Tris(triazolyl)triazine via click-chemistry: A C3 electron-deficient core with liquid crystalline and luminescent properties

Article abstract of DOI:10.1021/ol902900y

(Figure Presented) The synthesis of a novel core based on tris(triazolyl)triazine has been accomplished by a one-pot procedure that combines a 3-fold deprotection of alkyne groups and click chemistry of the aromatic alkyne and azide precursors. The procedure is straightforward for the preparation of functional materials for organic electronics. Indeed, compounds with low reduction potential are obtained. These compounds also show liquid crystalline behavior, displaying columnar mesophases at room temperature, and are luminescent in the visible region both In solution and in thin films.

Full text of DOI:10.1021/ol902900y

ORGANIC
LETTERS
2010
Vol. 12, No. 7
1404-1407
Tris(triazolyl)triazine via Click-Chemistry:
A C3 Electron-Deficient Core with Liquid
Crystalline and Luminescent Properties
Eduardo Beltra´n,† Jose´ Luis Serrano,‡ Teresa Sierra,† and Raquel Gime´nez*,†
´
Area de Qu´ımica Orga´nica, Dpto de Qu´ımica Orga´nica y Qu´ımica F´ısica, Facultad de
Ciencias-Instituto de Ciencia de Materiales de Arago´n and Instituto de Nanociencia de
Arago´n, UniVersidad de Zaragoza-CSIC, 50009 Zaragoza, Spain
rgimenez@unizar.es
Received December 17, 2009
ABSTRACT
The synthesis of a novel core based on tris(triazolyl)triazine has been accomplished by a one-pot procedure that combines a 3-fold deprotection
of alkyne groups and “click chemistry” of the aromatic alkyne and azide precursors. The procedure is straightforward for the preparation of
functional materials for organic electronics. Indeed, compounds with low reduction potential are obtained. These compounds also show liquid
crystalline behavior, displaying columnar mesophases at room temperature, and are luminescent in the visible region both in solution and in
thin films.
Since the first report of the Cu(I)-catalyzed 1,3-dipolar
cycloaddition of azides to alkynes,1 a “click-chemistry”
reaction, the number of applications has grown dramatically
to cover a wide range of fields from synthetic chemistry to
biomedicine and materials science.2 Reactions proceed
regioselectively and give high yields in reasonable reaction
times and with tolerance to a large number of functional
groups. Most of these cycloaddition reactions involve
aliphatic azides and terminal alkynes, with less evidence for
the use of aromatic azide precursors.2a,3 Exploitation of
aromatic click-chemistry opens up the path to novel organic
conjugated systems for organic electronics. We have focused
our attention on the versatile 1,3,5-triazine scaffold as it could
be triply functionalized with triazole rings by click-chemistry
to give the C3-symmetrical system 2,4,6-tris(triazolyl)-1,3,5-
triazine (TTT). This azaheterocyclic core, to the best of our
knowledge, has not been reported to date and the structure
consists of a highly electron-deficient system that could be
† Instituto de Ciencia de Materiales de Arago´n.
‡ Instituto de Nanociencia de Arago´n.
(1) (a) Tornøe, C. W.; Meldal, M. Peptidotriazoles: Copper(I)-catalyzed
1,3-dipolar cycloadditions on solid-phase, Peptides 2001; Proc. Am. Pept.
Symp.; American Peptide Society and Kluwer Academic Publishers: San
Diego, CA, 2001; pp 263-264. (b) Tornøe, C. W.; Christensen, C.; Meldal,
M. J. Org. Chem. 2002, 67, 3057–3064. (c) Rostovtsev, V. V.; Green, L. G.;
Fokin, V. V.; Sharpless, B. K. Angew. Chem., Int. Ed. 2002, 41, 2596–
2599.
(3) (a) Li, Y.; Flood, A. H. Angew. Chem., Int. Ed. 2008, 47, 2649–
2652. (b) Binder, W. H.; Sachsenhofer, R. Macromol. Rapid Commun. 2008,
29, 952–981. (c) Gallardo, H.; Bortoluzzi, A. J.; Pereira de Oliveira Santos,
(2) (a) Meldal, M.; Tornøe, C. W. Chem. ReV. 2008, 108, 2952–3015.
(b) Kolb, R. W.; Sharpless, K. B. Drug DiscoVery Today 2003, 8 (24),
1128–1137.
D. M. Liq. Cryst. 2008, 35, 719–725
.
10.1021/ol902900y  2010 American Chemical Society
Published on Web 03/02/2010

of interest for n-type organic semiconductors.4 Furthermore,
extension of the conjugation of the new heterocyclic core
with aromatic rings should give molecules with luminescent
properties.5 Moreover, this core shows clear potential for
multifunctional materials. Indeed, appropriate substitution at
the periphery of the molecules can provide access to novel
discotic liquid crystals, which are interesting from the point
of view of their one-dimensional self-organizing ability. Disc-
like aromatic compounds that form columnar liquid crystals
have great potential in the field of organic semiconductors
due to the overlap of π-orbitals, which results in uniaxial
charge carrier mobility with performances approaching those
of conjugated polymers.6
azides to alkyne monomers to occur at the same time as
alkyne deprotection.9 Taking advantage of this feature, we
tested a one-pot reaction in which the alkyne groups of
intermediate 1 are triply deprotected and made to react in
situ with the corresponding aromatic azide 2-4 in the
presence of catalytic Cu(I) (Scheme 1). The reactions were
carried out at room temperature to give the TTT derivatives
5-7 in moderate yields (45-51%). The 1H NMR spectra of
5-7 display one singlet for the proton of the heterocyclic
rings at about 9.17 ppm, which is consistent with the
formation of 1,4-disubstituted triazole rings.
All TTT compounds exhibit liquid crystalline behavior
(Table 1, Figure 1) and are thermally stable above their
clearing point according to thermogravimetric analysis. 6 and
7 display textures that are typical of hexagonal columnar
phases (Colh) by polarized optical microscopy (Figures 1b,c).
5 does not show a characteristic texture in pure state (Figure
1a), but a Colh mesophase is displayed in miscibility tests
with 6. The X-ray diffraction patterns only show a sharp
maximum in the low-angle region in all cases. Although this
pattern does not unambiguously confirm the hexagonal
symmetry of the mesophase, it rules out other columnar
symmetries like rectangular or oblique phases. Similar
behavior has also been found in other structures previously
described as hexagonal columnar phases,10 and it is due to
a minimum in the form factor, which precludes the observa-
tion of peaks in this angle region.
Scheme 1. Synthesis of TTT Compounds 5-7
5 has only three lateral chains and displays a monotropic
columnar mesophase, which is quite unusual.6b The as-
obtained product is a crystalline solid at room temperature
and this melts to an isotropic liquid at 150 °C. On cooling,
the compound does not crystallize but becomes glassy, and
depending on the cooling rate, a mesophase starts growing
at 123 °C (broad peak with maximum at 107 °C, Figure 1a).
In the second heating process there is a cold recrystallization
at 120 °C, with the material again becoming crystalline above
this temperature until the melting point. An X-ray diffraction
study was performed on a sample cooled from the isotropic
liquid and slowly allowed to reach room temperature. Only
a maximum at low angles and a diffuse halo at wide angles
were observed. Assuming that the low-angle maximum
should correspond to the d(10) reflection of a hexagonal
arrangement gives rise to a cell parameter (a) of 35.6 Å.
6, which contains six n-decyloxy terminal chains, showed
a mesophase with homeotropic and birrefringent domains
(Figure 1b). Only one peak was observed in the DSC
thermograms both on heating and cooling, indicating that
the mesophase is thermodynamically stable at room tem-
perature. The X-ray diffraction analysis is consistent with a
hexagonal columnar mesomorphism (Colh) that has a cell
parameter of 37.2 Å. Additionally, in the wide-angle region
a diffuse halo at 4.5 Å typical of the liquid-like order between
the aliphatic chains and a relatively narrow diffuse ring at
3.5 Å are observed. In partially oriented patterns, this latter
The work reported here involved the straightforward
synthesis of TTT derivatives and the study of their liquid
crystalline, optical, and redox properties. Aromatic azides
and the precursor 2,4,6-tris(ethynyl)-1,3,5-triazine are re-
quired for the synthesis of the TTT core by click-chemistry
(Scheme 1). The latter compound is quite unstable and, for
this reason, we prepared and stored the corresponding TMS-
protected compound, 2,4,6-tris[(trimethylsilyl)ethynyl]-1,3,5-
triazine (1).7
The orthogonality of click-chemistry, which allows mul-
tiple chemical transformations to occur in solution without
interference,8 enables the click cycloaddition of aliphatic
(4) (a) Shirota, Y.; Kageyama, H. Chem. ReV. 2007, 107, 953–1010.
(b) Kulkarni, A. P.; Tonzola, C. J.; Babel, A.; Jenekhe, S. A. Chem. Mater.
2004, 16, 4556–4573. (c) Hughes, G.; Bryce, M. R. J. Mater. Chem. 2005,
15, 94–107.
(5) Molecular Fluorescence; Valeur, B., Ed.; Wiley: Weinheim, Ger-
many, 2002.
(6) (a) Sergeyev, S.; Pisula, W.; Geerts, Y. H. Chem. Soc. ReV. 2007,
36, 1902–1929. (b) Laschat, S.; Baro, A.; Steinke, N.; Giesselmann, F.;
Ha¨gele, C.; Scalia, G.; Judele, R.; Kapatsina, E.; Sauer, S.; Schreivogel,
A.; Tosoni, M. Angew. Chem., Int. Ed. 2007, 46, 4832–4887. (c) Kato, T.;
Yasuda, T.; Kamikawa, Y.; Yoshio, M. Chem. Commun. 2009, 729–739.
(7) Sonoda, M.; Inaba, A.; Itahashi, K.; Tobe, Y. Org. Lett. 2001, 3,
2419–2421.
(8) (a) Malkoch, M.; Thibault, R. J.; Drockenmuller, E.; Voit, B.; Russell,
T. P.; Hawker, C. J. J. Am. Chem. Soc. 2005, 127, 14942–14949. (b)
Chittaboina, S.; Xie, F.; Wang, Q. Tetrahedron Lett. 2005, 46, 2331–2336.
(9) Thibault, R. J.; Takizawa, K.; Lowenheilm, P.; Helms, B.; Mynar,
J. L.; Frechet, J. M.; Hawker, C. J. J. Am. Chem. Soc. 2006, 128, 12084–
12085.
(10) (a) Zheng, H.; Xu, B.; Swager, T. M. Chem. Mater. 1996, 8, 907–
911. (b) Cavero, E.; Uriel, U.; Romero, P.; Serrano, J. L.; Gime´nez, R.
J. Am. Chem. Soc. 2007, 129, 11608–11618. (c) Barbera´, J.; Puig, L.;
Serrano, J. L.; Sierra, T. Chem. Mater. 2004, 16, 3308–3317.
(11) Eaton, D. F. Pure Appl. Chem. 1998, 60, 1107–1114.
Org. Lett., Vol. 12, No. 7, 2010
1405

Table 1. Thermal Properties, X-ray Diffraction Data, and Optical Properties for 5-7
phase transitiona temp (°C) and
enthalpies (kJ mol-1
)
a (Å)b
c (Å)b
λ
(nm) (log ε)
λ
(nm)c
416
λ
(nm)c
em
Φd
THF
THF
bulk
compd
abs
em
5
I 107 (2.1) Colh 50 g
35.6
257 (4.33), 289 (4.47), 299sh
416
0.35
g 55 Colh 120 (-16.9)e Cr 150 (26.1) I
I 185 (4.0) Colh 18 g
6
7
37.2
32.9
3.5
3.5
264 (4.69), 304 (4.62)
448
471
450
481
0.43
0.18
g 28 Colh 194 (5.3) I
I 172 (10.6) Colh 18 g
266 (4.62), 301 (4.63)
g 23 Colh 183 (10.6) I
a DSC data for first cooling and second heating processes, 10 deg/min, peak temperatures, I ) isotropic liquid, Colh ) hexagonal columnar mesophase,
g ) glass transition, Cr ) crystal. b Hexagonal cell parameters. c Excitation wavelength at 300 nm. d Quantum yields in cyclohexane solutions relative to
diphenylanthracene (Φ ) 0.9 in cyclohexane solution).11 e Cold crystallization.
wavelengths are located in the UV region, at around 300
nm, and are attributed to π-π* transitions due to the high
absorption coefficients. All TTT compounds are luminescent,
emitting in the blue-green part of the visible spectrum (Figure
2 and abstract figure). A remarkable red-shift in the emission
maximum is observed as the number of peripheral alkoxy chains
increases. The compounds also show luminescence in thin films
at room temperature with emission wavelengths similar to those
of the solutions. The large Stokes shifts are consistent with a
strong charge transfer character and/or a conformational relax-
Figure 1. Polarized optical photomicrographs of mesophases for 5
at 92 °C (a), 6 at 25 °C (b), and 7 at 145 °C (c).
ation in the excited state, of higher polarity when increasing
the number of alkoxy chains. This trend has also been observed
in other luminescent mesogens.10b
reflection is reinforced along the column axis, which is
indicative of the periodic stacking of the cores at a mean
distance of 3.5 Å, typical of π-stacked systems.
7, which contains nine n-decyloxy peripheral chains, also
exhibits a hexagonal columnar mesophase that is stable at
room temperature, with a cell parameter of 32.9 Å and a
stacking distance of 3.5 Å.
It can be seen from the results in Table 1 that the cell
parameter of the hexagonal lattice (a) increases from 5 to 6,
but decreases in 7. This is unexpected given the increasing
size of the individual molecules (from three peripheral chains
to nine) but it can be explained in terms of the number of
molecules per unit cell, Z. Indeed, a reasonable density value
in the range 0.9-1.1 g·cm-3 is obtained if we consider two
molecules per unit cell (Z ) 2) for 5 and 6. In both cases,
the column must consist of stacked discs formed by two TTT
molecules. However, for 7, the Z value calculated corre-
sponds to one molecule per unit cell and a density value of
0.9 g·cm-3 (Table S1, SI). These findings can be accounted
for by the strong tendency of the triazine core to give
columnar assemblies regardless of the number of surrounding
flexible tails. Three or six tails are not sufficient to fill
efficiently the space around the column and this is overcome
by the stacking of pairs of molecules in 5 and 6. For 7, nine
peripheral tails efficiently fill the space around the column
and stabilize a columnar mesophase with only one molecule
per stacking unit.10c
Figure 2. Emission spectra of 5-7 in THF solution (dashed lines)
and thin films at rt (solid lines).
Discotic aromatic compounds that form columnar liquid
crystals have great potential in the field of organic electronics
(12) (a) Eichhorn, H. J. Porphyrins Phthalocyanines 2000, 4, 88–102.
(b) van de Craats, A.; Warman, J. M. AdV. Mater. 2001, 13, 130–133. (c)
Lehmann, M.; Kestemont, G.; Aspe, R. G.; Buess-Herman, C.; Koch,
M. H. J.; Debije, M. G.; Piris, J.; de Haas, M. P.; Warman, J. M.; Watson,
M. D.; Lemaur, V.; Cornil, J.; Geerts, Y. H.; Gearba, R.; Ivanov, D. A.
Chem.sEur. J. 2005, 11, 3349–3362. (d) Gearba, R.; 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. (e) An, Z.; Yu, J.; Jones,
S. C.; Barlow, S.; Yoo, S.; Domercq, B.; Prins, P.; Siebbeles, L. D. A.;
Kippelen, B.; Marder, S. R. AdV. Mater. 2005, 17, 2580–2583.
The UV-vis absorption and photoluminescence of 5-7
were measured in tetrahydrofuran (THF) solutions. The
results are summarized in Table 1. The maximum absorption
1406
Org. Lett., Vol. 12, No. 7, 2010

the LUMO energy levels as the difference between -4.8
eV and Ered (eV). Thus, LUMO energy levels of -3.52 eV
for 5 and -3.47 eV for 6 and 7 were calculated from the
first reduction potentials. These reduction potentials and
LUMO values are consistent with an enhancement of the
electron-deficient nature of the aromatic core due to the
presence of the triazolyl rings. Other triazine derivatives have
been reported to show higher reduction potentials,13 for
example, 2,4,6-triphenyl-1,3,5-triazine shows a reversible
reduction potential at -2.09 V vs FOC and this corresponds
to a LUMO energy of -2.71 eV.13a
Table 2. Cyclic Voltammetry results for 5-7
compd
Ered (V)a
Eopt (eV)c
LUMO (eV)
sol
5
6
7
-1.28, -2.24b
-1.33, -2.15b
-1.33
3.66
3.50
3.53
-3.52
-3.47
-3.47
a E1/2 reduction potentials. b Ep of the irreversible process. c Calculated
from solution absorption edges.
as one-dimensional semiconductors.5 In particular, columnar
liquid crystals with nitrogenated aromatic cores have an
electron-accepting character for which n-type semiconducting
properties have been described.4,12 In this respect, TTT
compounds should be reasonably good electron-accepting
molecules since the introduction of the three triazole rings
should increase further the electron-deficient nature of the
triazine ring.13 The redox behavior of the TTT compounds
was studied by cyclic voltammetry in oxygen-free dichlo-
romethane solutions. All potentials are referenced against
ferrocene/ferrocenium (FOC) as the internal standard (Table
2). 5-7 did not show any oxidation process at positive
potentials from 0 to +2.2 V. However, 5 showed a reversible
reduction process at a half-wave potential of -1.28 V. In
addition, an irreversible reduction process was found at
-2.24 V. 6 showed the same behavior, with reversible and
irreversible reduction processes (-1.33 and -2.15 V,
respectively). In contrast, only one reversible reduction
process was found, at a half-wave potential of -1.33 V, for
7. Under the premise that the energy level of ferrocene/
ferrocenium is 4.8 eV below vacuum level,14we obtained
It is interesting to note that the LUMO values obtained
are also considerably lower than that of tris(8-hydroxyquino-
line)aluminum (Alq3), the most commonly used molecular
electron-transport hole-blocking material for OLEDs (ELU
-
MO(Alq3) ) -2.9 eV),15 and similar to those of other well-
known electron-accepting materials.16 Consequently, prom-
ising characteristics for electron injection from standard
cathode metals such as magnesium-silver alloy (work
function of Mg ) -3.7 eV)15 can be envisaged for these
materials.
In summary, novel electron-deficient organic compounds
can be prepared by a straightforward synthesis through click-
chemistry. The materials show good liquid crystalline
properties, exhibiting ordered columnar mesophases. TTT
compounds also show luminescence from the blue to green
region of the visible spectrum depending on the peripheral
substitution. Electrochemical measurements confirmed the
electron-deficient nature of this class of compounds and their
potential for electron transport. Further studies on these
molecules for multifunctional materials are in progress.
Acknowledgment. We thank the MICINN (Spain) and
FSE (UE) (projects MAT2006-13571-CO2-01 and MAT2009-
14636-CO3-01), the CSIC-I3 (project 200860I054), and the
Gobierno de Arago´n (Research group E04) for financial
support. E.B. thanks the FPI program of MICINN (Spain)
for a fellowship.
(13) (a) Fink, R.; Frenz, C.; Telakkat, M.; Schmidt, H. W. Macromol-
ecules 1997, 30, 8177–8181. (b) Bock, H.; Babeau, A.; Seguy, I.; Jolinat,
P.; Destruel, P. ChemPhysChem. 2002, 6, 532–535. (c) Pieterse, K.;
Lauritsen, A.; Schenning, A. P. H. J.; Vekemans, J. A. J. M.; Meijer, E. W.
Chem.sEur. J. 2003, 9, 5597–5604. (d) Chen, H.-F.; Yang, S.-J.; Tsai,
Z.-H.; Hung, W.-Y.; Wang, T.-C.; Wong, K. T. J. Mater. Chem. 2009, 19,
8112–8118.
(14) This value is estimated on the basis of a rather crude approximation,
neglecting solvent effects, using the standard electrode potential for a normal
hydrogen electrode (NHE) at about -4.6 eV (Bard, A. J.; Faulkner, L. R.
Electrochemical Methods-Fundamentals and Applications; Wiley: New
York, 2001; p 634) on the zero vacuum level scale and a value of 0.2 V vs
NHE for the potential of FOC.
Supporting Information Available: Detailed experimen-
tal procedures and characterization data, and X-ray diffrac-
tograms with tabulated data, UV-vis absorption spectra, and
CV plots. This material is available free of charge via the
Internet at http://pubs.acs.org.
(15) Parker, I. D. J. Appl. Phys. 1994, 75, 1656–1666.
(16) Schmidt, R.; Oh, J. H.; Sun, Y.-S.; Deppisch, M.; Krause, A.-M.;
Radacki, K.; Braunschweig, H.; Ko¨nemann, M.; Erk, P.; Bao, Z.; Wu¨rthner,
F. J. Am. Chem. Soc. 2009, 131, 6125–6228.
OL902900Y
Org. Lett., Vol. 12, No. 7, 2010
1407