Discotic liquid crystalline materials for potential nonlinear optical applications: Synthesis and liquid crystalline behavior of 1,3,5-triphenyl-2,4, 6-triazine derivatives containing achiral and chiral alkyl chains at the periphery

Article abstract of DOI:10.1016/j.tetlet.2003.11.085

As a novel approach to nonlinear optical materials, new octupolar discotic liquid crystalline materials 1,3,5-triphenyl-2,4,6-triazine derivatives containing achiral alkyl chains (5a) and chiral alkyl chains (5b) at the periphery were synthesized. The former exhibits an ordered hexagonal columnar mesophase, whereas the latter displays a rectangular columnar mesophase. The negative exciton splitting observed in the CD spectrum of a thin film of 5b suggests that it has a left-handed helical structure within the column.

Full text of DOI:10.1016/j.tetlet.2003.11.085

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 1019–1022
Discotic liquid crystalline materials for potential nonlinear optical
applications: synthesis and liquid crystalline behavior of 1,3,5-
triphenyl-2,4,6-triazine derivatives containing achiral and
chiral alkyl chains at the peripheryq
Hyoyoung Lee,a, Dongwoo Kim,a Hyung-Kun Lee,a Wenfeng Qiu,a Nam-Keun Oh,b
Wang-Cheol Zinb and Kimoon Kima,*
aNational Creative Research Initiative Center for Smart Supramolecules and Department of Chemistry, Division of Molecular and
Life Sciences, Pohang University of Science and Technology, San 31 Hyojadong, Pohang 790-784, South Korea
bDepartment of Materials Science and Engineering, Pohang University of Science and Technology, San 31 Hyojadong,
Pohang 790-784, South Korea
Received 27 October 2003; revised 16 November 2003; accepted 19 November 2003
Abstract—As a novel approach to nonlinear optical materials, new octupolar discotic liquid crystalline materials 1,3,5-triphenyl-
2,4,6-triazine derivatives containing achiral alkyl chains (5a) and chiral alkyl chains (5b) at the periphery were synthesized. The
former exhibits an ordered hexagonal columnar mesophase, whereas the latter displays a rectangular columnar mesophase. The
negative exciton splitting observed in the CD spectrum of a thin film of 5b suggests that it has a left-handed helical structure within
the column.
ꢀ 2003 Elsevier Ltd. All rights reserved.
In conjunction with developing nonlinear optical (NLO)
materials for optoelectronics and photonics, there has
been growing interest in the second order nonlinear
optical properties of octupolar molecules in recent
years.1 Octupolar NLO molecules have several advan-
tages over traditional dipolar NLO molecules. First,
dipolar molecules have the strong tendency to adopt
centrosymmetric packing in the solid states nullifying
bulk nonlinearity whereas octupolar molecules lacking
permanent dipole moments have better chances to adopt
noncentrosymmetric packing. Second, the b value of a
dipolar molecule increases to a maximum value and then
decreases as the bond-length alternation increases,
whereas that of an octupolar molecule increases steadily
as the extent of the charge transfer increases,2 which
provides a firm basis for designing NLO molecules with
large b values. For the last decade, therefore, consider-
able research efforts have been focused on the design
and synthesis of octupolar molecules with large b values,
and a number of organic and metal-organic octupolar
molecules whose b values match those of the best-
known dipolar NLO chromophores have been
reported.3 The biggest challenge is however the macro-
scopic organization of such octupolar molecules in a
noncentrosymmetric environment leading to large bulk
second order NLO properties. Building noncentrosym-
metric octupolar networks in crystalline solids using
weak intermolecular interactions,4 and orientation of
octupolar chromophores in polymers or sol–gel matrices
using all optical poling5 have been attempted with only a
moderate success. Our long interests in NLO materials6
and liquid crystalline materials7 led us to take a novel
approach to this problem, which involves the synthesis
of a discotic liquid crystalline material comprising a flat,
octupolar chromophore as a core and long, chiral alkyl
tails at the periphery, which lead to a helical columnar
structure.8 We anticipated that such octupolar discotic
liquid crystalline materials with noncentrosymmetric
structures may have large bulk second order NLO
properties because Langmuir–Blodgett films composed
Keywords: Nonlinear optical materials; Discotic liquid crystals; Left-
handed helix.
q Supplementary data associated with this article can be found, in the
online version, at doi:10.1016/j.tetlet.2003.11.085
* Corresponding author. Tel.: +82-54-279-2113; fax: +82-54-279-8129;
e-mail: kkim@postech.ac.kr
Present address: ETRI, 161 Gajeong-Dong, Yuseong-Gu, Daejon
305-350, Republic of Korea.
0040-4039/$ - see front matter ꢀ 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2003.11.085

1020
H. Lee et al. / Tetrahedron Letters 45 (2004) 1019–1022
of a supramolecular array of a chiral helicene had been
reported to show highly enhanced NLO properties.9 We
chose the octupolar molecule 1,3,5-triphenyl-2,4,6-tri-
azine as a discotic mesogen because the NLO properties
of its derivatives have been well studied.10 Here we
report the synthesis of a novel discotic liquid crystalline
material derived from 1,3,5-triphenyl-2,4,6-triazine that
displays a rectangular columnar phase in which the
molecules are stacked along the columns with a left-
handed helical structure.11
which was then reacted with 2 to yield 3b in 79% yield.
The conversion of 3b to its boronic acid derivative 4b
was achieved by lithiation with tert-butyl lithium fol-
lowed by treating with trimethyl borate. The Suzuki-
coupling reaction of 4b with cyanuric chloride gave 5b in
33% yield.17
Both 5a and 5b show enantiotropic liquid crystalline
behavior as revealed by differential scanning calorimetry
(DSC) and polarized optical microscopy (POM). Their
phase behavior is summarized in Table 1. Compound 5a
exhibits a mesophase from 36 to 145 ꢁC on heating and
from 145 to 26 ꢁC on cooling, whereas 5b shows a
mesophase from )15 to 56 ꢁC on heating and below
41 ꢁC on cooling. For the latter, the transition from
mesophase to crystalline phase on cooling was not
observed down to )20 ꢁC. Compared to 5a, 5b exhibits
not only much lower phase transition temperatures but
also much smaller enthalpy changes associated with the
phase transitions, which is probably due to the loose
packing of the branched alkyl chains in the latter. The
POM image of 5a in mesophase reveals a pseudo-focal
conic texture (Fig. 1a), which is characteristic of hexa-
gonal columnar phases, whereas that of 5b shows a
dendritic texture, which is often observed in rectangular
columnar phase (Fig. 1b).18
The 1,3,5-triphenyl-2,4,6-triazine derivative 5a having
achiral alkyl chains at the periphery was synthesized in
five steps (Scheme 1). 3,4,5-Trihydroxy-1-bromobenzene
2 was prepared in two steps from commercially available
2,6-dimethoxyphenol: bromination12 followed by depro-
tection of the methyl groups gave 2 in 64% yield.
Alkylation of 2 with bromoalkane in DMF in the
presence of potassium carbonate and a catalytic amount
of potassium iodide produced 3a in 81% yield. Lithia-
tion of 3a with tert-butyl lithium at )78 ꢁC followed by
treating with trimethyl borate and 3 N HCl gave aryl
boronic acid 4a.13 Finally, the Suzuki-coupling reaction
of 4a and cyanuric chloride in toluene using Pd(PPh3)4
as the catalyst yielded 5a in 52% yield.14;15
The synthesis of 1,3,5-triphenyl-2,4,6-triazine derivative
5b having chiral alkyl chains at the periphery was car-
ried out similarly. Hydrogenation of (S)())-b-citronellol
with 10% Pd/C in dry ethyl acetate followed by bro-
mination gave (3S)-1-bromo-3,7-dimethyloctane,16
To obtain more information on the structures of 5a and
5b in mesophase X-ray diffraction (XRD) studies were
carried out using synchrotron radiation. The X-ray
pattern of the mesophase of 5a taken at 80 ꢁC displays a
strong peak and a weak peak in the small-angle region
p
with reciprocal Bragg spacings in a ratio of 1: 3. These
peaks were assigned to the (100) and (110) reflections of
a hexagonal arrangement with a lattice constant
ꢀ
a ¼ 28.0 A. The lattice parameter decreases with
increasing temperature as usual. On the other hand, the
X-ray pattern of 5b taken at room temperature
ꢀ
shows three peaks at 23.1, 20.7, and 11.6 A, which were
assigned to the (200), (110), and (020) reflections,
respectively, of a rectangular columnar structure with
8b
ꢀ
ꢀ
lattice parameters of a ¼ 46.4 A and b ¼ 23.1 A (Table
2).
The effect of the chiral alkyl tails attached to the core on
the structure of 5b in the mesophase was investigated by
CD spectroscopy. First of all, the UV spectra of thin
films of 5a and 5b are essentially identical with a broad
absorption at 318 nm (Fig. 2), which confirms that the
alkyl tails do not affect the absorption of the triazine
core in 5a and 5b. Most importantly, the CD spectrum
of a thin film of 5a is featureless whereas that of 5b
exhibits strong circular dichroism (Fig. 2) with a nega-
tive exciton splitting centered around 318 nm, which
Scheme 1. Reagents and conditions: (i) NaH, NBS, CHCl3, )45 ꢁC; (ii)
BBr3, MC, rt; (iii) RBr, K2CO3, KI, DMF, reflux; (iv) t-BuLi,
B(OMe)3, THF, )78 ꢁC; (v) C3N3Cl3, Na2CO3, Pd(PPh3)4, toluene,
reflux.
Table 1. Phase transition temperatures (ꢁC) and enthalpies (kJ molꢀ1, in parentheses) of 5a and 5ba
Compound
Heating
Cooling
5a
5b
K 35.6 (15.6) Colhd 145.3 (1.66) I
K )15.2 (2.45) ꢁColr 55.8 (0.40) I
I 144.7 (1.05) Colhd 25.7 (15.7) K
I 41.5 (0.56) ꢁColr
a K ¼ crystalline phase; Colhd ¼ hexagonal columnar mesophase; I ¼ isotropic phase.
ꢁColr ¼ chiral rectangular columnar mesophase.

H. Lee et al. / Tetrahedron Letters 45 (2004) 1019–1022
1021
Figure 1. Polarized optical micrograph (·160) of (a) 5a at 88 ꢁC and (b) 5b at rt.
Table 2. X-ray diffraction data for 5a and 5b (lattice constants, measured and calculated lattice spacings, and proposed indexing)
qa (nmꢀ1
)
dobs (A)
dcal (A)
hkl
b
ꢀ
Lattice constants (A)
ꢀ
ꢀ
Compound
T (ꢁC)
5a
80
a ¼ 28:0
2.59
4.53
2.72
3.03
5.44
24.2
13.9
23.1
20.7
11.6
24.3
13.9
23.1
20.7
11.6
100
110
200
110
020
5b
rt
a ¼ 46:4
b ¼ 23:1
a
ꢀ
q ¼ 4p sin h=k, k ¼ 1:54019 A.
b d ¼ 2p=q.
Figure 2. CD (top, scale at left) and UV–vis absorption spectra (bot-
tom, scale at right) of thin films of 5a (dot) and 5b (solid) deposited on
a quartz substrate by spin coating.
Figure 3. Schematic representations of (a) the hexagonal columnar
mesophase of 5a and (b) the rectangular columnar mesophase with a
left-handed helix of 5b. The illustrations are idealized; the discs may be
laterally shifted toward each other and may not have the same dis-
tance.
suggests that the discotic molecules are stacked along
the column, while tilted with respect to the column axis,
to form a left-handed helix within the column (Fig.
3b).19 No circular dichroism observed with a solution of
5b, and the decreasing circular dichroism of a film of 5b
with increasing temperature support that the optical
activity of 5b is not derived from the stereogenic center
of the chiral alkyl tails, but from the helical arrangement
of the molecules induced by the chiral alkyl tails.20
the periphery, which displays a rectangular columnar
mesophase. Furthermore, the negative exciton splitting
observed in the CD spectrum of a thin film of 5b sug-
gests that it has a left-handed helical structure within the
column. The noncentrosymmetric arrangement of 5b in
the liquid crystalline film should result in nonzero bulk
second order NLO properties, which are currently being
investigated. With the expected NLO properties as well
as the liquid crystalline behavior at room temperature
5b may find useful applications in optoelectronics and
photonics.
In summary, as part of a novel approach to organizing
octupolar molecules in a noncentrosymmetric environ-
ment, we designed and synthesized a new octupolar
discotic liquid crystalline material, 1,3,5-triphenyl-2,4,6-
triazine derivative (5b) containing chiral alkyl chains at

1022
H. Lee et al. / Tetrahedron Letters 45 (2004) 1019–1022
Schouten, P. G.; Warman, J. M.; Kentgens, A. P. M.;
Devillers, M. A. C.; Meijerink, A.; Picken, S. J.; Sohling,
Acknowledgements
U.; Schouten, A.-J.; Nolte, R. J. M. Chem. Eur. J. 1995, 1,
We gratefully acknowledge the Creative Research Ini-
tiative Program of the Korean Ministry of Science and
Technology (MOST) for support of this work, and the
Brain Korea 21 Program of the Korean Ministry of
Education for graduate studentship to D.K. and
H.-K.L. Experiments at the Pohang Accelerator Labora-
tory were supported in part by MOST and POSTECH.
ꢁ
171; (b) Barbera, J.; Iglesias, R.; Serrano, J. L.; Sierra, T.;
de la Fuente, M. R.; Palacios, B.; Perez-Jubindo, M. A.;
ꢁ
ꢁ
Vazquez, J. T. J. Am. Chem. Soc. 1998, 120, 2908; (c)
Engelkamp, H.; Middelbeek, S.; Nolte, R. J. M. Science
1999, 284, 785; (d) Serrano, J. L.; Sierra, T. Chem. Eur. J.
2000, 6, 759.
9. (a) Verbiest, T.; Elshocht, S. V.; Kauranen, M.; Helle-
mans, L.; Snauwaert, J.; Nuckolls, C.; Katz, T. J.;
Persoons, A. Science 1998, 282, 193; (b) Fox, J. M.; Katz,
T. J.; Elshocht, S. V.; Verbiest, T.; Kauranen, M.;
Persoons, A.; Thongpanchang, T.; Krauss, T.; Brus, L.
J. Am. Chem. Soc. 1999, 121, 3453.
References and notes
1. (a) Zyss, J. J. Chem. Phys. 1993, 98, 6583; (b) Zyss, J.;
Ledoux, I. Chem. Rev. 1994, 94, 77; (c) Brasselet, S.; Zyss,
J. J. Opt. Soc. Am. B 1998, 15, 257; (d) Ledoux, I.; Zyss,
J. C. R. Physique 2002, 3, 407.
2. (a) Lee, Y.-K.; Jeon, S.-J.; Cho, M. J. Am. Chem. Soc.
1998, 120, 10921; (b) Lee, H.; An, S.-Y.; Cho, M. J. Phys.
Chem. B 1999, 103, 4992; (c) Cho, B. R.; Park, S. B.; Lee,
S. J.; Son, K. H.; Lee, S. H.; Lee, M.-J.; Yoo, J.; Lee,
Y. K.; Lee, G. J.; Kang, T. I.; Cho, M.; Jeon, S.-J. J. Am.
Chem. Soc. 2001, 123, 6421.
3. (a) Dhenault, C. J.; Ledoux, I.; Samuel, I. D. W.; Zyss, J.;
Bourgault, M.; Le Bozec, H. Nature 1995, 374, 339; (b) del
Rey, B.; Keller, U.; Torres, T.; Rojo, G.; Agullo-Lopez,
F.; Nonell, S.; Marti, C.; Brasselet, S.; Ledoux, I.; Zyss, J.
J. Am. Chem. Soc. 1998, 120, 12808; (c) McDonagh, A. M.;
Humphrey, M. G.; Samoc, M.; Luther-Davies, B.; Hou-
brechts, S.; Wada, T.; Sasabe, H.; Persoons, A.
J. Am. Chem. Soc. 1999, 121, 1405; (d) Cho, B. R.; Lee, S.
J.; Lee, S. H.; Son, K. H.; Kim, Y. H.; Doo, I.-Y.; Lee, G. J.;
Kang, T. I.; Lee, Y. K.; Cho, M.; Jeon, S.-J. Chem. Mater.
2001, 13, 1438; (e) Cho, B. R.; Piao, M. J.; Son, K. H.;
Lee, S. H.; Yoon, S. J.; Jeon, S.-J.; Cho, M. Chem. Eur. J.
2002, 8, 307; (f) Le Bouder, T.; Maury, O.; Bondon, A.;
Costuas, K.; Amouyal, E.; Ledoux, I.; Zyss, J.; Le Bozec,
H. J. Am. Chem. Soc. 2003, 125, 12284.
10. (a) Ray, P. C.; Das, P. K. Chem. Phys. Lett. 1995, 244,
€
153; (b) Wortmann, R.; Glania, C.; Kramer, P.; Matsch-
iner, R.; Wolff, J. J.; Kraft, S.; Treptow, B.; Barbu, E.;
€
€
Langle, D.; Gorlitz, G. Chem. Eur. J. 1997, 3, 1765;
(c) Wolff, J. J.; Siegler, F.; Matschiner, R.; Wortmann, R.
Angew. Chem., Int. Ed. 2000, 39, 1436.
11. The synthesis of liquid crystalline 1,3,5-triphenyl-2,4,6-
triazine derivatives has been reported recently: (a) Lee,
C.-H.; Yamamoto, T. Tetrahedron Lett. 2001, 42, 3993; (b)
Lee, C.-H.; Yamamoto, T. Bull. Chem. Soc. Jpn. 2002,
615; (c) Meier, H.; Holst, H. C.; Oehlhof, A. Eur. J. Org.
Chem. 2003, 4173, Except one, they all have achiral alkyl
chains at the periphery. Even in the case of the one with
chiral alkyl chains, however, a normal hexagonal colum-
nar mesophase was observed by POM and DSC studies.
12. Foley, J. W. U.S. Patent 4,182,912, 1979.
13. Stewart, D. S.; McHattie, G. S.; Imrie, C. T. J. Mater.
Chem. 1998, 8, 47.
14. Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457.
15. Compound 5a: 1H NMR (300 MHz, CDCl3): d 7.98 (s,
6H), 4.07–4.18 (m, 18H), 1.77–1.93 (m, 18H), 1.50–1.54
(m, 18H), 1.28 (m, 108H), 0.88 (m, 27H); 13C NMR
(75 MHz, CDCl3): d 171.1, 153.4, 142.6, 131.2, 107.7, 73.8,
69.4, 32.2, 30.6, 29.9, 29.8, 29.7, 29.6, 26.4, 26.3, 22.9, 14.3;
MS (FAB): m=z [M+Hþ] 1715.0. Anal. Calcd for
4. (a) Desiraju, G. R. Acc. Chem. Res. 2002, 35, 565; (b)
Evans, O. R.; Lin, W. Acc. Chem. Res. 2002, 35, 511.
C
111H195O9N3: C, 77.70; H, 11.46; N, 2.45. Found: C,
77.89; H, 11.25; N, 2.48.
ꢁ
5. (a) Chaput, F.; Riehl, D.; Levy, Y.; Boilot, J.-P. Chem.
16. Palmans, A. R. A.; Vekemans, J. A. J. M.; Havinga, E. E.;
Meijer, E. W. Angew. Chem., Int. Ed. 1997, 36, 2648.
17. Compound 5b: 1H NMR (500 MHz, CDCl3): d 7.99 (s,
6H), 4.04–4.25 (m, 18H), 1.92–1.96 (m, 9H), 1.60–1.86 (m,
9H), 1.50–1.63 (m, 18H), 1.29–1.35 (m, 27H), 1.15–1.24
(m, 27H), 0.93–0.99 (m, 27H), 0.85–0.88 (m, 54H); 13C
NMR (125 MHz, CDCl3): d 171.22, 153.42, 142.57,
131.23, 107.61, 72.10, 67.58, 39.61, 39.52, 37.76, 37.62,
36.68, 30.14, 29.93, 28.23, 25.02, 24.96, 22.92, 22.82, 19.83;
MS (FAB): m=z [M+Hþ] 1715.8. Anal. Calcd for
Mater. 1993, 5, 589; (b) Fiorini, C.; Nunzi, J.-M. Chem.
Phys. Lett. 1998, 286, 415; (c) Delaire, J. A.; Nakatani, K.
Chem. Rev. 2000, 100, 1817.
6. (a) Lee, J.-H.; Kim, K.; Kim, J.-H.; Kim, J.-J. Bull. Korean
Chem. Soc. 1992, 13, 268; (b) Kang, S. H.; Kim, J.; Hahn,
J. H. Mater. Res. Bull. 1997, 32, 1127; (c) Olbrechts, G.;
Wostyn, K.; Clays, K.; Persoons, A.; Kang, S. H.; Kim, K.
Chem. Phys. Lett. 1999, 308, 173.
7. (a) Kim, S. J.; Kang, S. H.; Park, K.-M.; Kim, H.; Zin,
W.-C.; Choi, M.-G.; Kim, K. Chem. Mater. 1998, 10,
1889; (b) Kang, S. H.; Kim, M.; Lee, H.-K.; Kang, Y.-S.;
Zin, W.-C.; Kim, K. Chem. Commun. 1999, 93; (c) Kang,
S. H.; Kang, Y.-S.; Zin, W.-C.; Olbrechts, G.; Wostyn, K.;
Clays, K.; Persoons, A.; Kim, K. Chem. Commun. 1999,
1661; (d) Lee, H.-K.; Lee, H.; Ko, Y. H.; Chang, Y. J.; Oh,
N.-K.; Zin, W.-C.; Kim, K. Angew. Chem., Int. Ed. 2001,
40, 2669.
C111H195O9N3: C, 77.70; H, 11.46; N, 2.45. Found: C,
77.28; H, 11.48; N, 2.45.
18. Dierking, I. Textures of Liquid Crystals; Wiley-VCH:
Weinheim, 2003.
19. Harada, N.; Nakanishi, K. Circular Dichroic Spectro-
scopy. Exiton Coupling in Organic Stereochemistry; Uni-
versity Science Books: Mill Valkey, CA, 1983.
20. At the moment we do not understand why the S
configuration in the side chains leads to a left-handed
helix.
8. Helical discotic mesophases have been reported recently:
(a) van Nostrum, C. F.; Bosman, A. W.; Gelinck, G. H.;