H-bonded complexes containing 1,3,4-oxadiazole derivatives: Mesomorphic behaviour, photophysical properties and chiral photoinduction

Article abstract of DOI:10.1039/c4tc00886c

Two series of new V-shaped acids derived from 1,3,4-oxadiazoles are described. These acids were used to prepare supramolecular complexes through hydrogen bonding with 2,4-diamino-6-dodecylamino-1,3,5-triazine in a 3:1 ratio, respectively. The formation of the complexes was studied by infrared and NMR techniques. The thermal behaviour and mesomorphic properties of all the complexes were investigated by polarized light optical microscopy, differential scanning calorimetry and X-ray diffraction. Hexagonal and rectangular columnar mesophases were observed for all complexes at room temperature, without evidence of crystallization. The results of circular dichroism studies allowed us to propose that in the liquid crystalline state these materials adopt a helical columnar organization and this chirality can be controlled by irradiation with CPL. Furthermore, the complexes display strong blue luminescence in solution and in the mesophase. the Partner Organisations 2014.

Full text of DOI:10.1039/c4tc00886c

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Materials Chemistry C
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PAPER
H-bonded complexes containing 1,3,4-oxadiazole
derivatives: mesomorphic behaviour,
photophysical properties and chiral
photoinduction†
Cite this: J. Mater. Chem. C, 2014, 2,
7029
a
b
c
Andre A. Vieira, Emma Cavero, Pilar Romero, Hugo Gallardo, a Jose Luis Serranob
*
´
´
c
*
and Teresa Sierra
Two series of new V-shaped acids derived from 1,3,4-oxadiazoles are described. These acids were used to
prepare supramolecular complexes through hydrogen bonding with 2,4-diamino-6-dodecylamino-1,3,5-
triazine in a 3 : 1 ratio, respectively. The formation of the complexes was studied by infrared and NMR
techniques. The thermal behaviour and mesomorphic properties of all the complexes were investigated
by polarized light optical microscopy, differential scanning calorimetry and X-ray diffraction. Hexagonal
and rectangular columnar mesophases were observed for all complexes at room temperature, without
evidence of crystallization. The results of circular dichroism studies allowed us to propose that in the
liquid crystalline state these materials adopt a helical columnar organization and this chirality can be
controlled by irradiation with CPL. Furthermore, the complexes display strong blue luminescence in
solution and in the mesophase.
Received 30th April 2014
Accepted 25th June 2014
DOI: 10.1039/c4tc00886c
www.rsc.org/MaterialsC
expression of supramolecular chirality in functional materials.4
On one hand, the higher level of order associated with a helical
Introduction
disposition of the molecules along the column can give rise to
an improvement in the anisotropic properties, pointing to the
application of these materials as semiconductors, photo-
conductors, organic light emitting diodes and photovoltaic
cells.2f,g,5 On the other hand, supramolecular chirality can be
achieved and/or controlled by tailoring the molecular structure
(chiral information encoded in the constituent molecules),6
intermolecular interactions (i.e. hydrogen bonding, p-stacking,
etc.)7 or an external source such as light.8
We have already demonstrated the versatility of the supra-
molecular system based on 2-alkylamino-4,6-diamino-1,3,5-
triazine and V-shaped acids to obtain functional columnar
liquid crystals with well-dened columnar organization. These
structures arise due to the synergistic action of the p-stacking
tendency of the melamine-derived core and the strong lateral
interactions between V-shaped molecules. In these materials an
inherent helical architecture is proposed and this arises from
the propeller-like conformation of the complex.9 On the basis of
this helical architecture supramolecular chirality is achieved in
functional self-organized materials. Firstly, chiral information
present in the stereogenic centres of peripheral hydrocarbon
tails is transmitted to the columnar architecture and supra-
molecular optical activity in the mesophase is detected by CD
spectroscopy.10,11 Secondly, the incorporation of the 2,5-
diphenyl-1,3,4-oxadiazole moiety into the V-shaped acid
provided luminescent columnar liquid crystals with helical
Self-organized structures based on liquid crystals have great
potential for the implementation of functional so materials due
to the combination of unique features such as molecular order
and mobility.1 In particular, columnar liquid crystals offer myriad
possibilities for molecular design and a high level of structural
control to attain well dened 1D organization, with an additional
two-dimensional order of the columns.2 Furthermore, the use of
non-covalent interactions to stabilize columnar arrangements
may lead to highly consistent materials through rather simple
synthesis and purication procedures, in which the supramo-
lecular organization remains stable at room temperature appro-
priate for applications. In particular, p-stacking and hydrogen-
bonding allow the construction of well-dened columnar
arrangements in an easy, dynamic and reproducible way.3
In a further step, columnar liquid crystals provide the basis
to achieve helical supramolecular architectures that allow the
a
´
´
Departamento de Quımica, INCT-Catalise, Universidade Federal de Santa Catarina –
´
UFSC, 88040-900 Florianopolis-SC, Brazil. E-mail: hugo@qmc.ufsc.br; Tel: +55 48
37219544
b
´
´
Departamento de Quımica Organica, Facultad de Ciencias, Institute of Nanoscience of
Aragon (INA), Universidad de Zaragoza, C/Pedro Cerbuna 12, 50009-Zaragoza, Spain
c
´
´
´
Instituto de Ciencia de Materiales de Aragon, Dep. Quıımica Organica, Facultad de
Ciencias, Universidad de Zaragoza – CSIC, C/Pedro Cerbuna 12, 50009 Zaragoza,
Spain. E-mail: tsierra@unizar.es; Tel: +34 976 762276
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4tc00886c
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architectures, which in turn led to supramolecular activity when tetrazoleheterocycles 2 by Huisgen13 1,3-dipolar cycloaddition
chiral tails were present in the V-shaped acid.10 Additionally, using sodium azide and ammonium chloride in DMF. The
photoresponsive features established by the ability of the azo- tetrazole compounds 2 were reacted with freshly prepared
benzene-derived systems to follow the helicity of circular methyl 4-(chlorocarbonyl)benzoate in pyridine to afford the 3,5-
polarized light11 have been veried in these materials.
disubstituted 1,3,4-oxaziazole14 4 in good yields (68–93%). The
As a continuation of this research, we describe here the carbonyl group in the para position with respect to the tetrazole
synthesis and characterization of novel luminescent V-shaped ring probably inhibits the progress of this reaction. The acid
acids derived from 2,5-diphenyl-1,3,4-oxadiazole, and their intermediates 5 were obtained in quantitative yield by hydro-
corresponding H-bonded complexes with 2,4-diamino-6-dode- lysis of the ester function of compound 4 with potassium
cylamino-1,3,5-triazine in a 3 : 1 ratio, respectively. Two series hydroxide in ethanol/water. Compounds 7 were obtained by
of complexes were prepared and they differ in the structure of diesterication of triisopropylsilyl 3,5-dihydroxybenzoate (6)
the V-shaped acid, namely a symmetrical (containing oxadi- with the respective acid 3 using N,N0-dicyclohexylcarbodiimide
azole groups in both arms) and an unsymmetrical one (bearing (DCC) and 4-(N,N-dimethylamino)pyridinium 4-toluenesulfo-
an oxadiazole group (X) in one arm and an azobenzene group nate (DPTS) in dichloromethane (Scheme 2).
(A) on the other). As in previous studies, these complexes have a
The desired nal acids 8 (XX) were obtained in good yields
non-discotic structure that favours columnar mesomorphism (74–97%) by removal of the triisopropylsilyl group with tetra-n-
with helical superstructures. Furthermore, the presence of butylammonium uoride in dichloromethane (see ESI†). The
oxadiazole promotes luminescent properties in the building acids 11 (XA) were obtained by the esterication of the corre-
blocks and this is transmitted to the bulk. Chirality in the sponding phenol 9 bearing the azo group, which was reported
peripheral tails leads to helical superstructures that are biased previously,11a with acid 5 and subsequent removal of the trii-
towards optically active materials by the transfer of molecular sopropylsilyl group.
chirality to the supramolecular organization. The presence of
The structures and purities of compounds XX and XA were
1
azobenzene groups allows the control of chirality using circu- fully characterized and veried by H and 13C NMR spectros-
larly polarized light as an external chiral stimulus. The combi- copy, elemental analysis, and MALDI-TOF mass spectrometry
nation of these properties should lead to multifunctional self- (see ESI†).
organized materials with dened supramolecular architectures.
Aer the preparation and characterization of the nal acids
(XX and XA) and the melamine (M), the complexes M-XX and
M-XA were prepared by dissolving both compounds in tetrahy-
drofuran (Scheme 3) in a proportion of 1 mole of melamine to 3
moles of acid. The solvent was evaporated by stirring the
mixture at 40 ꢀC, on a rotary evaporator, to prevent precipitation
of the acid.
Results and discussion
Synthesis
The melamine M, 2,4-diamino-6-dodecylamino-1,3,5-triazine,
was prepared by the reaction of dodecylamine with 2,4-diamino-
6-chloro-1,3,5-triazine using sodium hydrogen carbonate as a
base.12
The V-shaped acids X(S)10*X(S)10*, X(R)10*X(R)10*,
X12X12, X(S)10*A(S)10*, X(S)10*A12, X12A(S)10* and X12A12
were prepared following the synthesis pathway outlined in
Schemes 1 and 2.
The formation of the complexes M-XX and M-XA was studied
by IR and NMR (see ESI†). The IR spectra (Fig. S1†) showed
differences between the acids XX and XA and their complexes
M-XX and M-XA, especially in regions corresponding to the
carbonyl, O–H and N–H groups responsible for the interactions
between molecules.
All of the 1H NMR spectra (Fig. 1–3 and S3–S10†) clearly
indicate the formation of complexes, where it is assumed that
The starting materials in the synthesis route were the
O-alkylbenzonitriles 1, which were used to obtain the respective
Scheme 1 Synthesis route of the intermediate acids 5.
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Scheme 2 Synthesis of the final acids 8 and 11 (XX and XA).
there is a rapid equilibrium between the complex and its broadband) aer formation of the supramolecular complex
components.9 Large displacements were observed for the M-X(S)10*X(S)10* in CD2Cl2. The protons of the N-methylene
chemical shis of the ve NH groups of M. The downeld shis group of the alkyl chain in the triazine are slightly deshielded,
of these hydrogen atoms can be seen in Fig. 1: 4.92 and 4.81 while the proton of the central benzene ring para to the
ppm before the complexation and 6.79, 6.46 and 5–7 ppm (very carboxylic acid group is shied upeld.
Scheme 3 General preparation of the complexes M-XX and M-XA from 3 moles of acid (XX and XA) and 1 mole of triazine (M).
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615 ꢁ 18 L molꢂ1 in CD2Cl2 and 161 ꢁ 8 L molꢂ1 in THF-d8 were
calculated by nonlinear curve tting of the chemical shis.
The stoichiometry, however, became 3 : 1 in the bulk mate-
rial when the molar amount of the acid was three times that of
the melamine. The 13C CPMAS spectra of solid samples helped
to conrm the complex stoichiometry (Fig. 3). The shielding of
the carboxylic acid carbon in the solid spectrum of the complex,
in contrast to that observed in solution (Fig. 2), can be attrib-
uted to p stacking of the central aromatic core.
Thermal properties
All of the acids XX and XA and nal complexes M-XX and M-XA
were investigated by polarized optical microscopy (POM) and
differential scanning calorimetry (DSC) in order to determine
their thermal and thermodynamic properties and meso-
morphism. None of the acids used to form these complexes are
liquid crystalline (see ESI†). The investigation of the thermal
stability of these new V-shaped acids by thermogravimetric
analysis (TGA) indicated that they did not exhibit weight loss at
Fig. 1 Protons of the melamine, the methylene of the alkyl chain of
the melamine and the hydrogen of the central aromatic ring of the acid
X(S)10*X(S)10* are displaced after formation of the complex
(500 MHz, CD2Cl2, 25 ꢀC).
Likewise, variations in the chemical shis of the 13C signals
of carbon atoms involved in the formation of the supramolec-
ular structure were also clearly observed (Fig. 2). The 13C signals
of the triazine core (167.5, 166.8, 166.5 ppm) were shied to
lower frequencies by 4 or 5 ppm (162.8, 160.9, 159.9 ppm,
respectively) and the 13C signals of the carboxylic acid (Z) and of
the aromatic central ring (M, N, O, P) were shied markedly
aer complexation.
Additional experiments to study the formation of these
complexes in solution were performed. Diffusion ordered
spectroscopy (DOSY) revealed a unique diffusion coefficient for
the complexes that was different from those of the acid and
melamine (Fig. S3–S5†). Moreover, we previously reported
similar systems and showed that the stoichiometry of these
complexes in solution is 1 : 1.15 In this present work, the
continuous variation method was applied to 1H NMR experi-
ments on the complex M-X(S)10*X(S)10*. Titration experiments
in solution in CD2Cl2 and THF-d8 showed that all NH proton
signals and the N–CH2 signals of the alkyl chain of the mela-
mine M, as well as the hydrogen atoms of the central aromatic
ring of the acid X(S)10*X(S)10*, were displaced upon increasing
the proportion of acid in the solution while keeping
the melamine concentration constant. Binding constants of
ꢀ
temperatures below 290 C under a nitrogen atmosphere. The
transition temperatures and enthalpy values of the complexes
are given in Table 1.
When observed under the optical microscope, all the
complexes appear as homogenous samples, and a uniform
texture develops on cooling from the isotropic liquid in all
cases. This and the appearance of a sharp peak in the cooling
DSC scan (see ESI†), corresponding to the transition I-meso-
phase, support the correct formation of the acid–melamine
complexes. The mesophases give in all cases poorly dened
textures between crossed polarizers such as those shown in
Fig. 4 and the ESI.† Nevertheless, these textures are reminiscent
of those observed for previously described propeller-like
complexes which are structurally related. A slight difference in
textures between complexes containing acids XX and complexes
containing acids XA was detected. For the former, homeotropic
zones appeared on cooling and this indicated a uniaxial
columnar arrangement such as in the hexagonal columnar
Fig. 2 Main shifts in 13C NMR spectra after formation of the complex Fig. 3 13C-CPMAS spectra of acid X(S)10*A(S)10* (bottom), M-X(S)
M-X(S)10*A(S)10* (125 MHz, CD2Cl2, 25 ꢀC).
10*A(S)10* complex (middle) and M triazine (top).
7032 | J. Mater. Chem. C, 2014, 2, 7029–7038
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Table 1 Thermal properties and lattice parameters of the complexes M-XX and M-XA
b
˚
Complexes
Phase
Temp.a (ꢀC)
147.6
DH [kJ molꢂ1
12.8
]
Phase
Colh
Colh
Colh
Colr
Colr
Colr
Colr
Temp.a (ꢀC)
87.9
Phase
Parameter lattice (A)
M-X(S)10*X(S)10*
M-X(R)10*X(R)10*
M-X12X12
I
I
I
I
I
I
I
g
g
g
a ¼ 50.2
h ¼ 3.4
149.1
19.4
87.9
a ¼ 49.7
h ¼ 3.4
181.4
42.9
92.2
a ¼ 47.5
h ¼ 3.5
M-X(S)10*A(S)10*
M-X(S)10*A12
M-X12A(S)10*
M-X12A12
93.3
5.8
a ¼ 96.1
b ¼ 70.4
a ¼ 82.2
b ¼ 78.6
a ¼ 78.6
b ¼ 77.0
a ¼ 98.6
b ¼ 78.9
107.0
10.0
113.9
14.8
119.5
16.0
Transition temperatures were determined by DSC on the second cooling scan at 10 ꢀC minꢂ1. I ¼ isotropic liquid; Colh ¼ hexagonal columnar
a
mesophase; Colr ¼ rectangular columnar mesophase. b All X-ray diffraction experiments were carried out at room temperature.
mesophase (Fig. 4a and S11†). This feature was not observed for the presence of two 2,5-diphenyl-1,3,4-oxadiazole arms, which
the latter compound (Fig. 4b and S11†), which displayed promotes stronger intermolecular interactions.
textures similar to those observed for previously described
Branched chains derived from citronellol, in M-X(S)10*X(S)
rectangular columnar mesomorphic complexes.11 In any case, 10* and M-X(R)10*X(R)10*, led to a decrease of ca. 30 ꢀC in the
the type of two-dimensional arrangement was further clearing temperatures with respect to their achiral homologue
conrmed by X-ray diffraction (see below).
M-X12X12 with linear chains.
All of the complexes formed by the melamine derivative, M,
Likewise, a decrease in the clearing temperatures was also
and the acids XX and XA appeared as homogenous materials observed for the M-XA complexes bearing branched chains
and showed liquid crystalline behaviour, with wide mesophase (M-X(S)10*A(S)10*, M-X(S)10*A12, and M-X12A(S)10*)
temperature ranges. This nding also supports the formation of compared with their linear analogue M-X12A12, with the
H-bonded complexes since the V-shaped acids were not meso- decreases ranging from 12.5–26 ꢀC and the difference being
genic (see ESI†). According to the thermograms registered by more pronounced when both tails in the acid were derived from
DSC, the mesophase remained stable until room temperature, citronellol.
without crystallization (aer slow cooling from the isotropic
state), for complexes containing unsymmetric acids XA.
Complexes derived from symmetric acids XX show a glass
transition at around 90 ꢀC, and this signies that the meso-
Structural characterization of the mesophase
The mesophases of the complexes M-XX and M-XA were studied
phase is frozen in a glassy state. This behavior reects the
strength of the hydrogen bonding interactions between the acid
and melamine and is consistent with previously published
studies.9–11,15,16
The complexes formed with XX acids show higher clearing
temperatures than the complexes formed with XA acids, and
this is probably due to the symmetric structure of the acid, in
and their parameters were measured by X-ray diffraction at
room temperature (Tables 1 and S1†). As mentioned above,
these materials did not crystallize on slow cooling. Accordingly,
for all the complexes, X-ray experiments were carried out on
samples slowly cooled from the isotropic liquid, so that the
mesophase could develop completely.
The reections observed in the small-angle X-ray scattering
(SAXS) patterns obtained for the complexes M-XX were consis-
tent with a hexagonal columnar (Colh) mesophase (Table S1†).
The SAXS patterns of M-X(S)10*X(S)10* (Fig. 5a) and M-X(R)
10*X(R)10* showed a set of three low-angle sharp maxima with
a reciprocal spacing ratio 1 : O3 : O7 and the patterns of
M-X12X12 contained a set of two low-angle sharp maxima with a
reciprocal spacing ratio 1 : 2. With the respective absence of the
reection (20) and the reection (11),17 these maxima can be
assigned to the (10), (11), (21) and (10), (20) reections,
respectively, of a two-dimensional hexagonal lattice. In the
wide-angle X-ray scattering (WAXS) patterns, two diffuse
maxima were observed for all complexes (Fig. S13†). The inner
maximum corresponds to the conformationally disordered
Fig. 4 Polarized optical photomicrographs taken at room tempera-
ture for (a) M-X12X12 and (b) M-X12A(S)10*.
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presence of azobenzene in the V-shaped acids confers on the
complex the ability to respond to light as an external stimulus.
Indeed, the photoinduced E/Z isomerization of azobenzene is a
well-documented phenomenon, which proceeds reversibly
upon irradiation with UV and visible light.18 When this process
is driven by circularly polarized light (CPL), chirality can be
induced into liquid crystalline organization.8,19 This phenom-
enon has been described for calamitic and B-type liquid crys-
talline organization, whereas, for columnar mesophases, it is
still limited to the present type of propeller-like complexes.
The optical activity of the mesophase was studied for all of
the chiral complexes by Circular Dichroism (CD) on thin lms
(ca. 300 nm to 1 mm thickness measured by prolometry)
prepared by casting a dichloromethane solution of the corre-
sponding complex onto an amorphous quartz plate and
subsequent thermal treatment as for X-ray experiments. For
every complex, six CD spectra were recorded at six different
orientations measured by rotating the sample cells by 60ꢀ
around the light beam. The spectra were almost identical for all
orientations of the sample, and this rules out the prevalence in
the observed signals of possible linear dichroism effects due to
the macroscopic orientation.20
The study of the chiroptical properties of these materials had
two objectives. First, it was planned to determine whether the
chirality of the chiral tails is transmitted to the supramolecular
organization of the mesophase and whether this can be related
to a helical disposition of the complexes along the column.
Second, it was intended to explore the possibility of inducing
and tuning supramolecular chirality in chiral and non-chiral
mesophases by irradiation with circular polarized light (CPL) as
a result of the presence of azobenzene groups.
The complex M-X12X12 was CD silent due to the absence of
chirality in its chemical structure, whereas M-X(S)10*X(S)10*
and M-X(R)10*X(R)10*, bearing chiral alkoxylic chains, dis-
played non-zero CD spectra (Fig. 6). The appearance of a
Cotton effect corresponding to the 1,3,4-oxadiazole chromo-
phore is consistent with helical stacking in the columnar
arrangement, as previously reported for structurally similar
complexes derived from a melamine core and three V-shaped
acids.9–11 The handedness of this helical arrangement is biased
towards a given chiral sense that is determined by the
conguration of the stereogenic centre. Thus, the CD spectra
of the complexes formed by both enantiomeric acids, M-X(S)
10*X(S)10* and M-X(R)10*X(R)10*, present signals of the
opposite sign, which indicates enantiomeric helical handed-
ness for both materials. The CD spectra of complexes M-XA
(see Fig. 7 and S14†) show optical activity for complexes that
bear either one or two chiral tails in the acid. The sign of the
signal is always positive when one or two (S)10* tails are
present in the acid and negative when (R)10* is present. It was
Fig. 5 SAXS diagram of the mesophase of M-X(S)10*X(S)10* (a) and
M-X12A(S)10* (b).
aliphatic chain and is, usually observed in liquid-crystalline
˚
˚
compounds. The outer maximum observed at 3.4 A and 3.5 A,
respectively, conrms the long range correlation order of the
columnar mesophase and represents the mean stacking
distance between the molecular cores along the column
(h parameter of the Colh mesophase).
From the values of the stacking parameter (h) it was possible
to determine the number of complexes per unit cell (Z).
Considering a density close to 0.85 g cmꢂ3, which is a reason-
able value for organic materials of this type, the Z value is 1 for
all the three complexes, which means that there is one complex
per columnar stratum.9
The reections observed in the SAXS patterns obtained for all
complexes M-XA were consistent with rectangular columnar
(Colr) mesophases (Table S1†). As an example, a set of ve
maxima was observed for M-X12A(S)10* (Fig. 5b). These
maxima can be assigned to the (11), (02), (12), (32) and (62)
reections of the two-dimensional rectangular lattice. For all
the complexes, odd-values can be obtained for h + k ruling out
the C2mm symmetry, and this is consistent with a P2gg
symmetry for the rectangular lattice. WAXS patterns showed the
diffuse maximum corresponding to the disordered aliphatic
tails, but a second high angle halo was absent for all of the
materials, meaning that the correlation along the column in the
mesophase is only short range and there is no regular stacking
distance. Nevertheless, on the basis of our previous studies, and
assuming densities of around 0.85–0.9 g cmꢂ3, the stacking
distances along the column can be estimated between 3.3 and
˚
3.85 A, and this allows us to estimate the number of complexes
per unit cell (Z). The Z values obtained are 4, which indicate two
molecules per column, and this is consistent with values
obtained for similar systems described previously.9,10,11c
Chiroptical properties
It has been proposed in previous studies that this type of also observed that the presence of a single chiral tail in the V-
mesogen, consisting of a melamine derivative surrounded by shaped acids (XA) is sufficient to induce CD-active columnar
three V-shaped acids, can adopt a propeller-like conformation organization. These observations are independent of the
that yields columnar architectures with inherent helical orga- chemical nature of the rigid arm in which the tail is located,
nization. The helical sense can be controlled by the presence of i.e., either the oxadiazole (X) group or the azobenzene (A)
chiral tails in the acid partner and can be detected by circular group, as it was also seen for other complexes derived from
dichroism measurements in the mesophase.9 Moreover, the unsymmetric V-shaped acids.11a
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deduced from these experiments that there is a strong depen-
dency of the response of the complexes to CPL with the position
of the chiral tail and that the presence of the chiral tail in the
azobenzene moiety is less favorable.
In order to corroborate the possibility of not only controlling
the chirality of the systems but also of inducing optical activity
into an achiral material, complex M-X12A12 was irradiated with
CPL.
The CD spectra corresponding to different irradiation
processes are shown in Fig. 8. As expected, the untreated sample
did not show a signal due to the absence of chiral tails. Under
irradiation with le-handed CPL (l-CPL) and right-handed CPL
(r-CPL), non zero CD signals appeared, with opposite sign. The
intensity of the signals was dependent on the irradiation time.
This means that an amplication of the chirality occurs, and
this probably arises from an increase in the population of azo-
benzene chromophores that absorb light within a helical
disposition.
Fig. 6 CD spectra recorded on thin films in the mesophase (25 ꢀC) of
the complexes M-X(S)10*X(S)10* and M-X(R)10*X(R)10*.
Photophysical properties
The photophysical properties of these complexes, M-XX and
M-XA, were investigated in dilute solutions in dichloromethane
and in thin lms. The UV-vis absorption and uorescence
spectroscopy data are summarized in Table 2.
For M-XX, as shown in Fig. 9, all materials showed similar
absorption spectra between 280 and 360 nm, with an intense
absorption maximum at 318 nm.
These absorption bands are assigned to p–p* transitions of
the 1,3,4-oxadiazole heterocyclic due to their high molar
absorption coefficients (3 ꢃ 20 000 molꢂ1 cmꢂ1). All of these
compounds displayed strong blue emission in solution (lmax.
FL. 370–530 nm) with intense emission at 418 nm. These
complexes showed an excellent photoluminescence quantum
yield (fFL) in dichloromethane solution that is quite similar to
the value for standard quinine sulfate.21
All of the M-XX complexes also exhibited uorescence in thin
Fig. 7 CD spectra recorded in the mesophase (25 ꢀC) of the complex
M-X(S)10*A12.
lms. As
a consequence of their glassy behaviour, the
Experiments aimed at controlling the chirality of the meso-
phase were performed by irradiating the columnar M-XA
complexes with CPL. Thin lms of the complexes were irradi-
ated with CPL (488 nm, Ar + laser line, 20 mW cmꢂ2) at room
temperature.
For the complex with only one chiral tail in the 2,5-diphenyl-
1,3,4-oxadiazole moiety (M-X(S)10*A12), it was possible to
amplify its original chirality under right-CPL irradiation and
also to induce the opposite signal by irradiating with le-CPL
(Fig. 7).
For the complex bearing two chiral tails (M-A(S)10*X(S)10*) a
change was observed in the sign upon irradiation with orthog-
onal CPLs but an increase in the intensity of the CD signal was
not detected (Fig. S14a†). Finally, for the complex that incor-
porates only one chiral tail in the azo moiety (M-X12A(S)10*) the
CD spectrum showed Cotton effects, but signicant changes
Fig. 8 CD spectra corresponding to successive irradiation on a cast
film of complex M-X12A12. The irradiation times are given with respect
were not observed upon CPL irradiation (Fig. S14b†). It was to the previous irradiation step.
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Paper
Table 2 Optical properties of the complexes M-XX and M-XA
Abs. lmax./nm
Fl. lmax./nm
Sol.a
Filmb Sol.a Filmb fFL
Egd/eV
c
Compound
M-X(S)10*X(S)10*
M-X(R)10*X(R)10* 318
M-X12X12
M-X(S)10*A(S)10*
M-X(S)10*A12
M-X12A(S)10*
M-X12A12
318
314
314
314
423
420
423
421
420
420
424
414
414
414
—
—
—
0.97 3.7
0.96 3.7
1.00 3.7
318
326, 368 320
327, 369 322
326, 368 317
327, 370 315
—
—
—
—
—
—
—
—
—
a
CH2Cl2 solution (10ꢂ5 mol Lꢂ1). b Measurements in the lm. c Relative
d
to quinine sulfate (fFL
absorption spectra of the lms.
¼
0.546) in CH2Cl2. Determined from
Fig. 10 Normalized absorbance (solid lines) and emission (dashed
lines) spectra of the compounds M-X(S)10*X(S)10* in the as-prepared
film (black) and mesomorphic glass (red).
Fig. 9 Absorbance (solid lines) and normalized emission (dashed lines)
spectra of the compounds M-XX in solution in CH2Cl2 (10ꢂ5 mol Lꢂ1).
Fig. 11 Emission spectra of the compounds M-XX and M-XA in
solution (10ꢂ5 M).25
preparation of the lms was easy, and lms of high quality were
obtained by casting from THF solution on to a quartz plate.
Related with the observations from the X-ray experiments
commented above, uorescence spectra were recorded on as-
prepared lms and on lms heated to the isotropic phase
followed by slow cooling to ambient temperature (Fig. 10).
As a rst observation, it is clear that this class of complex,
M-XX, is able to maintain the intrinsic uorescence from the
1,3,4-oxadiazole in thin lms, since absorption and uores-
cence proles similar to those recorded in solution (Fig. 9) were
obtained for the lms (Fig. 10).
Small differences in the absorbance and uorescence spectra
of the differently treated lms were observed. Whereas the
absorption maximum is red-shied for the thermally treated
lm, the uorescence maximum is blue-shied. The effect of re-
absorption may change the spectral distribution of their pho-
method reported in the literature.23 The optical band gaps of
these compounds (Eg) varied from 3.6 to 3.7 eV.
The photophysical properties of M-XA complexes were also
investigated in CH2Cl2 solutions and in thin lms. The UV-
visible and uorescence data are summarized in Table 2.
The UV-visible spectra of the complexes in solution showed
bands corresponding to the different chromophores present in
the molecule, i.e., oxadiazole (X) and azobenzene (A), at around
330 nm and 360 nm, respectively (Fig. S15 in the ESI†). These
compounds also displayed a weak blue emission at around 420
nm in solution (Fig. 11), but the presence of azobenzene groups
led to the loss of uorescence in thin lms.24
Conclusions
toluminescence spectra. These can be explained by means of In summary, luminescent V-shaped acid derivatives of hetero-
the inner-lter effect which associates the blue shi to a cyclic 1,3,4-oxadiazoles (symmetrical XX and unsymmetrical
decrease in the lm thickness.22 This behaviour should be XA) with different alkyl chains (chiral and non-chiral) were
related to the degree of organization of the material, which is synthesized. These compounds were used to form supramo-
higher for the thermally treated samples (see X-ray diffraction lecular complexes through hydrogen bonding with three acid
studies).
molecules around the 4-diamino-6-dodecylamino-1,3,5-triazine
The optical band gaps (Eg) of all compounds were deter- (M) core. None of the V-shaped acids presented meso-
mined by their corresponding absorption in thin lms, using a morphism. In contrast, all of the complexes M-XX and M-XA
7036 | J. Mater. Chem. C, 2014, 2, 7029–7038
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exhibited liquid crystalline behaviour. M-XX presented hexag-
onal columnar mesomorphism whereas rectangular columnar
mesophases were observed for M-XA complexes. In any case, the
mesomorphic organization remains at room temperature and is
frozen below the glass transition in the M-XX complexes.
All of the complexes bearing chiral tails derived from citro-
nellol showed optical activity in the mesophase. For azobenzene
containing complexes, the possibility of switching reversibly the
supramolecular chirality of the material was demonstrated
when the chiral tail was located in the oxadiazole arm of the
V-shaped acid. Indeed, the optical activity of thin lms can be
amplied and switched between opposite signs for the complex
with only a single chiral tail in the oxadiazole moiety. For the
complex with chiral tails in both the oxadiazole and azobenzene
arms amplication was not observed but switching between two
opposite CD signals could be achieved. In contrast, none of
these effects was visible for complexes bearing the chiral tail in
the azobenzene moiety. More interestingly, supramolecular
chirality is induced in thin lms of the achiral complex
M-X12A12.
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Acknowledgements
We thank the following institutions for nancial support:
CAPES, CNPq/PRONEX, MINECO (projects MAT38538-CO2-01,
MAT2011-27978-CO2-01 and CTQ2012-35692), Gobierno de
7 (a) T. Ishi-i, R. Kuwahara, A. Takata, Y. Jeong, K. Sakurai and
S. Mataka, Chem.–Eur. J., 2006, 12, 763; (b) M. Peterca,
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P. A. Heiney, J. Am. Chem. Soc., 2006, 128, 6713; (c)
D. Franke, M. Vos, M. Antonietti, N. A. J. M. Sommerdijk
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Gestel, A. R. A. Palmans, B. Titulaer, J. A. J. M. Vekemans
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Aragon (project E04) and the EU (FEDER and FSE founding).
Thanks are due to: Nuclear Magnetic Resonance, Mass Spec-
trometry, Elemental Analysis and Thermal Analysis Services of
the CEQMA, Universidad de Zaragoza-CSIC (Spain).
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infrequent in the X-ray patterns of hexagonal columnar 25 Solution concentrations: M-X12X12, 4.0 ꢄ 10ꢂ5 M; M-X(S)
mesophases and it is due to a minimum in the structure
factor, which precludes the observation of peaks at the
corresponding diffraction angles. See: (a) W. Pisula,
Z. Tomovic, Ch. Simpson, M. Kastler, T. Pakula and
10*X(S)10*, 4.5 ꢄ 10ꢂ5 M; M-X(R)10*X(R)10*, 3.0 ꢄ 10ꢂ5
M; M-X12A12, 3.4 ꢄ 10ꢂ5 M; M-X12A(S)10*, 4.2 ꢄ 10ꢂ5 M;
M-X(S)10*A(S)10*, 3.0 ꢄ 10ꢂ5 M; M-X(S)10*A12, 4.1 ꢄ
10ꢂ5 M.
7038 | J. Mater. Chem. C, 2014, 2, 7029–7038
This journal is © The Royal Society of Chemistry 2014