Multifunctional supramolecular dendrimers with an s-triazine ring as the central core: Liquid crystalline, fluorescence and photoconductive properties

Article abstract of DOI:10.1002/chem.201402646

Novel liquid crystal (LC) dendrimers have been synthesised by hydrogen bonding between an s-triazine as the central core and three peripheral dendrons derived from bis(hydroxymethyl)propionic acid. Symmetric acid dendrons bearing achiral promesogenic units have been synthesised to obtain 3:1 complexes with triazine that exhibit LC properties. Asymmetric dendrons that combine the achiral promesogenic unit and an active moiety derived from coumarin or pyrene structures have been synthesised in order to obtain dendrimers with photophysical and electrochemical properties. The formation of the complexes was confirmed by IR and NMR spectroscopy data. The liquid crystalline properties were investigated by differential scanning calorimetry, polarising optical microscopy and X-ray diffractometry. All complexes displayed mesogenic properties, which were smectic in the case of symmetric dendrons and their complexes and nematic in the case of asymmetric dendrons and their dendrimers. A supramolecular model for the lamellar mesophase, based mainly on X-ray diffraction studies, is proposed. The electrochemical behaviour of dendritic complexes was investigated by cyclic voltammetry. The UV/Vis absorption and emission properties of the compounds and the photoconductive properties of the dendrons and dendrimers were also investigated.

Full text of DOI:10.1002/chem.201402646

DOI: 10.1002/chem.201402646
Full Paper
&
Liquid Crystals
Multifunctional Supramolecular Dendrimers with an s-Triazine
Ring as the Central Core: Liquid Crystalline, Fluorescence and
Photoconductive Properties
Madalina Bucos¸,[a] Teresa Sierra,[a] Attilio Golemme,*[c] Roberto Termine,[c] Joaquꢀn Barberꢁ,[a]
Raquel Gimꢂnez,[a] Josꢂ Luis Serrano,[b] Pilar Romero,*[a] and Mercedes Marcos*[a]
Abstract: Novel liquid crystal (LC) dendrimers have been
synthesised by hydrogen bonding between an s-triazine as
the central core and three peripheral dendrons derived from
bis(hydroxymethyl)propionic acid. Symmetric acid dendrons
bearing achiral promesogenic units have been synthesised
to obtain 3:1 complexes with triazine that exhibit LC proper-
ties. Asymmetric dendrons that combine the achiral prome-
sogenic unit and an active moiety derived from coumarin or
pyrene structures have been synthesised in order to obtain
dendrimers with photophysical and electrochemical proper-
ties. The formation of the complexes was confirmed by IR
and NMR spectroscopy data. The liquid crystalline properties
were investigated by differential scanning calorimetry, polar-
ising optical microscopy and X-ray diffractometry. All com-
plexes displayed mesogenic properties, which were smectic
in the case of symmetric dendrons and their complexes and
nematic in the case of asymmetric dendrons and their den-
drimers. A supramolecular model for the lamellar meso-
phase, based mainly on X-ray diffraction studies, is pro-
posed. The electrochemical behaviour of dendritic com-
plexes was investigated by cyclic voltammetry. The UV/Vis
absorption and emission properties of the compounds and
the photoconductive properties of the dendrons and den-
drimers were also investigated
Introduction
by divergent or convergent iterative methods and many den-
drimers have been built by the convergent union of dendrons.
Although the preparation of dendrimers has improved over
the past decade,[8] the larger and more complex structures still
require major synthetic effort and this is often plagued by low
yields and high polydispersity.[9] An alternative strategy is
based on supramolecular chemistry, which allows the prepara-
tion of the dendrimers by a self-assembly process based on
non-covalent interactions,[10] such as metal coordination
chemistry,[11] hydrogen-bonding interactions[5] or electrostatic
interactions.[12] In addition, dendrimers functionalised with
promesogenic units can self-organise into liquid crystalline
phases. Liquid crystal dendrimers present an attractive option
for the design of functional materials that retain the inherent
properties of dendrimers and provide well-organised materials
that are able to respond to external stimuli.[13]
Dendrons and dendrimers are an important subclass within
the dendritic polymer family.[1–4] The interest in these com-
pounds is due to their special structural characteristics, which
give them unique properties that differ from those of conven-
tional polymers. These materials have a unique structure that
grows from a focal point with a defined number of radially at-
tached branches as a function of the generation and peripheral
functional groups. The increasing interest in these kinds of ar-
chitectures arises from their applications in drug delivery, catal-
ysis and advanced materials.[1–7] Dendrimers can be obtained
[a] M. Bucos¸, Dr. T. Sierra, Dr. J. Barberꢀ, Dr. R. Gimꢁnez, Dr. P. Romero,
Dr. M. Marcos
Departamento de Quꢂmica Orgꢀnica
In 1998, Lehn and Zimmerman et al. reported the prepara-
tion of a dendrimer consisting of three H-bonded dendrons,
which self-organised to give a thermotropic discotic liquid
crystal.[14] Since then, a large number of supramolecular liquid
crystal dendrimers obtained by the self-assembly of dendrons
have been reported.[13e] Recently, we studied a new type of
liquid crystal dendrimer functionalised with carbazole groups
as hole-transporting moieties. These compounds were pre-
pared by hydrogen bonding between a triazine core (M), as an
electron-transporting unit, and three peripheral carboxylic acid
dendrons derived from a bifunctionalised bis(hydroxymethyl)-
propionic acid (bis-MPA).[15] As a continuation of our research
Facultad de Ciencias-Instituto de Ciencia de Materiales de Aragꢃn
Universidad de Zaragoza-CSIC, 50009 Zaragoza (Spain)
E-mail: promero@unizar.es
mmarcos@unizar.es
[b] Prof. Dr. J. L. Serrano
Departamento de Quꢂmica Orgꢀnica
Facultad de Ciencias-Instituto de Nanociencia de Aragꢃn
Universidad de Zaragoza, 50009 Zaragoza (Spain)
[c] Dr. A. Golemme, Dr. R. Termine
LASCAMM CR-INSTM, CNR-IPCF UOS CS-LiCryL
Dipartimento CTC. Universitꢄ della Calabria, 87036 Rende (Italy)
E-mail: a.golemme@unical.it
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201402646.
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programme in this area, we present here four new families of
liquid crystal dendrimers synthesised by hydrogen bonding be-
tween the triazine central core (M) and three peripheral bifunc-
tionalised dendrons derived from bis(hydroxymethyl)propionic
acid (bis-MPA). The structures of the dendrons and dendrimers
are shown in Scheme 1. Two types of symmetric bis-MPA-de-
rived dendrons were synthesised. The first type is formed by
the first- and second-generation dendrons functionalised with
an achiral promesogenic unit (5-[4-(4-butoxybenzoyloxy)phe-
nyloxy]pentanoic acid) (Ac-A), the second is formed by a first-
generation dendron containing a chiral promesogenic unit (5-
[4-(4-((S)-2-methylbutoxy)benzoyloxy)phenoxy]pentanoic acid)
(Ac-A*). Two types of bifunctional first-generation dendrons
bearing a unit of the achiral promesogenic unit A and
a pyrene (Py: 1-pyrenebutyric acid; Ac-Py) or coumarin (Cou:
7-(diethylamino)coumarin-3-carboxylic acid; Ac-Cou) moiety
were also prepared in order to study their electrochemical and
photophysical properties.
The triazine core (M) was selected in this work due to its
ability to recognise other molecules by the donation and ac-
ceptance of hydrogen bonds,
a characteristic that plays a key
role in the self-organisation of
molecules and thus facilitates
the formation of liquid crystals
in low molecular weight com-
pounds as well as in polymers or
dendrimers.[16,17] Coumarin and
pyrene were chosen to play the
role of fluorophores in the den-
drimer. Pyrene is a widely used
probe due to its high fluores-
cence efficiency and excimer for-
mation[18] and coumarin is bio-
compatible and highly valuable
as a fluorescent chemosensor in
a variety of fields.[19]
The aim of the work described
here was to exploit mesomor-
phism as
a tool to organise
these functional dendrimers and
to evaluate the luminescence
and photoconductivity proper-
ties of these materials.
Results and Discussion
Synthesis of promesogenic
units
The achiral Ac-A and chiral Ac-
A* ester derivatives were pre-
pared by previously reported
procedures[15] (Supporting Infor-
mation Scheme S3). 1-Pyrenebu-
tyric acid was purchased from
Sigma–Aldrich and it was used
without further purification. 7-
(Diethylamino)coumarin-3-car-
boxylic acid was prepared by the
method described by Duan
et al.[20] A short flexible spacer
(1,4-butylene) was introduced in
compounds Ac-A, Ac-A* and Ac-
Py in order to decorrelate the
mesogenic units from the rest of
the molecule, and thus facilitate
Scheme 1. Schematic representation of: a) the promesogenic and functional units (Ac-A, Ac-A*, Ac-Py and Ac-
Cou), carboxylic acid dendrons of the first and second generation, and c) synthetic route to first generation of
supramolecular dendrimers. Similarly, the synthesis of the second-generation dendrimers was made starting from
the dendron Ac-G2A4.
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the molecular arrangement of
these units within the
mesophase.
Synthesis of dendrons
Carboxylic acid dendrons were
prepared by DCC esterification
between monomer 2,2-bis(hy-
droxymethyl)propionic acid (bis-
MPA) protected by
a benzyl
ester group and the appropriate
promesogenic acid. The benzyl
ester group was subsequently
removed efficiently by catalytic
hydrogenolysis without affecting
the ester bonds of the polyester
backbone.[21]
The
synthetic
routes followed to prepare the
asymmetric and symmetric car-
boxylic acids are shown in
Scheme 2 and Supporting Infor-
mation Scheme S2, respectively.
Typical procedure for the
synthesis of the functionalised
dendrimers
The hydrogen-bonded com-
plexes were prepared by mixing
a CH2Cl2 solution of the appro-
priate amount of each compo-
nent (the triazine and the car-
boxylic acid dendron derivative
in a 1:3 ratio) and slowly evapo-
Scheme 2. Synthetic route for bifunctionalised carboxylic acid dendrons. I) DCC, DPTS, CH2Cl2; II) Pd(OH)2/C 20%,
cyclohexene/THF.
rating the solvent by stirring at
room temperature. The general
synthetic route for dendrimers is
shown in Scheme 1.[16b]
FT-IR characterisation
Synthetic details and full characterisation data for all prome-
sogenic units, dendrons, dendrimers and their intermediates
are provided in the Supporting Information.
The IR spectra of the complexes differed from those of the
dendrons and triazine, especially in the regions corresponding
to the carbonyl COOH and NꢀH groups, which are responsible
for the interactions between molecules. As an example (Sup-
porting Information Figure S7), the stretching absorption at
1707 cmꢀ1 for carbonyl groups in dendron Ac-G1ACou is re-
placed by the shoulder at 1701 cmꢀ1 for complex M-
G1ACouA. Significant modification of the bands corresponding
to the free NꢀH groups of the triazine was also observed in
the supramolecular dendrimer, and this proves that association
takes place through the carboxylic acid group of the dendrons.
Similar behaviour was observed for all the complexes (Support-
ing Information Figures S4–S8).
Structural characterisation
The structural characterisation of the compounds was carried
out by elemental analysis and spectroscopic methods: Fourier
transform infrared spectroscopy (FTIR) and NMR spectroscopy.
The data are gathered in the Supporting Information. High-res-
olution mass of dendrons and their precursors by MALDI-TOF
mass spectrometry are collected in Table S3 and Figures S1–S3
in the Supporting Information.
The formation and stability of intermolecular H-bonding as-
sociations between the triazine and the dendrons were studied
by infrared spectroscopy on neat samples as KBr pellets and
by NMR spectroscopy in deuterated solvents. The results con-
firmed the proposed structures of these materials.
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NMR spectroscopy characterisation
The chemical structures of the dendrons and the complexes
1
were confirmed by one-dimensional H and 13C NMR spectros-
1
1
copy and by two-dimensional H-1H COSY, DOSY, H-1H NOESY,
1H-13C HSQC and 1H-13C HMBC experiments. The 1H and 13C
spectra of the dendrons and two-dimensional 1H-1H NOESY
spectrum for M-G1A*2, 1H-13C HSQC spectrum for M-G1APy
1
and H-13C HMBC spectrum for Ac-G1ACou are provided in the
Supporting Information (Figures S9–S21).
The 1H NMR spectra recorded on CDCl3 solutions clearly
show the formation of complexes, assuming that there is
a rapid equilibrium between the complex and its components;
1
the H NMR spectra of the triazine (M), bifunctionalised den-
drons (Ac-G1ACou and Ac-G1APy) and complexes are shown
in Figure S22 (in the Supporting Information). All NH proton
signals of the triazine derivative M are shifted downfield upon
complexation as these protons are involved in the hydrogen
bonds. The signals for the protons of the N-methylene group
of the triazine alkyl chain are shifted to slightly higher field. In
the dendron molecules, the acidic proton signals are very
Figure 1. Expansions of 13C NMR spectra for complexes M-G1ACou and
M-G1APy related to their dendrons or triazine (125 MHz, CDCl3, 258C).
We previously reported similar systems[22] and demonstrated
that the stoichiometry of these complexes in solution is 1:1. Ti-
tration NMR spectroscopy experiments in [D2]dichloromethane
allowed the estimation of the binding constant (718(ꢁ98)mꢀ1
at 258C) for the complexation of the triazine derivative and
the Ac-G1A2 dendron (Figure 2). This value was calculated by
nonlinear curve fitting of the chemical shifts.
1
broad and are barely visible in the H spectrum but they can
be clearly observed in NOESY experiments due to molecular
exchange with amino protons (Supporting Information Fig-
ure S19). The protons close to the H bond experience slight
displacements. For example, the methyl group signals move
by around ꢀ0.03 ppm and the diastereotopic methylene (Hs)
signals by +0.04/+0.02 ppm. The change in the shape of
these AB system signals after complexation is particularly note-
worthy. The shift variations of the signals of the main protons
involved in complex formation are listed in Table S2 in the Sup-
porting Information.
DOSY and NOESY experiments were also employed to deter-
mine whether hydrogen-bonding interactions are established
between the triazine derivative M and the carboxylic acids. The
self-diffusion of a chemical species in a solvent depends on its
molecular size and hydrodynamic volume. According to this
principle, the association of components leads to changes in
the molecular diffusion coefficient and this can be used to
detect the presence of a complex formed by hydrogen bond-
ing in solution. Since there is fast exchange between the com-
plex and the components on the NMR time scale, this value
corresponds to the apparent diffusion coefficient of the com-
plex and is a weighted average of the diffusion coefficients of
both the bound and free components in the solvent. It is clear
that these are the only coefficients measured for complexes
and they are practically similar to those of the corresponding
acids and much lower than those of the corresponding tri-
azines (Supporting Information Figure S23).
Variations in the chemical shifts of the 13C signals of carbon
atoms involved in the formation of the supramolecular struc-
ture were also clearly observed. For example, the 13C signals of
the triazine core are shifted to lower frequencies by 4 or
5 ppm and the 13C signals of the carboxylic acid groups and of
the methylene groups in the b-position to the carboxylic acid
group are shifted by +0.2–0.8 ppm after complexation
(Figure 1).
Figure 2. 1H NMR titration spectra of the complex M-G1A2 (500 MHz,
[D2]dichloromethane, 258C). The concentration of acid Ac-G1A2 was
increased while keeping the triazine concentration [M] constant at 2.55 mm.
The stoichiometry, however, was 3:1 in the bulk material
when the molar amount of the acid was three times that of
the triazine.[23] 13C CPMAS spectra of solid samples helped to
confirm this stoichiometry (Supporting Information Figure S24).
Thermal stability of the dendrimers and dendrons
The thermal stability of the dendrimers and dendrons was
studied by thermogravimetric analysis (TGA) under a nitrogen
atmosphere. All of the studied compounds exhibited good
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thermal stability and in all cases the 5% weight loss
point is detected at temperatures more than 408C
above the isotropisation process. The dendrimer and
dendrons of the second generation derived from Ac-
A are slightly more stable than the homologous first
generation compounds. Furthermore, the dendrons
and dendrimers bearing the pyrene moiety are ther-
mally more stable than the coumarin derivatives.
Table 1. Mesomorphic behaviour, transition temperatures [8C] and X-ray diffraction re-
sults for dendrons and dendrimers under investigation. All of the diffractograms were
recorded at room temperature.
Compound
Thermal
data[a]
Mesophase
X-ray study
Layer
spacing d(ꢄ)[c]
S per chain
[ꢄ2][d]
[e]
Ac-G1A2
Ac-G2A4
Ac-G1A*2
Ac-G1APy
Ac-G1ACou
M-G1A2
g ꢀ3 SmC 75 SmA 81 I
g 10 SmC 91 N 94 I
g 4 SmC 25 C 82 I
g 14 N 90 I[b]
SmC
SmC
SmC
–
–
–
40
–
–
–
42.6
41.8
42.0
–
38
[e]
–
–
–
g 12 N 50 I[b]
–
Liquid crystalline properties
g 13 SmC 56 I
g 9 SmC 85 I
SmC
SmC
SmC
–
37.8
38.5
39.4
–
M-G2A4
The mesomorphic behaviour of the compounds was
analysed by polarised optical microscopy (POM), dif-
ferential scanning calorimetry (DSC) and X-ray diffrac-
tion (XRD). Three cycles were carried out in DSC ex-
periments and data were taken from the second
cycle. In some cases, the isotropization temperatures
were taken from POM observations because transi-
tion peaks were not detected in DSC curves. The
identification of the liquid-crystal phase was carried
out on the basis of POM observations and was con-
M-G1A*2
M-G1APy
M-G1ACou
g ꢀ1 SmC 20 I
g 8 N 50 I[b]
g 14 N 60 I[b]
–
–
–
[a] Data obtained by DSC in the second heating process and taken at the maximum
of the peak obtained at 108Cminꢀ1; g: glass; C=crystal phase; N=nematic meso-
phase; SmC=smectic C mesophase; SmA=smectic A mesophase; I=isotropic liquid.
[b] Optical data. [c] Smectic layer spacing. [d] Calculated cross-sectional area per chain
(equivalent to the cross-sectional area per mesogenic unit) projected onto the smectic
plane. [e] Crystallises under the conditions of the X-ray diffraction experiment.
firmed by X-ray diffraction. The transition temperatures, meso-
phase type and the most relevant X-ray data for the dendrons
and dendrimers synthesised are gathered in Table 1.
Only two precursor molecules exhibit liquid crystalline be-
haviour. The pure 2,4-diamino-6-dodecylamino-1,3,5-triazine
(M) exhibits a monotropic smectic A mesophase (Cr 1038C
(SmA 398C) I).[16b] 5-[4-(4-Butoxybenzoyloxy)phenyloxy]penta-
noic acid (Ac-A) showed a nematic mesophase (C 91 N 104 I),
identified by its schlieren texture and by the characteristic
droplet optical texture observed when the nematic phase
began to form on cooling the isotropic liquid. X-ray studies
confirmed the nematic phase.
As can be seen from the results in Table 1, all of the den-
drons and dendrimers synthesised behave as glassy materials
at low temperatures (g transition in the range ꢀ3 to 148C) and
all of them are liquid crystalline. Some representative examples
of the DSC traces are shown in the Supporting Information
(Figures S25–S27). Dendron Ac-G1A*2 exhibits semicrystalline
behaviour, showing a glass transition below room temperature
(Table 1). This behaviour has been previously described in den-
drimers.[24] The DSC thermograms of the Ac-G2A4 dendron
shows a crystal-to-mesophase transition in the first cycle,
whereas, in the second and following scans only a glass transi-
tion appears.
Figure 3. POM microphotograph of the smectic C mesophase of the
dendron Ac-G1A2 taken at 478C on cooling from the isotropic phase.
served by optical microscopy on applying mechanical stress to
the sample (Supporting Information Figure S28c and d) and
was confirmed by X-ray diffraction. DSC scans show only
a glass transition and the nematic-to-isotropic liquid transition
temperature were determined by POM.
With the exception of the M-G1ACou complex, the meso-
phase temperature range is slightly smaller for the dendrimers
than for the corresponding dendrons.
Dendrons derived from acids Ac-A and Ac-A* exhibit a smec-
tic C phase at room temperature (Figure 3). In addition, den-
drons Ac-G1A2 and Ac-G2A4 show a smectic A or a nematic
mesophase, respectively, at higher temperatures and over
a short temperature range. The corresponding supramolecular
dendrimers exhibit, in all cases, a smectic C mesophase from
room temperature. The mesophase was identified by optical
microscopy (Supporting Information Figure S28), where a blur-
red schlieren texture and a broken fan-shaped texture were
observed. On the other hand, the bifunctional dendrons and
dendrimers bearing coumarin or pyrene moieties exhibit only
nematic mesophases. The nematic nature of the phase was ob-
Structural characterisation of the mesophases
All of the compounds and complexes were investigated by X-
ray diffraction with the aim of confirming the type of meso-
phase and determining the structural parameters. The meso-
phases were studied at room temperature and the data ob-
tained are gathered in Table 1. In the case of dendrons Ac-
G1A2 and Ac-G1A*2 the X-ray study was not possible due to
the tendency of these two compounds to crystallise under the
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conditions of the diffraction experiments. This tendency to
crystallise is clearly illustrated for Ac-G1A*2 by the presence of
an exothermic transition peak (cold crystallisation) close to
room temperature detected by DSC in the heating process. In
the case of Ac-G1A2 crystallisation was not observed by opti-
cal microscopy or by DSC; however, the sample crystallised
after a few minutes when kept at room temperature during ex-
posure to the X-rays. The tendency to crystallisation persists at
higher temperatures, which precluded the X-ray characterisa-
tion of these two compounds under any set of conditions.
For all the remaining compounds and complexes, the X-ray
patterns confirmed the nature of the mesophase assigned by
optical microscopy (Table 1). Thus, the diffractograms of Ac-
G1APy, Ac-G1ACou and their triazine complexes recorded in
the nematic mesophase contain only diffuse scattered intensi-
ty, as expected given the absence of long-range positional
order and the existence of long-range orientational order only
(Supporting Information Figure S29). However, the orientation-
al order is not reflected in the X-ray patterns due to the lack of
macroscopic alignment in the bulk sample. The diffuse scatter-
ing arises from the short-range order of the fluctuating molec-
ular positions in the nematic mesophase.
merise by H bonding between the carboxyl group,[25] a process
that generates the mesogenic entity.
The fact that the layer spacing is significantly shorter than
twice the molecular length is due to a combination of two
phenomena: 1) the conformational freedom of the chains as
they are in a molten state in the mesophase, which reduces
the effective molecule length; 2) the tilt in the smectic
mesophase.
The layer spacing for the complexes (Table 1) is similar to
that measured for the dendron Ac-G2A4. In particular, this
compound and its triazine complex have practically the same
layer spacing. This is consistent with the fact that the triazine
complex is a trimer of the dendron, in which the three mole-
cules associated with the triazine core are statistically oriented
in two opposite directions and are tilted with respect to the
smectic layer (Figure 5). This arrangement must yield an overall
molecular length very similar to that of the dimeric dendron.
Thus, in the smectic mesophase the molecules of M-G1A2 and
M-G1A*2 and the molecules of M-G2A4 contain, respectively,
six or twelve mesogenic units statistically oriented in each
direction.
This kind of molecular arrangement is similar to that previ-
ously found by some of us for other series of covalent,[26]
ionic[27] and H-bonded[15] dendrimers that show smectic meso-
phases. The fact that the measured layer spacing is similar for
the two dendron generations (compare d values for M-G1A2
and M-G2A4 in Table 1) supports our previous finding that,
upon increasing the generation, the dendritic branches largely
expand in the direction of the smectic plane. This produces
a broadening of the molecule without a significant increase in
length.
On the other hand, the smectic nature of the mesophases
exhibited by Ac-G2A4 and complexes M-G1A2, M-G2A4 and
M-G1A*2 was confirmed by the presence of one or two sharp
reflections in the small-angle region in addition to a diffuse
scattering band in the wide-angle region (Figure 4). When two
Additional support for this structural model and for the
tilted nature of the mesophase of pure Ac-G2A4 and of the
three smectic complexes can be obtained from simple cross-
section calculations. The molecular cross-sectional area A in ꢄ2
can be deduced as A=V/d, where V is the molecular volume in
ꢄ3 and d is the experimentally measured layer spacing in ꢄ.
For a density of 1 gcmꢀ3, which is typical for organic com-
pounds, the molecular volume can be calculated using the fol-
lowing equation: V=Mꢅ1024/NA, where M is the molar mass in
g and NA is Avogadro’s number. On combining the two equa-
tions, the cross-sectional area of the molecule can be obtained
as A=Mꢅ1024/(dꢅNA).
Figure 4. X-ray diffractograms of: a) M-G1A2, b) M-G1A*2, and c) M-G2A4
recorded in the smectic C phase at room temperature after cooling from the
isotropic liquid.
small-angle maxima are detected, they are in the reciprocal
spacing ratio 1:2. This confirms the lamellar organisation and
the two maxima are, respectively, the first and second order re-
flections from the periodically stacked layers. The large-angle
halo arises from the interferences between the molten chains
and other short-range intra- and intermolecular interferences
in the direction perpendicular to the long axes of the mole-
cules. Although this kind of pattern could correspond to either
a smectic A or a smectic C organisation, the room-temperature
mesophases were assigned as smectic C on the basis of the
optical textures (vide supra) and the structural parameters de-
duced from the X-ray measurements (vide infra). The experi-
mentally measured layer spacing (d in Table 1) for Ac-G2A4 is
larger than the length of a single molecule in its most-extend-
ed conformation and shorter than twice its length. This is ex-
pected considering that the carboxylic acids are known to di-
In the case of Ac-G2A4 the cross-sectional area S per meso-
genic unit (or per chain) can be obtained by dividing the total
cross-section A by the number of mesogenic units oriented in
each direction in the dimer, that is, four. This gives a value of
about 40 ꢄ2 (Table 1).
Similar values of S close to 42 ꢄ2 were obtained for the
cross-sectional area per mesogenic unit in the case of the
three smectic complexes. In this case, since each complex con-
tains three molecules of the dendritic acid with their mesogen-
ic units statistically oriented in each direction, S is obtained by
dividing the total molecular cross-section by three for the first
generation (G1) and by six for the second generation (G2). The
hydrocarbon chain in the triazine does not affect these calcula-
tions because it is embedded in the central dentritic sublayer
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layer packing in the mesophase, both for pure Ac-G2A4 and
the complexes
It is interesting to note that the large S values do not pre-
clude the possibility that, in addition to tilting, there is some
degree of interdigitation or partial intercalation between the
terminal hydrocarbon chains of molecules in neighbouring
layers. The threefold symmetry of the supramolecular dendrim-
ers seems a priori to favour columnar mesomorphism, al-
though the flexibility of the bis-MPA units allows the formation
of a calamitic superstructure, as proposed in the models sug-
gested by the X-ray studies, and only calamitic phases were
observed.
The main difference between the symmetric and asymmetric
derivatives is the type of mesophase that they exhibit, which is
mainly lamellar for the former and nematic for the latter. Logi-
cally, the presence of two bulky groups, such as the pyrene
and coumarin units, in the dendron and dendrimer derivatives
introduces steric hindrance to the parallel molecular organisa-
tion necessary to obtain the lamellar phase. Nevertheless, the
intermolecular interactions of the neighbouring promesogenic
units are strong enough to maintain the nematic order.
Optical properties
The UV/Vis absorption and fluorescence spectra of the pyrene
and coumarin dendrons Ac-G1APy, Ac-G1ACou, and M-G1APy,
M-G1ACou dendrimers were measured in dilute dichlorome-
thane solution and in thin films (Figure 6). The data are collect-
Table 2. Photophysical data for dendrons and dendrimers.
Compound
labs [nm]
labs [nm]
lem [nm]
lem [nm]
(CH2Cl2)
(film)
(CH2Cl2)
(film)
Ac-G1APy
M-G1APy
Ac-G1ACou
M-G1ACou
344
344
425
425
346
348
416
417
377
377
458
455
466
468
531
530
ed in Table 2. Pyrene-containing dendrons and dendrimers
show essentially the same spectra in dilute solutions and this
is attributed to the characteristic absorption and emission
bands of the pyrene chromophore in the monomeric form.
However, the emission spectra of thin films show, in addition
to the monomer emission, a broad emission band centred at
466–468 nm and this is due to excimer emission (Figure 6a).[29]
In addition, in the absorption spectra of the thin films some
line broadening and a slight red-shift is observed, which indi-
cates some monomer pre-association. This indicates that the
pyrene units must be spatially close to each other in order to
exhibit the excimer emission band.
Figure 5. Proposed arrangement of the H-bonded complexes in the smectic
C mesophase of: a) M-G1A2, and b) M-G2A4. The rings represent the tri-
azine cores and the rectangles represent the mesogenic units.
(Figure 5). The S values of 40–42 ꢄ2 obtained for both the acid
and the complexes are too large for an orthogonal (smectic A)
mesophase and are consistent with a tilted smectic meso-
phase, for which we can expect a larger value for the cross-sec-
tional area of the mesogenic unit projected onto the smectic
plane.[28] Moreover, the coherence between the S values ob-
tained for all the compounds supports the aforementioned
structural model proposed for the molecular conformation and
The coumarin dendron and dendrimers also display similar
behaviour. In solution the absorption spectra show two bands
at about 262 and 425 nm and these are typical of the coumar-
in chromophore.
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Electrochemical properties
Cyclic voltammetry was performed on the pyrene and coumar-
in dendrons Ac-G1APy, Ac-G1ACou and their respective den-
drimers M-G1APy, M-G1ACou, in order to calculate the energy
level of the highest occupied molecular orbital (HOMO) of the
dendrons and dendritic complexes containing the electroactive
species pyrene and coumarin (Table 3). The measurements
Table 3. Cyclic voltammetry data for dendrons and dendrimers.
Compound
Eox [V]
HOMO [eV][a]
Ac-G1APy
M-G1APy
Ac-G1ACou
M-G1ACou
1.35
1.35
1.27
1.27
ꢀ5.69
ꢀ5.69
ꢀ5.61
ꢀ5.61
[a] EHOMO =ꢀe[EoxꢀE1/2(Fc/Fc+)+4.8 V].
were performed in dichloromethane solution under an argon
atmosphere using a 3-electrode cell with a glassy carbon rod
as the working electrode, Pt wire as the counter electrode and
Ag/AgCl as the reference electrode. The potential was cycled
between 0 and 2 V at a scan rate of 100 mVsꢀ1 at 258C. The
solutions were 10ꢀ4 m in electroactive material and 0.1m in tet-
rabutylammonium hexafluorophosphate as supporting electro-
lyte. Ferrocene (Fc) was used as the internal reference (E1/2(Fc/
Fc+)=0.46 V; Figure 7).
The dendron and dendrimer containing the pyrene unit
showed similar behaviour, with two oxidation waves and the
first oxidation potential at 1.35 V corresponding to the oxida-
tion of the pyrene unit to its radical cation form (Figure 7a and
b). For the coumarin-containing compounds three oxidation
waves were observed in the case of the dendron and two
waves for the dendrimer (Figure 7c and d). The first oxidation
potential was found at lower values than for the pyrene com-
pounds, at 1.27 V. In all cases the anodic peaks of all com-
pounds are much more intense than the cathodic peaks. This
marked asymmetry in the redox couple indicates that these
species undergo irreversible oxidation.
Figure 6. Absorption and fluorescence spectra in solution (CH2Cl2), and fluo-
rescence spectra in thin films for: a) M-G1APy (dashed line: emission in solu-
tion; grey line: emission in film; black line: absorbance), and b) M-G1ACou
(grey line: emission in film; dashed line: emission in solution; black line:
absorbance).
The highest absorption band broadens and shows a blue-
shift in the thin-film spectra as a consequence of packing ef-
fects in the condensed phase. The emission spectra in solution
display a band at 455–458 nm and this is related to the lumi-
nescence of the coumarin unit. However, in the thin film the
emission is much broader and is considerably red-shifted at
530 nm (Figure 6b).
Pyrene and coumarin compounds are all electron-donating
and the coumarin derivatives are more easily oxidised. HOMO
levels, which are related to the hole injection/transporting
properties of the compounds, can be estimated from the first
oxidation wave and the values are in the range expected for
hole conductors.
The results suggest that the coumarin units in the thin film
of both dendron and dendrimer are aggregated, and this is
consistent with results previously described for thin films of
pure coumarin derivatives or for host materials doped at high
concentrations.[19,30]
Photoconductivity studies
Thus, the novel pyrene- and coumarin-containing dendrons
and dendrimers exhibit quite different emission spectra in
dilute solution and thin films. Thin films show emission charac-
teristics typical of close packed pyrene or coumarin chromo-
phores as spectra with dye–dye interactions are observed in all
cases.
Photoconductivity measurements as a function of the applied
electric field were performed on the two dendrons Ac-G1APy
and Ac-G1ACou and on their respective dendrimers M-G1APy
and M-G1ACou. Experimental details are included in the Sup-
porting Information. Since photogeneration depends on the
intensity I of the incident radiation, and the analysed substan-
ces have a different light absorption coefficient a, all of the ex-
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field dependence for the photo-
conductivity, which is usually at-
tributed to an increase in photo-
generation quantum efficiency
with higher fields,[32] could be
taken as an indication that in
the case of M-G1APy and M-
G1ACou good levels of photo-
generation can be achieved
even at low fields. In turn, this
might be attributed to the si-
multaneous presence of an elec-
tron-acceptor unit (the triazine)
and
a unit with a relatively
strong donor character (coumar-
in or pyrene). Further studies will
be necessary to confirm this hy-
pothesis. Even if the dendrimers
were not doped in order to in-
crease photogeneration efficien-
cy, their photoconduction is
comparable to those measured
in other non-crystalline organic
photoconductors used as the
active element in optoelectronic
devices.[33]
Figure 7. Voltammograms for: a) Ac-G1APy, b) M-G1APy, c) Ac-G1ACou, and d) M-G1ACou.
The dendrons Ac-G1APy and
Ac-G1ACou show a photocon-
ductivity that is orders of magni-
tude lower than the photoconductivity of their respective den-
drimers. Not surprisingly, it appears that the presence of a mel-
amine core group, in addition to pyrene or coumarin, improves
the photoconductivity, probably by improving the electron
transport. Further conclusions cannot be drawn from the data
in terms of the structure–property relation.
Conclusion
We have designed and synthesised supramolecular liquid crys-
talline dendrimers with an s-triazine core and fluorescent den-
drons incorporated in the periphery. The compounds were pre-
pared using an easy and versatile method that involves
H bonding of dendritic carboxylic acids with 2,4-diamino-6-do-
decyl-1,3,5-triazine.
Figure 8. Normalised photoconductivity as a function of applied electric
field for Ac-G1APy, Ac-G1ACou, M-G1APy and M-G1ACou. For the Ac-
G1ACou data, the experimental error was much higher than in the other
cases due to a much higher dark current.
All synthesised complexes displayed mesogenic properties;
smectic in the case of monofunctionalised dendrons and their
complexes, and nematic in the case of bifunctionalised den-
drons and dendrimers.
perimental results are presented in terms of the normalised
photoconductivity sph/(Ia) in order to compare the results ob-
tained for the different compounds. The observed photocon-
ductivity as a function of the applied electric field for all four
substances is shown in Figure 8.
The novel pyrene- and coumarin-containing dendrons and
dendrimers exhibit quite different emission spectra in dilute
solution and in thin films. Thin films show emission characteris-
tics typical of close-packed pyrene or coumarin chromophores
as spectra with dye–dye interactions are observed in all cases.
HOMO levels, which are related to the hole injection/transport-
ing properties of the compounds, can be estimated from the
first oxidation wave, and they show values in the range of hole
conductors. The dendrimers show a higher photoconductivity
The curves for M-G1APy and M-G1ACou show a similar pat-
tern of photoconductivity as a function of field. In particular,
the photoconductivity does not vary significantly (it increases
slightly) with the applied field. This behaviour can be attribut-
ed to the dependence of the charge mobility, m, on the applied
1
=
field,[31] usually modelled as lnm/E . The lack of a stronger
2
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when compared to the dendrons, an effect that could be as-
cribed to two different factors, both a consequence of the
presence of triazine. In fact, the triazine not only introduces
a possible increase in electron mobility, but also may increase
the efficiency of photogeneration due to the interaction
between triazine and coumarin/pyrene. This latter effect is also
suggested by the weak field dependence of the photo-
conductivity.
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Acknowledgements
This work was supported by the seventh FP THE PEOPLE PRO-
GRAMME, The Marie Curie Actions, ITN, no. 215884-2, the
MINECO, Spain, (under Projects: CTQ2012-35692 and MAT2012-
38538-CO3-01), which included FEDER funding, and the
Aragꢆn Government-FSE (Project E04). A.G. and R.T. acknowl-
edge support from the European Community’s Seventh Frame-
work Program (FP7 2007–2013) through the MATERIA Project
(PONa3_00370) and from MIUR through the PRIN 2012JHFYMC
project. Thanks are given to the Nuclear Magnetic Resonance,
Mass Spectrometry, Elemental Analysis and Thermal Analysis
Services of the CEQMA, Universidad de Zaragoza-CSIC (Spain).
M.B. acknowledges support from the EU through an ESR
fellowship
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Received: March 18, 2014
Published online on && &&, 0000
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FULL PAPER
&
Liquid Crystals
M. Bucos¸, T. Sierra, A. Golemme,*
R. Termine, J. Barberꢀ, R. Gimꢁnez,
J. L. Serrano, P. Romero,* M. Marcos*
&& – &&
Multifunctional Supramolecular
Dendrimers with an s-Triazine Ring as
the Central Core: Liquid Crystalline,
Fluorescence and Photoconductive
Properties
Branch and bind: Luminescence and
photoconductivity properties of new
supramolecular liquid crystalline den-
drimers (see structure) with an s-1,3,5-
triazine core and fluorescent dendrons
incorporated in the periphery are de-
scribed.
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