Synthesis of liquid crystalline materials based on 1,3,5-triphenylbenzene and 2,4,6-triphenyl-1,3,5-s-triazine

Article abstract of DOI:10.1016/j.tetlet.2008.07.021

C₃-symmetric alkyloxy/aryloxy polyether dendrimers have been synthesized from 1,3,5-triphenylbenzene and 2,4,6-triphenyl-1,3,5-s-triazine and their liquid crystalline properties have been studied. C₃-symmetric derivatives with n-hexy

Full text of DOI:10.1016/j.tetlet.2008.07.021

Tetrahedron Letters 49 (2008) 5419–5423
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of liquid crystalline materials based on 1,3,5-triphenylbenzene
and 2,4,6-triphenyl-1,3,5-s-triazine
a
b
Sambasivarao Kotha a, , Dhurke Kashinath , Sandeep Kumar
*
a Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India
b Raman Research Institute, C.V. Raman Avenue, Sadashivanagar, Bangalore 560 080, India
a r t i c l e i n f o
a b s t r a c t
Article history:
C3-symmetric alkyloxy/aryloxy polyether dendrimers have been synthesized from 1,3,5-triphenyl-
benzene and 2,4,6-triphenyl-1,3,5-s-triazine and their liquid crystalline properties have been studied.
C3-symmetric derivatives with n-hexyl and n-dodecyl chains at the periphery show mesophase proper-
ties when they are complexed with trinitrofluorenone at different temperatures.
Ó 2008 Elsevier Ltd. All rights reserved.
Received 29 April 2008
Revised 23 June 2008
Accepted 3 July 2008
Available online 6 July 2008
Keywords:
1,3,5-Triphenylbenzene
s-Triazine
Dendrimers
Liquid crystals
Polyaromatic hydrocarbons have attracted considerable interest
owing to their physical1 and biological properties.2 Among these,
biphenyl-based polyaromatic compounds have gained significance
due to their utility as building blocks in preparing hyper-branched
polymers3 and developing materials with photoelectric and lumi-
nescent properties.4 In several cases, the 1,3,5-triphenylbenzene
core has been used to develop organic light emitting diodes
(OLEDs).5 Triphenylbenzene has also been used in the synthesis
of dendrimers6 and fullerene fragments.7 1,3,5-s-Triazine and its
derivatives are another class of interesting compounds which show
various biological properties. Many triazine derivatives show anti-
bacterial, antifungal, and antiviral8 properties and the majority of
triazine derivatives have been used in the cosmetic industry.9
Along with these, much attention has been paid to these molecules
for the preparation of liquid crystalline materials and for the devel-
opment of OLEDs.10
Though there are reports of utilizing 1,3,5-triphenylbenzene
and the s-triazine moiety for the synthesis of materials science ori-
ented molecules, very few reports are available, where 1,3,5-tri-
phenylbenzene and 2,4,6-triphenyl-1,3,5-s-triazine units have
been used for the development of OLEDs and liquid crystalline
materials.11 Thus, considering the importance of these molecules,
and in continuation of our interest in C3-symmetric polyaromatic
compounds12 and liquid crystalline materials,13 herein we report
a simple and general method for the preparation of polyether den-
drimers with triphenylbenzene and triphenyl-s-triazine units as
central units. Some of the dendrimers reported here show meso-
morphic behavior when complexed with trinitrofluorenone (TNF).
Toward this goal, we prepared initially central cores 3 and 4 by
an acid catalyzed cyclotrimerization reaction starting from p-
hydroxyacetophenone 1 and p-hydroxybenzonitrile 2 (Scheme
1).14,15 Thus, p-hydroxyacetophenone was treated with silicon
OH
O
CH3
CN
b
a
X
X
X
OH
OH
HO
OH
2
1
X = C 3 81%; X = N 4 93%
Scheme 1. Preparation of the central cores 3 and 4. Reagents and conditions: (a)
SiCl4, EtOH, rt, 12 h, 81%; (b) F3CSO3H, CHCl3, rt, 12 h, 93%.
Cl
Cl
Br
C6H13
O
OC6H13
C12H25
O
OC12H25 BnO
OBn
R
R
R
R = H 5
R = OC6H13
R = H 7
R = OC12H25
R = H 9
R = OBn 10
6
8
* Corresponding author. Tel.: +91 22 257 6 7160; fax: +91 22 2572 3480.
E-mail address: srk@chem.iitb.ac.in (S. Kotha).
Figure 1. Structures of the dendritic wedges 5–10.
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.07.021

5420
S. Kotha et al. / Tetrahedron Letters 49 (2008) 5419–5423
R
OC6H13
C6H13
O
O
X
Cl
3
K2CO3
X
X
+
or
acetone
reflux 12 h
O
OC6H13
C6H13
4
R
OC6H13
R
O
O
R = H 5
R = OC6H13
C6H13
O
R
6
X = C; R = H 11
X = N; R = H 12
OC6H13
OC6H13
X = C; R = OC6H13 13
X = N; R = OC6H13 14
Scheme 2. Preparation of polyether dendrimers 11–14.
tetrachloride (SiCl4) in absolute ethanol at room temperature to
give the cyclotrimerized product 3 in 81% yield. Similarly, treat-
ment of p-hydroxybenzonitrile 2 with trifluoromethanesulfonic
acid (F3CSO3H) in dry chloroform at room temperature generated
the cyclotrimerized product 4 in 93% yield (Scheme 1).
In a separate route, the dendritic wedges (chlorides and bro-
mides) 5–10 were synthesized starting from 3,5-dihydroxybenzoic
acid, gallic acid, and alkyl/aryl halides (n-hexyl chloride, n-dodecyl
chloride, and benzyl bromide) (Fig. 1).16 We then focused our
attention toward coupling these dendritic wedges with central
cores 3 and 4 under phase-transfer catalysis (PTC) conditions.
Thus, the trihydroxyl compounds 3 and 4 were reacted with
dendritic wedge 5 or 6 (derived from the 3,5-dihydroxy benzoic
acid, gallic acid, and n-hexylchloride) in the presence of K2CO3 in
acetone for 12 h to give polyether dendrimers 11, 12, 13, and 14
in 72%, 81%, 86%, and 78% yields, respectively (Scheme 2). Dendri-
mers with a dodecyl group at the periphery (15–18) were also syn-
thesized using the same reaction conditions in good yields.17
Polyether dendrimers with a benzyl group at the periphery (19–
22) were prepared using the 18-crown-6 as a phase-transfer cata-
lyst in DMF under refluxing conditions. All the final dendrimers
(Table 1) were characterized from spectral data.18
Having demonstrated the synthesis of polyether dendrimers
with n-hexyl, n-dodecyl, and benzyl groups at the periphery, we
focused our attention toward the physical properties of these
derivatives. The thermal behavior of all the materials and their
charge transfer (CT) complexes with trinitrofluorenone (TNF)
were investigated by polarizing optical microscopy (POM) and
differential scanning calorimetry (DSC). Thermal data obtained
from the DSC heating and cooling cycles are collected in Table 2.
Compound 13 was semi-solid at room temperature. It showed a
broad melting peak centered at about 59 °C in the first heating run
of the DSC. On cooling from the isotropic phase, it stayed in opti-
cally transparent state up to room temperature. The DSC cooling
Table 1
Structures of the peripheral moieties of dendrimers 11–22
Central core = 3 Ar=
Yield
(%)
Central core = 4 Ar=
Yield
(%)
11
13
72
86
12
14
81
78
C6H13
O
OC6H13
C6H13
O
O
OC6H13
C6H13
OC6H13
OC6H13
C6H13
O
OC6H13
OC6H13
15
17
19
21
88
70
63
40
16
18
20
22
91
73
76
85
C12H25
O
OC12H25
C12H25
O
O
OC12H25
C12H25
OC12H25
OC12H25
C12H25O
OC12H25
OC12H25
Table 2
Phase transition temperatures (peak temperature/°C) and associated enthalpy
changes (J/g in parentheses) of alkyloxy polyether dendrimers (13, 14, 17 and 18)
and their 1:1 molar CT complexes with TNF
Compound
Number
Heating scan
Cooling scan
13
13:TNF
14
14:TNF
17
SS 59.4 (18.8) I
Viscous oil
SS 66.0 (23.0) I
SS 56.2 (5.5) Col 77.6 (6.5) I
S 42.7 (48.8) I
No transition
PhH2CO
PhH2CO
OCH2Ph
PhH2CO
PhH2CO
OCH2Ph
OCH2Ph
No transition
No transition
No transition
17:TNF
18
SS 54–70 (0.8) I
Cr 57.1 (49.8) I
I 61.7 (0.7) Col 54.5 (0.4) Col
I 31.8 (36.9) Cr
OCH2Ph
18:TNF
SS 50.6 (24.1) Col 101.5 (0.6) I I 96.9 (1.2) Col
OCH2Ph
OCH2Ph
S = solid, SS = semi-solid, Cr = crystals, Col = columnar phase, I = isotropic.

S. Kotha et al. / Tetrahedron Letters 49 (2008) 5419–5423
5421
Figure 2a. DSC traces for compound 14 on heating and cooling (scan rate
5 °C minÀ1).
Figure 3b. DSC traces for compound 17:TNF complex on heating and cooling (scan
rate 5 °C minÀ1).
Probably a weak transition would be noticed by loading a sufficient
amount; say 5–7 mg of the sample.
The glassy-solid 17 displayed a melting transition at about
43 °C. On cooling, its behavior was similar to 13 and 14, that is,
no transition up to room temperature (Fig. 3a). The DSC of its CT
complex with TNF showed a very broad and weak transition con-
sisting of two or three peaks in the 55 °C to 70 °C temperature
range (Fig. 3b). Upon cooling, the POM showed the appearance of
columnar phase texture (Figs. 6a and 6b) at about 55 °C, which re-
mained stable up to room temperature. The DSC cooling run exhib-
ited two transitions; one centered at 61.7 °C and the other at 54 °C.
The second transition could be due to a columnar phase to
Figure 2b. DSC traces for compound 14:TNF complex on heating and cooling (scan
rate 5 °C minÀ1).
cycle confirmed this observation, which did not show any transi-
tion down to room temperature. Upon doping with TNF (about
1:1 molar ratio), it changed to a red viscous oil which was not
liquid crystalline. Compound 14 was also semi-solid at ambient
temperature and on heating displayed a broad melting transition
at about 66 °C (Fig. 2a). As with 13, this material did not show
any transition up to room temperature. The yellow-orange semi-
solid TNF complex of 14 melted at about 56 °C to a mesophase,
which cleared at about 78 °C (Fig. 2b). Upon cooling, columnar
phase appeared at about 55 °C under POM (Fig. 5) but, surprisingly,
the DSC did not display any transition in the cooling run indicating
the second order nature of this transition. This could also be be-
cause of the small amount (about 2 mg) loaded in the DSC cup.
Figure 4a. DSC traces for compound 18 on heating and cooling (scan rate
5 °C minÀ1).
Figure 3a. DSC traces for compound 17 on heating and cooling (scan rate
5 °C minÀ1).
Figure 4b. DSC traces for compound 18:TNF complex on heating and cooling (scan
rate 5 °C minÀ1).

5422
S. Kotha et al. / Tetrahedron Letters 49 (2008) 5419–5423
Figure 7a. POM image of compound 18:TNF at 97 °C (crossed polarizers, magni-
fication 200).
Figure 5. POM image of compound 14:TNF at 55 °C (crossed polarizers, magnifi-
cation 500).
columnar phase transition, but no change in optical texture was
observed at this transition. The exact nature of the two columnar
phases could only be deduced from detailed XRD studies.
The crystalline compound 18 melted at about 50 °C and crystal-
lized at about 31 °C on cooling (Fig. 4a). Its 1:1 molar CT complex
with TNF was an orange-red semi-solid at room temperature. The
DSC showed two endothermic transitions in the first heating cycle
(Fig. 4b). The first one centered at about 50 °C was due to the melt-
ing of the solid to a columnar phase and the second, centered at
101 °C was a columnar phase to isotropic phase transition. Upon
cooling, the columnar phase appeared at 97 °C (Figs. 7a and 7b).
This phase remained stable down to room temperature and no
Figure 7b. POM image of compound 18:TNF at 97 °C (crossed polarizers, magni-
fication 500).
crystallization peak was observed on cooling. As the compound
did not crystallize, the second heating run of the DSC was devoid
of the melting transition and only a mesophase to isotropic transi-
tion at 100.4 °C was observed. The isotropic phase to columnar
phase transition appeared at the same temperature on second
cooling.
In conclusion, we have demonstrated a simple synthesis of
polyether dendrimers with 1,3,5-triphenylbenzene and 2,4,6-tri-
phenyl-1,3,5-s-triazine central cores with long alkyl chains at the
peripheries and their utility for the development of liquid crystal-
line materials.
Figure 6a. POM image of compound 17:TNF at 55 °C (crossed polarizers, magni-
fication 200).
Acknowledgments
We greatly acknowledge DST (NSTI), New Delhi, for the finan-
cial support. D.K. thanks CSIR, New Delhi, for the award of research
fellowship.
References and notes
1. (a) Romagnoli, B.; van Baal, I.; Price, D. W.; Harwood, L. M.; Hayes, W. Eur. J. Org.
Chem. 2004, 4148; (b) Romagnoli, B.; Harwood, L. M.; Hayes, W. Tetrahedron
Lett. 2003, 44, 37; (c) Engel, G. D.; Gade, L. H. Chem. Eur. J. 2002, 8, 4319; (d)
Stone, L.; Smith, D. K.; McGrail, P. T. J. Am. Chem. Soc. 2002, 124, 856; (e) Zhang,
W.; Cao, X.-Y.; Zi, H.; Pei, J. Org. Lett. 2005, 7, 959; (f) Shirota, Y. J. Mater. Chem.
2000, 10, 1 and references cited therein.
2. (a) Covell, D. G.; Huang, R.; Wallqvist, A. Mol. Cancer Ther. 2007, 6, 2261; (b)
Basnet, A.; Thapa, P.; Karki, R.; Na, Y.; Jahng, Y.; Jeong, B.-S.; Jeong, T. C.; Lee, C.
S.; Lee, E.-S. Bioorg. Med. Chem. 2007, 15, 435; (c) Tabacco, A.; Bardelli, F.;
Meiattini, F.; Tarli, P. Clin. Chim. Acta 1980, 104, 405.
3. Zeng, S.; Zimmerman, S. C.; Kolotuchin, S. V.; Reichert, E. C. E.; Ma, Y.
Tetrahedron 2002, 58, 825.
4. Miller, T. M.; Neenan, T. X.; Zayas, R.; Bair, H. E. J. Am. Chem. Soc. 1992, 114, 1018.
5. Pang, J.; Marcotte, E. J.-P.; Seward, C.; Brown, R. S.; Wang, S. Angew. Chem., Int.
Ed. 2001, 40, 4042.
Figure 6b. POM image of compound 17:TNF at 55 °C (crossed polarizers, magni-
fication 500).

S. Kotha et al. / Tetrahedron Letters 49 (2008) 5419–5423
5423
6. (a) Mongin, O.; Brunel, J.; Porrés, L.; Blanchard-Desce, M. Tetrahedron Lett. 2003,
44, 2813; (b) Brunel, J.; Ledoux, I.; Zyss, J.; Blanchard-Desce, M. Chem. Commun.
2001, 923; (c) He, Q.; Haung, H.; Yang, J.; Lin, H.; Bai, F. J. Mater. Chem. 2003, 13,
1085.
7. (a) Gómez-Lor, B.; Echavarren, A. M. Org. Lett. 2004, 6, 2993; (b) Gómez-Lor, B.;
Gonzalez-Cantalapiedra, E.; Ruiz, M.; de Frutos, Ó.; Cárdenas, D. J.; Santos, A.;
Echavarren, A. M. Chem. Eur. J. 2004, 10, 2601.
8. (a) Srinivas, K.; Srinivas, U.; Rao, V. J.; Bhanuprakash, K.; Kishore, K. H.; Murty,
U. S. N. Bioorg. Med. Chem. Lett. 2005, 15, 1121. and references cited therein; (b)
Srinivas, K.; Srinivas, U.; Bhanuprakash, K.; Kishore, K. H.; Murty, U. S. N.; Rao,
V. J. Eur. J. Med. Chem. 2006, 41, 1240.
9. (a) Uli, O.; Lim, H. W. Basic Clin. Dermat. 2007, 38, 279; (b) Couteau, C.;
Pommier, M.; Paparis, E.; Coiffard, L. J. M. Pharmazie 2007, 62, 449; (c) Uli, O.;
Bernd, H. Cosmet. Toiletries 2004, 119, 61.
10. (a) Shirota, Y.; Kuwabara, H.; Inada, T.; Nakada, H.; Yonemoto, Y.; Kawami, S.;
Imai, K. Appl. Phys. Lett. 1994, 65, 807; (b) Bulovic, N.; Gu, G.; Burrows, P. E.;
Forrest, S. R. Nature 1996, 380, 29; (c) Shen, Z.; Burrows, P. E.; Bulovic, N.;
Forrest, S. R.; Thompson, M. E. Science 1997, 276, 2009; (d) Cammidge, A. N.;
King, A. M. Tetrahedron Lett. 2006, 47, 5569.
11. (a) Lee, C.-H.; Yamamoto, T. Tetrahedron Lett. 2001, 42, 3993. and references
cited therein; (b) Chérioux, F.; Mailotte, H.; Audebert, P.; Zyss, J. Chem.
Commun. 1999, 2083; (c) Shu, W.; Valiyaveettil, S. Chem. Commun. 2002,
1350.
12. (a) Kotha, S.; Chakraborty, K.; Brahmachary, E. Synlett 1999, 1621; (b) Kotha, S.;
Kashinath, D.; Lahiri, K.; Sunoj, R. B. Eur. J. Org. Chem. 2004, 4003.
13. (a) Kumar, S. Chem. Soc. Rev. 2006, 35, 83; (b) Bisoyi, H. K.; Kumar, S.
Tetrahedron Lett. 2007, 48, 4399; (c) Kumar, S.; Bisoyi, H. K. Angew. Chem., Int.
Ed. 2007, 46, 1501; (d) Kumar, S.; Pal, S. K.; Kumar, P. S.; Lakshminarayanan, V.
Soft. Mater. 2007, 2, 896.
16. (a) Balagurusamy, V. S. K.; Ungar, G.; Percec, V.; Johansson, G. J. Am. Chem. Soc.
1997, 119, 1539; (b) Hawker, C. J.; Fréchet, J. M. J. J. Am. Chem. Soc. 1990, 112,
7638.
17. Preparation of compound 17:
A
mixture of
5
(20 mg, 0.05 mmol), K2CO3
and 3,4,5-tris-n-
(100 mg, 0.73 mmol), 18-crown-6
(0.02 equiv),
dodecyloxybenzyl chloride (184 mg, 2.5 mmol) in dry acetone (10 mL) was
refluxed for 24 h. At the conclusion of reaction (TLC monitoring), it was cooled
to rt, diluted with water (10 mL), and extracted with EtOAc (3 Â 10 mL).
Organic layers were washed with brine, water, dried over MgSO4, and
concentrated under reduced pressure to give the crude product which was
purified by silica gel column chromatography. Elution of the column with 2%
EtOAc–petroleum ether mixture gave the desired 1st generation dendrimer 17
(96 mg, 70%) as a sticky solid. Rf 0.3 (2% EtOAc–petroleum ether).
18. Spectral data for compound 17: 1H NMR (300 MHz CDCl3): d 0.87 (t, J = 6 Hz,
27H, terminal CH3), 1.25–1.29 (m, 144H, alkyl –CH2–), 1.42-1.47 (br s, 18H,
alkyl –CH2–), 1.70–1.80 (heptet, J = 6.9 Hz, 18H, Ar-O–CH2–CH2–), 3.99 (m, 18
H, Ar-O–CH2–CH2–), 5.00 (s, 6H, Ar-O–CH2-Ar), 6.65 (s, 6H, ortho Ar-H of
dendritic wedge), 7.08 (d, J = 9 Hz, 6H, Ar-H of core), 7.63 (d, J = 8.7 Hz, 6 H, Ar-
H of core) 7.66 (s, 3H, Ar-H of central benzene ring). 13C NMR (75.4 MHz
CDCl3): d 14.21 (terminal CH3), 22.77, 26.19, 29.49, 29.73, 29.78, 29.85, 30.42,
32.01, 69.21 (meta Ar-O–CH2–CH2–), 70.64 (para Ar-O–CH2–CH2–), 73.51 (Ar-
O–CH2-Ar), 106.23, 115.24, 129.98, 128.41, 131.95, 134.13, 138.04, 141.86,
153.41, 158.63. Mass (MALDI-TOF): 2284.; Spectral data for compound 18:1H
NMR (300 MHz CDCl3): d 0.88 (t, J = 5.4 Hz, 27H, terminal CH3), 1.26 (br s,
144H), 1.42–1.47 (br s, 18H), 1.70–1.80 (heptet, J = 6.8 Hz, 18H, Ar-O–CH2–
CH2–), 3.93–4.01 (m, 18H, Ar-O–CH2–CH2–), 5.06 (br s, 6H, Ar-O–CH2-Ar), 6.66
(s, 6H, ortho Ar-H of dendritic wedge), 7.13 (br s, 6H, Ar-H of core), 8.72 (br s,
6H, Ar-H of core). 13C NMR (75.4 MHz CDCl3): d 14.19 (terminal CH3), 22.76,
26.18, 26.21, 29.44, 29.47, 29.50, 29.73, 29.78, 29.84, 30.42, 32.00, 69.22 (meta
Ar-O–CH2–CH2–), 73.51 (para Ar-O–CH2–CH2–), 106.26, 114.85, 129.29,
130.95, 131.40, 138.11, 153.43, 162.43, 170.72. Mass (MALDI-TOF):
2288.
14. Elmorsy, S. S.; Khalil, A. G. M.; Girges, M. M.; Salama, T. A. J. Chem. Res. (S) 1997,
232.
15. Iyoda, M.; Fukuda, M.; Yoshida, M.; Sasaki, S. Chem. Lett. 1994, 2369.