A new class of discotic mesogens derived from tris(N-salicylideneaniline)s existing in C3h and Cs keto-enamine forms

Article abstract of DOI:10.1021/jo0712650

(Chemical Equation Presented) Two series of a unique class of columnar liquid crystals derived from tris(N-salicylideneaniline)s [TSANs] in which the proton and the electron interact with each other through the H-bonding environment are reported. The synt

Full text of DOI:10.1021/jo0712650

A New Class of Discotic Mesogens Derived from
Tris(N-salicylideneaniline)s Existing in C3h and Cs Keto-Enamine
Forms
Channabasaveshwar V. Yelamaggad,* Ammathnadu S. Achalkumar, D. S. Shankar Rao, and
S. Krishna Prasad
Centre for Liquid Crystal Research, Jalahalli, Bangalore, 560 013, India
yelamaggad@gmail.com
ReceiVed June 20, 2007
Two series of a unique class of columnar liquid crystals derived from tris(N-salicylideneaniline)s [TSANs]
in which the proton and the electron interact with each other through the H-bonding environment are
reported. The synthesis is carried out by condensing 1,3,5-triformylphloroglucinol with the respective
dialkoxyanilines or trialkoxyanilines. 1H NMR and 1H-1H COSY NMR studies revealed their existence
as an inseparable mixture of two keto-enamine tautomeric forms with C3h and Cs rotational symmetries
instead of the expected enol-imine form. The influence of the number of peripheral alkoxy tails on the
columnar mesomorphic behavior is investigated by using polarizing optical microscopy, differential
scanning calorimetry, and X-ray scattering. The fluid/glassy columnar states probed for a number of
representative compounds confirmed the D6h (hexagonal) or D2h (rectangular) symmetry of the columns.
The electronic absorption and emission characteristics of these compounds have been studied in both
mesomorphic and solution states. Of special interest, the photoluminescence spectra of solution and fluid/
glassy two-dimensional structure evidently disclose the promising light generating capability of these
new discotics systems.
Introduction
the early 1970s,2 the calamitics were recognized as novel
materials for the display technology, and presently, they form
the backbone of the flat panel display industry. The discotic
LCs, discovered by Chandrasekhar et al. in 1977,3 are becoming
increasingly interesting from both scientific and application
viewpoints;4 they are slowly entering into the mainstay of
organic molecular electronics. In general, disc-shaped mesogens
are made of flat or nearly flat (two-dimensional) molecules
Thermally enforced self-assembly of shape-anisotropic mol-
ecules leads to the formation of liquid crystal (LC) phases
wherein the molecules are mobile, yet possess orientational and/
or one- or two-dimensional positional order.1 Owing to this
unique order-mobility feature, they are finding increasing
applications in several important technological fields.1e Con-
ventionally, the anisometric molecules employed to stabilize LC
phases are either rod-like (calamitic) or disc-like (discotic). In
(2) Gray, G. W.; Harrison, K. J.; Nash, J. A. Electron. Lett. 1973, 9,
130-131.
(3) Chandasekhar, S.; Sadashiva, B. K.; Suresh, K. A. Pramana 1977,
9, 471-480.
* Address correspondence to this author. Phone: 91-80-2838 1119. Fax: 91-
80-28382044.
(1) (a) Chandrasekhar, S. Liquid Crystals, 2nd ed.; Cambridge University
Press: Cambridge, UK, 1992. (b) Kato, T. Science 2002, 295, 2414-2418.
(c) Tschierske, C. Annu. Rep. Prog. Chem. Sect. C 2001, 97, 191-267. (d)
de Gennes, P. G.; Prost, J. The Physics of Liquid Crystals; Oxford Science
Publication: Oxford, UK, 1993. (e) Bahadur, B. Liquid Crystals: Applica-
tions and Uses; World Scientific: Singapore, 1990; Vols. 1-3.
(4) (a) Bushby, R. J.; Lozman, O. R. Curr. Opin. Collidal Interface Sci.
2002, 7, 343-354. (b) Kumar, S. Liq. Cryst. 2004, 31, 1037-1059. (c)
Kumar, S. Chem. Soc. ReV. 2006, 35, 83-109. (d) Warman; J. M; Van De
Craats, A. M. Mol. Cryst. Liq. Cryst. 2003, 396, 41-72. (e) Van De Craats,
A. M.; Warman, J. M.; Fechtenkotter, A.; Brand, J. D.; Harbison, M. A.;
Mullen, K. AdV. Mater. 1999, 11, 1469-1472.
10.1021/jo0712650 CCC: $37.00 © 2007 American Chemical Society
Published on Web 10/04/2007
8308
J. Org. Chem. 2007, 72, 8308-8318

New Discotics DeriVed from Tris(N-salicylideneaniline)s
a
SCHEME 1
a Reagents and conditions: (i) 70% HNO3, NaNO2, CH2Cl2, rt, 1 h (50-60%); (ii) H2 (1 atm, balloon), 10% Pd/C, THF, rt, 12 h (85-94%).
a
SCHEME 2
a Reagents and conditions: (i) 5a-i, ethanol, reflux, 6 h (60-75%); (ii) 6a-i, ethanol, reflux, 6 h (65-87%). bThe ratio of C3h and Cs isomers was
1
determined by comparing the integral areas of amine protons Ha (C3h isomer) vs Hc, He, and Hg (Cs isomer) in H NMR.
having4 3-, 4-, or 6-fold rotational symmetry, to which are
attached, via ester, or ether, or thioether links, several alkyl
chains of the same or even different lengths. Owing to the
attractive interactions between similar molecular fragments, i.e.,
aromatic cores and flexible paraffinic tails, an effective nano-
segregation occurs among them, and thus the columnar phase
is preferred over the nematic (N) phase. The Col phases,
characterized by indefinitely long molecular columns, aggregat-
ing into two-dimensional (2D) lattices with different symmetries,
are of great significance. This is because these one-dimensional
(1D) Col stacks facilitate smooth charge-transport (as high as
1.0 cm2 V-1 S-1) along the column axis4 that can be initiated
either by making a charge transfer (CT) complex or by
photolysis. In addition, they offer the possibility of combining
several physical properties with the orientational control of the
molecular order, self-healing of structural defects, and the ease
of processability when compared with inorganic semiconductors
or zone refined single crystals or conductive polymers used in
electronic applications.5 Thus, the anisotropy in electrical
conduction of fluid Col phase can be well exploited in molecular
electronic devices such as one-dimensional conductors,6 pho-
toconductors,7 molecular wires and fibers,8 organic light emitting
diodes (OLEDs),9 field-effect transistors, and photovoltaic
(5) (a) Heeger, A. J. Angew. Chem., Int. Ed. 2001, 40, 2591-2611. (b)
Kertesz, M.; Choi, C. H.; Yang, S. J. Chem. ReV. 2005, 105, 3448-3481.
(c) Pope, M.; Swenberg, C. E. Electronic Processes in Organic crystals
and Polymers; Oxford University Press; Oxford, UK, 1999. (d) Mullen,
K.; Wenger, G. Electronic Materials: The Oliogomer Appraoch; Wiley-
VCH: New York, 1998. (e) McQuade, D. T.; Pullen, A. E.; Swager, T. M.
Chem. ReV. 2000, 100, 2537-2574.
J. Org. Chem, Vol. 72, No. 22, 2007 8309

Yelamaggad et al.
TABLE 1. Phase Transition Temperaturesa (°C) and Corresponding Enthalpies (kJ/mol) of the DT-n Seriesf
phase sequence
compd DT-n
heating
Cr 128.5 [39.2] I
cooling
I 104 [30.7] Cr
DT-4
DT-5
DT-6
DT-7
DT-8
DT-10
DT-12
DT-16
DT-8B
Cr 104 [5.6]b Colh 119.3 [3.3] I
Cr 84.9 [32.6]b Colr 131.7 [3.6] I
Cr 75.3 [24.7] Colr 138 [4.2] I
Cr 75.6 [39.8] Colr 136.4 [5.3] I
Cr 82.6 [49] Colr 141.9 [5.2] I
Cr 82.6 [77.1] Colr 133.9 [7.1] I
Cr 73.4 [53.9] Colr 127.3 [7.1] I
Colh 109.9 [2.7] I
I 117 [3.3] Colh 57.3 [1.4] Cr
I 129.8 [4] Colrc,d
I 136.3 [4.1] Colrc,d
I 135 [5.2] Colr 57 [1.1] Xe
I 140.9 [5.8] Colrc,d
I 132.7 [6.5] Colrc,d
I 126.2 [6.9] Colr 51.5 [62.9] Cr
c
I 105.5 [2.6] Colh
a Peak temperatures in the DSC thermograms obtained during the first heating and cooling cycles at a rate of 5 deg/min. b DT-5 and DT-6 show additional
Cr-to-Cr transitions during the first heating cycle at 96.5 [24.2] and 65.5 [1.1], respectively. c In DSC thermogram(s) of first/subsequent cooling cycle(s), no
other signatures were found until -60 °C; similarly, in microscopic observation the texture of the Col phase remains unaltered until room temperature
(30 °C). d The Col phase loses its fluidity below ∼60 °C as it perhaps freezes into a glassy state. e This phase supercools until -60 °C. f Cr ) crystal; Colh
) hexagonal columnar phase; Colr ) rectangular columnar phase; I ) isotropic liquid; X ) perhaps a glass.
TABLE 2. The Results of Indexation of XRD Profiles at a Given
cells.10 The performance of such electronic devices critically
Temperature (T) of Phases Displayed by the Compounds of the
DT-n Series
depends on the high charge-carrier mobility. Consequently, there
have been intensive investigations to find discotics with the Col
phase at room temperature or Col glasses possessing high charge
mobility.
d (Å) d (Å)
Miller
lattice
compd T (°C) phase measd calcd index hkl parameters (Å)
DT-5
90
70
30
Colh
Colr
X
22.7
13.1
11.3
8.5
7.5
6.8
6.5
4.5
3.32
27.0
20.8
13.5
7.8
7.4
6.7
6.2
4.5
3.3
22.6
13.1
11.3
8.6
7.6
6.6
100
110
200
210
300
220
220
a ) 26.1
c ) 3.32
Recently, based on the work of MacLachalan et al.,11 we have
discovered that an altogether new class of disc-like organic
molecules derived from tris(N-salicylideneanilines) [TSANs],
existing in an inseparable mixture of two keto-enamine forms
with C3h and Cs rotational symmetries, display Col phase(s)
solely.12 These discogens are rather unique: First, because the
central core of these systems, which is the size of polyaromatics,
is a flat electron-accepting (n-type) entity featuring multiple
strong intramolecular hydrogen (H) bonding between the 2°-
amine protons and oxygens of the cyclohexanetrione. Second,
the individual column is resulting from the self-assembly of a
mixture of two geometrical isomers (having C3h and Cs
symmetry in varying proportions) in close (core-core) spatial
proximity. Third, owing to the possibility of contiguous three-
proton transfer and a simultaneous rearrangement of the
π-orbitals in these systems, a coupling between motions of the
proton and the conduction electron might be produced, which
is a key phenomenon for many proposed molecular electronic
devices. Furthermore, in a later study, we noted that these
materials possess some promising electronic (inherent fluores-
cence) property due to extended π-conjugations.12b In this
context, there have been several recent innovations where the
electronic properties of TSANs, in particular the C3h isomer,
can be effectively altered by changing the peripheral substitu-
6.6
001
110
200
220
430
340
250
060
DT-8
27.1
20.8
13.5
7.8
7.5
6.8
6
a ) 41.5
b ) 35.7
c ) 3.3
001
110
200
220
430
050
530
060
27.1
19.9
13.5
7.7
7.4
6.7
27.1
19.9
13.8
7.6
7.6
6.9
a ) 39.7
b ) 37.1
c ) 3.26
6.2
6.0
4.4
3.26
26.1
4.9
3.5
27.0
4.8
001
100
DT-8B
90
22
Colh
Colh
26.1
27
a ) 30.1
c ) 3.5
001
100
a ) 31.2
c ) 3.39
(6) (a) Adam, D.; Schuhamcher, P.; Simmerer, J.; Haussling, L.;
Siemensmeyer, K.; Etzbach, K. H.; Ringsdorf, H.; Haarer, D. Nature 1994,
371, 141-143. (b) Fontes, E.; Heiney, P. A.; De Ju, W. H. Phys. ReV. Lett.
1988, 61, 1202-1205.
(7) Chandrasekhar, S.; Balagurusamy, V. S. K. Proc. R. Soc. London,
Ser. A 2002, 458, 1783-1794.
3.39
001
tions. For example, Lee and co-workers have determined that
stereoelectronic control of self-assembly of C3h isomers can
enhance their fluorescence efficiency in solutions.13a Subse-
quently, it was also shown that by manipulating a mutually
reinforcing H-bond network in the C3h isomer, a 2D conjugated
system with fast and reversible conformational switching, and
thus fluorescence switching, can be obtained.13b These results
are quite significant given the fact that the beneficial self-
organizing nature of discotics into large and well-organized
(8) (a) Osburn, E. J.; Schmidt, A.; Chau, L. K.; Chen, S. Y.; Smolenyak,
P.; Armstrong, N. R.; O’Brian, D. F. AdV. Mater. 1996, 8, 926-928. (b)
Percec, V.; Glodde, M.; Bera, T. K.; Miura, Y.; Shiyanovskaya, I.; Singer,
K. D.; Balagurusamy, V. S. K.; Heiney, P. A.; Schnell, I.; Rapp, A.; Spiess,
H.-W.; Hudsonk, S. D.; Duank, H. Nature 2002, 419, 384-387.
(9) Christ, T.; Glusen, B.; Greiner, A.; Kettner, A.; Sander, R.; Stumpflen,
V.; Tsukruk, V.; Wendorff, D. J. AdV. Mater. 1997, 9, 48-52.
(10) Schmidt-Mende, L.; Fechtenkotter, A.; Mullen, K.; Moons, E.;
Friend, R. H.; MacKenzie, J. D. Science 2001, 293, 1119-1126.
(11) Chong, J. H.; Sauer, M.; Patrick, B. O.; MacLachlan, M. J. Org.
Lett. 2003, 21, 3823-3826.
(12) (a) Yelamaggad, C. V.; Achalkumar, A. S.; Rao, D. S. S.; Prasad,
S. K. J. Am. Chem. Soc. 2004, 126, 6506-6507. (b) Yelamaggad, C. V.;
Achalkumar, A. S. Tetrahedron Lett. 2006, 47, 7071-7075.
(13) (a) Riddle, J. A.; Lathrop, S. P.; Bollinger, J. C.; Lee, D. J. Am.
Chem. Soc. 2006, 128, 10986-10987. (b) Jiang, X.; Bollinger, J. C.; Lee,
D. J. Am. Chem. Soc. 2006, 128, 11732-11733.
8310 J. Org. Chem., Vol. 72, No. 22, 2007

New Discotics DeriVed from Tris(N-salicylideneaniline)s
domains has been shown to improve the OLED characteristics.
However, the vast majority of discotics cannot be used as the
active emissive layer in OLEDs owing to their low luminescence
efficiency. This has led to many attempts to design the
intrinsically luminescent disc-like mesogens, which self-as-
semble to form the fluid Col structure with oriented lumino-
phores.14 In this respect, the Col phase derived from TSANs
should further benefit from emission efficiency due to its
intrinsic 2D π-conjugation wherein the central cyclohexane-
1,3,5-trione is electron deficient while the peripheral aromatic
cores bearing multiple alkoxy tails are electron donating. In
addition, TSANs have proved promising in fabricating novel
molecular materials. MacLachlan et al.15a have revealed that
upon appending with additional functional groups, TSANs may
form H-bonded capsules, clefts, and chains while Riddle and
co-workers15b have shown that sterically hindered TSANs could
be used as mechanically coupled biconcave molecules, which
can release the clatherated guest molecules.
romethane to achieve the corresponding 3,4-dialkoxynitroben-
zenes (3a-i) and 3,4,5-trialkoxynitrobenzenes (4a-i). The
catalytic (Pd-C/H2) hydrogenation of these nitro derivatives
furnished the key 3,4-dialkoxyanilines (5a-i) and 3,4,5-
trialkoxyanlines (6a-i)17b in almost quantitative yields. Finally,
as shown in Scheme 2, the condensation reaction between 1,3,5-
triformylphloroglucinol11,12 and 4 equiv of alkoxyanilines 5a-i
and 6a-i in refluxing ethanol afforded the requisite DT-n and
TT-n series of discotics, respectively, in reasonably good yields.
The spectroscopic data as well as the results of elemental
analyses were found to be consistent with the proposed
molecular structure of all the intermediates and final compounds.
1
The H NMR spectra of both the DT-n and TT-n series of
discotic TSANs showed the expected complex pattern in the
downfield region and thus evidenced their occurrence as a
mixture of the above-mentioned two keto-enamine tautomeric
forms. (See the experimental procedures section as well as the
Supporting Information for details.)
Mesomorphic Behavior. The phase behavior of the synthe-
sized TSANs was established with the aid of a polarizing optical
microscope (POM), a differential scanning calorimeter (DSC),
and X-ray diffraction (XRD). A sample placed between a clean
untreated glass slide and a cover slip was used for the POM
study. The preliminary mesophase assignment was based on
the two inherent properties displayed by mesogenic TSANs:
the birefringence and fluidity. The signatures observed in DSC
thermograms due to phase transitions of all the samples were
consistent with those of the optical experiments. In general, the
phase transition temperatures deduced from calorimetric mea-
surements of the first heating and cooling cycles at a rate of 5
deg/min are presented. However, when the thermal signatures
were not observed in DSC thermograms, the transition temper-
atures have been taken from the microscopic observations. It is
important to mention here that the phase transitions and their
temperatures in both optical and calorimetric studies are highly
reproducible for any number of heating/cooling runs demon-
strating that TSANs possess excellent thermal stability. The
details of these investigations with analysis of the results of
both series of compounds are presented in the sections to follow.
DT-n Series. The phase sequence, transition temperatures,
and associated enthalpies of TSANs with two exterior alkoxy
tails on each benzene ring belonging to the DT-n series are
summarized in Table 1. As can be seen, all but one of the
homologues are liquid crystalline; the first member DT-4 with
n-butyloxy tails does not exhibit mesomorphism implying that
the peripheral chain melting (disordering) temperature is higher
than the temperature at which stacking of the TSAN cores
becomes unfavorable.18 In other words, the chain length is not
enough to increase the disorder at the periphery of the molecule
to stabilize the mesophase. Other members of the series exhibit
enantiotropic monomesomorphic behavior, in particular, the Col
phase that was preliminarily characterized by the optical
textures.
In essence, the aforementioned studies clearly imply that
TSANs have a truly versatile molecular architecture in which
both electronic as well as novel molecular material features (self-
organizing ability, fluidity, molecular recognition, etc.) can be
imbued. Nonetheless, to reveal their in-depth and/or other
physicochemical properties, elaborative synthetic studies are
essential. In this direction, the understanding of their mesomor-
phic behavior, as mentioned earlier, is at a rather primitive
stage.12 Seeking an elaborative study, a large number of TSANs
have been obtained; here we present the first detailed investiga-
tion on the synthesis, molecular structural characterization, phase
behavior, and photophysical properties of two series of TSANs
differing in their peripheral aromatic core substitutions. These
two sets of TSANs possessing two and three alkoxy tails on
each of the three benzene rings linked to the central cyclohex-
ane-1,3,5-trione have been abbreviated as the DT-n series and
the TT-n series, respectively, where DT and TT refer to
Dialkoxy-TSANs and Trialkoxy-TSANs, respectively, while n
indicates the number of carbon atoms in the alkoxy chains.
Results and Discussion
Synthesis and Molecular Structural Characterization. The
reaction sequences adopted to prepare the intermediates and the
target DT-n and TT-n series of compounds are depicted in
Schemes 1and 2, respectively. As shown in Scheme 1, 1,2-
dialkoxybenzenes (1a-i) and 1,2,3-trialkoxybenzenes (2a-i)16
were subjected to the two-phase nitration17a by using an aqueous
solution of nitric acid (70%) and sodium nitrite in dichlo-
(14) (a) Ecoffet, C.; Markovitsi, D.; Jallabert, C.; Strzelecka, H.; Veber,
M. Thin Solid Films 1994, 242, 83-87. (b) Marguet, S.; Markovitsi, D.;
Goldmann, D.; Janietz, D.; Praefcke, K.; Singer, D. J. Chem. Soc., Faraday
Trans. 1997, 93, 147-155. (c) Cormier, R. A.; Gregg, B. A. Chem. Mater.
1998, 10, 1309-1319. (d) Rohr, U.; Schlichting, P.; Bohm, A.; Gross, M.;
Meerholz, K.; Brauchle, C.; Mullen, K. Angew. Chem., Int. Ed. 1998, 37,
1434-1437. (e) Ito, S.; Herwig, P. T.; Bohme, T.; Rabe, J. P.; Rettig, W.;
Mullen, K. J. Am. Chem. Soc. 2000, 122, 7698-7706. (f) Benning, S. A.;
Hassheider, T.; Keuker-Bauman, S.; Bock, H.; Salla, F. D.; Frauenheim,
T.; Kitzerow, H. S. Liq. Cryst. 2001, 28, 1105-1113. (g) Attias, A. J.;
Cavalli, C.; Donnio, B.; Guillon, D.; Hapiot, P.; Malthete, J. Chem. Mater.
2002, 14, 375-384.
On cooling from the isotropic liquid, all the LC TSANs,
except DT-8B (as we shall discuss later), displayed a striking
fan-like growth pattern that eventually coalesces to a mosaic
texture with interspersed bright and dark areas as shown in
Figure 1, panels a and b. This texture remains unaltered until
room temperature for TSANs DT-6, DT-7, DT-8, DT-10, and
DT-12, although the phase could not be mechanically sheared
(15) (a) Sauer, M.; Yeung, C.; Chong, J. H.; Patrick, B. O.; MacLachlan,
M. J. J. Org. Chem. 2006, 71, 775-788. (b) Riddle, J. A.; Bollinger, J. C.;
Lee, D. Angew. Chem. Int. Ed. 2005, 44, 6689-6693.
(16) See the Supporting Information.
(17) (a) Zinsou, A.; Veber, M.; Strzelecka, H.; Jallabert, C.; Fourre, P.
New J. Chem. 1993, 17, 309-313. (b) Percec, V.; Schlueter, D.; Ronda, J.
C.; Johansson, G. Macromolecules 1996, 29, 1464-1472.
(18) Collard, D. M; Lillya, C. P. J. Am. Chem. Soc. 1991, 113, 8577-
8583.
J. Org. Chem, Vol. 72, No. 22, 2007 8311

Yelamaggad et al.
still diffuse second reflection corresponding to a distance of
3.32 Å arises due to the stacking of the TSAN cores within a
column. Thus, the X-ray data in conjunction with the textural
pattern suggest that the mesophase is indeed the hexagonal
columnar (Colh) phase. Interestingly, the XRD patterns obtained
for the fluid Col (at 70 °C) and immobile X (at 30 °C) phases
of compound DT-8 were found to be qualitatively identical.
They showed several sharp Bragg peaks in the low-angle region
besides two broad reflections at wide angles corresponding to
the distances in the range of ∼4.5 to ∼3.3 Å that relates to the
liquid-like order of the peripheral tails and the stacking distance
of TSANs within the same column, respectively. The first three
sharp reflections of Col and X are in the ratio of 1:0.76:0.5 and
1:0.73:0.5, respectively, and can be assigned to (110), (200),
and (220) reflections from a rectangular lattice. Thus, the higher
temperature fluid phase is a rectangular Col (Colr) phase, while
this structure seems to be frozen in the X phase. Importantly,
the core-core separation value, viz. 3.26 Å of the frozen Col
structure, is near the lowest values observed for a Col phase
with high charge carrier ability reported for the case of a
complex with two phthalocyanine cores linked by a lutetium
atom.19 It is important to mention here that the core-core
stacking in a Col structure bears a great significance because
this is dependent on the magnitude of correlations between the
cores, which in turn is dependent on the size of the core. Such
a correlation peak is not observed for the benzene hexanoates,20
where the core size is smaller, whereas for the triphenylene
(having a larger core) systems it is commonly observed.
Therefore, it is likely that strong intramolecular H-bonding leads
to the formation of an effective larger core of TSAN, which
could be considered as a close structural surrogate to aromatic
cores (see a later section for details).
Interestingly, TSAN DT-8B, with peripheral branched alkoxy
chains, displayed the Col phase over a wide thermal interval
(∼160 °C) well below and above room temperature as estab-
lished by several complementary techniques, described below.
In fact, the initiative of synthesizing the TSAN DT-8B molecule
and its trialkoxy analogue TT-8B (see later section for details)
was based on the widespread theme that when branched alkyl
chains are introduced at the peripheral region of discotic
molecules, the liquid-like disorder is enhanced. In particular,
branched alkyl chains facilitate the widening of the mesophase
thermal range and lowering of melting points without altering
the type of the mesophase.21 Compound DT-8B, placed between
an ordinary glass slide and cover slip, could be readily spread
around the slide at room temperature by mechanical shearing.
The spread-over samples showed a highly birefringent texture,
clearly indicating that the compound is already in the LC state.
On cooling at a rate of 5 deg/min from their isotropic (I) phase,
it showed a mesophase at 105 °C. Notably, the optical texture
remains unchanged until room temperature and no sign of
crystallization was noticed even after maintaining the samples
at low temperature or even when sheared. The DSC thermo-
grams obtained during the several heating and cooling scans
corroborated the microscopic observations and further supported
FIGURE 1. Photomicrographs of the textures observed for the Col
phase of DT-5 at 69 °C (a) and DT-12 at 119 °C (b).
FIGURE 2. Intensity vs 2θ profile derived from the 2D XRD pattern
of the Colh phase at 90 (a) and 22 °C (b) for compound DT-8B.
below ∼60 °C. The DSC thermograms of these TSANs obtained
during the first cooling or second heating cycles, except for
DT-8, show only a peak due to I-Col or Col-I transition,
respectively. On the other hand, the thermograms of the first
cooling (till - 60 °C) and second heating (from -60 °C) cycles
of TSAN DT-816 possessed additional signatures at 57 (∆H )
1.1 kJ/mol) and 72 °C (∆H ) 1.1 J/g), respectively, that can
be ascribed to a transition from the Col phase to an immobile
phase (hereafter referred to as X). In the optical study, no
textural change was seen across the Col-X phase transition.
However, as we shall see later, the XRD study revealed that
the X phase possesses the features of a Col structure having a
rectangular symmetry. Apparently, both optical and calorimetric
studies suggest that TSANs DT-6, DT-7, DT-8, DT-10, and
DT-12 form the glassy Col phase at room temperature; this
behavior is notable given the fact that such a two-dimensional
(2D) structure enables the charge migration with the concomitant
freezing of ionic impurities.8b
The general observation that discotics exhibit two main types
of Col phases with D6h (hexagonal) and D2h (rectangular)
symmetries, the phases of TSANs DT-5 and DT-8, as repre-
sentative cases was probed with the help of powder XRD
experiments. The results of indexing the powder pattern of the
phase(s) are summarized in Table 2. The low-angle region (0
< 2θ < 5 °) of the diffractogram obtained16 for the Col phase
of DT-5 at 90 °C displays a strong reflection corresponding to
a Bragg spacing (d) of 22.7 Å in addition to a set of weak
reflections: the five low-angle sharp maxima with the ratio of
their reciprocal spacings being 1:x3:x4:x7:x9 (within ex-
perimental limits). These are indexed to (100), (110), (200),
(210), and (300) reflections of a quasi-2D hexagonal lattice with
a spacing of about 26 Å. In the wide-angle region of the
diffractogram two broad reflections were observed indicating
that the ordering within the columns is not long-range. The first
peak corresponding to a distance of 4.5 Å is diagnostic of the
liquid-like disorder of the chains, while the relatively sharp but
(19) Van de Crats, A. M.; Warman, J. M.; Hasebe. H.; Naito, R.; Ohta,
K. J. Phys. Chem. B 1997, 101, 9224-9232.
(20) Safinya, C. R.; Clark, N. A.; Liang, K. S.; Varady, W. A.; Chiang,
L. Y. Mol. Cryst. Liq. Cryst. 1985, 123, 205-216.
(21) (a) Schouten, P. G.; Warman, J. H.; de Haas, M. P.; van Nostrum,
C. F.; Gelinck, G. G.; Nolte, R. J. M.; Copyn, M. J.; Zwikker, J. W.; Engel,
M. K.; Hanack, M.; Chang, Y. H.; Ford, W. T. J. Am. Chem. Soc. 1994,
116, 6880-6894. (b) Kumar, S.; Rao, D. S. S.; Prasad, S. K. J. Mater.
Chem. 1999, 9, 2751-2754.
8312 J. Org. Chem., Vol. 72, No. 22, 2007

New Discotics DeriVed from Tris(N-salicylideneaniline)s
TABLE 3. Phase Transition Temperaturesa (°C) and Corresponding Enthalpies (kJ/mol) of the TT-n Series
phase sequence
compd
TT-n
heating
cooling
TT-4
TT-5
TT-6
TT-7
Cr 176.1 [6.6] Colr 219.1 [0.8] Colh 246 [8.1] I
Cr 168.5 [1] Colr 186.5 [0.4] Colh 228.6 [7.9] I
Cr 55.4 [8.4] Colr 173.4b Colh 214.6 [10.3] I
Cr 54.2 [10.1] Colr 156.2b Colh 197 [10] I
Cr 57.2 [8.3] Colr 164.4b Colh 186.2 [13.7] I
Cr 64.2 [8.3] Colr 158.5b Colh 175.2 [14.7] I
Cr 52.9 [50.9] Colr 148.5b Colh 163.5 [22.2] I
Cr 59.9 [136.1] Colr 96.9 [7.5] Colh 128.1 [1.6] I
Colr 68b Colh 109.9 [7.5] I
I 245 [7.8] Colh 218 [0.7] Colr 171.6 [6.6] Cr
I 226.7 [7.7] Colh 185.5 [0.5] Colr 69.7 [0.8] Cr
I 210.2 [8] Colh 171.5b-d Colr
I 195.3 [9.5] Colh 154.4b-d Colr
TT-8
I 185.1 [12.7] Colh 163b Colr 56 [7.6] Cr
I 173.6 [14.4] Colh 157.7b Colr 62.9 [11.2] Cr
I 162.1 [15.1] Colh 147.5b Colr 44.8 [86] Cr
I 126.2 [4.4] Colh 92.8 [8.8] Colr 53.8 [62.3] Cr
I 106.3 [38.1] Colh 64b,c Colr
TT-10
TT-12
TT-16
TT-8B
a Peak temperatures in the DSC thermograms obtained during the first heating and cooling cycles at 5 deg/min. b The phase transition observed with a
polarizing microscope was too weak to be detected by DSC. c No other phase transition was observed until -60 °C. d The Col phase loses its fluidity below
∼60 °C as it perhaps freezes into a glassy state
FIGURE 3. (a) Optical photomicrograph of the texture of the Colh phase obtained for TT-4 at 242 °C. (b) Microphotograph showing the appearance
of the Colr phase with tiny needles (see right portion) over the mosaic pattern (see left portion) of the Colh phase of TT-4 at 218 °C. (c) The
intensity vs 2θ profile extracted from the XRD pattern of the Colr phase of TT-4 at 190 °C.
the repeatability of the transitions. For example, the DSC trace
obtained during the first heating cycle from -60 °C showed
only one endothermic peak corresponding to the Col-Iso
transition.16 In the subsequent cooling scan (down to -60 °C),
as expected, a sharp exothermic peak due to an Iso-Col
transition was seen. Furthermore, identical DSC profiles are
obtained for the second (or any number of subsequent) heating-
cooling runs. These results clearly demonstrate that the phase
transitions are highly reproducible and thus TSAN DT-8B
exhibits the Col phase over a much broader thermal range
through room temperature. To elucidate the symmetry of the
Col phase, powder XRD investigation was carried out at 90 °C
and ambient temperature (22 °C); as expected, the diffracto-
grams showed an identical pattern as shown in Figure 2. The
wide-angle region of both high- and low-temperature profiles
consisted of a diffuse reflection at 4.86 and 4.76 Å, respectively
(Table 2), that can be attributed to the average distance between
the liquid-like (molten) aliphatic side chains. Of special
significance, a sharp Bragg (100) peak with spacing of 26.1
and 27 Å at 90 and 22 °C, respectively, was observed in the
small-angle region. The absence of the expected multiple
reflections in this Col phase can be ascribed to a minimum in
the form factor.22c,23 Though the occurrence of only one (100)
peak precludes the unequivocal assignment of the mesophase
structure, the situation is reminiscent of several reports wherein
such a pattern has been assigned to a Colh phase.22c,23 Further,
the absence of a core-core separation peak implies that the
discs within a column are not well organized when compared
to the normal chain analogue DT-8.
TT-n Series. Table 3 depicts the phase sequence, transition
temperatures, and associated enthalpies of the TT-n series of
compounds. It is seen that all the TSANs display dimesomorphic
behavior involving a transition from the lower temperature Colr
to the higher temperature Colh phase. In comparison with their
dialkoxy analogues (DT-n series), these TSANs having three
alkoxy chains on each benzene ring, except a few, show lower
melting (Cr-Col) points and higher clearing (Col-I) temperatures.
This clearly indicates that the presence of three consecutive
alkoxy tails in TSANs favors the Col assembly over wide
thermal range; the effective space-filling and a greater core-
core packing within a column may be the cause for such a
behavior. The first member TT-4 exhibits two enantiotropic Col
phases, in contrast to the non-mesomorphic dialkoxy analogue,
DT-4; this comparison, in part, supports our aforementioned
assumption concerning the effective space-filling and a greater
core-core packing. Upon cooling the sample TT-4 slowly from
the isotropic phase, a mesophase appears with a dendritic growth
(Figure 3a) that quickly fills the field of view to furnish a mosaic
pattern. The dendritic textural pattern suggests the mesophase
(23) (a) Hayer, A.; de Halleux, V.; Kohler, A.; El-Garoughy, A.; Meijer,
E. W.; Barbera, J.; Tant, J.; Levin, J.; Lehmann, M.; Gierschner, J.; Cornil,
J.; Geerts, Y. H. J. Phys. Chem. B. 2006, 110, 7653-7659. (b) Barbera, J.;
Gimenez, R.; Serrano, J. L. Chem. Mater. 2000, 12, 481-489 and references
cited therein. (c) Carfagna, C.; Roviello, A.; Sirigu, A. Mol. Cryst. Liq.
Cryst. 1985, 122, 151-160. (d) Gramsbergen, E. F.; Hoving, H. J.; de Jeu,
W. H.; Praefcke, K.; Kohne, B. Liq. Cryst. 1986, 1, 397-400. (e)
Metersdorf, H.; Ringsdorf, H. Liq. Cryst. 1989, 5, 1757-1772. (f) Zeng,
H.; Carroll, P. J.; Swager, T. M. Liq. Cryst. 1993, 14, 1421-1429.
(22) (a) Destrade, C.; Foucher, P.; Gasparoux, H.; Nguyen, H. T.; Levelut,
A. M.; Malthete, J. Mol. Cryst. Liq. Cryst. 1984, 106, 121-146. (b) Artal,
M. C.; Toyne, K. J.; Goodby, J. W.; Barbera, J.; Photinos, D. J. J. Mater.
Chem. 2001, 11, 2801-2807. (c) Alavarez, L.; Barbera, J.; Puig, L.; Romero,
P.; Serrano, J. L.; Sierra, T. J. Mater. Chem. 2006, 16, 3768-3773.
J. Org. Chem, Vol. 72, No. 22, 2007 8313

Yelamaggad et al.
FIGURE 5. 1D intensity vs 2θ profiles obtained for the Colh phase
(a) and the Colr phase (b) of the compound TT-8B at 90 and 50 °C,
respectively.
FIGURE 4. Photomicrographs of the optical textures obtained for the
Colh phase at 181 °C (a) and the Colr phase at 156 °C (b) of compound
TT-8.
hexagonal lattice with a spacing of about 24 Å. Figure 3c gives
the 1D intensity vs the 2θ profile derived from the 2D pattern
of the low-temperature Col phase. Notably, the low-angle region
of the diffractogram comprises eight strong reflections, the
maxima, which are best assigned to a rectangular columnar
arrangement with cell parameters a ) 43 Å and b ) 23 Å. In
addition to these, a diffuse peak with its center around 4.3 Å
was observed in the wide-angle region, which is associated with
the liquid-like correlation between aliphatic chains. The (001)
reflection at about 3.5 Å indicates the periodic stacking of the
molecular cores within the columns. Thus, XRD study suggests
that the lower temperature mesophase of TT-4 is the Colr phase,
although it displays an uncharacteristic optical texture.
The middle and higher homologues, viz., TT-5 to TT-8, TT-
10, TT-12, and TT-16, as mentioned earlier, show similar phase
behavior; the occurrence of the dimesomorphic Colr-Colh
sequence was identified for all these compounds by textural
observation. As a representative case, the textural patterns of
the Colh and Colr phases of TT-8 are shown in Figure 4, parts
a and b, respectively. The unequivocal assignment of the
structures of these two mesophases of TT-8, again as a
representative case, was achieved by the XRD studies and the
results are summarized in Table 4. The XRD profile of the high-
temperature phase showed five sharp reflections in the low-
angle region.16 The reciprocal spacings of these reflections have
the ratio 1:x3:x4:x7:x12, which could be indexed as (100),
(110), (200), (210), and (220) reflections of a hexagonal lattice.
Apart from these reflections, the diffractogram showed two
broad reflections in the wide-angle region demonstrating that
the ordering within the columns is not long-range. Similarly,
the XRD pattern of the low-temperature mesophase showed two
broad diffuse scatterings as well as four sharp Bragg reflections
which could be, however, indexed to a rectangular 2D lattice
of the Col phase. Thus, analysis of the results of these
complementary studies indicates that TSANs comprising nine
peripheral alkoxy chains stabilize hexagonal and rectangular Col
phases in a dimesomorphic sequence. It is worth mentioning
here that for compounds TT-6 and TT-7 the Col phase seems
to freeze into a glassy state.
TABLE 4. The Results of Indexation of XRD Profiles at a Given
Temperature (T) of Col Phases Formed by the TSANS of the TT-n
Series
d (Å) d (Å)
Miller
lattice
compd T (°C) phase measd calcd index hkl parameters (Å)
TT-4
230
190
Colh
20.5
11.8
10.3
7.8
6.7
4.4
20.5
11.8
10.2
7.7
100
110
200
210
220
a ) 23.6
c )3.42
5.9
3.42
001
200
110
210
020
220
510
600
610
Colr
21.7
20.1
18.3
11.8
10.9
8.0
21.5
20.3
15.7
11.5
10.2
8.1
a ) 43.0
b ) 23.0
c ) 3.52
7.4
6.7
7.2
6.8
4.4
3.52
001
100
110
200
210
220
TT-8
175
150
Colh
24.5
14.1
12.2
9.3
24.5
14.2
12.3
9.3
a ) 28.3
c ) 3.54
6.7
7.1
4.5
3.54
25.7
24.6
9.4
001
200
110
420
720
Colr
25.7
24.6
9.5
a ) 51.3
b ) 28.0
c ) 3.49
6.7
6.5
4.5
3.49
24.8
4.9
001
100
TT-8B
90
50
Colh
Colr
24.76
a ) 28.6
c ) 3.55
3.55
001
200
110
25.4
23.7
4.9
25.4
23.7
a ) 47.3
b ) 30.2
c ) 3.57
3.57
001
to be a Col phase. On further cooling, a transition to another
Col phase occurs that was evident from the sharp change in the
optical texture; tiny needles that grow on the mosaic texture of
the higher temperature Col phase (Figure 3b) coalesce to yield
a nonspecific texture. The transition temperature obtained was
corroborated by the DSC measurements. The XRD data given
in Table 4 confirm that the high- and low-temperature meso-
phases are Colh and Colr phases, respectively. The diffraction
pattern obtained for the high-temperature Col phase consisted
of a set of sharp reflections in the low-angle region besides two
diffuse peaks at about 4.4 and 3.4 Å in the wide angles. The
low-angle peaks are in the ratio of 1:0.58:0.5:0.38 which are
indexed as (100), (110), (200), and (210) reflections of a 2D
Compound TT-8B also displayed columnar behavior; unlike
its dialkoxy analogue DT-8B, it showed two columnar phases
which we describe as follows. On cooling from the isotropic
phase, it showed a mesophase with a texture having fan-shaped
defects and homeotropic digitated stars, which is typical of the
Colh phase.16 On further cooling, a transition to another
mesophase occurs via a textural change in which a transient
needle pattern coalesces to a grainy texture. In this case also,
the Col phase, especially the lower temperature phase, was found
to be stable over a wide thermal range through room temper-
ature. As summarized in Table 4, the analyses of the XRD
patterns revealed that the high-temperature phase is Colh with
8314 J. Org. Chem., Vol. 72, No. 22, 2007

New Discotics DeriVed from Tris(N-salicylideneaniline)s
FIGURE 6. The C3h and Cs geometrical forms of TSANs and their pseudostructural resemblance to the triphenylene and benz[d]anthracenylium
systems, respectively.
FIGURE 7. Graphical representation of the phase behavior (while heating) of the DT-n (a) and TT-n (b) series of TSANs.
the lattice constant a ) 28.6 Å (Figure 5a) and the low-
temperature phase is Colr with the lattice constants a ) 47.3 Å
and b ) 30.2 Å. (Figure 5b). The core-core separation peak
was found to be centered around 3.6 Å.
Packing Within the Columns and Overview of the Phase
Behavior. Tris(N-salicylideneaniline)s realized in the present
study match the design concept of discotics; their C3h and Cs
tautomers form shape-persistent central rigid cores that are
surrounded by flexible tails. The intrinsic intramolecular H-
bonding among the H-atoms of the 2°-amine and carbonyl
groups of the central cyclohexane-1,3,5-trione ring imposes
shapes to the tautomeric forms. Importantly, depending upon
the structures of the tautomers, the extent (number) of intramo-
lecular H-bonding varies, and thus, their overall shapes differ.
For example, in the C3h isomer all the carbonyl groups are
involved in H-bonding, and it bears a resemblance to the
FIGURE 8. Schematic representation of the self-organization of C3h
triphenylene (Figure 6); consequently it has been regarded as a
pseudo-triphenylene.13a On the other hand, the Cs isomer
resembles a four-ring noncircular aromatic core, the benz[d]-
anthracenylium, if two of the three carbonyl groups of the central
core are participating in the H-bonding. However, owing to the
presence of peripheral alkoxy tails in the benz[d]anthracenylium-
like structures, it may be reasonable to consider that the overall
shape of the Cs isomer is disk-like. Eventually, the coassembly
of both shape-persistent rotational isomers into columns and
their further organization leads to the formation of a fluid
macroscopic structure having a 2D lattice of different sym-
metries. Furthermore, it is apparent that the extent of Col
mesomorphism of TSANs is susceptible to the nature and the
number of alkoxy chains at the exterior position: TSANs with
two alkoxy tails on each of the three exterior benzene rings
linked to the central cyclohexanetrione core exhibit Colh or Colr
phases, while their trialkoxy analogues stabilize a dimesomor-
phic sequence involving a transition from hexagonal to rectan-
gular Col phases. The majority of TSANs having three exterior
alkoxy chains show lower melting (Cr-Col) points and higher
and Cs tautomers of TSANs into phase(s) during cooling from their
isotropic liquid state.
clearing (Col-I) temperatures when compared with those of
dialkoxy compounds. The effective space-filling and a greater
core-core packing within a column of trialkoxy derivatives may
be the reason for their above-mentioned behavior. The graphical
representations of phase behavior and thus the thermal range
of the mesophases of DT-n and TT-n series are given in Figure
7, parts a and b, respectively. The general phase behavior of
the TSANs has been schematically shown in Figure 8.
Optical Absorption and Fluorescence Behavior. As dis-
cussed before, owing to the presence of multiple chromophores
and possible π-conjugations between the peripheral region and
the central core, both series of discotics TSANs are expected
to exhibit promising photophysical properties. Thus, UV-vis
absorption and fluorescence characteristics of some of the
representative compounds randomly selected from both the
series, viz., DT-8, TT-8, and TT-8B, were studied in solution
as well as in the mesophase. Indeed, the THF solution absorption
J. Org. Chem, Vol. 72, No. 22, 2007 8315

Yelamaggad et al.
FIGURE 9. Absorption and emission spectra in THF solution (top panels) and in the thin films of the mesophases (lower panels) obtained for
DT-8 (a), TT-8 (b), and TT-8B (c). The inset shows the pictures of solutions as seen before (LHS) and after the illumination (RHS) of light of 365
nm. Note that for thin films of the Col phase, the spectra were recorded as a function of temperature (see RHS traces of lower panels) (LHS )
left-hand side; RHS ) right-hand side.)
TABLE 5. Photophysical Properties of TSANS DT-8, TT-8, and TT-8B
THF solutiona
emissionb,c
glassy film
emissionb,c
TSANs
absorptionb
Stokes shift
absorptionb
Stokes shift
DT-8
TT-8
TT-8B
334, 416
334, 416
332, 416
481
484
480
65
65
64
341, 424
339, 418
336, 414
539
545,612
531, 616
123
138
202
a Micromolar solutions in THF. b Wavelengths (nm). c The excitation wavelength λex ) 416 nm.
spectra were recorded for all the compounds and their pattern
looked alike, as expected. As shown in the upper and lower
panels of Figure 9a-c, UV-vis spectra (blue trace) of the three
investigated TSANs in solution and mesophase respectively
showed two absorption maxima centered around 335 and 416
nm corresponding to π-π* and n-π* transitions. Upon
irradiating these solutions with one of the absorption maxima,
viz., 416 nm, a fluorescence maximum was observed at around
482 nm with a negligible shoulder (orange traces in the upper
panels of Figure 9a-c). The Stokes shift, which is the difference
between the spectral positions of the band maxima of the
absorption and fluorescence arising from the same electronic
transition, was found to be in the range of 64-65 nm (Table
5). The occurrence of luminescence at a longer wavelength than
the absorption is in agreement with the general observation. As
shown in the insets, the green light is visually perceivable in
the emissive state.
The absorption behavior of the fluid Col phase as well as its
frozen state in the form of a thin film were investigated. The
sample under investigation was held between the two cover slips
and cooled slowly (1 deg/min) from the isotropic liquid state
until room temperature. To begin with, the emission spectra
were recorded at room temperature. As already mentioned
above, the frozen Col structures of DT-8 and TT-8 and the
room-temperature Col phase of TT-8B show two absorption
bands analogous to those in the solution state. The apparent
small red shift in the two absorption maxima of the Col structure
is indicative of the intimate overlap of the fluorophores in such
states. When excited at 416 nm, two broad bands with
fluorescence maxima are observed at around 530 and 600 nm
for compounds TT-8 and TT-8B (see red traces of the lower
panels b and c of Figure 9), while for TSAN DT-8 only a
fluorescence maximum at around 539 nm was obtained. The
fluorescence spectra of these thin films at different temperatures
starting from the room temperature/glassy Col phase for the
above-mentioned excitation wavelength were measured and have
been shown in the right-hand side of the lower panels a-c of
Figure 9. As can be seen, the intensity of the emission peak
decreases progressively with the increase in the temperature that
can be attributed to breaking of larger columnar stacks into
smaller ones and thermally activated radiationless processes.24
Notably, when compared with the emission behavior of solution,
the films of the neat Col phase exhibit a strong bathochromic
(red) shift that can be attributed to the strong co-facial proximity
of C3h and Cs isomers within the Col structure as well as the
formation of excimers/aggregates. Generally, the excimers are
regarded as molecular dimers or a stoichiometric complex with
associated excited electronic states, dissociative ground states,
and structureless emission spectra.25 Furthermore, the spectral
pattern with two emission maxima of the trialkoxy compounds
TT-8 and TT-8B as compared to only a broad fluorescence
maximum of dialkoxy TSAN DT-8 perhaps reflects on the
dissimilar arrangement of the discs with six or nine peripheral
alkoxy tails within the columns; however, it must be mentioned
here that it is rather difficult to comment on the precise nature
of the associated forms. Thus, it is apparent that the TSANs
realized in the present investigation are capable of generating
the light in both solution and mesophase states. This is especially
(24) (a) Kleppinger, R.; Lillya, C. P.; Yang, C. J. Am. Chem. Soc. 1997,
119, 4097-4102. (b) Festag, R.; Kleppinger, R.; Soliman, M.; Wendorff,
J. H.; Lattermann, G.; Staufer, G. Liq. Cryst. 1992, 11, 699-710.
(25) (a) Birks, J. B. The Photophysics of Aromatic Excimers. In The
Exiplex; Gordon, M., Ware, W. R., Eds.; Academic Press: New York, 1975;
p 39. (b) Duan, X. F.; Wang, J.-L.; Pei, J. Org. Lett. 2005, 19, 4071-4074.
8316 J. Org. Chem., Vol. 72, No. 22, 2007

New Discotics DeriVed from Tris(N-salicylideneaniline)s
was poured into water and extracted with dichloromethane (4 ×
100 mL). The combined extracts were washed with 5% aqueous
NaHCO3 solution, water, and brine then dried over anhyd Na2SO4
and concentrated. The crude product obtained was purified by
column chromatography on silica gel (60-120 mesh). Elution with
hexanes followed by 10% dichloromethane-hexanes yielded 3b,
which was further purified by recrystallization from ethanol. Rf 0.51
(30% CH2Cl2-hexanes); white solid; mp 50-52 °C; yield 1.6 g,
54%; IR (KBr pellet) νmax in cm-1 3020, 2960, 2875, 1517, 1340,
remarkable in the later situation wherein the 2D order with short
intra-core distance facilitates motion of charges with impounding
motion of the ionic impurities.8b Nonetheless, detailed investiga-
tions involving the annealing of films, film morphology,
fluorescence efficiency, solvent effects, etc. are required to
further elucidate the photophysical properties of TSANs quan-
titatively.
Summary. The synthesis and phase behavior of a novel class
of discogens composed of TSANs are described. The principle
objective of this investigation was to validate our recent
demonstration that with the space-filling design concept, TSANs
can be enforced to display columnar mesomorphism wherein
the individual columns are built by the co-facial self-assembly
of two shape-persistent rotational isomers formed of intramo-
lecular H-bonding. Thus, several discotic TSANs belonging to
two series each consisting of six and nine peripheral n-alkoxy/
branched tails have been accomplished. In each series, the length
of the n-alkoxy tail has been varied to learn about the correlation
between structure and property. Our study using a combination
of optical polarizing microscope, differential scanning calorim-
eter, and X-ray diffraction revealed that the mesomorphic
behavior is sensitive to the number of exterior n-alkoxy tails
rather than the length. Notably, several of them with n-alkoxy
tails form a glassy Col structure in which the core-core
separation is quite small. Of special significance, TSANs
comprising branched alkoxy tails display the Col phase over a
broad thermal range through room temperature. The absorption
and fluorescence properties have been studied in the Col phase
and in solution. The observed light emissive characteristic of
the Col phase appears to be promising from the point of
electronic device application given the fact that in such structures
the proton and electron interact with each other through the
H-bonding environment. In essence, owing to the ease of
synthesis, structural diversity, columnar behavior, and electronic
properties, discotic TSANs may find a place in electronic device
application.
1
1217, 778, and 669; H NMR (CDCl3, 200 MHz) δ 7.87 (dd, 1H,
J ) 9.0 Hz, 2.6 Hz, Ar), 7.72 (d, 1H, J ) 2.6 Hz, Ar), 6.88 (d, 1H,
J ) 9.0 Hz, Ar), 4.07 (q, 4H, J ) 6.2 Hz, 2 × OCH2), 1.27-1.92
(m, 12H, 6 × CH2), 0.99 (t, 6H, J ) 7.4 Hz, 2 × CH3); MS (FAB+)
m/z for C16H25NO4, calcd 295.2, found 295.9. Anal. Calcd for
C16H25NO4: C, 65.06; H, 8.53, N, 4.74. Found: C, 65.3; H, 8.8;
N, 4.4.
3,4-Dipentyloxyaniline (5b). To a solution of nitro compound
3b (2.95 g, 10 mmol, 1 equiv) in dry THF (20 mL) was added
10% Pd-C (0.3 g) with stirring under hydrogen atmosphere
(balloon) for 12 h (monitored by TLC). The reaction mixture was
filtered through a celite bed. Evaporation of the solvent yielded
the amine that was passed through a very short basic alumina
column, using a mixture of ethylacetate-hexanes (10%) as the
eluent. The evaporation of solvent yielded pure amine 5b as a
viscous liquid; Rf 0.53 (30% EtOAc-hexanes); yield 2.4 g, 90%;
IR (KBr pellet) νmax in cm-1 3360, 2931, 2869, 1512, 1468, 1227,
1
1180, 896, 748, and 599; H NMR (CDCl3, 200 MHz) δ 6.73 (d,
1H, J ) 8.4 Hz, Ar), 6.30 (d, 1H, J ) 2.4 Hz, Ar), 6.21 (dd, 1H,
J ) 8.4 Hz, 2.4 Hz, Ar), 3.92 (q, 4H, J ) 6.8 Hz, 2 × OCH2),
3.43 (br s, 2H, NH2), 1.35 -1.87 (m, 12H, 6 × CH2), 0.89 (t, 6H,
J ) 6.0 Hz, 2 × CH3); MS (FAB+) m/z for C16H27NO2, calcd 265.2,
found 265.9. Anal. Calcd for C16H27NO2: C, 72.41; H, 10.25; N,
5.28. Found: C, 72.4; H, 10.3; N, 5.3.
3,4,5-Tripentyloxynitrobenzene (4b). The experimental pro-
cedure was as described for the preparation of 3b. Quantities: 2b
(3.2 g, 9.5 mmol, 1 equiv), sodium nitrite (0.09 g, 1.3 mmol, 0.14
equiv), and 70% HNO3 (1.3 mL, 28.5 mmol, 3 equiv). Rf 0.52 (30%
CH2Cl2-hexanes); a pale yellow liquid; yield 2.18 g, 60%; IR (KBr
pellet) νmax in cm-1 2956, 2934, 2872, 1617, 1524, 1493, 1467,
1390, 1339, 1234, 1115, 852, 794, 682, and 668; 1H NMR (CDCl3,
200 MHz) δ 7.47 (s, 2H, Ar), 4-4.1 (m, 6H, 3 × OCH2), 0.91-
1.88 (m, 27H, 9 × CH2, 3 × CH3); MS (FAB+) m/z for C21H35-
NO5, calcd 381.3, found 381.9. Anal. Calcd for C21H35NO5: C,
66.11; H, 9.25; N, 3.67. Found: C, 66.5; H, 9.4; N, 3.9.
Experimental Section
General. For general experimental details and instrumentation,
see the Supporting Information. The mesogenic compounds were
investigated for their liquid crystalline behavior employing an
optical polarizing microscope equipped with a programmable hot
stage and differential scanning calorimeter. X-ray diffraction studies
were carried on powder samples in Lindemann capillaries with Cu
KR radiation, using an Image Plate Detector equipped with a double
mirror focusing optics. UV-vis spectra were recorded for solution
and mesophase states. The above-mentioned optical polarizing
microscope equipped with a programmable hot stage and differential
scanning calorimeter was used to determine the melting points of
non-mesomorphic compounds. Fluorescence emission spectra in
solution and mesophase state were recorded with luminescence
spectrometers.
All the 3,4-dialkoxynitrobenzenes (3a-h) and 3,4,5-trialkox-
ynitrobenzenes (4a-h) and the corresponding anilines 5a-h and
6a-h were prepared following the experimental procedure de-
scribed below for the syntheses of 3b, 4b, 5b, and 6b as
representative cases.
3,4-Dipentyloxynitrobenzene (3b). To an ice-cooled solution
of 1,2-dipentyloxybenzene (1b) (2.5 g, 10 mmol, 1 equiv) in
dichloromethane (15 mL) was added NaNO2 (0.1 g, 1.4 mmol, 0.14
equiv) with stirring. To this ice-cooled suspension was added a
solution of 70% HNO3 (1.4 mL, 30 mmol, 3 equiv) in dichlo-
romethane (12 mL); this mixture was allowed to attain room
temperature and stirring was continued for 1 h. The reaction mass
3,4,5-Tripentyloxyaniline (6b). The experimental procedure was
as described for the preparation of 5b. Quantities: 4b (3.8 g, 10
mmol, 1 equiv), 10% Pd-C (0.38 g, 10% weight of the starting
material), dry THF (15 mL). Rf 0.44 (30% EtOAc-hexanes); gray
solid; mp 81-83 °C; yield 3.08 g, 88%; IR (KBr pellet) νmax in
cm-1 3331, 2956, 2933, 1600, 1504, 1456, 1230, 1108, 1050, 756,
1
and 585; H NMR (CDCl3, 200 MHz) δ 5.91 (s, 2H, Ar), 3.81-
3.94 (m, 6H, 3 × OCH2), 3.5 (br s, 2H, NH2), 1.28-1.82 (m, 18H,
9 × CH2), 0.92 (t, 9H, J ) 6.4 Hz, 3 CH3); MS (FAB+) m/z for
C21H37NO3, calcd 351.3, found 351.8. Anal. Calcd for C21H37NO3:
C, 71.75; H, 10.61; N, 3.98. Found: C, 71.6; H, 10.5; N, 4.3.
All the target dialkoxy TSANs (DT-n series) and trialkoxy
TSANs (TT-n series) were prepared following the experimental
procedure described below for the synthesis of DT-5 and TT-5 as
representative cases.
(2E,4E,6E)-2,4,6-Tris((3,4-dipentyloxyphenylamino)methyl-
ene)cyclohexane-1,3,5-trione (C3h isomer) and (Z)-2,4,6-Tris((3,4-
dipentyloxyphenylamino)methylene)cyclohexane-1,3,5-trione (Cs
isomer) (DT-5). A mixture of triformylphloroglucinol (0.084 g,
0.4 mmol, 1 equiv) and compound 5b (0.72 g, 2.7 mmol, 6.75
equiv) in absolute ethanol (15 mL) was heated to reflux under an
inert atmosphere for 2 h with vigorous stirring. The pale yellow
solid separated upon cooling the reaction mixture was collected
by filtration, repeatedly washed with ethanol, and air-dried. The
crude product DT-5 was further purified by repeated recrystalli-
J. Org. Chem, Vol. 72, No. 22, 2007 8317

Yelamaggad et al.
hexanes); a yellow solid, yield 0.39 g, 80%; UV-vis: λmax
zations in a mixture of absolute ethanol-dichloromethane (9:1).
Rf 0.54 (30% EtOAc-hexanes); a yellow solid; yield 0.29 g, 75%;
UV-vis: λmax ) 415.97 nm, ꢀ ) 5.4792 × 104 L mol-1 cm-1; IR
(KBr pellet) νmax in cm-1 3450, 2930, 2859, 1621, 1591, 1522,
)
415.06 nm, ꢀ ) 7.3046 × 104 L mol-1 cm-1; IR (KBr pellet) νmax
in cm-1 3452, 2956, 2871, 1622, 1588, 1508, 1459, 1289, 1230,
1113, 987, and 824; 1H NMR (CDCl3, 400 MHz) δ 13.46 (d, J )
13 Hz, dCNH), 13.28 (d, J ) 13.2 Hz, dCNH), 13.01 (d, J )
13.2 Hz, dCNH), 12.88 (d, J ) 13.4 Hz, dCNH) (these four
resonances are due to 3H), 8.62-8.78 (m, 3H, dCHN), 6.48 (s,
6H, Ar), 3.92-4.03 (m, 18H, 9 × OCH2), 0.92-1.85 (m, 81H, 27
× CH2, 9 × CH3); MS (FAB+) m/z for C72H113N3O12 (M + 2),
calcd 1211.8, found 1212.5. Anal. Calcd for C72H111N3O12: C,
71.43; H, 9.24; N, 3.47. Found: C, 71.2; H, 9.4; N, 3.6.
1
1459, 1262, 1136, 988, and 820; H NMR (CDCl3, 400 MHz) δ
13.47 (d, J ) 13.0 Hz, dCNH), 13.36 (d, J ) 13.2 Hz, dCNH),
13.01(d, J ) 13.2 Hz, dCNH), 12.97 (d, J ) 13.2 Hz, dCNH)
(these four resonances are due to 3H), 8.63-8.8 (m, 3H, dCHN),
6.81-6.91 (m, 9H, Ar), 3.98-4.10 (m, 12H, 6 × OCH2), 0.92-
1.85 (m, 54H, 18 × CH2, 6 × CH3); MS (FAB+) m/z for
C57H82N3O9 (M + 1), calcd 952.6, found 952.8; Anal. Calcd for
C57H81N3O9: C, 71.89; H, 8.57; N, 4.41. Found: C, 71.5; H, 8.7;
N, 4.4.
(2E,4E,6E)-2,4,6-tris((3,4-Trialkyloxyphenylamino)methylene)-
cyclohexane-1,3,5-trione (C3h isomer) and (Z)-2,4,6-Tris((3,4-
trialkyloxyphenylamino)methylene)cyclohexane-1,3,5-trione (Cs
isomer) TT-5. The experimental procedure was as described for
the preparation of DT-5. Quantities: triformylphloroglucinol (0.084
g, 0.4 mmol, 1 equiv), 6b (0.95 g, 2.7 mmol, 6.75 equiv), and
absolute ethanol (15 mL). The pale yellow solid separated upon
cooling the reaction mixture was collected by filtration, repeatedly
washed with ethanol, and air-dried. The crude product TT-5 was
further purified by repeated recrystallizations in a mixture of
absolute ethanol-dichloromethane (9:1). Rf 0.59 (30% EtOAc-
Supporting Information Available: General experimental
details, instrumentation, structural characterization data, H NMR
1
spectra of all the target molecules and representative intermediates
(1e, 3e, 5e, 2e, 4e, and 6e), 1H-1H COSY NMR spectra of DT-8,
TT-8, and TT-8B, 13C NMR of DT-8 and TT-8, photomicrographs
of the optical textures of DT-5 and TT-8B, DSC traces of DT-8,
TT-8, and TT-8B, and XRD patterns of DT-5, DT-8, and TT-8.
This material is available free of charge via the Internet at
http://pubs.acs.org.
JO0712650
8318 J. Org. Chem., Vol. 72, No. 22, 2007