M solution.
Electrochemical properties
The cyclic voltametry (CV) measurements of DiHDBP(6,6,6)
was performed in dichoromethane for the anodic and tetrahy-
drofuran for the cathodic scan containing 0.05 M tetrabuty-
lammonium perchlorate as the supporting electrolyte and a scan
rate of 50 mV sꢂ1. Platinum wire was used as the counter and
working electrode with Ag/Ag+ (0.01M AgNO3) as the reference
electrode, and ferrocene used as the internal standard. Fig. 7
shows DiHDBP(6,6,6) possesses two quasi-reversible oxidation
half-wave potentials (E1/2) at 0.95 and 1.08 V. Also, two revers-
ible reduction half-wave potentials at ꢂ1.48 V and ꢂ1.92 V were
observed. DiHDBP(6,6,6) has the dual ability of donating and
accepting electrons at the disc. The Monomer 6 (hexahexyloxy-
dibenzo-[a,c]phenazine) was found to possess an oxidation half-
wave potential at 1.04 V, and two reduction half-wave potentials
at ꢂ1.69 V and ꢂ2.09 V were observed. Ferrocene/ferrocenium
(Fc/Fc+) redox couple was used as an internal standard for all the
CV measurements. The corresonding HOMO/LUMO can be
We next used scanning electron microscopy (SEM) and
transmission electron microscopy (TEM) to examine the meso-
and nanoscale structures of the xerogels (Fig. 5). The SEM image
of the xerogel prepared from hexane solution (16 mg mLꢂ1) and
onto a silicon wafer substrate shows irregular fibrous self-
assembled morphology. TEM measurements were further
Fig. 5 Gel behavior of DiHDBP(6,6,8); (a) SEM images of xerogel
prepared from hexane solution (16 mg mLꢂ1) onto the silicon wafer
substrate, (b) TEM image was obtained from hexane solution (1 ꢁ 10ꢂ5
M) aged for 18 h and drop-cast onto a copper grid, (c) expanded TEM
images, (d) electron diffraction pattern from a single fiber, (e) photograph
of gel under natural light, (f) photograph of gel under UV lamp (excited
with 365 nm light).
Fig. 6 Normalized UV–vis absorption and fluorescence emission spec-
trum of DiHDBP(6,6,6), DiHDBP(6,10,6) and Monomer 6. The fluores-
cence spectrum was obtained upon excitation at 284nm (at 298 K, in 10ꢂ5
M CH2Cl2 solution).
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calcium hydride. Nuclear magnetic resonance spectra were
recorded on a Varian Unity-INOVA-500 MHz spectrometer
using CDCl3 as solvent. Chemical shifts are reported in ppm
relative to residual CDCl3 (d ¼ 7.26, 1H; 77.0, 13C). Mass spectra
were obtained on FT-MS on Bruker APEX II or MALDI-TOF
on Bruker Autoflex III TOF/TOF. Elemental analyses were
carried out on Heraeus CHN–O Rapid Elementary Analyzer and
Elementar vario EL III. Differential scanning calorimetry (DSC)
was measured on a Perkin-Elmer Pyris 1 with heating and
cooling rates of 10 ꢀC minꢂ1. Polarized optical microscopy
(POM) was carried out on a OLYMPUS CX41 with a Mettler
FP90/FP82HT hot stage system. X-Ray powder diffraction
(XRD) data were collected on the wiggler beamline BL17A of the
National Synchrotron Radiation Research Center (NSRRC),
Taiwan, using a triangular bent Si (111) monochromator and
ꢀ
a wavelength of 1.33367 A. The sample in a 1 mm capillary was
mounted on the Huber 5020 diffractometer. An air stream heater
is equipped at BL17A beamline and the temperature controller is
programmable by a PC with a PID feed back system. UV–vis
spectra were obtained on an HP 8453 diode-array spectropho-
tometer. Photoluminescence (PL) spectra were measured using
a Hitachi F-4500 luminescence spectrometer. Cyclic voltamme-
try (CV) measurements were undertaken on a BAS 100 B/W
electrochemical analyzer. Scanning electron microscopy (SEM)
was carried out by using the JSM-6701F, Field Emission Scan-
ning Electron in National University of Singapore. Transmission
electron microscopy (TEM) was carried out by using the JEM-
2100F, Field Emission Electron Microscopy at 200 keV in
Nanyang Technological University in Singapore.
Fig.
7 Cyclic voltammogram and differential pulse-voltammetry
diagrams of Monomer 6 (top line) and DiHDBP(6,6,6) (10ꢂ3M) (bottom
line) in TBAPF6 (0.05 mol Lꢂ1) solutions in degassed anhydrous solvent.
The reductions were recorded in THF and the oxidations in CH2Cl2,
using Ag/AgNO3 standard electrode. (Scan rate: 50 mV sꢂ1).
further derived using the first oxidation and reduction wave,
using the following equation: HOMO/LUMO ¼ ꢂ4.8 ꢂ Eox/Ered
whereby Eox/Ered is the onset potential of the first oxidation or
reduction wave versus Fc/Fc+. The HOMO and LUMO energy
level for DiHBDP(6,6,6) was estimated to be ꢂ5.23 eV and ꢂ2.93
eV, respectively. Interestingly, the HOMO energy of Monomer 6,
(ꢂ5.36 eV) is lower than DiHBDP(6,6,6), while the LUMO
energy of Monomer 6 (ꢂ2.73 eV) is higher than DiHBDP(6,6,6).
This indicates that there is a slight increase in the interaction
between the individual discotic core for the dimer in the 10ꢂ3
M
Synthesis of compounds
solution. This is indicative of a folded conformation of the dimer
at this concentration.
2-Hexyloxyphenol 1, 3,30,4,40-tetrakisalkoxybenzil 5, and
2,3,6,7-tetrakis(alkoxy)phenanthrene-9,l0-dione 6 were synthe-
sized according to literature procedures.36,37
Conclusion
In summary, we have presented a new family of discotic liquid
crystal dimers, DiHDBPs based on dibenzo[a,c]phenazine that
preferentially form into a folded conformation. From the XRD
studies, it is reasonable to assume these mesogens adopt the folded
conformation in the liquid crystalline state. This has enabled
liquid crystalline formation for the shortest spacer known to date.
In the columnar phase, the dimeric molecules stack predomi-
nantly in an intra-columnar manner to form columns. As such,
peripheral side chains longer than the spacer can also be liquid
crystalline. A rich polymesomorphism from Colh to Colr was also
observed and they form glasses on cooling to room temperature
that can last for a few hours to months. Interestingly, they also
form organogels, indicating the great ability of DiHDBPs to form
a 3-D network. These findings may contribute to the development
of polymer like stable ordered self-assembly materials important
for future applications in devices.
1,x-Bis(2-alkoxyphenoxyl)alkane (2). In a two-neck round-
bottom flask (100 mL) fitted with a condenser with a nitrogen
inlet containing ethanol (50 mL) were added 2-hexyloxyphenol (1
equivalent) and potassium carbonate (2.5 equivalent) and the
mixture stirred at room temperature for 1 h. After this time, the
dibromoalkane (1.1 equivalents) was added, and the reaction
mixture was refluxed for one day. The solid residue from the
reaction mixture was filtered, and the filtrate was evaporated to
dryness. The crude product was then dissolved in dichloro-
methane and the dichloromethane layer washed with water and
brine. The solvent was dried over MgSO4 and evaporated. The
crude product was further purified by column chromatography
on silica gel (hexane–ethyl acetate 15 : 1 as eluent) or recrystal-
lized from 1 : 1 dichloromethane–methanol. The yield of 2a is
91%, 2b is 93%, and 2c is 89%.
ꢀ
1
2a: Mp: 72 C. H NMR (500 MHz, CDCl3): d 6.89 (s, 8H),
4.03–3.98 (m, 8H), 1.87–1.79 (m, 8H), 1.59–1.55 (m, 4H), 1.50–
1.44 (m, 4H), 1.37–1.32 (m, 8H), 0.92–0.89 (m, 6H). 13C NMR
(125 MHz, CDCl3): d 149.21, 149.13, 121.05, 120.96, 114.11,
114.02, 69.21, 69.11, 31.58, 29.30, 29.26, 25.85, 25.68, 22.61,
14.02. Mass (EI): m/z 470 [M+]. Anal. calcd for C30H46O4: C,
76.55; H, 9.85. Found: C, 76.30; H, 9.78.
Experimental section
Materials and methods
All reactions were conducted under an atmosphere of nitrogen in
oven-dried glassware. Dichloromethane was distilled over
1710 | J. Mater. Chem., 2011, 21, 1704–1712
This journal is ª The Royal Society of Chemistry 2011

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ꢀ
1
2b: Mp: 68 C. H NMR (500 MHz, CDCl3): d 6.89 (s, 8H),
4.00 (t, J ¼ 6.5 Hz, 8H), 1.84–1.79 (m, 8H), 1.51–1.44 (m, 8H),
1.37–1.33(m, 16H), 0.92–0.89 (m, 6H). 13C NMR (125 MHz,
CDCl3): d 149.21, 120.98, 114.08, 114.07, 69.20, 69.10, 31.61,
29.57, 29.43, 29.34, 29.29, 26.04, 25.71, 22.62, 14.03. Mass (EI):
m/z 526 [M+]. Anal. calcd for C34H54O4: C, 77.52; H, 10.33.
Found: C, 77.12; H, 10.21.
bis(alkoxy)phenyl)-7-(hexyloxy)quinoxalin-6-yloxy)-hexane. To
a solution of the corresponding amine 4 and benzyl 5 or 6 in
dichloromethane (50 mL) was added acetic acid (1 mL). The
reaction mixture was stirred under nitrogen for 24 h. Removal of
the solvent in vacuo yielded a crude product that was rigorously
purified by chromatographic separation on silica gel using ethyl
acetate–n-hexane 1 : 5 as eluent and recrystallized from acetone
or ethanol–dichloromethane. The yield of DiQX(6,6,6) is 34%,
DiQX(6,6,12) is 30%, DiHDBP(6,6,6) is 30%, DiHDBP(6,6,8) is
30%, DiHDBP(6,6,10) is 28%, DiHDBP(6,6,12) is 33%,
DiHDBP(6,10,6) is 29% and DiHDBP(6,12,6) is 26%.
ꢀ
1
2c: Mp: 51 C. H NMR (500 MHz, CDCl3): d 6.88 (s, 8H),
4.00–3.98 (m, 8H), 1.84–1.78 (m, 8H), 1.49–1.43 (m, 8H), 1.36–
1.28 (m, 20H), 0.93–0.89 (m, 6H). 13C NMR (125 MHz, CDCl3):
d 149.21, 120.98, 114.08, 69.25, 31.61, 29.62, 29.44, 29.33, 29.29,
26.04, 25.71, 22.62, 14.03. LRMS (ESI): m/z 577 [M + Na+];
HRMS (ESI) m/z 577.4236 [M + Na+] (577.4233 calculated for
C36H58NaO4 [M + Na+]).
DiQX(6,6,6): 1H NMR (500 MHz, CDCl3): d 7.40 (s, 2H), 7.39
(s, 2H), 7.07 (d, J ¼ 8.5 Hz, 4H), 7.00 (s, 4H), 6.82 (d, J ¼ 8.5 Hz,
4H), 4.21 (t, J ¼ 6.5 Hz, 4H), 4.17 (t, J ¼ 6.5 Hz, 4H), 3.99 (t, J ¼
6.5 Hz, 8H), 3.80 (t, J ¼ 6.5 Hz, 8H), 2.03–1.66 (m, 24H), 1.52–
1.25 (m, 64H), 0.91–0.88 (m, 30H). 13C NMR (125 MHz, CDCl3):
d 152.34, 152.30, 150.36, 150.34, 149.23, 148.53, 148.52, 137.90,
137.86, 132.20, 122.46, 115.20, 113.17, 107.15, 69.07, 69.02,
68.96, 68.81, 31.53, 31.46, 31.42, 29.10, 28.99, 28.75, 28.68, 25.78,
25.58, 25.58, 25.56, 22.53, 22.52, 22.50, 13.93. Mass (MALDI-
TOF): m/z 1680.347 [M + H+]. Anal. calcd for C106H158N4O12: C,
75.76; H, 9.48; N, 3.33. Found: C, 75.61; H, 9.15; N, 3.05.
1,x-Bis(2-alkoxy-4,5-dinitro-phenoxyl)alkane (3). To a two
neck round-bottom flask containing dichloromethane (50 mL),
ꢀ
acetic acid (50 mL), and compound 2 (2 g) cooled to 0 C was
added slowly 65% nitric acid (6 mL). The reaction was allowed to
warm to room temperature anꢀd stirred for a further 1 h. The
mixture was again cooled to 0 C, 100% nitric acid (3 mL) was
added slowly, the mixture was allowed to warm to room
temperature and stirred overnight. After completion of the
reaction, the reaction mixture was poured into ice–water. The
yellow precipitate was collected by filtration and washed with
ice–water to remove the residual acid. The yield of 3a is 66%, 3b is
68%, and 3c is 65%.
3a: Mp: 153 ꢀC. 1H NMR (500 MHz, CDCl3): d 7.30 (d, J ¼ 5
Hz, 4H), 4.14–4.09 (m, 8H), 1.94–1.85 (m, 8H), 1.59–1.25 (m,
16H), 0.92–0.89 (m, 6H). 13C NMR (125 MHz, CDCl3): d 151.77,
151.52, 136.64, 136.34, 107.85, 107.82, 70.17, 69.81, 31.32, 28.60,
25.45, 25.42, 22.50, 13.94. Mass (FAB): 650 [M+]. Anal. calcd for
C30H42N4O12: C, 55.38; H, 6.51; N, 8.61. Found: C, 55.21; H,
6.55; N, 8.56.
1
DiQX(6,6,12): H NMR (500 MHz, CDCl3): d 7.41 (bs, 4H),
7.09–7.06 (m, 4H), 6.99 (d, J ¼ 8.5 Hz, 4H), 6.83 (d, J ¼ 8.5 Hz,
4H), 4.23–4.16 (m, 8H), 3.99 (t, J ¼ 6.5 Hz, 8H), 3.82–3.78 (m,
8H), 2.00–1.63 (m, 24H), 1.55–1.26 (m, 160H), 0.94–0.87 (m,
30H). 13C NMR (125 MHz, CDCl3): d 152.50, 150.36, 149.39,
148.62, 148.60, 137.84, 132.16, 122.59, 115.30, 113.27, 107.11,
69.19, 69.09, 68.96, 31.92, 31.54, 31.50, 29.74, 29.70, 29.68, 29.66,
29.64, 29.47, 29.43, 29.38, 29.36, 29.24, 29.13, 28.85, 28.75, 26.02,
25.90, 25.67, 25.63, 22.68, 22.57, 14.10, 14.00. Mass (MALDI-
TOF): m/z 2354.133 [M + H+]. Anal. calcd for C154H254N4O12: C,
78.59; H, 10.88; N, 2.38. Found: C, 78.49; H, 10.95; N, 2.40.
DiHDBP(6,6,6): 1H NMR (500 MHz, CDCl3): d 8.74 (s, 2H),
8.69 (s, 2H), 7.73 (s, 4H), 7.53 (s, 2H), 7.50 (s, 2H), 4.35–4.24 (m,
24H), 2.06–1.94 (m, 24H), 1.73–1.38 (m, 64H), 0.97–0.92 (m,
30H). 13C NMR (125 MHz, CDCl3): d 152.65, 150.97, 150.94,
149.24, 139.27, 138.91, 125.65, 125.60, 123.96, 107.92, 107.86,
106.63, 106.31, 69.56, 69.08, 69.01, 68.96, 68.71, 31.77, 31.68,
31.54, 29.35, 29.30, 28.93, 28.89, 25.83, 25.80, 25.73, 25.64, 22.63,
22.61, 14.03, 14.01. Mass (MALDI-TOF): m/z 1676.090 [M +
H+]. Anal. calcd for C106H154N4O12 : C, 75.95; H, 9.26; N, 3.34.
Found: C, 75.70; H, 9.32; N, 3.06.
DiHDBP(6,6,8): 1H NMR (500 MHz, CDCl3): d 8.76 (bs, 2H),
8.67 (bs, 2H), 7.71 (d, J ¼ 3 Hz, 4H), 7.60 (bs, 2H), 7.56 (bs, 2H),
4.37–4.25 (m, 24H), 2.10–1.95 (m, 24H), 1.77–1.31 (m, 96H),
0.94–0.88 (m, 30H). 13C NMR (125 MHz, CDCl3): d 152.93,
151.24, 151.15, 149.38, 149.35, 138.67, 138.57, 125.84, 125.70,
108.06, 107.91, 106.30, 69.60, 69.59, 69.21, 69.07, 68.83, 31.85,
31.56, 29.51, 29.48, 29.42, 29.38, 29.34, 28.92, 28.90, 26.20, 26.17,
25.75, 25.58, 22.69, 22.62, 14.10, 14.02. Mass (MALDI-TOF): m/
z 1901.381 [M + H+]. Anal. calcd for C122H186N4O12: C, 77.09;
H, 9.86; N, 2.95. Found: C, 77.44; H, 10.04; N, 2.73.
ꢀ
1
3b: Mp: 134 C. H NMR (500 MHz, CDCl3): d 7.30 (s, 4H),
4.10 (t, J ¼ 6.5 Hz, 8H), 1.89–1.86 (m, 8H), 1.59–1.26 (m, 24H),
0.93–0.90 (m, 6H). 13C NMR (125 MHz, CDCl3): d 151.76,
151.70, 136.50, 136.43, 107.86, 107.85, 70.17, 70.12, 31.35, 29.68,
29.45, 29.23, 28.68, 28.62, 25.78, 25.45, 22.51, 13.95. Mass
(FAB): m/z 706 [M+]. Anal. calcd for C34H50N4O12: C, 57.78; H,
7.13; N, 7.93. Found: C, 57.48; H, 7.22; N, 7.83.
ꢀ
1
3c: Mp: 135 C. H NMR (500 MHz, CDCl3): d 7.29 (s, 4H),
4.10 (t, J ¼ 6.5 Hz, 8H), 1.89–1.84 (m, 8H), 1.58–1.30 (m, 28H),
0.93–0.90 (m, 6H). 13C NMR (125 MHz, CDCl3): d 151.76,
151.72, 136.44, 107.86, 70.17, 70.15, 31.35, 29.56, 29.53, 29.24,
28.67, 28.62, 25.79, 25.45, 22.51, 13.95. LRMS (ESI): m/z 757 [M
+ Na+]; HRMS (ESI) m/z 757.3632 [M + Na+] (757.3636 calcu-
lated for C36H54N4NaO12 [M + Na+]).
1,x-Bis(2-alkoxy-4,5-diamino-phenoxyl) (4). In a two-neck
flask containing dinitro compound 3 (1 equivalent) and 10% Pd
on carbon (0.1 g) in ethanol (35 mL) was added hydrazine
monohydrate (10 equivalent) slowly, and the reaction mixture
was refluxed for 2 days under nitrogen. The reaction solution was
filtered and the filtrate was evaporated under vacuum. The
diamine is unstable and had to be used in situ.
DiHDBP(6,6,10): 1H NMR (500 MHz, CDCl3): d 8.73 (s, 2H),
8.69 (s, 2H), 7.74 (s, 4H), 7.51 (s, 2H), 7.48 (s, 2H), 4.35–4.25 (m,
24H), 2.07–1.94 (m, 24H), 1.75–1.29 (m, 128H), 0.93–0.86 (m,
30H). 13C NMR (125 MHz, CDCl3): d 152.63, 152.61, 150.97,
150.94, 149.30, 139.45, 139.06, 139.02, 130.87, 128.79, 125.67,
125.63, 124.17, 107.99, 107.93, 106.76, 106.46, 69.66, 69.08,
1,x-Bis (2,3,6,7-tetrakis(alkoxy)-12-(hexyloxy)dibenzo[a,c]-phe
nazin-11-yloxy)alkane (DiHDBPs) and 1,6-bis(2,3-bis(3,4-
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 1704–1712 | 1711

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5 S. Sergeyev, W. Pisula and Y. H. Geerts, Chem. Soc. Rev., 2007, 36,
1902.
6 S. Laschat, A. Baro, N. Steinke, F. Giesselmann, C. Hagele, G. Scalia,
R. Judele, E. Kapatsina, S. Sauer, A. Schreivogel and A. Tosoni,
Angew. Chem., Int. Ed., 2007, 46, 4832.
7 J. W. Goodby, I. M. Saez, S. J. Cowling, V. Gortz, M. Draper,
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69.04, 68.99, 68.72, 31.93, 31.55, 29.70, 29.63, 29.55, 29.53, 29.52,
29.44, 29.39, 28.97, 28.91, 26.20, 26.18, 25.75, 25.67, 22.69, 22.62,
14.11, 14.02. Mass (MALDI-TOF): m/z 2126.692 [M + H+].
Anal. calcd for C138H218N4O12: C, 77.99; H, 10.34; N, 2.64.
Found: C, 77.50; H, 9.99; N, 2.30.
DiHDBP(6,6,12): 1H NMR (500 MHz, CDCl3): d 8.77 (s, 2H),
8.65 (s, 2H), 7.71 (s, 4H), 7.56 (s, 2H), 7.50 (s, 2H), 4.35–4.25 (m,
24H), 2.07–1.94 (m, 24H), 1.75–1.28 (m, 160H), 0.94–0.87 (m,
30H). 13C NMR (125 MHz, CDCl3): d 152.55, 150.91, 150.87,
149.21, 139.35, 138.98, 138.93, 125.62, 125.57, 124.08, 124.06,
107.92, 107.86, 106.70, 106.31, 69.56, 69.03, 68.97, 68.92, 68.65,
31.92, 31.55, 29.74, 29.68, 29.56, 29.53, 29.44, 29.38, 28.93, 28.90,
26.19, 26.16, 25.73, 25.61, 22.68, 22.61, 14.09, 14.00. Mass
(MALDI-TOF): m/z 2348.694 [M + H+]. Anal. calcd for
C154H250N4O12: C, 78.72; H, 10.72; N, 2.38. Found: C, 78.37; H,
10.20; N, 2.01.
DiHDBP(6,10,6): 1H NMR (500 MHz, CDCl3): d 8.75 (d, J ¼
2.0 Hz, 4H), 7.74 (s, 4H), 7.55 (bs, 4H), 4.34 (t, J ¼ 6.5 Hz, 8H),
4.28–4.25 (m, 16H), 2.03–1.94 (m, 24H), 1.61–1.36 (m, 72H),
0.96–0.93 (m, 30H). 13C NMR (125 MHz, CDCl3): d 152.83,
151.04, 149.34, 139.23, 138.89, 125.70, 123.87, 107.97, 106.57,
106.38, 69.61, 69.14, 69.07, 31.68, 31.58, 29.51, 29.35, 29.29,
28.96, 28.89, 26.08, 25.84, 25.81, 25.75, 22.65, 22.62, 14.07, 14.04.
Mass (MALDI-TOF): m/z 1732.615 [M + H+]. Anal. calcd for
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2.5 Hz, 4H), 7.72 (s, 4H), 7.50 (s, 4H), 4.33 (t, J ¼ 6.5 Hz, 8H),
4.25 (t, J ¼ 6.5 Hz, 16H), 2.00–1.93 (m, 24H), 1.63–1.36 (m,
76H), 0.96–0.91 (m, 30H). 13C NMR (125 MHz, CDCl3):
d 152.65, 150.89, 149.23, 139.34, 139.00, 125.59, 124.08, 107.89,
106.69, 106.34, 69.56, 69.05, 68.98, 31.67, 31.66, 31.57, 29.61,
29.43, 29.35, 29.29, 28.96, 28.89, 26.08, 25.83, 25.79, 25.74, 22.63,
22.62, 22.61, 22.57, 14.04, 14.02. Mass (MALDI-TOF): m/z
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1760.282 [M
112H166N4O12).
+
H+]. (1759.2502 [M+] calculated for
€
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C
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Acknowledgements
We gratefully acknowledge the National Sciences Council of Taiwan
(NSC) for funding and National Synchrotron Radiation Research
Center (NSRRC) for carrying out XRD experiments. Thanks are
also gratefully expressed to Jey-Jau, Lee for lots of assistance with
XRD. And we also gratefully acknowledge Zou Changji for TEM in
Nanyang Technological University in Singapore.
ꢁ
34 (a) A. K. Rappe, C. J. Casewit, K. S. Colwell, W. A. Goddard III and
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