Synthesis and characterization of a new class of liquid-crystalline, highly luminescent molecules containing a 2,4,6-triphenyl-1,3,5-triazine unit

Article abstract of DOI:10.1016/S0040-4039(01)00640-2

A new class of 2,4,6-triphenyl-1,3,5-triazine derivatives having long alkoxyl side chains have been synthesized by a Pd(0)/Cu(I)-catalyzed carbon-carbon coupling reaction. These compounds behave as liquid-crystalline materials and show quantum yields high

Full text of DOI:10.1016/S0040-4039(01)00640-2

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 3993–3996
Synthesis and characterization of a new class of
liquid-crystalline, highly luminescent molecules containing a
2,4,6-triphenyl-1,3,5-triazine unit
Chi-Han Lee* and Takakazu Yamamoto*
Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
Received 21 February 2001; revised 11 April 2001; accepted 13 April 2001
Abstract—A new class of 2,4,6-triphenyl-1,3,5-triazine derivatives having long alkoxyl side chains have been synthesized by a
Pd(0)/Cu(I)-catalyzed carbonꢀcarbon coupling reaction. These compounds behave as liquid-crystalline materials and show
quantum yields higher than 50% in photoluminescence. © 2001 Elsevier Science Ltd. All rights reserved.
In recent decades, there has been much interest in the
synthesis and characterization of compounds contain-
ing a 1,3,5-triazine unit,1 owing to their interesting
properties, such as liquid-crystalline and nonlinear opti-
cal properties, as well as their applications in other
fields.2–4 Among these materials, disk-like liquid crys-
tals are particularly gathering attention.3 The liquid-
crystalline materials are of great interest, especially with
respect to their unique optical properties.
synthesis of the p-conjugated 2,4,6-triphenyl-1,3,5-tri-
azines with a more rigid and larger p-conjugation sys-
tem, which shows highly photoluminescent and
liquid-crystalline properties. Some of the new 1,3,5-tria-
zine derivatives 1 reported in this paper exhibit liquid-
crystalline properties, and all of them give green–blue
photoluminescence with high quantum yields (53–63%).
Synthesis of the p-conjugated 2,4,6-triphenyl-1,3,5-tria-
zine derivatives 1 (Scheme 1) is possible due to the
We herein report synthesis of a new class of 2,4,6-
triphenyl-1,3,5-triazine derivatives (1) with terminal
alkynes having long alkoxyl groups.
OR
OR
The conjugated diphenylacetylene backbone4 is a rigid
rod that can act as a mesogenic unit, and the synthe-
sized 2,4,6-triphenyl-1,3,5-triazine derivatives with the
long alkoxyl groups behave as liquid-crystalline materi-
als. Long alkoxyl and alkyl groups play an important
role in the generation of large nonlinear optical
coefficients5 and are essential for molecular assembly.6
It is known that various optical properties (such as
electroluminescent and nonlinear optical properties)
associated with the delocalized p-electron system are
enhanced due to the orientation of molecules induced
by liquid-crystalline ordering.7 Triazine is known to
serve as a core of liquid-crystalline materials.3 However,
to our knowledge, no report has been published for the
C
1a R = n-C9H19
1b R = n-C10H21
1c R = n-C11H23
1d R = n-C12H25
1e R = n-C13H27
1f R = n-C14H29
C
N
N
N
C
C
C
C
RO
RO
1
OR
OR
Keywords: 2,4,6-triphenyl-1,3,5-triazine derivatives; coupling reaction;
liquid crystals; luminescent.
Scheme 1. Structure of 2,4,6-triphenyl-1,3,5-triazine deriva-
tives.
* Corresponding authors. Tel.: +81-45-924-5220; fax: +81-45-924-
5276; e-mail: tyamamot@res.titech.ac.jp
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)00640-2

3994
C.-H. Lee, T. Yamamoto / Tetrahedron Letters 42 (2001) 3993–3996
RO RO
RO RO C C H
HO
HO
a)
b), c)
COCH3
COCH3
2
2a R = n-C9H19
2b R = n-C10H21
2c R = n-C11H23
2d R = n-C12H25
2e R = n-C13H27
2f R = n-C14H29
Br
Br
e)
d)
N
N
1
yield/%:
N
88 (1a), 88 (1b),
88 (1c), 89 (1d),
89 (1e), 89 (1f)
CN
Br
Br
3
Scheme 2. (a) NaOH, 1-bromoalkane, DMF, reflux; (b) POCl3, DMF, CH2Cl2, reflux; (c) NaOH, H2O, 1,4-dioxane, reflux; (d)
CF3SO3H, CHCl3, room temperature; (e) terminal phenylalkynes, Pd(PPh3)4, CuI, NEt3, toluene, 55°C, 15 h. For the detailed
reaction conditions, see Ref. 12.
Compounds 1d and 1e did not show liquid-crystalline
development of a palladium-catalyzed carbonꢀcarbon
cross-coupling reaction; 8–10 namely, in the present case,
the cross-coupling reaction of terminal alkynes 2 having
disubstituted long alkoxyl groups with 2,4,6-tris(p-bro-
mophenyl)triazine 3 (Scheme 2). Terminal phenylalky-
nes 2 having long alkoxy groups were prepared in three
steps from 3,4-dihydroxyacetophenone, as previously
reported.11
behavior, either.
In contrast to the cases of compounds 1a, 1d, 1e and 1f,
1b and c showed mesogenic properties. When a crys-
talline sample of 1b was heated, it showed two
endothermic peaks at 85.6 and 99.4°C before melting
into the isotropic state. Compound 1b exhibited a
columnar hexagonal disordered (Dhd) mesophase.3a,c
The transitions were enantiotropic, and two exothermic
peaks were observed at 66.3 and 102.7°C in the DSC
thermogram when the sample was cooled. Compound
1c had mesogenic properties similar to those of com-
pound 1b, as shown in Table 1. Slow cooling of the
sample from the isotropic liquid to the mesophase gave
rise to a mosaic texture and homeotropic areas.
The compounds 1a–f were synthesized by a Pd(0)/Cu
(I)-catalyzed coupling reaction of the corresponding
terminal alkynes 2a–f with 2,4,6-tris(p-bromo-
phenyl)triazine 3 prepared from p-bromobenzonitrile
according to the reported method2e at 55°C in toluene/
NEt3 (Scheme 2). All compounds 112 are readily soluble
in organic solvents such as tetrahydrofuran, chloroform
and dichloromethane, and can be characterized by
NMR spectroscopy. All results of the spectroscopic
studies and elemental analysis were consistent with the
proposed molecular structures. Compounds 1a–f are
air-stable solids that can be stored at ambient tempera-
ture for a longer period than 3 months.
The UV–vis absorption peak of compound 1b is shifted
to a longer wavelength (umax=365 nm) from that of
1,3,5-triazine (umax=272 nm) due to the expansion of
the p-conjugation system. Other compounds show UV–
vis absorption peaks at essentially the same places
(Table 2). The UV–vis absorption peaks of 1a–f are
shifted by 1–5 nm to longer wavelengths in the solid
film, presumably due to intermolecular p–p interaction
The phase assignments13 and transition temperatures of
compounds 1a–f were studied by differential scanning
calorimetry (DSC) and polarized optical microscopy
(POM). The heating and cooling rates were 5°C/min.
The DSC thermograms of compounds 1a–f were mea-
sured by heating and subsequent cooling. Results of the
DSC and POM experiments are summarized in Table 1.
Table 1. Phase transition temperatures of 2,4,6-triphenyl-
1,3,5-triazine derivatives
Compound
Phase transition temperature (°C)a
K 1D12.7 K1 1D21.8 K2
1a
1b
1c
The POM observations revealed that 1a, 1d, 1e and 1f
were not a liquid-crystalline material. The DSC peak at
higher temperature is responsible for the melting and
crystallization transitions, while that at lower tempera-
ture is associated with crystal–crystal (k–k) transitions
in the solid state. When 1a and 1f were cooled from the
isotropic state, they solidified and the crystalline phase
displayed mosaic textures.14 However, no birefringent
phases were observed upon heating or cooling, indicat-
ing that 1a and 1f were not a liquid-crystalline material.
85.6
99.4
K X Dhd X I
66.3
92.0
102.7
K X Dhd 1X02.7 I
65.6
76.0
1d
1e
1f
K 1D03.1I
K 1D10.0I
K 7D5.5 K1 D K2
95.8
a K, K1, K2: crystal; Dhd, columnar hexagonal discorded; I: isotropic.

C.-H. Lee, T. Yamamoto / Tetrahedron Letters 42 (2001) 3993–3996
Table 2. UV–vis and photoluminescence data of 1a–f
3995
Compound
CHCl3 solution
ucm (nm)
Cast film
ucm (nm)
bf (%)a
umax (nm)
umax (nm)
1a
1b
1c
1d
1e
1f
364
365
363
363
364
363
477
476
472
477
472
474
369
370
364
365
365
365
515
507
504
504
503
504
53
54
54
54
61
63
a Quantum yield on chloroform.
between the planar core p-systems assisted by the side-
chain crystallization effect of the long alkoxyl group.
Similar bathochromic shifts in the solid have been
reported for transition metal complexes with organic
ligands having long alkoxyl side chains.15
We thank Mr. Y. Sano and Professor M. Matsumoto
(Kyushu Sangyo University) for the DSC, and Dr. Y.
Muramatsu (Tokyo Institute of Technology) for the
photoluminescent measurement.
References
The emission spectroscopic data of compounds 1a–f are
also listed in Table 2. Compound 1b exhibits a sharp
emission peak (uem) at 476 nm; the peak position essen-
tially agrees with the onset position of the UV–vis
absorption band as usually observed for p-conjugated
molecules and polymers.16 Similar results were observed
with compounds 1a, 1c, 1d, 1e and 1f. As shown in
Table 2, compounds 1a–f give large photoluminescence
quantum yields of 53–63%. Such high quantum yield
and the chemical stability of 1a–f indicate the potential
applicability of the compounds in optical devices such
as a dye laser. The quantum yield of 1a–f is higher than
that (40–45%) of previously reported.2d The peak posi-
tion indicates that 1 is suited to emission of blue light.
1. Monroe, B. M.; Weed, G. C. Chem. Rev. 1993, 93, 435.
2. (a) Ky Hirschberg, T. H. K.; Brunsveld, L.; Ramzi, A.;
Vekemans, J. J. M.; Sijbesma, R.; Meijer, E. W. Nature
2000, 407, 167; (b) Wolff, J.; Siegler, F.; Matschiner, R.;
Wortmann, R. Angew. Chem., Int. Ed. 2000, 39, 1436; (c)
Ghanashyam Acharya, S. N.; Srinivasa Gopalan, R.;
Kulkarni, G. U.; Venkatesan, K.; Bhattacharya, S. Chem.
Commun. 2000, 1351; (d) Pohlers, G.; Scaiano, J. C.
Chem. Mater. 1997, 9, 1353; (e) Cherioux, F.; Maillotte,
H.; Audebert, P.; Zyss, J. Chem. Commun. 1999, 2083; (f)
Brasselet, S.; Cherioux, F.; Audebert, P.; Zyss, J. Chem.
Mater. 1999, 11, 1915; (g) Murata, Y.; Endo, K. (Mit-
subishi Chemical Industries Ltd.), Jpn. Kokai Tokkyo
Koho Jpn., 292, 860, 1999; [Chem. Abstr. 1999, 131,
305223p].
3. (a) Lo, W.-J.; Hong, Y.-L.; Lin, R.-H.; Hong, J.-L. Mol.
Cryst. Liq. Cryst. 1997, 308, 133; (b) Wang, Y.-H.; Hong,
Y.-L.; Yen, F. S.; Hong, J.-L. Mol. Cryst. Liq. Cryst.
1996, 287, 109; (c) Janietz, D. J. Mater. Chem. 1998, 8,
265; (d) Goldmann, D.; Janietz, D.; Schmidt, C.; Wen-
dorff, J. H. Liq. Cryst. 1998, 25, 711.
4. (a) Malthete, J.; Leclercq, M.; Dvolaitzky, M.; Gabard,
J.; Billard, J.; Pontikis, V.; Jacques, J. Mol. Cryst. Liq.
Cryst. 1973, 23, 233; (b) Takatsu, H.; Takeuchi, K.;
Sasaki, M. Mol. Cryst. Liq. Cryst. 1986, 141, 27.
5. Tsibouklis, J.; Wernick, A. R.; Shand, J. A.; Milburn, G.
H. W. Liq. Cryst. 1988, 3, 1393.
6. (a) Merkushev, E. B.; Novikov, A. N.; Makarchenko, S.
S.; Moskalchuk, A. S.; Glushkova, V. V.; Kogai, T. I.;
Polyakova, L. G. Zh. Org. Khim. 1975, 11, 1259; (b)
Imamoto, T.; Koto, H. Chem. Lett. 1986, 967; (c) Hatzi-
grigoriou, E.; Varvoglis, A.; Bakola-Christianpoulou, M.
J. Org. Chem. 1990, 55, 315; (d) Ray, D. G.; Koser, G. F.
J. Am. Chem. Soc. 1990, 112, 5672.
The photoluminescence peak is shifted to a longer
wavelength in the solid cast film (cf. Table 1). Such a
redshift in the solid-phase has been observed with vari-
ous p-conjugated compounds and polymers16 and is
associated with the formation of an excimer-like
adduct. For the present compounds, the molecular
assembly assisted by the side-chain crystallization will
promote the formation of the excimer-like adduct. The
intensity of the photoluminescence is weakened in the
solid phase, as is usually observed with aromatic com-
pounds forming an excimer or excimer-like adduct.
As described above, a new class of 2,4,6-triphenyl-1,3,5-
triazine derivatives, 1a–f, having long alkoxyl side
chains can be easily synthesized. These compounds are
highly luminescent and some of the compounds behave
as liquid crystals. The present findings that the 2,4,6-
triphenyl-1,3,5-triazine-based core can serve as a new
mesogenic unit and that the compounds obtained have
highly luminescent properties are expected to pave the
way for the development of new optical devices using
the luminescent and liquid–crystalline materials.
7. (a) Yu, L.; Bao, Z. Adv. Mater. 1994, 6, 156 and refer-
ences cited therein; (b) Maddux, T.; Li, W.; Yu, L. J. Am.
Chem. Soc. 1997, 119, 844; (c) Yamamoto, T.; Sug-
anuma, H.; Maruyama, T.; Muramatsu, Y.; Arai, M.;
Komarudin, D.; Ooba, N.; Tomaru, S.; Sasaki, S.; Kub-
ota, K. Chem. Mater. 1997, 9, 1217.
Acknowledgements
We gratefully acknowledge support for this work from
the JSPS (Japan Society for the Promotion of Science).
8. (a) Dieck, H. A.; Heck, R. F. J. Organomet. Chem. 1975,
93, 259; (b) Sonogashira, K.; Tohda, Y.; Hagihara, N.

3996
C.-H. Lee, T. Yamamoto / Tetrahedron Letters 42 (2001) 3993–3996
Tetrahedron Lett. 1975, 4467; (c) Davis, S. G. Organo-
and brine and then dried over anhydrous Na2SO4 and
evaporated. The product was purified by column chro-
matography (SiO2, eluent, hexane/CHCl3=1/1) and
recrystallization (hexane 100%) to afford a pale yellow
powder of pure 1b in 88% yield. 1H NMR (300 MHz,
CDCl3) l 0.89 (t, 18H, J=6.4 Hz), 1.28 (br, 84H), 1.85
(quint, 12H, J=6.4 Hz), 4.03 (t, 12H, J=6.6 Hz), 6.86 (d,
3H, J=8.2 Hz), 7.10 (s, 3H), 7.15 (d, 3H, J=8.2 Hz),
7.70 (d, 6H, J=8.6 Hz), 8.75 (d, 6H, J=8.6 Hz). IR
(KBr) 2200 cm−1. Anal. calcd for C105H147N3O6: C, 81.50;
H, 9.58; N, 2.72. Found: C, 81.35; H, 9.75; N, 2.86.
13. Gray, G. W.; Goodby, J. W. Smectic Liquid Crystals,
Textures and Structures; Hill: London, 1984.
14. Mosaic textures were reported to appear in smectic
phases of hexagonal structures, see: Demus, D. In Liquid
Crystals: Application and Uses; Bahadur, B., Ed.; World
Scientific: Singapore, 1990; Vol. 1, p. 1.
15. (a) Ohta, K.; Ando, M.; Yamamoto, I. J. Porphyrins
Phthalocyanines 1999, 3, 249; (b) Ohta, K.; Ando, N.;
Yamamoto, I. Liq. Cryst. 1999, 26, 663. The transition
metal (e.g. Pd) complexes also exhibit liquid-crystalline
behavior.
transition Metal Chemistry: Syntheses; Pergamon:
Oxford, UK, 1982; (d) Takahashi, S.; Kuroyama, Y.;
Sonogashira, K.; Hagihara, N. Synthesis 1980, 627.
9. (a) Heck, R. F. Palladium Reagents in Organic Syntheses;
Academic Press: New York, 1990; (b) Hartley, F. A. The
Chemistry of Platinum and Palladium; Applied Science
Publishers: London, 1973; p. 327.
10. (a) Osakada, K.; Sakata, R.; Yamamoto, T. J. Chem.
Soc., Dalton Trans. 1997, 1265; (b) Yamamoto, T. Bull.
Chem. Soc. Jpn. 1999, 72, 621.
11. (a) Kitamura, T.; Lee, C. H.; Taniguchi, Y.; Fujiwara,
Y.; Matsumoto, M.; Sano, Y. J. Am. Chem. Soc. 1997,
119, 619; (b) Kitamura, T.; Lee, C. H.; Taniguchi, H.;
Matsumoto, M.; Sano, Y. J. Org. Chem. 1994, 59, 8053.
12. All new compounds showed satisfactory spectroscopic
and analytical data. Typical procedure for product 1b: A
mixture of 2,4,6-tris(p-bromophenyl)triazine 3 (66 mg,
0.12 mmol), Pd(PPh3)4 (14 mg, 0.012 mmol), CuI (2.3 mg,
0.012 mmol), 3,4-didecyloxyphenylacetylene (2b) (164 mg,
0.396 mmol), freshly distilled triethylamine (1 mL) and
anhydrous toluene (15 mL) was degassed and stirred at
55°C for 15 h under argon. After cooling to room
temperature, the mixture was evaporated, washed with
saturated aqueous NH4Cl, and extracted with chloro-
form. The combined extracts were washed with water,
16. Yamamoto, T.; Yamada, W.; Takagi, M.; Kizu, K.;
Maruyama, T.; Ooba, N.; Tomaru, S.; Kurihara, T.;
Kaino, T.; Kubota, K. Macromolecules 1994, 27, 6620.
.