Functional star-shaped tris(triazolyl)triazines: Columnar liquid crystal, fluorescent, solvatofluorochromic and electrochemical properties

Article abstract of DOI:10.1039/c2jm15648b

Columnar liquid crystals with a C₃ star-shaped heteroaromatic nitrogen-rich core, i.e. 2,4,6-tris(1′,2′,3′-triazol-4′- yl)-1,3,5-triazine (TTT), that show luminescence and solvatofluorochromism, and are also electron-acceptors are described. Th

Full text of DOI:10.1039/c2jm15648b

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PAPER
Functional star-shaped tris(triazolyl)triazines: columnar liquid crystal,
fluorescent, solvatofluorochromic and electrochemical properties†
a
Eduardo Beltran, Jose Luis Serrano, Teresa Sierra and Raquel Gimenez
ab
a
a
*
ꢀ
ꢀ
ꢀ
Received 3rd November 2011, Accepted 8th February 2012
DOI: 10.1039/c2jm15648b
Columnar liquid crystals with a C3 star-shaped heteroaromatic nitrogen-rich core, i.e. 2,4,6-tris(10,20,30-
triazol-40-yl)-1,3,5-triazine (TTT), that show luminescence and solvatofluorochromism, and are also
electron-acceptors are described. The core has been extended with polyalkoxybenzoyloxyphenyl groups
situated at the 1 position of the 1,2,3-triazole rings to obtain molecules with three, six or nine n-
decyloxy chains (series TB) or three, six or nine (S)-3,7-dimethyloctyloxy chains (series TB*). The
preparation of the target compounds involved a copper-catalysed alkyne–azide cycloaddition
(CuAAC) ‘‘click chemistry’’ procedure with a 1,3,5-triazine precursor and aromatic azides and this
proved to be a versatile way to functionalise the periphery of the heteroaromatic core. Comparison of
these compounds and those bearing polyalkoxyphenyl substituents (series T) has led to a deeper
understanding of the self-assembly of the compounds with the TTT core. It was found that they self-
organise into columnar mesophases, in which two molecules with a low-symmetry conformation
occupy on average a columnar stratum. These molecules tend to adopt a polar conformation instead of
the apolar C3 conformation and arrange in an antiparallel assembly. For some compounds the
molecules in this arrangement have enough flexibility to tilt upon lowering temperature or by applying
an electric field. Interestingly, the hexagonal columnar arrangement is preserved in a glassy state at
room temperature. Furthermore, the mesophases show luminescence in the blue region of the visible
spectrum depending on the peripheral substitution and molecular structure. We also report a novel
property of TTT compounds (series T, TB and TB*) as they are able to act as solvatofluorochromic
probes, a property that allowed us to estimate the overall polarity of the liquid crystalline medium. An
aromatic azide precursor was also found to be mesomorphic and exhibited a monolayer SmA
mesophase.
In this regard, it is of interest to investigate novel molecular
Introduction
structures, with different electronic and optical properties, that can
form columnar mesomorphism. Significant effort has been focused
on electron-rich cores for which high hole mobilities (‘‘p-type
semiconduction’’, that is, holes are more easily injected than elec-
trons from the electrodes) comparable to amorphous silicon have
been achieved.9,10 These structures mainly comprise triphenylenes,
phthalocyanines and hexaperihexabenzocoronenes. However, very
few of these have been reported to show the electron mobility
(‘‘n-type semiconduction’’, that is, electrons are more easily injected
than holes) necessary for the preparation of charge-balanced
devices.9,11,12 Azaaromatic cores are candidates for columnar liquid
crystals with high electron affinity13–22 as the replacement of a CH
group by nitrogen atoms in the aromatic core of the mesogen is one
strategy to obtain low LUMO levels and to bring about n-type
semiconducting properties.23,24
Columnar liquid crystals are a class of soft matter in which
molecular units self-organise to form arrays of columnar stacks.1,2
Organic p-conjugated molecules with columnar mesophases are
the subject of intense research as in each individual column the
interior can include co-facially stacked aromatic cores surrounded
by hydrocarbon chains. This configuration can yield materials with
efficient charge or energy transport in organic electronic devices
such as organic field-effect transistors (OFETs), photovoltaic solar
cells and electroluminescent devices.3–8
aDepartamento de Quꢀımica Orgaꢀnica, Facultad de Ciencias—Instituto de
Ciencia de Materiales de Aragoꢀn (ICMA), CSIC—Universidad de
Zaragoza, 50009 Zaragoza, Spain. E-mail: rgimenez@unizar.es; Fax:
+34 976761209; Tel: +34 976762277
b
Instituto de Nanociencia de Aragoꢀn (INA), Universidad de Zaragoza,
50009 Zaragoza, Spain
Motivated by the design of novel multifunctional materials
based on columnar liquid crystals we have focused on star-sha-
ped discotics25–35 due to their appealing structure and proper-
ties,36 based on the 1,3,5-triazine core. The 1,3,5-triazine ring is
† Electronic supplementary information (ESI) available: Detailed
experimental procedures and characterisation data, TGA data,
calculations from XRD data, POM studies, CD spectra, emission
spectra for TB* series and CV plots. See DOI: 10.1039/c2jm15648b
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an electron-accepting unit that is able to form columnar meso-
phases with appropriate substitution at the 2,4,6-posi-
tions.16,22,37–44 We recently reported the synthesis of the
unprecedented heteroaromatic nitrogen-rich 2,4,6-tris(10,20,30-
triazol-40-yl)-1,3,5-triazine (TTT) core.45 The synthetic procedure
included a one-pot deprotection–cycloaddition reaction, one of
the few examples in which the copper-catalysed alkyne–azide
cycloaddition (CuAAC) protocol has been applied to columnar
liquid crystals,46,47 and this was the first use of triazine and
aromatic azide precursors.45 This method allowed us to obtain
multifunctional structures that exhibit very stable liquid crys-
talline columnar phases and luminescence from systems with
high electron affinity. We report here the synthesis and the full
characterisation (liquid crystalline, redox and optical properties
in solution and in the liquid crystal phase) of novel compounds
based on the nitrogen-rich TTT core. The versatility of the
synthetic procedure has allowed us to extend the core with pol-
yalkoxybenzoyloxyphenyl groups situated at the 1 position of the
1,2,3-triazole rings to obtain star-shaped molecules with three,
six or nine n-decyloxy chains (series TB) or three, six or nine
(S)-3,7-dimethyloctyloxy chains (series TB*) (Scheme 1). The TB
and TB* series both have longer side arms (a more marked star-
shape) than series T45 (Scheme 1). These compounds show
a marked tendency to form stable hexagonal columnar meso-
phases. The self-assembly of both, the T and TB series, is dis-
cussed here. It was found that in most cases two molecules self-
organise into a columnar stratum and that these molecules tend
to adopt a polar conformation, instead of the intuitively expected
apolar C3 conformation, and arrange in an antiparallel assembly.
We also report
a novel property of tristriazolyltriazine
compounds in that they act as a solvatofluorochromic probe,
which allowed us to estimate in a qualitative way the overall
polarity of the liquid crystalline media.
Results and discussion
Synthesis
Compounds from series TB and TB* were synthesised by
a copper-catalysed alkyne–azide cycloaddition (CuAAC) ‘‘click
reaction’’ in which the trimethylsilyl-protected trialkyne 2,4,6-
tris[(trimethylsilyl)ethynyl]-1,3,5-triazine reacted with the
aromatic azides in ‘‘one-pot’’ procedure. In the reaction the
alkyne groups are triply deprotected and made to react in situ
with the aromatic azides. The reactions were carried out at room
temperature and gave the desired products in moderate yields
(28–51%) (Scheme 1). The 1H NMR spectra of the final
compounds display one singlet for the proton of the triazole rings
at 9.2–9.3 ppm, which is consistent with the formation of 1,4-
disubstituted-1,2,3-triazole rings. The additional chemical char-
acterisation is available in the ESI†. The synthetic procedure,
which was designed for the preparation of series T,45 proved to be
fully applicable to aromatic azides containing an ester group.
Aromatic azides were obtained by esterification with
DCC/DMAP of polyalkoxylated benzoic acids with 4-azido-
phenol, which was previously prepared by diazotization of 4-
aminophenol followed by reaction with sodium azide.48 4-Azi-
dophenol was successfully esterified under these conditions with
yields in the range from 77% to almost quantitative. All char-
acterisation data for the new aromatic azide precursors are
available in the ESI†.
Thermal properties. Liquid crystalline properties
Series TB and TB* were studied by differential scanning calo-
rimetry (DSC), thermogravimetric analysis (TGA) and polarised
optical microscopy (POM). The liquid crystalline properties were
confirmed by X-ray diffraction (XRD) (Table 1).
Series TB and TB* display hexagonal columnar mesophases
over wide temperature ranges when they bear six (TB6 and
TB6*) or nine (TB9 and TB9*) terminal chains. TB6 and TB6*
show in their first heating cycle a peak at 162 ꢀC and 155 ꢀC
which corresponds to a partial melting process. For all
compounds the DSC curves were reproducible after the first
heating and they show only one peak corresponding to the
clearing point and a glass transition, which freezes the meso-
morphic order at room temperature (Fig. 1). The exception to
this behaviour is TB9*, for which fluidity remains at room
temperature and a glass transition was not observed upon
ꢀ
cooling down to ꢁ20 C (Table 1). It is remarkable that these
compounds exhibit very similar transition temperatures regard-
less of the different number of terminal chains (TB6 vs. TB9) or
whether the structure of the terminal chain is linear or branched
(TB6 vs. TB6*).
Scheme 1 Molecular structures of series TB, TB* and T.45 O(S)Cit ¼
(S)-3,7-dimethyloctyloxy. Synthetic procedure for the preparation of TB
and TB*. (a) (i) NaNO2 (aq), HCl, HOAc, H2O and (ii) NaN3 (aq); (b)
polyalkoxybenzoic acids, DCC, DMAP, dichloromethane. (c) Sodium
ascorbate, CuSO4$5H2O, TBAF 1 M in THF, THF/H2O.
Compound TB6 exhibits a fern-like texture by POM on
cooling from the isotropic liquid and this becomes glassy at
64 ꢀC, with the texture maintained but a remarkable decrease in
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Table 1 Phase transitions and XRD parameters for series TB and TB*
Phase transition Ta/ꢀC (DH/kJ molꢁ1
)
Mesophase Tb/ꢀC
hk
Parameterse
ꢁ
dmeas/A
ꢁ
dcalc/A
Compound
TB3
TB6
Cr 218c Idec
I 146 (0.8) Colh 64 g
g 74 Colh 151 (0.7) I
—
—
I 149 (2.5) Colh 58 g
g 71 Colh 151 (2.2) I
—
—
g(Colh), 20 ꢀ
—
—
36.9
4.6 (br)
37.4
4.6 (br)
35.2
20.0
4.6 (br)
36.0
—
10
—
10
—
10
11
—
10
—
—
10
20
—
10
11
21
—
—
36.9
—
37.4
—
35.2
20.3
—
36.0
—
—
33.6
16.8
—
32.5
18.8
12.3
—
—
a ¼ 42.6 A
ꢁ
C
2
2
2
ꢁ
S ¼ 1572.2 A
Colh, 120 ꢀ
—
C
a ¼ 43.2 A
ꢁ
ꢁ
S ¼ 1615.7 A
g(Colh), 20 ꢀ
C
a ¼ 40.3 A
ꢁ
TB9
ꢁ
—
—
S ¼ 1407.3 A
—
a ¼ 41.6 A
Colh, 130 ꢀ
C
ꢁ
—
—
2
2
2
ꢁ
—
—
4.6 (br)
—
33.6
S ¼ 1496.5 A
TB3*
TB6*
Cr 212c Idec
—
a ¼ 39.2 A
g(Colh), 20 ꢀCd
ꢁ
I 145 (1.2) Colh 57 g
g 66 Colh 149 (1.3) I
—
I 115 (2.8) Colh
Colh 120 (3.0) I
ꢁ
—
—
17.2
S ¼ 1329.1 A
4.6 (br)
32.5
18.8
12.4
4.6 (br)
—
a ¼ 37.7 A
Colh, 20 ꢀC
ꢁ
TB9*
ꢁ
—
—
—
S ¼ 1229.9 A
—
—
—
—
a
DSC data for the first cooling and second heating cycles. Onset temperatures: Cr ¼ crystal, Colh ¼ hexagonal columnar mesophase, g or g(Colh) ¼
b
c
glassy hexagonal columnar phase, I ¼ isotropic liquid. Phase and temperature of the XRD experiment. DSC data for the first heating cycle,
d
e
dec ¼ decomposition. Sample heated at 153 ꢀC for 5 minutes and rapidly cooled down to room temperature. a ¼ (2/O3) ((10) + (11)O3 + (20)O4
+ (21)O7 + .)/(nreflections).
Fig. 1 DSC thermogram of compound TB6.
the optical birefringence observed for the light propagating
perpendicular to the columns (Fig. 2a and b). A similar effect has
been observed for compound TB6*, which shows a fan-shaped
and spherulitic texture (Fig. 2c and d). The DSC traces only show
a Tg-like transition on cooling the mesophase. However, it is
evident that the decrease in the optical birefringence is not only
related to the freezing of the side chains, but indicates a more
profound reorganization of the molecules. A similar phenom-
enon has been described for transitions from a hexagonal
columnar phase to a cone-like polar columnar phase in self-
assembled bent-core polycatenars due to deformation of flexible
flat-disc superstructures into cones.49,50
Fig. 2 Microphotographs of the textures observed by POM for TB6 (a)
140 ꢀC, (b) 45 ꢀC at the same area as (a); for TB6* (c) 135 ꢀC, (d) 43 ꢀC at
the same area as (c), (e) ITO planar alignment cell without applied field at
156 ꢀC, (f) ITO planar alignment cell with applied field, 19 V mmꢁ1 at 156
ꢀC at the same area as (e); and for TB9 (g) 150 ꢀC (h) 137 ꢀC.
A decrease in the optical birefringence with a change to
a homeotropic texture was also observed on applying an electric
field near the isotropic liquid (Fig. 2e and f). Although the polar
columnar phases can be switched by applying electric fields,49,51
a spontaneous polarization signal was not observed, probably
due to sample instability. We were therefore unable to confirm
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that a change from a columnar to a polar columnar phase takes
place on applying a voltage, but it is interesting to note that the
compound is E-field responsive, as this property has recently
been described as an efficient way to prepare macroscopically
oriented uniaxial materials.52 By comparison, the application of
an electric field under similar experimental conditions to
compound T6 induced a texture with fewer and smaller areas of
homeotropic orientation than for TB6, indicating that ester
groups in the structure of TB6 are beneficial to the reorientation
process.
had one sharp reflection in the low-angle region and a broad
diffuse halo in the high-angle region, which is characteristic of
ꢁ
the liquid-like order of the hydrocarbon chains at 4.6 A (Fig. 3).
This pattern, together with texture observed by POM, led us to
propose that the low-angle halo corresponds to the (1 0) reflec-
tion of a hexagonal columnar mesophase,53–55 giving cell
ꢁ
ꢁ
parameters of 43.2 A for TB6 and 41.6 A for TB9. The absence of
other reflections at low diffraction angles apart from the (1 0)
reflection is common in diffraction patterns of hexagonal
columnar mesophases41,53–56 and it is due to a minimum in the
form factor, which precludes the observation of peaks in this
region. In the case of TB9, this situation was further confirmed
by the XRD experiment at room temperature, in which two
reflections in the low-angle region in a (1 : 1/O3) ratio were
observed. These reflections correspond to the (1 0) and (1 1)
reflections of a hexagonal columnar organisation in the glass.
For compound TB6* two reflections were observed in the
small angle region in a (1 : 1/O4) ratio and the typical halo cor-
responding to the liquid-like arrangement of the aliphatic chains
Compound TB9 shows a spherulitic texture with homeotropic
domains (Fig. 2g and h). Compound TB9*, bearing nine chiral
terminal chains, exhibits a hexagonal columnar mesophase with
the lowest clearing point of the series. The mesophase of TB9*
remains stable at room temperature and appears with a homeo-
tropic texture in both the heating and cooling processes.
On the other hand, compounds TB3 and TB3*, bearing only
ꢀ
three terminal chains, are crystalline solids that melt at 218 C
and 212 ꢀC, respectively. The melting transition is not repro-
ducible because, at such high temperatures, it is associated with
a decomposition process that is also evidenced in DSC.
ꢁ
was evident at 4.6 A. Despite the absence of the (1 1) reflection,
and also based on POM studies22,57 and miscibility studies (ESI†,
Fig. S3), we assume that the first reflection observed is the (1 0)
reflection of a 2D-hexagonal lattice with a cell parameter of
A direct comparison between series TB and series T, which was
reported previously,45 led us to conclude that TTT derivatives
show a marked tendency to self-assemble into columns in spite of
the different sizes of the molecule, which is elongated with
alkoxyphenyl groups or alkoxybenzoyloxyphenyl groups. One
difference between the two series is that compound T3, with only
three decyloxy tails and shorter star branches, shows a mono-
tropic columnar mesophase whereas compounds TB3 and TB3*
do not exhibit mesomorphism. Another difference between the
two families concerns the enthalpy and transition temperature
values, which are higher for series T than for series TB, indicating
that the more rigid and smaller core leads to stabilization of the
columnar arrangement. Nevertheless, TB compounds display
higher glass transition temperatures than those in series T,
a finding of particular interest for the applicability of the
compounds as these vitrification processes involve freezing of the
columnar order.
ꢁ
39.2 A. In the case of compound TB9*, three reflections were
observed in the low-angle region in a (1 : 1/O3 : 1/O7) ratio and
these allowed us to assign the 2D-hexagonal symmetry to the
ꢁ
mesophase with a cell parameter of 38.3 A. As expected, the cell
parameters of the TB compounds are larger than the parameters
obtained for TB* compounds and this is a consequence of the
shorter length of the 3,7-dimethyloctyloxy chains in comparison
with the decyloxy tails.58
Mesophases for compounds in series T were previously
described as hexagonal columnar.45 Compounds with six and
nine decyloxy tails (T6 and T9, respectively) showed a regular
intracolumnar distance along the columns, as revealed by the
ꢁ
reflection corresponding to c ¼ 3.5 A. On considering the XRD
parameters and the molecular volume, and assuming a density of
Azide precursors were not liquid crystalline except for A1,
which bears one decyloxy terminal chain. Upon cooling the
ꢀ
isotropic melt, droplet growth was observed by POM at 70 C.
This observation is typical of nematic mesophases but the
temperature interval is very narrow and the texture quickly
evolved to a myelinic texture with homeotropic areas of a smectic
A mesophase (ESI†, Fig. S1). XRD studies allowed us to
ꢁ
measure a layer spacing of 28.2 A for the smectic A mesophase,
which is in accordance with the molecular length. It seems
reasonable that the rod-shape and the axial dipolar moment of
molecule A1 favor mesomorphism. In contrast, the analogous
compound A1* is not liquid crystalline, which means that the
presence of the shorter, branched chiral chain is sufficient to
prevent mesomorphic behaviour.
Self-organisation in the mesophase
XRD experiments allowed us to confirm the liquid crystalline
properties of the compounds (Table 1). Compounds TB6 and
TB9, with six and nine decyloxy tails, respectively, both gave
a similar diffraction pattern at high temperatures. Each pattern
Fig. 3 XRD patterns of compounds TB6, TB9, TB6* and TB9*.
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around 1 g cmꢁ3, which is typical for liquid crystals, we estimated
that the number of molecules per columnar stratum (Z) was two
for T6 and one for T9.45 In the case of series TB and TB* we did
not find evidence for regular stacking along the columns (c) in the
XRD experiments, but a similar estimation of the number of
molecules per columnar stratum yields a value of Z ¼ 2 for mean
It is interesting to note that, whereas the C3 conformation is
non-polar, lowering of the symmetry by rotation along the
triazine–triazole bond induces polarity that can yield an anti-
parallel dipolar arrangement with a neighbour occupying the
same slice. This leads to a model for the hexagonal columnar
mesophase with two molecules per unit cell. For example, the
estimated column diameter for two molecules of TB6 in this
ꢁ
stacking distances of 3.7–6.2 A (ESI†, Table S1). It is reasonable
ꢁ
to believe that mean stacking distances should be higher than
ꢁ
3.5 A in the TB and TB* series, and increase on increasing the
conformation without considering any overlap is 55 A assuming
an all-trans conformation for the terminal chains.64 The differ-
ꢁ
ence with the experimental value (a parameter) of about 10 A
number of lateral chains as the measured a parameter decreases
but the molecular volume increases and there are greater spatial
requirements of the polyalkoxybenzoate terminal groups.
Taking into account the data discussed above, we can consider
now a model for the packing of two molecules in each column
stratum of the hexagonal columnar mesophase, which applies to
compounds TB6, TB9, TB6*, and TB9* as well as to T3 and T6.
In a columnar mesophase the molecules should be able to adopt
a conformation that allows reasonably ordered stacking into
columns. The possibility of considering a C3-symmetric confor-
mation is not plausible as we would expect a much higher cell
parameter for the compounds with Z ¼ 2 than that experimen-
tally measured by XRD, even considering folding of the chains
surrounding the core. Given that there is free rotation along the
triazine–triazole bond, we propose that these molecules are in
a low symmetry conformation that yields an approximately ‘half-
disc shape’ (Fig. 4). Bent conformations of mesogens have also
been proposed for flexible star-shaped columnar liquid crystals
with a benzene central core and polyester side arms.59–63
could be explained by the proportion of gauche conformation in
the terminal chains in order to surround the core and fill the free
volume efficiently. Then, in the simplest model each TB6 mole-
cule in the non-C3 conformation would fill a half-disc that
interacts with another molecule to form a disc-shaped dimer
(Fig. 4), a situation consistent with the value Z ¼ 2 and the cell
parameters obtained by X-ray diffraction. This could be an
oversimplified model for the TB and TB* series because we do
not observe at the XRD patterns the intracolumnar order. In
fact, aggregation of antiparallel molecules could take place in
such a way that the molecules are not in the same plane without
the need to occupy a discrete disc, but formed by segregation of
the nitrogen-rich aromatic rings and benzoate groups in the core
of the columns, surrounded by the aliphatic chains at the
periphery.
Optical properties
Chiroptical properties
Circular dichroism (CD) spectra were measured on thin films of
the hexagonal columnar mesophase frozen at room temperature
in a effort to prove the existence of supramolecular chirality for
compounds TB6* and TB9*, which contain six and nine (S)-3,7-
dimethyloctyloxy chiral tails, respectively. These chiral
compounds could feasibly induce supramolecular chiral organi-
sation in the mesophase.39,58,63,65–67 Only TB6* showed a rather
weak signal in the absorption region of the core (275 nm,
296 nm). The band was not observed in the isotropic liquid state,
which can be attributed to a barely preferential chiral organisa-
tion of the chromophores in the columnar mesophase induced by
the chiral peripheral tails (Fig. 5). The spectrum recorded for
TB9* was CD silent, likely due to the high congestion of chiral
peripheral tails that hampers a preferential helical handedness
for the stacking of the cores.
UV-Vis absorption and photoluminescence studies
The UV-Vis absorption and emission spectra of series TB and
TB* were measured in tetrahydrofuran (THF) and cyclohexane
in diluted solutions (10ꢁ6 to 10ꢁ7 M) to ensure that the samples
were molecularly dissolved. Spectra were also recorded on thin
films at room temperature for systems that displayed the glassy
hexagonal columnar phase in the case of the liquid crystalline
compounds. The non-mesomorphic compounds TB3 and TB3*
were measured as thin films prepared by drop-casting from
dichloromethane solutions (Table 2).
Compounds absorb in the UV region with maxima located
below 300 nm. These bands are attributed to p–p* transitions
due to the high absorption coefficients. All derivatives are
Fig. 4 Schematic drawing for the idealised arrangement of TB6 mole-
cules to give a hexagonal columnar mesophase formed by disc-shaped
dimers. This model also applies for TB9, TB6*, TB9*, T3 and T6.
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Significant differences are observed in the fluorescence
quantum yields, with much lower values obtained for series TB
(F < 0.06) than for series T (F ¼ 0.18–0.43).45 This may be due to
the free bond rotation of the benzoate group, which leads to
extended non-radiative pathways competing for the relaxation of
the excited state.69
In the film state all of the compounds are luminescent, with
lower emission maxima for series TB than for series T, which
show a blue-green luminescence in the mesophase. There is no
difference in the emission maxima wavelengths between TB6 and
TB9 and their corresponding chiral analogues TB6* and TB9*,
respectively. This is consistent with a structural organisation of
the fluorophores in the columnar phase similar for both pairs of
compounds, chiral and achiral. This would be in agreement with
the weak CD signal observed for TB6* and the CD silent spec-
trum recorded for TB9*, which indicate the weakness and
absence, respectively, of a high order degree in their respective
columnar organisations related to a helical disposition of the
molecules.
Fig. 5 CD (black) and absorption spectra (blue) of TB6* recorded in
a thin film at room temperature.
luminescent in solution in the near UV-blue region of the spec-
trum. Series TB and TB* show similar absorption and emission
spectra, indicating that the peripheral alkoxy tails, being linear or
branched, do not have any influence on the optical properties.
But series TB and TB* show emission behaviour different from
series T due to the benzoyloxy-extended side arms (Table 3).
In series TB a slight blue-shift in the emission maximum is
observed on increasing the number of alkoxy chains from three
(TB3) to six (TB6) and nine (TB9) in THF solutions, but
a marked red-shift occurs in cyclohexane solutions, with the
emission maximum reaching the visible region of the spectrum.
This means that in cyclohexane the excited state decreases in
polarity and in charge transfer character on increasing the
number of peripheral alkoxy chains.
Solvatofluorochromism
TTT derivatives consist of organic molecular structures in which
electron-donating groups (alkoxy terminal groups) are linked
through a conjugated system to an electron-accepting fragment
(triazine ring). This kind of system, where intramolecular charge
transfer can occur, is susceptible to showing solvatochromic
properties.16,68,70–72 A measurable and different solvatochromic
emission effect, also called solvatofluorochromism,71 was
observed when the optical properties were studied in cyclohexane
(apolar solvent) and THF (more polar solvent) for the series TB
and TB* (Fig. 6, left and Table 2) and series T (Fig. 6, right and
Table 3).
The compounds in series T show an emission peak centered at
416 nm in cyclohexane solutions regardless of the number of
decyloxy chains. However, a red-shift of the emission maximum
is observed in THF solutions on changing from three (T3) to six
(T6) and nine (T9) long tails. This finding is consistent with
a strong charge transfer character and/or a conformational
relaxation in the excited state, which increases in polarity on
increasing the number of alkoxy chains.
For TB and TB* the emission wavelength is lower in THF
than in cyclohexane and blue-shifts were observed for TB6
(ꢁ57 nm) and TB9 (ꢁ83 nm) on increasing the polarity of the
solvent (negative solvatochromism). In this case the ground
state is more polar than the excited state. In contrast, the
Table 2 UV-Vis absorption and emission data
Compound
labsTHF/nm (log 3)
labschex/nm (log 3)
lemTHF/nma
lemchex/nma
lemfilm/nma
Fb
c
c
TB3
TB6
TB9
TB3*
TB6*
TB9*
281 (5.22)
—
371
358
358
370
359
356
—
409
425
427
410
425
427
—
280 (4.96), 294 (4.96)
287 (5.23)
281 (4.82)
280 (4.78), 294 (4.76)
287 (5.20)
274 (4.87), 295 (4.84)
284 (4.91)
—
415
441
0.02
0.01
—
0.06
0.03
c
c
—
274 (4.97), 295 (4.98)
284 (4.85)
416
441
a
b
Excitation wavelength at the absorption maximum. Quantum yields in cyclohexane relative to 9,10-diphenylanthracene (F ¼ 0.9 in cyclohexane
c
solution).68 Low solubility.
Table 3 UV-Vis absorption and emission data
Compound
labsTHF/nm45
labschex/nm
lemTHF/nm45
lemchex/nm
lemfilm/nm45
T3
T6
T9
257, 289, 299sh
264, 304
266, 301
249, 284, 300sh
262, 302
265, 300sh
416
448
471
416
416
416
416
450
481
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Fig. 6 Emission spectra for series TB (left) and T (right) in THF (solid lines) and cyclohexane (dashed lines). TB3 in cyclohexane is not plotted due to
low solubility.
opposite effect was found for series T, where a marked red-shift
in emission maxima for T6 (+32 nm) and T9 (+55 nm) was
observed on increasing the polarity of the solvent (positive
solvatochromism). This means that the excited state is more
polar than the ground state and the system will be better
stabilized by polar solvents.
One limitation of solvatofluorochromism is that it only gives
an indication of the overall polarity of the probe environment.
As a result, although we can infer that TB molecules are more
dipolar than T molecules in the condensed phase and that the
overall polarity of the liquid crystal phases that consist of TB as
a macroscopic medium is higher than for series T, a direct
comparison with the symmetry and conformation of the mole-
cules in the mesophase cannot be made. However, the results are
consistent with the fact that TB6 films can be oriented under
electric fields more easily than T6.
The emission spectra of thin films at room temperature (in the
glassy hexagonal columnar phase for the liquid crystalline
compounds) for series TB are more similar to the spectra in
cyclohexane solution than those in THF, whereas for series T
the emission spectra in thin films match those in THF solutions.
Solvatochromic properties enable information to be gained
about the polarity of the media. This method has been used to
successfully characterise not only solvents but also systems such
as solid surfaces, stationary phases for chromatography, or
supramolecular media such as cyclodextrins and micelles.70 The
fact that the mesogenic compounds of series T and TB show
solvatofluorochromic properties means that they can act as
probes to characterise the polarity of the columnar hexagonal
mesophases. Comparison of the spectra obtained from solutions
and films allows us to estimate the polarity of the environment
of the fluorophores, i.e., of the intrinsic liquid crystal. This
allows us to conclude that in the mesophase the polarity of the
excited state is low and similar to the apolar cyclohexane in TB6
and TB9, and more polar and similar to THF in T6 and T9. In
other words, the TB molecules have a more polar ground state
than T molecules in the mesophase, and their dipole moment
decreases in the excited state.
Redox properties
Cyclic voltammetry (CV) studies of compounds TB/TB* were
performed in oxygen free dichloromethane solutions. The elec-
trochemical data are summarised in Table 4. Compounds did not
show any oxidation process on scanning from 0 to +2.2 V. All
compounds showed a reversible reduction wave at around
ꢁ1.65 V, relative to the Ag/AgCl reference electrode, irrespective
of the number of alkoxy chains. For comparative purposes and
LUMO calculations the values have been also referenced to the
ferrocene/ferrocenium couple (FOC), and were about ꢁ2.13 V.
This corresponds to LUMO energy levels of around ꢁ2.67 eV.
Series TB has similar electron-acceptor properties with respect
to other triazine derivatives such as 2,4,6-triphenyl-1,3,5-triazine,
which has a reduction potential of ꢁ2.09 V versus FOC and
a LUMO energy of ꢁ2.71 eV.73 In comparison with the previ-
ously reported series T,45 for which a first reduction potential of
Table 4 Electrochemical behaviour
red
(E1/2 vs.
Ag/AgCl)/V
a
red
(E1/2 vs. FOC /V
Compound
Egb/eV
LUMOc/eV
TB3
TB6
TB9
TB3*
TB6*
TB9*
ꢁ1.65
ꢁ1.65
ꢁ1.65
ꢁ1.63
ꢁ1.64
ꢁ1.65
ꢁ2.13
ꢁ2.13
ꢁ2.13
ꢁ2.11
ꢁ2.12
ꢁ2.13
3.89
3.86
3.86
3.87
3.84
3.84
ꢁ2.67
ꢁ2.67
ꢁ2.67
ꢁ2.69
ꢁ2.68
ꢁ2.67
a
E1/2 ¼ 0.48 V vs. Ag/AgCl. b Optical bandgap calculated from THF solution absorption edges. c Calculated as the difference between ꢁ4.8 eV and E1/
red
2
vs. FOC.
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ꢁ1.28 eV or ꢁ1.33 eV versus FOC and LUMO values of ꢁ3.47 V
or ꢁ3.52 V was found, series TB showed more negative reduction
potentials, higher LUMOs and higher optical bandgaps.
Electrochemistry shows that the reduction wave is indepen-
dent of the number of alkoxy tails. The fact that we observe the
same voltammogram is due to that adding alkoxy peripheral
chains at the meta position of the external aromatic ring does
not affect significantly the reduction process. The redox
processes are ground-state properties, and are related to the
absorption spectra, which in fact is almost the same for all
compounds. The emission properties, that is, the ones not
related to the ground state but to the excited state, are the ones
that are changed with the number of peripheral chains, indi-
cating that the redox properties of the excited state would be
also different.
Finally, as a proof of principle, the CuAAC ‘click chemistry’75
procedure designed for the 1,3,5-triazine core proved to be
a versatile way to decorate the periphery of this acceptor core
with triazole functional spacers76 and aromatic donors. This
approach is useful to obtain octupolar columnar liquid crystals
with interesting optical and electronic properties.
Acknowledgements
We thank the MICINN (Spain) and FEDER (European Union)
under the project MAT2009-14636-CO3-01, and the Gobierno
ꢀ
de Aragon (Research group E04) for financial support. E.B.
thanks the FPI program of MICINN (Spain) for a fellowship.
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