Thermotropic properties and molecular packing of discotic tristriazolotriazines with rigid substituents

Article abstract of DOI:10.1002/chem.201400034

Tristriazolotriazines with a threefold dialkoxyaryl substitution have been prepared by Huisgen reaction of cyanuric chloride and the corresponding tetrazoles. Although these dyes show a negative or inverted solvatochromism of the UV/Vis absorption, their

Full text of DOI:10.1002/chem.201400034

DOI: 10.1002/chem.201400034
Full Paper
&
Liquid Crystals
Thermotropic Properties and Molecular Packing of Discotic
Tristriazolotriazines with Rigid Substituents
Thorsten Rieth,[a] Tomasz Marszalek,[b] Wojciech Pisula,*[b] and Heiner Detert*[a]
Dedicated to Professor Rudolf Zentel on the occasion of his 60th birthday
Abstract: Tristriazolotriazines with a threefold dialkoxyaryl
substitution have been prepared by Huisgen reaction of cya-
nuric chloride and the corresponding tetrazoles. Although
these dyes show a negative or inverted solvatochromism of
the UV/Vis absorption, their fluorescence is strongly positive
solvatochromic. These discotic fluorophores are also emis-
sive in their solid state and in their broad liquid-crystalline
mesophase. The structural study indicates that the thermo-
tropic properties and organization of these systems can be
well tuned by the steric demand of the aryl groups. Depend-
ing on the substituents, the compounds showed either
a pure crystalline phase or a highly complex helical super-
structure with a characteristic liquid-crystalline phase at ele-
vated temperatures. Changing the steric demand of the at-
tached aryls allowed controlling the discs arrangement
within the columnar helix, which is of great importance for
the molecular orbital overlap.
formation of liquid crystals.[5,6] Discotic LCs of this type are
known to self-assemble into columnar superstructures driven
by p stacking of the core and local phase separation between
the aromatic center and the aliphatic rim. Due to an efficient p
overlap of neighboring molecules in columnar aggregates,
these materials are enabled to one-dimensional charge-carrier
transport,[4] which qualifies these discotic LCs as semiconduct-
ing materials in organic-field-effect transistors and photovolta-
ics.[7] To ensure high mobilities of the charge carriers, a defect-
free and close intracolumnar packing of the molecules are an
essential prerequisite. As an additional factor, the arrangement
of the building blocks towards each other plays a role for the
molecular orbital overlap and thus for the electron tunneling
between two adjacent discs. This local intermolecular organiza-
tion is controllable by the molecular design of the shape and
symmetry of the aromatic core, as well as the architecture and
functionality of the substituents.[25b]
Introduction
Thermotropic liquid crystals (LC) form an important category
of soft matter, molecular order and dynamics of which are in-
termediate between the isotropic fluid and that of a crystal.[1,2]
Compounds forming thermotropic mesophases are generally
composed of a rigid core and flexible side chains with a careful-
ly balanced size ratio. The core usually consists of an elongat-
ed, rod-like unit including aromatic rings. With flexible aliphatic
chains, these mesogenes form calamitic liquid crystals (LCs).
Bending the linear core results in banana-shaped biaxial mole-
cules.[4] Molecules with a two-dimensional, disc-like core form
another class of thermotropic LCs. Commonly, discotic LCs
have a large p-conjugated aromatic system substituted with
aliphatic side chains with typically high symmetry.[1,4] The core
is usually an extended polycyclic aromatic hydrocarbon, such
as triphenylene or hexabenzocoronene, but star-like com-
pounds consisting of a small central ring and three-to-six p-
conjugated arms can also meet the steric requirements for the
The vast majority of semiconducting discotic LCs are based
on electron-rich polycyclic aromatic hydrocarbons, such as tri-
phenylene,[8] hexabenzocoronene, and even larger p systems
with a strong preference for hole-conduction.[9] Discotic LCs
with an n-type, electron-transporting core are still rather
scarce.[10] Functionalization of intrinsically p-type acenes with
electron-withdrawing groups (EWGs) is a common strategy to
realize useful n-type materials.[11] Similarly, the exchange of
benzene rings with electron-deficient heterocycles proved to
be a successful route. For example, hexa-azatriphenylenes[12]
are of the most promising candidates for electron-transporting
semiconductors. This core is composed of three electron-defi-
cient pyrazine units. A closely related polycyclic molecule was
obtained by a (formal) cyclocondensation of three 1,2,4-tria-
zoles to tris[1,2,4]triazolo[1,3,5]triazine, a heterocyclic system
[a] T. Rieth, Prof. Dr. H. Detert
Institute for Organic Chemistry
Johannes Gutenberg University Mainz
Duesbergweg 10–14, 55099 Mainz (Germany)
Fax: (+49)6131-39-25338
E-mail: detert@uni-mainz.de
[b] Dr. T. Marszalek, Dr. W. Pisula
Max Planck Institute for Polymer Research
Ackermannweg 10, 55128 Mainz (Germany)
Fax: (+49)6131-379-350
E-mail: pisula@mpip-mainz.mpg.de
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201400034. It contains detailed information
on the preparation of compounds 1–15, the analytical data, and spectra.
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with C3h symmetry.[13] Though known for over a century,[14] this
disc-shaped molecule has only recently been recognized as
a core for discotic LCs.[15,16] The synthesis of alkoxyaryl-tristria-
zolotriazines follows Huisgens et al. procedure: condensation
of three 5-phenyltetrazoles with cyanuric chloride followed by
ring transformation leads to 3,7,11-trisubstituted tri-
The synthesis of all required tetrazoles starts with the alkyla-
tion of catechols. Addition of hydrazoic acid to benzonitrile 9
gave 3,4-dioctyloxyphenyltetrazole 4 in 71% yield (Scheme 2).
Similarly, the synthesis of naphthalenyl tetrazole 5 starts with
alkylation of 2-bromonaphthalenediol 10.[18] A Rosenmund–von
Braun reaction gave nitrile 12 (46%), which was converted to
the tetrazole 5 (79%).
[1,2,4]triazolo[4,3-a:4,3-c:4,3-e][1,3,5]triazines
(TTTs).[17]
Six
alkoxy side chains on the rim guarantee excellent solubility in
common solvents. These discotic molecules with propeller-like
arranged aryl groups form broad thermotropic mesophases
with a hexagonal columnar phase in the liquid-crystalline state
and highly complex helical superstructures in their crystalline-
like phase. For these model compounds, it has been proven
that the phase formation, intermolecular packing, and helical
packing can be precisely tuned by the steric demand of the
aryl groups persisting a close p-stacking distance.
Herein, we present the synthesis of three star-shaped TTTs
1–3 with a threefold dialkoxyaryl-substitution.[6] The size of the
rigid triaryl-TTT is gradually increased, from a simple triphenyl
substitution to tris(b-naphthyl) and tris(4-biphenylyl).
Results and Discussion
Synthesis
Huisgens et al. method[17] is currently the only synthetic ap-
proach to TTTs, which allows to obtain this tetracyclic system
with carbon-based substituents. With minor modifications of
the original procedure, TTTs 1–3 were obtained by threefold
acylation of 5-aryltetrazoles 4–6 with cyanuric chloride 7. A
thermal elimination of nitrogen from an acyltetrazole unit led
to a 1,5-dipole that cyclizes to a 1,2,4-triazole annulated to the
central 1,3,5-triazine (Scheme 1). Simple triphenyl-TTT is a solid
Scheme 2. Synthesis of tetrazoles 4 and 5. Reagents and conditions: i) Br-
C8H17, K2CO3, DMF, 808C, 9: 87%, 11: 65%; ii) NaN3, Et3NꢁHCl, toluene, reflux,
4: 71%, 5: 79%; iii) CuCN, NMP, 2008C, 12: 46%.
A convergent strategy was chosen for the preparation of bi-
phenyl-substituted tetrazole 6. The final step is a Pd-catalyzed
Suzuki cross-coupling reaction of iodo-phenyl tetrazole 16 and
didecyloxyphenyl boronic acid 15.[19] Iodophenyl tetrazole 16
was prepared from iodobenzoic amide[20] and triazido chlorosi-
lane (89%)[21] Regarding the extension of the rigid core, the
longer decyl chains were attached to catechol. N-Bromosucci-
nimide (NBS) selectively brominated the 4-position of 13 (95%;
Scheme 3). A bromine/lithium exchange on 14 and subsequent
treatment with tri-iso-propyl borate followed by hydrolysis led
Scheme 1. Synthesis of TTTs. Reagents and conditions: i) collidine, xylenes,
RT–808C. 1: R=3,4-dioctyloxyphenyl: 79%; 2: R=5,6-dioctyloxynaphthalen-
2-yl: 72%; 3: R=3’,4’-didecyloxybiphenyl-4-yl: 52%.
with a high melting point (m.p. 3108C).[17] To obtain discotic
molecules with the ability to form thermotropic mesophases,
a substitution of the triphenyl-TTT with two or three flexible
side chains on terminal positions of all benzene rings is
required.
Scheme 3. Synthesis of biphenyl-tetrazole 6. Reagents and conditions:
i) NBS, CHCl3, reflux 14: 95%; ii) 1) nBuLi, diethyl ether, ꢀ208C; 2) B(O-iso-
propyl)3, ꢀ708C–RT, HCl (2 n), 15: 77%; iii) [Pd(PPh3)4], K2CO3, glyme/water
(1:1), N2, reflux, 6: 27%.
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to the required boronic acid 15 (77%). The Pd-catalyzed cou-
pling of 15 to tetrazole 16 gave the desired biphenylyl-tetra-
zole 6 in 27% yield. As described in Scheme 1, the tetrazoles
4–6 were used to obtain the TTTs 1–3.
indicate an aggregation of the chromophores even in highly
diluted (ca. 10ꢀ7 m) solution. Contrary to the negative or invert-
ed solvatochromism of their absorption spectra, increasing sol-
vent polarity shifted the fluorescence of all TTTs strongly to the
red. The effect of enhanced solvent polarity (cyclohexane to
ethanol) increased in the sequence TTT
1710 cmꢀ1)<TTT (Du˜solv =2908 cmꢀ1)<TTT
1
2
(Du˜solv
(Du˜solv
=
Optical properties
3
=
3051 cmꢀ1). These C3-symmetric, octupolar chromophores are
composed of three loosely coupled dipolar p systems. The sol-
vatochromism of the absorption spectra revealed a polar
ground state of the short dialkoxyphenyltriazole segment. The
inverted solvatochromism of the absorption of TTT 2 and TTT
3 can be attributed to a superposition of dipole stabilization
and a significantly enhanced polarizability of the extended
conjugated systems.[22] Excitation causes an intramolecular
charge transfer from the donor groups to the acceptor triazole.
Accordingly, the larger conjugated systems are more dipolar
and their stabilization by solvent dipoles leads to a pronounced
positive solvatochromism.
The tristriazolotriazines 1–3 are colorless solids with a good
solubility in nonpolar solvents irradiated with UV, the pure
compounds, as well as their solutions, emit a violet fluores-
cence. The optical properties of TTTs 1–3 are presented in
Table 1. The maximum of the long-wavelength absorption
Table 1. Optical properties and solvatochromism.
Entry
Solvent
lmax [nm] loge lFmax [nm] Du˜solv [cmꢀ1
]
TTT 1 cyclohexane
310
310
4.63 370
4.66 373
4.62 389
4.61 391
4.61 395
388
4.53 398
4.63 417
4.58 448
4.61 459
4.56 453
428
4.86 394
4.91 404
4.85 433
4.35 445
4.70 445
422
5231
5448
6866
7869
7796
4808
5455
6600
8074
8701
8789
6221
5869
6112
7961
8297
8584
6978
toluene
dichloromethane 307
acetonitrile
ethanol
film
299
302
327
327
327
TTT 2 cyclohexane
toluene
Thermotropic properties
The mesomorphic properties of the three disc-like TTTs were
characterized by polarized optical microscopy (POM), differen-
tial scanning calorimetry (DSC), and wide- and small-angle X-
ray scattering (WAXS and SAXS). Investigations of TTT 1, 2, and
3 with a polarized optical microscope indicate pseudo-focal
conic fan-shaped textures by cooling (108Cminꢀ1) the meso-
genes from the isotropic liquid phase into an enantiotropic
liquid-crystalline phase. These fan-shaped textures were char-
acteristic for a discotic hexagonal columnar phase (Colh).[23]
Slow cooling (18Cminꢀ1) of TTT 1 and TTT 2 led to dendritic
growth, a characteristic behavior of columnar liquid-crystalline
phases.[23] Figure 1 shows the observed textures of the meso-
phases at fast and slow cooling.
dichloromethane 329
acetonitrile
ethanol
328
324
338
320
324
film
TTT 3 cyclohexane
toluene
dichloromethane 322
acetonitrile
ethanol
film
325
322
326
band of TTT 1 appears in the UV (lmax =310 nm, in cyclohex-
ane) and compound TTT 2 with the larger naphthyl substitu-
ents absorbs at longer wavelengths (lmax =327 nm). Due to the
twisted biphenyl linkage, the absorption spectrum of TTT 3
peaks at an intermediate position (lmax =320 nm). The fluores-
cence of all TTTs is separated from the absorption by large
Stokes shifts increasing with the size of the p systems. Though
the maxima of the fluorescence spectra of TTTs 1–3 in cyclo-
hexane are observed in the UV, their intense emissions extend
into the violet region, especially for the compounds with the
naphthalenyl and biphenylyl units. Similar to the solution, TTTs
1–3 are fluorescent in the solid state. Spin-coated films were
obtained from TTTs 1–3 from toluene solution on glass sub-
strates. Compared with solution, the absorption spectra in the
spin-coated films were broadened and slightly shifted to
longer wavelengths, and the shift decreased with increasing
core size.
An increasing polarity of the solvent affects the absorption
and emission spectra. Although the absorption of TTT 1 is neg-
atively solvatochromic (Du˜solvꢀ854 cmꢀ1 for cyclohexane to
ethanol), TTT 2 and TTT 3 displayed a weak but inverted solva-
tochromism. Furthermore, their absorption spectra were signif-
icantly broadened in the polar solvents. This and the smaller
absorption coefficients of TTTs 1–3 in ethanol and acetonitrile
Figure 1. POM textures (crossed-polarizers) of TTTs 1–3 in the mesophase.
a) TTT 1: fast cooling (10 Kminꢀ1), 2258C; b) TTT 1: slow cooling (1 Kminꢀ1),
2268C; c) TTT 2: fast cooling (10 Kminꢀ1), 1858C; d) TTT 3: fast cooling
(10 Kminꢀ1), 1708C. All scale bars=100 mm.
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Structural characterization
The structure investigation of the bulk supramolecular organi-
zation was performed on macroscopically aligned fibers pre-
pared by mechanical extrusion in the viscoelastic (liquid-crys-
talline phase, if present) state. For the measurements, the
fibers were mounted vertically toward the two dimensional de-
tector, which collected the scattered reflections in either in the
wide- or small-angle range. In our previous studies on discotic
LCs, this technique provided valuable information about the
molecular arrangement and order within the superstructure.[24]
The patterns for TTT 1 and TTT 2 recorded in their liquid-
crystalline phase at higher temperatures exhibit a typical hex-
agonal columnar organization with nontilted discs (Figure 3).
Figure 2. DSC traces (2nd cycle) of TTT 1 and TTT 2. Heating scans:
10 Kminꢀ1; cooling scans: TTT 1 20 Kminꢀ1, TTT 2 5 Kminꢀ1
.
In accordance to the observations by POM, DSC of TTT
1 and TTT 2 reveals two phase transitions during heating, from
crystalline (Cr) to columnar hexagonal (Colh) and from Colh to
isotropic (I; Figure 2). The second DSC heating curve of TTT
1 shows an endothermic transition into a broad columnar hex-
agonal discotic mesophase at 1008C and a second, less endo-
thermic transition at 2268C into the isotropic melt. Upon cool-
ing, the expected exothermic transitions were slightly shifted
to lower temperatures (I!Colh 2228C; Colh!Cr 928C), both
transitions occur with typical transition enthalpies of DHm =
19.9 kJmolꢀ1 and DHc =4.5 kJmolꢀ1. Similarly, TTT 2 shows two
endothermic transitions in the second heating curve (Cr!Colh
112 8C; Colh!I 1978C), but only one exothermic transition
upon cooling (i!Colh 1948C). The remarkably low value for
the melting enthalpy (1.3 kJmolꢀ1) and the lack of the signal
for the transition into the crystalline phase (Colh!Cr) were at-
tributed to a very slow crystallization of the highly viscous ma-
terial. The second DSC heating curve of TTT 3 shows only one
endothermic transition at T=1728C with a very small enthalpy
of DH=1.2 kJmolꢀ1. In the cooling scan, this transition was
shifted to 1678C. Similar to TTT 2, we assume that upon fast
cooling (5 Kminꢀ1) of the viscous liquid phase, TTT 3 does not
reach the thermal equilibrium, and a supercooled melt re-
mains. The transition temperatures and enthalpy values of all
DSC thermograms are given in Table 2.
Figure 3. 2DWAXS patterns of a) TTT 1 (at 1508C); b) TTT 2 (at 1408C) re-
corded in their liquid crystalline phase; and c) equatorial integration for TTT
1 (reflections are assigned by Miller’s indexes).
Equatorial reflections in the small-angle regime correspond to
the columnar assemblies, long-range one aligned parallel to
extrusion direction (fiber axis). From their positions, hexagonal
unit-cell parameters for the intercolumnar arrangement with
Table 2. Thermal data obtained by DSC for TTTs 1–3. Tm =transition tem-
perature from crystalline (Cr) to hexagonal columnar phase (Colh).
ahex =2.61 nm for TTT 1 and ahex =3.08 nm for TTT 2 were ex-
Entry
Tm [8C]
DH [kJmolꢀ1
]
Ti [8C]
DH [kJmolꢀ1
]
tracted. The low intensity of the 110 and 200 reflections is
characteristic for the liquid-crystalline short-range order of the
samples. Additionally, an amorphous halo at around 2q=208
suggests fluid-like, disordered alkyl substituents. Wide-angle re-
flections in the meridional plane were attributed to the nontilt-
ed intracolumnar packing of the molecules from which p-
stacking distances of 0.34 nm for TTT 1 and 0.36 nm for TTT 2
were determined. In both cases, these reflections are quite
weak, confirming the low intracolumnar order in the liquid-
TTT 1
TTT 2
TTT 3
100
112
–
19.9
1.3
–
226
197
172
4.5
2.5
1.2
Ti =transition temperature from hexagonal columnar (Colh) to isotropic
liquid phase; data taken from second heating curve (10 Kminꢀ1).
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crystalline phase. The inter- and intracolumnar spacings are
smaller for TTT 1 due to less bulky phenyl substituents in com-
parison to TTT 2. The naphthalene units hinder an approaching
of TTT 2 molecules, leading to a slightly larger p-stacking dis-
tance in the liquid-crystalline phase.
Upon cooling back the compounds to their crystalline
phase, the organization significantly changed as evident from
a high number of new and distinct reflections indicating
a highly ordered phase (Figure 4a and b). Compound TTT 1 re-
Figure 5. a) Schematic illustration of the dimer intracolumnar packing of
TTT 1 and Cerius2 simulation for a TTT 1 dimer with b) face-to-face packing
of nontilted discs; c) nontilting and 608 rotation between discs; and d) 608
rotation and tilting of 78/ꢀ78 (meridional reflection is indicated by dashed
circle).
the patterns (Figure 5b–d). Thereby, the exact tilting direction
is unclear. The simulated patterns confirm that such small mo-
lecular tilting angles still lead to pronounced meridional p-
stacking reflections, which are typically an evidence for nontilt-
ing. Additionally to the dimer and pentamer organization, off-
meridional scattering intensities observed in the small-angle
range suggest an even more complex intracolumnar packing
of the disc-like molecules (Figure 4c, d). The 011 peaks are lo-
cated on a scattering line, which corresponds to a spacing of
6.60 nm for TTT 1 and 5.40 nm for TTT 2 of molecules in identi-
cal positional order within the columnar stack. This suggests
that not only the molecules are rotated toward each other, but
that additionally the whole dimers and pentamers are twisted
by 12 and 88, respectively, toward each other (schematic illus-
tration for two dimers is given in Figure 5a). Therefore, 20 mol-
ecules are necessary for the helical pitch for TTT 1 and 15 for
TTT 2. From the reflections on the equatorial plane in the pat-
tern, the unit cells were identified as rectangular with a=2.40
and b=4.40 nm (Figure S7 in the Supporting Information) for
TTT 1 and a=2.80 and b=5.10 nm for TTT 2.
The extension of the phenyl (in TTT 1) to biphenyl (in TTT 3)
substituents inhibits the formation of a liquid-crystalline phase
and results only in a crystalline state or isotropic melt at high
temperatures of TTT 3 as already evident from the DSC data.
In contrast to TTT 1 and TTT 2, which were extruded in their
soft liquid-crystalline phase resulting in pronounced macro-
scopic alignment of the columnar stacks in the sample, the
high crystallinity of TTT 3 prevented a sufficient orientation of
domains for the measurement. Therefore, the pattern of TTT 3
displays a large number of isotropic rings, which are character-
istic for the crystalline nature of the compound (Figure 6), but
low macroscopic orientation of the structure. Based on these
results, it is only possible to determine a tilted arrangement of
the discs toward the columnar axis being typical for crystalline
phases of disc-shaped molecules. The wide-angle meridional
Figure 4. 2DWAXS and 2DSAXS patterns of a),c) TTT 1 and b),d) TTT 2 ob-
tained for the crystalline phase. Off-meridional reflections related to a long-
range helical organization are indicated by dashed circles in the 2DSAXS
patterns.
vealed a higher crystallinity, which is in agreement with signifi-
cantly larger enthalpy values of the phase transitions due to
the smaller steric influence of the phenyl substituents. The p-
stacking distance of the discs decreased to 0.33 nm for TTT 1,
which is quite small in comparison to other discotics,[4] while
this intracolumnar value remains unchanged for TTT 2 for
both phases. Additional meridional reflections in the middle
range correspond to spacings of 0.66 nm for TTT 1 and
1.80 nm for TTT 2 and might be an evidence for dimer and
pentamer (1.80/0.36 nm=5 molecules), respectively, formation
in the columnar stack. In the case of the dimer, the TTT 1 mole-
cules could be rotated by 608 towards each other leading to
staggered packing, as has been already observed for C3 sym-
metric discotics.[25] For TTT 2 in the pentamer arrangement, it
is assumed that the intermolecular rotation angle decreases to
248. However, in both cases, the corresponding reflections
emerge exactly on the meridional plane of the pattern instead
on the typical off-meridional range. Cerius2 simulations[26] sug-
gest that the discs might perform a slight alternating tilting of
around 78 and ꢀ78 toward the columnar axis resulting in the
appearance of the reflections exactly on the meridional axis of
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Conclusion
Discotic fluorophores based on the tristriazolotriazine core sub-
stituted with dialkoxyaryl groups have been prepared. The
thermotropic properties and the molecular packing of these
propeller-shaped compounds are controlled by the size of the
aromatic substituents. Depending on the substituents, the
compounds showed either a purely crystalline phase or highly
complex helical superstructure, and a liquid-crystalline phase
at higher temperatures. Compounds TTT 1 and TTT 2 formed
liquid-crystalline phases with a hexagonal columnar structure.
2D-WAXS and -SAXS analyses of their crystalline phases imply
the formation of dimers (TTT 1) and pentamers (TTT 2) with
a helical arrangement including 20 or 15 molecules, respective-
ly. The change in helical organization corresponds to a variation
in molecular arrangement and in orbital overlap that is crucial
for the charge-carrier hopping between discs. These com-
pounds serve as model systems for better understanding of
the design/structure relation. In future, these finding can be
transferred for the optimization of discotic semiconductors
with a high device performance.
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Keywords: fluorescence
· helical superstructures · liquid
crystals · mesophases · X-ray scattering
Chem. Eur. J. 2014, 20, 5000 – 5006
www.chemeurj.org
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Received: January 5, 2014
Published online on March 12, 2014
Chem. Eur. J. 2014, 20, 5000 – 5006
www.chemeurj.org
5006
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim