Tris(N-salicylideneanilines) [TSANs] exhibiting a room temperature columnar mesophase: synthesis and characterization

Article abstract of DOI:10.1016/j.tetlet.2006.07.102

Tris(N-salicylideneaniline) derivatives with peripheral branched alkyl chains, existing in their C_3h and C_s symmetric keto-enamine tautomeric forms, synthesized by condensing alkoxyanilines with 1,3,5-triformylphloroglucinol, display a technologically important columnar liquid crystal phase over a wide thermal interval (～160 °C) well below and above room temperature, as established by optical and calorimetric studies.

Full text of DOI:10.1016/j.tetlet.2006.07.102

Tetrahedron Letters 47 (2006) 7071–7075
Tris(N-salicylideneanilines) [TSANs] exhibiting
a room temperature columnar mesophase:
synthesis and characterization
C. V. Yelamaggad* and A. S. Achalkumar
Centre for Liquid Crystal Research, Jalahalli, Bangalore 560013, India
Received 10 May 2006; revised 12 July 2006; accepted 20 July 2006
Available online 14 August 2006
Abstract—Tris(N-salicylideneaniline) derivatives with peripheral branched alkyl chains, existing in their C3h and Cs symmetric keto-
enamine tautomeric forms, synthesized by condensing alkoxyanilines with 1,3,5-triformylphloroglucinol, display a technologically
important columnar liquid crystal phase over a wide thermal interval (ꢀ160 ꢀC) well below and above room temperature, as estab-
lished by optical and calorimetric studies.
ꢁ 2006 Elsevier Ltd. All rights reserved.
Liquid crystal (LC) phases are intermediate between the
anisotropic solid and isotropic liquid states. They are
liquid in their facile mobility, yet retain some of the
intermolecular and intramolecular order of crystals.
Conventionally, such LC phases consist of rod-like (cal-
mitic) or disk-like (discotic) molecules comprising cova-
lently bonded structurally incompatible entities namely,
flexible aliphatic chains and a rigid anisometric core.1
The parallel arrangement with long range orientational
order of such molecules leads to the formation of the
nematic (N) phase whereas microphase segregation of
incompatible regions promotes a smectic (Sm) or colum-
nar (Col) phase.2
stacks facilitates smooth charge-transport. This anisot-
ropy in conduction can be well exploited in molecular
electronic devices such as one-dimensional conductors,3
photoconductors,4 molecular wires and fibers,5 light
emitting diodes,6 field-effect transistors, and photovol-
taic cells.7 The performance of such electronic devices
depends critically on the high charge-carrier mobility.
The vast majority of disc-like mesogens, comprising
electron-rich (donor) aromatic cores such as triphenyl-
ene, dibenzopyrene, and hexabenzocoronene, etc.; are
good hole transporters, that is, the p-type materials.
However, examples of n-type discotics derived from
electron-deficient aromatic systems with electron-accept-
ing character are rather scarce. Examples include coron-
ene derivatives peripherally substituted with electron
withdrawing carboximide groups or aromatic systems
with electronegative nitrogen in the core such as tri-
azines or quinoxalines.8 Recently, electron deficient
heterocycles like hexaazatriphenylene9 and hexa-
azatrinaphthylene10 have been reported as new n-type
discogens.
In particular, Col phases, characterized by indefinitely
long molecular columns aggregating into two-dimen-
sional (2D) lattices with different symmetries, are of
great significance. This is because they allow the possi-
bility of combining several physical properties with the
orientational control of the molecular order, self-healing
of structural defects and ease of processability when
compared to the inorganic semiconductors or zone
refined single crystals currently used in electronic appli-
cations. The central aromatic core of a disc-like mole-
cule acts as a conducting unit, while surrounding
paraffinic tails serve as a mantle (insulator). Thus the
organization of such molecules in one-dimensional Col
Tris(N-salicylideneanilines) [TSANs], existing exclu-
sively in keto-enamine forms with C3h and Cs symme-
tries, undergo the multiple proton transfer process.11
From the point of view of applications, especially in
electronic devices involving n-type discogens, such an
inherent property of TSANs is highly promising. In fact,
we have recently demonstrated that the TSANs with
appropriate molecular design self-assemble to form
*
Corresponding author. Tel.: +91 8028381119; fax: +91
9028382044; e-mail: yelamaggad@yahoo.com
0040-4039/$ - see front matter ꢁ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.07.102

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C. V. Yelamaggad, A. S. Achalkumar / Tetrahedron Letters 47 (2006) 7071–7075
R2
R2
R3
R1
H2
R3
R1
N
H6
N
H1
O
H5
O
O
H
O
N
H
N
H7
N
H
R3
R2
H4
R3
R2
R3
R2
N
O
R3
R2
O
H8
H
H3
R1
R1
R1
R1
Cs
C3h
I: R1 = H; R2 = R3 = O-n-CnH2n+1
II : R1 = R2 = R3 = R = O-n-CnH2n+1
1 : R1 = H; R2 = R3 = 3,7-Dimethyloctyloxy
2 : R1 = R2 = R3 = 3,7-Dimethyloctyloxy
Figure 1. The molecular structure of tris(N-salicylideneanilines) [TSANs] existing in keto-enamine forms with C3h and Cs from previous (I and II)12
and present (1 and 2) investigations.
OH
Br
Col LC phases in which the proton and electron interact
with each other through the H-bonding environment.12
The central core of a TSAN, formed by strong intramo-
lecular hydrogen bonding between the enamine proton
and oxygen of the cyclohexanetrione, is almost the size
as triphenylene. The core–core separation observed in
i
R1
R3
R1
ii
R2
HO
˚
the Col phase of these molecules is 3.29 A. Remarkably,
HO
the magnitude of core–core separation further decreases
R1 = H
R1 = OH
6a: R1 = H; R2= R3 = 3,7-Dimethyloctyloxy
6b: R1= R2 = R3= 3,7-Dimethyloctyloxy
˚
to 3.26 A on freezing the Col structure. This value is
close to the shortest core–core separation observed hith-
erto.9a Small core–core separation leads to a better p–p*
overlap; the formation of a glassy columnar phase
ensures a defect-free alignment. These features make
them promising materials for a rapid intracolumnar
charge migration. However, ideal materials should have
a wide mesophase range with low melting and clearing
points to circumvent the difficulties involved in experi-
mental investigations.
iii
R1
R2
R
R3
5a: R1 = H; R2= R3 = 3,7-Dimethyloctyloxy
5b: R1= R2 = R3= 3,7-Dimethyloctyloxy
R = NO2
iv
4a: R1= R2 = 3,7-Dimethyloctyloxy
These features motivated us to design and synthesize
TSANs exhibiting a room temperature columnar meso-
phase (Fig. 1). A number of research groups have
recently shown that when branched alkyl chains are
introduced at the peripheral region of discotic mole-
cules, the liquid-like disorder is enhanced. This leads
to a widening of the mesophase thermal range and low-
ering of melting points without altering the type of the
mesophase.13 Following this strategy, we have designed
the first examples of TSANs that display a room temper-
ature Col fluid phase. The molecular structures of the
two TSANs, 1 and 2 existing in keto-enamine forms
with C3h and Cs symmetries, synthesized in the present
investigation are shown in Figure 1. As can be seen, in
these materials a 3,7-dimethyloctyloxy chain was used
as the branched peripheral tail.
R = NH2
4b: R1= R2 = R3= 3,7-Dimethyloctyloxy
H
O
O
H
v
O
H
H
1 ; C3h : Cs (1:1.3)
2 ; C3h : Cs (1:1.2)
O
H
O
O
H
3
Scheme 1. Synthesis of 1 and 2. Reagents and conditions: (i) 48% HBr,
H2SO4, (80%); (ii) 1-bromo-3,7-dimethyloctane, anhyd K2CO3, DMF,
80 ꢀC, 17 h, (6a: 82%; 6b: 85%); (iii) 70% HNO3, NaNO2, CH2Cl2, 1 h,
(5a: 75%; 5b: 80%); (iv) 10% Pd/C, H2, THF, 12 h, (4a: 80%; 4b: 85%)
and (v) 4a or 4b, EtOH, 80 ꢀC, 5 h, (1: 70%; 2: 75%).
1H NMR and H–1H COSY NMR spectra provided
1
The synthetic route to target molecules 1 and 2 is
depicted in Scheme 1. The pure liquid crystalline yellow-
ish products were characterized by 1H NMR, 13C NMR,
IR, UV–vis spectroscopy, FAB mass spectrometry and
elemental analysis.14
proof of the existence of 1 and 2 in their C3h and Cs
symmetric keto-enamine tautomeric forms in the ratio
of 1:1.3 and 1:1.2, respectively. The coupling between
the enamine protons and vinylic protons was clearly

C. V. Yelamaggad, A. S. Achalkumar / Tetrahedron Letters 47 (2006) 7071–7075
Table 1. Mesomorphic behaviour of tris(N-salicylideneanilines) 1 and 2
7073
TSANs (C3h:Cs)a
Phase sequenceb
Heating (ꢀC)
Cooling (ꢀC)
1 (1:1.3)
2 (1:1.2)
Col 109.9 (5.8) I
Col2 68d Col1 109.9 (4.1) I
I 105.5 (1.9) Colc
I 106.3 (20.7) Col1 64d Col2
c
Col = Columnar phase (Col1 and Col2 are different columnar phases); Iso = Isotropic phase.
a Ratio obtained from 1H NMR.
b Peak temperatures in the DSC thermograms obtained during the first heating and cooling cycles.
c No crystallization was observed till À60 ꢀC.
d The phase transition observed with a microscope was too weak to be detected by DSC.
elucidated from 1H–1H COSY NMR. These NMR
spectral data are in agreement with earlier reports.11,12
The mesomorphic behaviour of the two TSANs was
examined by observing optical textures as well as ascer-
taining the transition temperatures using polarizing
optical microscope (POM) (Leitz DMRXP) equipped
with a programmable hot stage (Mettler FP90). The
transition temperatures and associated enthalpies were
obtained from thermograms recorded on a differential
scanning calorimeter (DSC) (Perkin–Elmer DSC7) that
was calibrated previously using pure indium as the stan-
dard. The results obtained are shown in Table 1. These
compounds, 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 both compounds were
already in the LC state. On cooling at a rate of 5 ꢀC/
min from their isotropic phase, compound 1 showed a
mesophase at 105 ꢀC; the corresponding optical texture
consisted of pseudo-focal conic fan-shaped defects, a
typical feature for the columnar phase.15 Interestingly,
the optical texture remained unchanged till room tem-
perature as shown in Figure 2a and no sign of crystal-
lization was noticed even after maintaining the
samples at low temperature or even when sheared.
Upon cooling from the isotropic phase, sample 2
showed a mesophase (at 106 ꢀC) with a texture having
pseudo-focal conic fan-shaped defects and homeotropic
digitated stars (Fig. 2b), which is typical of the Col
phase (Col1). On further cooling, the texture of the
Col1 phase sharply transformed to another pattern com-
prising tiny needles (Fig. 2c) that coalesces to a grainy
Figure 2. Optical microphotographs (cross polarizers) of the textures
observed: (a) the Col phase of TSAN 1 at RT; (b) the Col1 phase of
TSAN 2 at 95 ꢀC and (c) the Col2 phase of TSAN 2 at 64 ꢀC.
texture. This indicates the presence of another Col
phase, which we abbreviate as Col2.12
In this case also, the Col phase was found to be stable at
room temperature. The DSC thermograms of both com-
pounds corroborated the microscopic observations. For
example, the DSC trace obtained during the first heating
cycle of TSAN 2 from À60 ꢀC showed only one endo-
thermic peak at 110 ꢀC corresponding to the Col–Iso
transition (Fig. 3h1). In the subsequent cooling scan
(down to À60 ꢀC), as expected, a sharp exothermic peak
due to an Iso–Col transition was seen (Fig. 3c1). As
shown in Fig. 3h2 and c2, identical DSC profiles were
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 the TSANs are quite stable.
Furthermore, these mesogens exhibited fluorescent prop-
erties. In the UV–vis spectra, both compounds showed
two absorption maxima at 329 nm (p–p*) and 416 nm
(n–p*). Excitation at 420 nm for both the compounds,
showed an emission spectrum with maximum of
482 nm. As a representative case, the absorption and
emission spectra of compound 2 in THF solution are
shown in Figure 4a and b, respectively. The difference

7074
C. V. Yelamaggad, A. S. Achalkumar / Tetrahedron Letters 47 (2006) 7071–7075
6. Christ, T.; Glusen, B.; Greiner, A.; Kettner, A.; Sander,
R.; Stumpflen, V.; Tsukruk, V.; Wendorff, D. J. Adv.
Mater. 1997, 9, 48–52.
7. Schmidt-Mende, L.; Fechtenkotter, A.; Mullen, K.;
Moons, E.; Friend, R. H.; MacKenzie, J. D. Science
2001, 293, 1119–1126.
8. Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schen-
ning, A. P. H. Chem. Rev. 2005, 105, 1491–1546, and the
references cited therein.
9. (a) Gearba, R. I.; Lehmann, M.; Levin, J.; Ivanov, D. A.;
Koch, M. H. J.; Barbera’, J.; Debije, M. G.; Piris, J.;
Geerts, Y. H. Adv. Mater. 2003, 15, 1614–1618; (b)
Chang, T.-H.; Wu, B.-Ru.; Chiang, M. Y.; Liao, S.-C.;
Ong, C. W.; Hsu, H.-F.; Lin, S.-Y. Org. Lett. 2005, 7,
4705–4708.
Figure 3. The DSC traces of TSAN 2 (at a rate of 5 ꢀC/min).
10. Kestemont, G.; de Halleux, V.; Lehmann, M.; Ivanov, D.
A.; Watson, M.; Geerts, Y. H. Chem. Commun. 2001,
2074–2075.
11. Chong, J. H.; Sauer, M.; Patrick, B. O.; Maclachlan, M. J.
Org. Lett. 2003, 21, 3823–3826.
12. Yelamaggad, C. V.; Achalkumar, A. S.; Shankar Rao, D.
S.; Prasad, S. K. J. Am. Chem. Soc. 2004, 126, 6506–
6507.
13. (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.; Shankar Rao, D. S.; Prasad, S. K. J.
Mater. Chem. 1999, 9, 2751–2754.
0.4
0.3
0.2
0.1
100
80
60
40
20
0
a
b
300
400
500
600
λ (nm)
Figure 4. (a) UV absorption and (b) emission (for the excitation at
420 nm) spectra of TSAN 2 in THF.
14. The requisite branched chain alkyl bromide, 1-bromo-3,7-
dimethyloctane was prepared by treating 3,7-dimethyloct-
anol with 48% hydrobromic acid in the presence of sulfuric
acid. The O-alkylation of catechol or pyragallol with the
alkylbromide was carried out by employing a William-
son’s ether synthesis protocol to obtain dialkoxy or
trialkoxy benzenes 6a or 6b, respectively. The nitration
of 6a or 6b using an aqueous solution of nitric acid (70%)
and catalytic amounts of sodium nitrite in dichlorometh-
ane generated the corresponding nitrobenzenes 5a or 5b,
which upon catalytic hydrogenation furnished anilines 4a
and 4b. 1,3,5-Triformylphloroglucinol 3 was prepared by
the Duff formylation of phloroglucinol as described by
Chong et al.11 Finally, the amines (4a and 4b, 0.57 mmol,
4 equiv) and 3 (0.14 mmol, 1 equiv) were taken in absolute
ethanol and heated at 80 ꢀC for 4 h.12 The solvent was
removed in vaccuo from the reaction mixture to give the
target compounds as dull yellow gummy masses. These
products were purified by repeatedly dissolving them in
dichloromethane and re-precipitating the LC mass by the
addition of a large excess of ethanol at 10–15 ꢀC. Data for
1: Rf = 0.72 (30% EtOAc–hexanes); A yellow liquid
crystalline gummy mass; yield = 60%; UV–vis (THF):
in the number of peripheral alkoxy chains does not affect
their absorption and emission characteristics noticeably,
as both show identical patterns.
In conclusion, we have reported the synthesis and char-
acterization of the first examples of TSANs (existing
exclusively in their C3h and Cs keto-enamine tautomeric
forms) exhibiting a technologically important columnar
mesophase over a large thermal range, well below and
above ambient temperature. Seemingly, the branched
alkyl tails at the peripheral region of the TSAN core,
prevent crystallization and thus the molecules stay in a
liquid crystalline state over a wide temperature range
including room temperature. In our opinion, these Col
LCs featuring proton and electron interaction through
the H-bonding environment are ideal candidates for
electronic device applications.
k
max = 416.09 nm, e = 7.4044 · 104 L molÀ1 cmÀ1
;
IR
(KBr pellet): mmax in cmÀ1 3450, 2924, 2853, 1621, 1592,
1458, 1018, and 829; 1H NMR (CDCl3, 400 MHz) d: 13.48
(d, J = 13.0 Hz, @CNH), 13.37 (d, J = 13.3 Hz, @CNH),
13.01 (d, J = 13.2 Hz, @CNH), 12.96 (d, J = 13.4 Hz,
@CNH) (these three resonances are due to 3H), 8.63–8.77
(m, 3H, @CHN), 6.82–6.9 (m, 9H, Ar), 4.03–4.11 (m, 12H,
6 · OCH2), 0.86–1.88 (m, 114H, 12 · CH, 24 · CH2,
18 · CH3); MS (FAB+): m/z for C87H142N3O9 (M+1),
Calculated for C87H142N3O9: 1373.1, Found: 1373.3;
References and notes
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University Press, 1994; (b) Kato, T. Science 2002, 295,
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Siemensmeyer, K.; Etzbach, K. H.; Ringsdorf, H.; Haarer,
D. Nature 1994, 371, 141–143.
Elemental analysis: calculated (found) (%):
C 76.1
(75.75), H 10.35 (9.95), N 3.06 (2.96). 2: Rf = 0.73 (30%
EtOAc–hexanes); A yellow liquid crystalline gummy mass;
yield: 72%; UV–vis (THF):
kmax = 416.10 nm, e =
5.605 · 104 L molÀ1 cmÀ1; IR (KBr pellet): mmax in cmÀ1
3448, 2923, 2852, 1627, 1591, 1459, 1308, 1120, 824; 1H
NMR (CDCl3, 400 MHz) d: 13.48 (d, J = 13.0 Hz,
5. 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.

C. V. Yelamaggad, A. S. Achalkumar / Tetrahedron Letters 47 (2006) 7071–7075
7075
@CNH), 13.31 (d, J = 13.2 Hz, @CNH), 13.01 (d,
J = 13.2 Hz @CNH), 12.89 (d, J = 13.4 Hz, @CNH)
(these four resonances are due to 3H), 8.63–8.8 (m, 3H,
@CHN), 6.49 (s, 6H, Ar), 3.99–4.05 (m, 18H, 9 · OCH2),
0.86–1.90 (m, 171H, 18 · CH, 36 · CH2, 27 · CH3);
MS (FAB+): m/z for C117H201N3O12 (M+), Calculated
for C117H201N3O12: 1840.5, Found: 1840.3; Elemental
analysis: calculated (found) (%): C 76.29 (75.70), H 10.99
(10.9), N 2.28 (2.76).
15. (a) Destrade, C.; Foucher, P.; Gasparoux, H.; Nguyen, H.
T.; Levelut, A. M.; Malthete, J. Mol. Cryst. Liq. Cryst.
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Goodby, J. W.; Barbera, J.; Photinos, D. J. J. Mater.
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