Liquid crystalline ionic dendrimers containing luminescent oxadiazole moieties

Article abstract of DOI:10.1021/ma202051c

Two novel series of dendrimers constituted by the ionic grafting of poly(propyleneimine) PPI-(NH₂)_x (x = 4, 8, 16, 32, 64) and poly(amidoamine) PAMAM-(NH₂)_x (x = 64) with carboxylic acids bearing an oxadiazole ring have been synthesized, and their liquid crystalline properties have been investigated. Series I is generated by the ionic attachment between the dendrimers and 1,3,4-oxadiazole-containing acids. Series II results from the ionic junction of the dendrimers to 1,2,4-oxadiazole-containing acids. The liquid crystalline behavior has been investigated by means of differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and X-ray diffractometry (XRD). The liquid crystal properties are significantly improved in the dendrimers compared to the mesogenic precursors. The structural parameters determined by X-ray diffraction reflect the different supramolecular organization built by each kind of oxadiazole-containing acid introduced. On the basis of these experimental results, a packing model is proposed based on a microsegregation phenomenon and a variable degree of interdigitation between the mesogenic units. The absorption and emission properties of the compounds have also been studied. To our knowledge, these are the first dendrimers reported up to date combining oxadiazole units and liquid crystal properties.

Full text of DOI:10.1021/ma202051c

Article
pubs.acs.org/Macromolecules
Liquid Crystalline Ionic Dendrimers Containing Luminescent
Oxadiazole Moieties
Silvia Hernandez-Ainsa,† Joaquín Barbera,*,† Mercedes Marcos,*,† and Jose Luis Serrano‡
́
́
́
†Departamento de Química Organ
Zaragoza-CSIC, C/Pedro Cerbuna 12, 50009 Zaragoza, Spain
‡Instituto de Nanociencia de Aragon
, Mariano Esquillor edif. I+D, 50018 Zaragoza, Spain
́ ́
ica, Facultad de Ciencias-Instituto de Ciencia de Materiales de Aragon, Universidad de
́
S
* Supporting Information
ABSTRACT: Two novel series of dendrimers constituted by
the ionic grafting of poly(propyleneimine) PPI-(NH2)x (x = 4,
8, 16, 32, 64) and poly(amidoamine) PAMAM-(NH2)x (x =
64) with carboxylic acids bearing an oxadiazole ring have been
synthesized, and their liquid crystalline properties have been
investigated. Series I is generated by the ionic attachment
between the dendrimers and 1,3,4-oxadiazole-containing acids.
Series II results from the ionic junction of the dendrimers to
1,2,4-oxadiazole-containing acids. The liquid crystalline
behavior has been investigated by means of differential
scanning calorimetry (DSC), polarizing optical microscopy
(POM), and X-ray diffractometry (XRD). The liquid crystal
properties are significantly improved in the dendrimers compared to the mesogenic precursors. The structural parameters
determined by X-ray diffraction reflect the different supramolecular organization built by each kind of oxadiazole-containing acid
introduced. On the basis of these experimental results, a packing model is proposed based on a microsegregation phenomenon
and a variable degree of interdigitation between the mesogenic units. The absorption and emission properties of the compounds
have also been studied. To our knowledge, these are the first dendrimers reported up to date combining oxadiazole units and
liquid crystal properties.
and this has been demonstrated to be a good strategy to avoid
the rotational symmetry in the molecule and to give a biaxial
nematic mesophase.7
INTRODUCTION
■
Dendrimers are highly branched macromolecules that have
attracted significant interest in material science and biomedi-
cine fields.1 Their unique architecture originates novel
properties, such as better solubility and lower viscosity than
their analogous linear polymers. Moreover, the high density of
functional groups contained in their structure allows the
introduction of a higher number of active units with specific
properties.
The 1,3,4- and 1,2,4-oxadiazole moieties have been
incorporated to many series of organic compounds due to
their interest as electron transporting materials or their
application in emission layers for OLEDS2 and in biology.3
These molecules have also been investigated in the liquid
crystal field. As a general term, they tend to generate
mesophases typical of rodlike mesogens such as the smectic
or the nematic phase,4 but some columnar mesophases5 have
also been described when these molecules are conveniently
designed to possess a disk shape.
Oxadiazole units have been extensively incorporated in
dendrons8 and dendrimers9 to study their luminescent
properties in nonmesomorphic materials. The highly branched
structure of the dendrimer may improve the luminescence
efficiency of the materials in films since their globular shape
reduces intermolecular interactions.10 Interestingly, light
harvesting effects have also been observed in luminescent
dendrimers.11 However, no works concerning the liquid
crystalline behavior of dendrimers bearing oxadiazole have
been reported. The well-defined architecture of dendrimers
enables for the specific and controlled location of the
oxadiazole moieties in the dendritic scaffold, and this feature
favors the supramolecular organization required to obtain
mesomorphic properties.
A simple method to obtain mesomorphic dendrimers is the
incorporation of appropriate molecules to the periphery of a
dendrimer.12 Although most of the works collected up to now
deal with covalent attachment, the ionic junction of molecules
More interesting is the occurrence of the biaxial nematic
mesophase for some of these derivatives.6 This mesophase, of
great interest in liquid-crystal electro-optical display technology,
has also been investigated in dendrimers. Thus, the side-on
attachment of the mesogen as a side group to siloxane
dendrimers hinders the rotation about the molecular long axis,
Received: September 8, 2011
Revised: December 14, 2011
Published: December 28, 2011
© 2011 American Chemical Society
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bearing a carboxylic group to the amine terminal groups of the
poly(propyleneimine) (PPI) and poly(amidoamine)
(PAMAM) dendrimers has demonstrated to be an easy and
versatile strategy to produce mesomorphic materials.13
Following this approach, we have prepared the two series of
dendrimers shown in Scheme 1. Series I is constituted by
EXPERIMENTAL SECTION
■
1. Materials and Methods. PPI-(NH2)x (x = 4, 8, 16, 32, and 64)
dendrimers were purchased from SyMO-Chem BV (Eindhoven, The
Netherlands), and Starburst-PAMAM-(NH2)64 dendrimer was manu-
factured by Dendritech. The rest of reagents were purchased from
Aldrich, and all of them were used as received. Anhydrous THF used
for dendrimer preparation was purchased from Scharlau and dried
using a solvent purification system.
Scheme 1. Schematic Representation and Nomenclature of
the Reported Dendrimers
The infrared spectra of all the compounds were obtained with a
Nicolet Avatar 360 FTIR spectrophotometer in the 400−4000 cm−1
spectral range using KBr pellets and NaCl cells. 1H NMR was
performed on a Bruker AVANCE 400 spectrometer and on a Bruker
AVANCE 300 spectrometer. 13C NMR was performed on a Bruker
AVANCE 400 spectrometer operating at 100 MHz and on a Bruker
AVANCE 300 spectrometer operating at 75 MHz. Elemental analyses
were performed using a Perkin-Elmer 240C microanalyzer.
Mesogenic behavior and transition temperatures were determined
using an Olympus DP12 polarizing optical microscope equipped with
a Linkam TMS91 hot stage and a CS196 central processor.
Differential scanning calorimetry (DSC) experiments were
performed in DSC TA Instruments Q-20 and Q-2000 equipments.
Samples were sealed in aluminum pans, and a scanning rate of 10 °C
min−1 under a nitrogen atmosphere was used. The equipment was
calibrated with indium (156.6 °C, 28.4 J g−1) as the standard. Three
thermal cycles were carried out. The mesophase transition temper-
atures were read at the maximum of the corresponding peaks.
Thermogravimetric analysis (TGA) was performed using a TA
Instruments TGA Q5000 at a rate of 10 °C min−1 under an argon
atmosphere.
The XRD experiments were performed in a pinhole camera (Anton-
Paar) operating with a point-focused Ni-filtered Cu Kα beam.
Lindemann glass capillaries with 0.9 mm diameter were used to
contain the sample, and when necessary, a variable-temperature
attachment was used to heat the sample. The patterns were collected
on flat photographic film perpendicular to the X-ray beam. Bragg’s law
was used to obtain the spacing.
UV−vis absorption spectra were measured with a UV4−200 from
ATI-Unicam using 10−5−10−6 M solutions in CHCl3 (HPLC grade).
Fluorescence spectra were measured with a Perkin-Elmer LS50B
fluorescence spectrometer using solutions in CHCl3 of ca. 0.01
absorbance (about 10−8−10−9 M) under excitation at the absorption
maximum. Films were prepared by casting of a solution of ∼1 mg/mL
in CHCl3 on a quartz plate.
2. Synthesis and Nomenclature.
2.1. Oxadiazole Acids. Carboxylic acids based on two oxadiazole
isomers (1,3,4- and 1,2,4-) have been used to prepare the dendrimers.
They are denoted 1,3,4-OXAn and 1,2,4-OXAn. The subscript “n”
indicates the length of the spacer between the rigid moiety and the
carboxyl group, namely n = 4 and 10. The synthesis of 1,3,4-OXAn and
1,2,4-OXAn was performed following previously described methods
(Scheme 2).6d,14
2.2. Ionic Dendrimers. Ionic dendrimers were synthesized by a
procedure previously described by us,13c following the method of
Crooks15 as schematically represented in Scheme 3a. Namely, x equiv
of the OXAn acid was dissolved in anhydrous THF. The mixture was
added to a solution of 1 equiv of PPI or PAMAM dendrimer of the
corresponding generation (PPI-(NH2)x or PAMAM-(NH2)x) in
anhydrous tetrahydrofuran (THF) and sonicated for 10 min. The
mixture was then slowly evaporated at room temperature and dried in
vacuum until the weight remains constant for ca. 12 h at 40 °C. These
compounds are named D-(1,3,4-OXAn)x or D-(1,2,4-OXAn)x, where D
denotes the type of dendrimer matrix (PPI or PAMAM), n the spacer
length of the acid, and x the number of terminal amine groups of the
dendrimers (related to the generation). The terminal chain of the
oxadiazole-based carboxylic acids is pentyl in all cases.
dendrimers coming from the ionic functionalization of PPI (x =
4, 8, 16, 32, and 64) and PAMAM (x = 64) with carboxylic
acids (1,3,4-OXAn) bearing the 1,3,4-oxadiazole moiety with
different spacer lengths (n = 4 and 10). One dendrimer coming
from the covalent attachment of the 1,3,4-OXA10 unit to the
third-generation PPI (x = 16) is also included in this series.
Series II is formed by ionic dendrimers constituted by the
functionalization of PPI (x = 16 and 64) with carboxylic acids
(1,2,4-OXAn) containing the 1,2,4-oxadiazole ring with differ-
ent spacer length (n = 4 and 10). Both dendritic scaffolds have
been chosen because of their easy functionalization by means of
the transfer proton reaction between their peripheral amine
groups and the carboxylic acid. Besides, most of the compounds
have been prepared by using the PPI dendrimer because this
dendrimer presents higher fluidity and thermal stability when
compared with the PAMAM core. These two properties are of
great importance for future applications of these materials.
The aim of this work is to study the liquid crystalline
behavior, looking for the nematic biaxial mesophase, and to
evaluate the luminescent properties in these dendrimers.
2.3. Covalent Dendrimer. Covalent dendrimer (PPI-(1,3,4-
OXA10)16-cov) belonging to series I was synthesized following a
method previously described,16 as schematically represented in
Scheme 3b. 1.03 equiv of 1,3,4-OXA10 acid was dissolved in THF
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Scheme 2. Synthetic Route and Nomenclature of Intermediates for 1,3,4-OXAn and 1,2,4-OXAn Acids
(10 mL), and a solution of carbonyldiimidazole (CDI) (1 equiv) in
THF (10 mL) was slowly added to the acid solution. The reaction
mixture was stirred for 1 h at room temperature under a stream of
argon to remove the CO2 formed. The mixture was transferred to a
Schlenk flask containing the PPI (1 equiv of terminal amino groups).
The mixture was heated to 40 °C for 72 h. The solvent was half
reduced in vacuum, and then 50 mL of water was added to yield a
precipitate, which was isolated by filtration. The product obtained was
purified by further washing with water and hot methanol to give a
white power after drying in vacuum (51% yield).
their purity. MALDI-TOF spectrometry was also employed to
characterize the covalent dendrimer, but the ionic nature of the
rest of the dendrimers prevents the use of this technique.
1.1. FT-IR Characterization. As observed in Figure 1a, the
stretching absorption at 1707−1699 cm−1 corresponding to the
carbonyl groups of 1,3,4-OXAn acids is replaced by a new band
corresponding to the asymmetric stretching absorption of the
carboxylate group at 1560−1550 cm−1 in ionic dendrimers of
series I. In the case of dendrimers of series II, the band of the
carbonyl group shifts from 1707 to 1705 cm−1 in the 1,2,4-
OXAn acids to 1567−1561 cm−1 in the carboxylate of the ionic
dendrimers (see Figure 1a).17 For PPI-(1,3,4-OXA10)16-cov a
band at 1641 cm−1 corresponding to the symmetric stretching
of the carbonyl group in the amide bond replaces the band of
RESULTS AND DISCUSSION
■
1. Characterization of the Dendrimers. IR and NMR
spectroscopy, together with elemental analysis, reveal the
correct formation of the dendrimers (series I and II) and
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Scheme 3. Schematic Representation of the (a) Ionic Synthetic Route and (b) Covalent Synthetic Route of the Dendrimers
Figure 1. FT-IR spectra: (a) PPI-(1,2,4-OXA10)16 (green), 1,2,4-OXA10 (red), PPI16 (black); (b) PPI-(1,3,4-OXA10)16-cov (blue), 1,3,4-OXA10
(red), PPI16 (black).
1
Figure 2. (a) H NMR spectra of PPI-(1,3,4-OXA10)16-cov (blue), PPI16 (black), and PPI-(1,3,4-OXA10)16 (red). (b) 13C NMR spectra of PPI-
(1,3,4-OXA10)16-cov (blue), 1,3,4-OXA10 (black), and PPI-(1,3,4-OXA10)16 (red).
the carboxyl group in the 1,3,4-OXA10 acid at 1707 cm−1 (see
Figure 1b). Moreover, a band at 3295 cm−1 corroborates the
presence of the amide NH group.
[1,3,4-OXAn]x and PPI-[1,2,4-OXAn]x compounds (series I and
II) reveals the complete formation of the salts in the ionic
compounds. In the same way, the shift of the signal from 3.30
p p m [ − N H C H 2 C H 2 N H 2 ] t o c a . 3 . 4 5 p p m
1
1.2. NMR Experiments. Dendrimers were studied by H
1
+
NMR and 13C NMR spectroscopy. In H NMR experiments
[−NHCH2CH2NH3 ] and the displacement from 2.70 ppm
+
(Figure 2a) the absence of signal at 2.70 ppm related to
[NHCH2CH2NH2] to ca. 3.05 ppm [NHCH2CH2NH3 ]
[−CH2−NH2] of PPI and the appearance of another signal at
indicates the salt formation in PAMAM derivatives (not
shown). In the covalent compound (PPI-[1,3,4-OXA10]16-
+
ca. 2.90 ppm corresponding to [−CH2−NH3 ] for ionic PPI-
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cov) the signal is shifted from 2.70 to 3.30 ppm, which indicates
the formation of the amide bond [−CH2−NHCO] (Figure 2a).
In the 13C NMR spectra (Figure 2b) the signal of the carbon
atom of the carboxyl group (COOH) of the acid shifts from
178.2 ppm (1,3,4-OXA10) to 179.2 ppm (PPI-(1,3,4-
OXA10)16), which indicates the formation of the carboxylate
(COO−). Likewise, the deprotonation of the carboxylic acid is
corroborated by the displacement of the signal corresponding
to the Cα to the carbonyl group (CH2−COOH) from 33.8 ppm
in the acid to 36.9 ppm in the dendrimer (CH2−COO−).
Carboxylate is also formed in the case of PAMAM dendrimers,
as denoted by the signal shift from 178.2 ppm (in case of the
1,3,4-OXA10 acid) to 179.5 ppm (asigned to the carboxylate
group in PAMAM-(1,3,4-OXA10)64) (not shown). The
formation of the amide group in the case of PPI-(1,3,4-
OXA10)16-cov is confirmed by the appearance of the signal at
173.6 ppm, assigned to the carbon of the amide group
(CH2NHCO) (see Figure 2b).
1.3. MALDI-TOF Analysis. The complete functionalization of
PPI-(1,3,4-OXA10)16-cov was checked by MALDI-TOF mass
spectrometry. The MALDI-TOF MS spectrum reveals the
presence of an intense peak corresponding to the fully
functionalized dendrimer (see Figure S1, Supporting Informa-
tion). Except for the peak appearing at 9925 that could
correspond to the dendrimer with one nonfunctionalized
amino group, the rest of the peaks must be associated with
statistical defects present in the original PPI dendrimer.18
2. Thermal Stability of the Dendrimers. The thermal
stability of the dendrimers was studied by TGA experiments
(see Table S1, Supporting Information).
The type of OXA moiety (1,3,4- or 1,2,4-) and the length of
the spacer in the OXAn moiety (n = 4, 10) exercise a marked
influence in the onset temperature of decomposition of these
dendrimers. Thus, dendrimers bearing the 1,3,4-OXAn moiety
possess the onset temperature of decomposition above 340 °C,
whereas those containing the 1,2,4-OXAn moiety exhibit lower
temperatures (between 275 and 305 °C). Likewise, when
comparing dendrimers with the same OXAn moiety, it is
observed that temperatures of decomposition for the longer
spacer (n = 10) compounds are at least 15 °C higher than those
bearing the shorter spacer (n = 4).
The type of bond affects the thermal stability of the
compounds. Hence, the temperature at which 5% of initial mass
is lost (T5%) is higher for the covalent compound (PPI-(1,3,4-
OXA10)16-cov) when compared with its homologous ionic
compound (PPI-(1,3,4-OXA10)16) and the rest of the
compounds (see Table S1).
belonging to series II (precursors of 1,2,4-OXAn) exhibit
enantiotropic mesomorphism (see Table 1 and Table S2).
Table 1. Temperatures and Enthalpies of Selected
a
Precursors
series
compound
thermal data
I
3
C 197 C′ 205 [20.1] I
b
I
1,3,4-OXA4
1,3,4-OXA10
10
C 186 [54.0] I; I 166 N 162 [50.4] C
I
C 175 [47.9] I
c
II
II
II
C 120 [0.6] C′ 154 [20.8] SmA N 217 [0.6] I
1,2,4-OXA4
1,2,4-OXA10
C 153 [26.7] SmA 190 [0.5] N 217 [1.0] I
C 142 [35.3] SmA 181 [0.7] N 193 [0.8] I
a
Temperatures (°C) at the maximum of the peak and enthalpy values
(kJ mol−1 in brackets) of the transitions obtained by DSC in the
second heating process, or in the second cooling process in the case of
the monotropic mesophases, performed at 10 °C/min. C, C′ =
crystalline phases, SmA = smectic A mesophase, N = nematic
b
mesophase, I = isotropic liquid. Thermal transition observed only by
c
POM. The SmA-N transition is hidden in the C′-SmA transition.
3.2. Liquid Crystal Properties of the Dendrimers. In spite of
the absence of mesomorphism or the occurrence of only
monotropic behavior in some OXAn acids, all dendrimers
exhibit liquid crystalline properties in addition to a number of
crystal-to-crystal transitions (Table 2).
The liquid crystalline temperature ranges oscillate between
30 and 50 °C, being slightly higher for PAMAM derivatives
when compared with their PPI homologues. This fact may be
connected with the different nature of the dendrimer scaffolds
because of the existence of hydrogen bonds in the structure of
the PAMAM core.
The covalent dendrimer (PPI-(1,3,4-OXA10)16-cov) presents
a shorter mesomorphic range than its homologous ionic
compound (PPI-(1,3,4-OXA10)16). Thus, the ionic attachment
enhances the tendency of these compounds to display liquid
crystal properties, a feature that is in good agreement with
other reported works.20
Dendrimers belonging to series I display higher melting and
isotropization temperatures when bearing the shorter spacer (n
= 4). This is probably due to the increase in the interactions
between the promesogenic moieties compared to the longer
spacer (n = 10).
Compounds of series II exhibit lower melting but higher
isotropization temperatures than their homologues belonging
to series I, thus presenting broader mesomorphic temperature
range (between 145 and 180 °C).
On the other hand, the generation of the dendrimer does not
produce remarkable influence in the melting and isotropization
temperatures of the compounds belonging to both series.
All of the compounds present smectic A mesomorphism as
revealed by the textures observed by POM (see Figure 3) and
confirmed by the patterns obtained by XRD (vide infra). A
hexagonal columnar mesophase has also been detected by XRD
for PPI-(1,3,4-OXA10)64 and PPI-(1,3,4-OXA4)64 at lower
temperatures. The occurrence of this type of mesomorphism
can be explained by the sterical hindrance at the periphery of
the fifth generation of the PPI dendrimer. In contrast, PAMAM
dendrimers show no columnar mesomorphism due to the
larger volume of the PAMAM dendritic architecture with
respect to the PPI one, which prevents this congestion.13c,21
4. X-ray Studies of the Compounds. The mesomorphic
behavior of the dendrimers and precursors has been confirmed
Also, the dendrimer scaffold exerts influence in the T5%, this
temperature being considerably higher for PPI than for
PAMAM derivatives. In fact, PAMAM dendrimers are well-
known to be less thermally stable than PPI dendrimers.19
3. Liquid Crystal Properties. The mesomorphic behavior
of the compounds was analyzed by POM, DSC, and X-ray
diffraction. Three cycles were carried out in DSC experiments,
and data were taken from the second cycle. In some cases, the
isotropization temperatures were taken from POM observa-
tions because no transition peaks were detected in the DSC
curves.
3.1. Liquid Crystal Properties of the Precursors. The type of
oxadiazole moiety plays a determinant role in the liquid crystal
behavior of the precursors. Indeed, precursors of series I
containing the 1,3,4-oxadiazole ring are not mesomorphic or
the mesomorphism is only monotropic, whereas those
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a
Table 2. Temperatures and Enthalpies of the Phase Transition of the Dendrimers
series
compound
thermal data
c
I
PPI-(1,3,4-OXA10)4
PPI-(1,3,4-OXA10)8
PPI-(1,3,4-OXA10)16
PPI-(1,3,4-OXA10)16-cov
PPI-(1,3,4-OXA10)32
PPI-(1,3,4-OXA10)64
PAMAM-(1,3,4-OXA10)64
PPI-(1,3,4-OXA4)4
C 110 [4.7]
C 98 [18.2]
C 92 [31.2]
C 109 [37.3]
C 91 [66.6]
C 91 [114.9]
C 83 [87.3]
C 121 [12.6]
C 111 [9.0]
C 110 [15.7]
C 94 [60.7]
C 123 [76.8]
C 121 [187.6]
C 43 [57.7]
C 65 [9.1]
C′ 130
C″ 137 [27.4]
SmA 164 [9.4]
SmA 163 [37.0]
SmA 176 [55.2]
SmA 189 [59.5]
SmA 185 [117.5]
SmA 175 [180.6]
SmA 176 [390.6]
SmA 183 [6.3]
SmA 184 [15.7]
SmA 190 [27.5]
SmA 183 [60.4]
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
c
I
C′ 122
C″ 130 [46.1]
I
C′ 128 [162.2]
C′ 146 [248.8]
C′ 131 [343.3]
C′ 124
I
I
c
c
Colh 140
I
C″ 133 [45.2]
I
C′ 125 [429.2]
C′ 140 [0.2]
C′ 148 [17.6]
C′ 151 [29.0]
C′149 [88.7]
I
C″ 156 [1.3]
I
PPI-(1,3,4-OXA4)8
I
PPI-(1,3,4-OXA4)16
PPI-(1,3,4-OXA4)32
PPI-(1,3,4-OXA4)64
PAMAM-(1,3,4-OXA4)64
PPI-(1,2,4-OXA10)16
PPI-(1,2,4-OXA10)64
PPI-(1,2,4-OXA4)16
PPI-(1,2,4-OXA4)64
I
c
Colh 170
d
I
SmA 190
I
SmA 191 [103.4]
c
d
II
II
II
II
C′ 67
C″83 [69.2]
SmA 234
d
C′ 95 [365.1]
SmA 240
d
C 65 [108.9]
C 59 [386.7]
SmA 237
d
SmA 240
a
Temperatures (°C) read at the maximum of the corresponding peaks and enthalpy values (kJ mol−1, in brackets) of the transitions obtained by DSC
corresponding to the second heating process performed at 10° C/min. The peaks are broad and transitions span several degrees below and above the
b
peak maximum. Two peaks are observed for this transition that probably correspond to overlapped melting of two crystalline phases (C′ and C″).
c
d
Mesophase and transition temperature identified by XRD. Temperature determined by POM. C, C′, C″= crystalline phases, SmA = smectic A
mesophase, Colh = hexagonal columnar mesophase, I = isotropic liquid.
Figure 3. POM textures of (a) PPI-(1,3,4-OXA10)32 at 150 °C in the second heating process in the SmA phase, (b) PPI-(1,3,4-OXA10)16-cov at 140
°C in the first cooling process in the SmA phase, (c) PPI-(1,3,4-OXA4)4 at 165 °C in the second heating process in the SmA phase, and (d) PPI-
(1,2,4-OXA4)64 at 170 °C in the first cooling process in the SmA phase.
by X-ray diffraction. Precursors exhibiting nematic meso-
morphism contain a diffuse scattering maximum at low angles
and a diffuse halo at high angles corresponding to the
intermolecular short-range interactions between molecules.
The high-angle scattering corresponds to a mean distance of
4.5−4.6 Å and is related to the mean distance between the
conformationally disordered alkyl chains and between aromatic
segments. The low-angle scattering arises from the short-range
interaction along the main molecular axes. In the SmA
mesophase (precursors of series II), this low angle scattering
is replaced by a sharp, strong peak corresponding to the layer
periodicity.
All of the dendrimers display SmA mesomorphism. In this
case, X-ray diffraction patterns are constituted by a diffuse halo
in the wide angle region, corresponding to the short-range
correlations between the mesogenic units and the conforma-
tionally disordered terminal chains coming from the oxadiazole
moieties and from the dendrimer branches, and by one or two
sharp maxima in the small angle region (see Figure S4a,b).
These sharp maxima evidence a long-range lamellar packing of
molecules, and when there are two small-angle maxima, they
correspond to the first and second layer orders. The layer
spacing of the SmA mesophase of each dendrimer was obtained
by applying Bragg’s law, and they are collected in Table 3.
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ammonium as a consequence of the mismatch between the
length of the different segments of the mesogenic units.
Therefore, the 1,3,4-OXA4 units do not interdigitate to such
a large extent, and the side-by-side arrangement of this aromatic
units is probably replaced by a side-by-side arrangement of the
hydrocarbon chains (Figure 4b). In both models there is
segregation of the aromatic units and the hydrocarbon chains in
different sublayers.
The layer thickness is also dependent on the nature of the
dendrimer matrix. Greater values are obtained for the two
PAMAM derivatives as it was expected due to its larger size
when compared to PPI, and the same relationship as for the
PPI derivatives is observed between the spacer length and the
layer periodicity.
The layer thickness of the ionic PPI-(1,3,4-OXA10)16
dendrimer (44.8 Å) and that of its analogous covalent PPI-
(1,3,4-OXA10)16-cov (47.4 Å) dendrimer do not significantly
differ from each other. The slightly higher spacing layer value
obtained for the covalent one gives evidence of a higher
mobility present in the case of the ionic compounds, where the
moieties can fluctuate in position and slightly penetrate in the
dendrimer. By contrast, in the covalent compound moieties
occupy fixed positions. The same tendency has been previously
described in other similar dendrimers.13a This enhanced
mobility may allow the ionic compound to reach the
mesomorphic state at lower temperatures than the covalent
analogue.
Table 3. X-ray Structural Parameters Data of Dendrimers
and 1,2,4-OXAn Acids
a
b
series
compound
T/°C phase structural parameters
I
I
I
I
I
I
PPI-(1,3,4-OXA10)4
PPI-(1,3,4-OXA10)8
PPI-(1,3,4-OXA10)16
PPI-(1,3,4-OXA10)16-cov
PPI-(1,3,4-OXA10)32
PPI-(1,3,4-OXA10)64
PPI-(1,3,4-OXA10)64
PAMAM-(1,3,4-OXA10)64
PPI-(1,3,4-OXA4)4
PPI-(1,3,4-OXA4)8
PPI-(1,3,4-OXA4)16
PPI-(1,3,4-OXA4)32
PPI-(1,3,4-OXA4)64
PPI-(1,3,4-OXA4)64
PAMAM-(1,3,4-OXA4)64
1,2,4-OXA10
170
165
163
160
161
130
150
148
175
168
168
168
168
180
168
175
175
165
165
170
170
SmA
SmA
SmA
SmA
SmA
Colh
SmA
SmA
SmA
SmA
SmA
SmA
Colh
SmA
SmA
SmA
SmA
SmA
SmA
SmA
SmA
d = 42.9
d = 43.9
d = 44.8
d = 47.4
d = 45.9
a = 49.4
d = 49.3
d = 56.0
d = 56.0
d = 54.1
d = 55.4
d = 56.1
a = 67.3
d = 53.6
d = 61.6
d = 38.5
d = 30.8
d = 47.4
d = 49.4
d = 40.4
d = 41.1
I
I
I
I
I
I
I
II
II
II
II
II
II
1,2,4-OXA4
PPI-(1,2,4-OXA10)16
PPI-(1,2,4-OXA10)64
PPI-(1,2,4-OXA4)16
PPI-(1,2,4-OXA4)64
a
Mesophases exhibited by the compounds at the given temperature.
d = layer spacing (Å) of the smectic phase. a = lattice constat of the
b
hexagonal columnar mesophase.
It is important to highlight how a subtle change in the
structure of the oxadiazole heterocycle, namely the 1,3,4- or
1,2,4-oxadiazole ring, produces great differences not only in the
mesomorphic behavior, as described before, but also in the
supramolecular packing of the dendrimers. Thus, dendrimers
bearing the 1,2,4-OXAn unit (series II) also exhibit layer
spacings consistent with interdigitation of the oxadiazole
moieties. However, in this case, no influence of the spacer
length is observed, and full interdigitation between molecules of
adjacent layers occurs in all of the dendrimers, as deduced from
the X-ray results. This fact clearly indicates that more effective
intermolecular side-by-side interactions are taking place in case
of the 1,2,4-OXA moieties, favoring their interdigitation
independently of the spacer length.
The hexagonal columnar phase exhibited by PPI-(1,3,4-
OXA10)64 and PPI-(1,3,4-OXA4)64 is identified by the existence
of two sharp reflections in the reciprocal ratio 1:√3 in the low
angle region of the patterns (see Figure S4c). In this case the
molecules adopt a disk shape where the dendrimer matrix is
located in the central part and the oxadiazole moieties are
disposed radially.
As observed in Table 3, the structural parameters of this
columnar hexagonal mesophase considerably change in an
unexpected way when going from the longer spacer moiety
dendrimer (a = 49.4 Å) to the shorter one (a = 67.3 Å). This
fact is mainly connected to the elongation of the dendrimer
matrix along the vertical axis of the discs. By applying
geometrical calculations taking into account the XRD
experimental data,13c we have estimated the height (h) of the
disks that constitute the columnar arrangement, being h = 34 Å
for PPI-(1,3,4-OXA10)64 and h = 16 Å for PPI-(1,3,4-OXA4)64.
These height values indicate that the dendrimer with the longer
spacer elongates twice along the vertical axis of the disk
compared to the dendrimer with the smaller spacer. Hence,
neighboring columns interpenetrate in a large extent in the case
of the PPI-(1,3,4-OXA4)64 (Figure 5a) compared to PPI-(1,3,4-
The length of the low-molecular-weight molecules obtained
by molecular modeling are 35.8 Å (1,3,4-OXA10), 28.7 Å (1,3,4-
OXA4), 35.0 Å (1,2,4-OXA10), and 28.2 Å (1,2,4-OXA4). These
data are in accordance with the experimental values obtained by
XRD for the mesomorphic 1,2,4-OXA acids (38.5 Å for 1,2,4-
OXA10 and 30.8 Å for 1,2,4-OXA4).
It has been previously described that this kind of ionic and
covalent dendrimers self-assembling in a smectic A layer adopt
a cylindrical shape, in which the substitutents are statistically
located upward and downward with respect to the dendrimeric
core.13c,22 The assumption of this location of the oxadiazole
moieties in the dendrimers here described should lead to layer
spacing being higher than twice the oxadiazole moieties length.
However, the XRD experimental layer spacing values are
significantly lower, suggesting some peculiarities in the
distribution of the oxadiazole units in the cylindrical model.
Furthermore, dendrimers of series I bearing the 1,3,4-OXA4
unit exhibit a layer spacing of 53.6−56.1 Å, whereas dendrimers
containing the same oxadiazole ring but a longer spacer (1,3,4-
OXA10) possess a shorter layer spacing (42.9−49.3 Å).
The most plausible explanation to justify these unexpected
experimental features is that some interdigitation of the
oxadiazole moieties belonging to adjacent layers takes place
(see Figure 4). In fact, the longer spacer present in the structure
of 1,3,4-OXA10 favors a more efficient interaction with
molecules of the neighboring layers. This packing is based on
a side-by-side arrangement of the aromatic moieties of adjacent
molecules in a head-to-tail fashion, and this arrangement allows
the electrostatic interactions between the carboxylate groups
and the ammonium groups of the dendrimer (Figure 4a). On
the other hand, interdigitation of 1,3,4-OXA4 with a side-by-
side arrangement of the aromatic moieties would preclude the
ionic interactions between the carboxylate groups and the
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Figure 4. Schematic representation of the molecular packing model of the ionic dendrimers in the SmA mesophase. (a) Interdigitated 1,3,4-OXA10
moieties. (b) Partially interdigitated 1,3,4-OXA4 moieties. (c) Schematic representation of the 1,3,4-OXA10 and 1,3,4-OXA4 acids. The thickness of
each sublayer, and hence the space filled by each molecule region, is approximately proportional to its mass.
to note that the interdigitation phenomenon between
molecules in neighboring columns follows the same trend
described for the low generation derivatives in the SmA phase.
Both effects (dendrimer matrix elongation and interdigitation)
are connected to each other and simultaneously contribute to
account for the difference in the hexagonal lattice constant a.
5. Optical Properties. The UV−vis absorption and
emission spectroscopic data of the OXAn acids and dendrimers
of series I and series II in CHCl3 and in casted films are
summarized in Table S3 (Supporting Information). All
compounds containing the same oxadiazole ring yielded
identical absorption and emission spectra. No remarkable
influence of the length of the spacer, the generation, or the type
of dendrimer has been found. Thus, dendrimers with the 1,3,4-
OXAn unit present the absorption maximum at 315−316 nm
and dendrimers with the 1,2,4-OXAn moiety exhibit the
absorption maximum at 295 nm (Figure 6). These absorption
bands are asigned to π → π* transitions produced in the
conjugated oxadiazole framework due to their high molar
Figure 5. Schematic representation of the disk packing, values of the
diameter a, and the height h of the dendrimer disks for (a) PPI-(1,3,4-
OXA4)64 and (b) PPI-(1,3,4-OXA10)64 in the hexagonal columnar
mesophase.
absorption coefficients ((4.7−3.3) × 104).4e
All of the compounds present emission in the UV region.
The emission spectra of the compounds are only dependent on
the type of oxadiazole ring. Compounds belonging to series I
exhibit the strongest emission maxima at 377−379 nm. Two
less intense maxima are found at ∼360 and ∼390 nm.
OXA10)64 (Figure 5b), and both situations fulfill the require-
ments for an efficient packing and space filling. It is interesting
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Regarding the luminescent properties, all the dendrimers
display emission in solution and in the solid state. Derivatives
bearing the 1,3,4-OXAn moiety present higher quantum yield
values compared to their homologous containing the 1,2,4-
OXAn unit.
It has been shown how the change in the oxadiazole
heterocycle isomer leads to remarkable differences in the
mesomorphism, structural parameters of the supramolecular
organization, and luminescent properties of these dendrimers.
Figure 6. UV absorption spectra in solution (CHCl3) (full line),
emission spectra in solution (CHCl3) (dashed line), and emission
spectra in film (dotted line). Red lines correspond to PPI-(1,3,4-
OXA4)32, and black lines correspond to PPI-(1,2,4-OXA10)64.
ASSOCIATED CONTENT
■
S
* Supporting Information
More details about the synthesis, characterization, thermal
stability data, POM textures, XRD patterns, and optical data of
the materials. This material is available free of charge via the
Internet at http://pubs.acs.org.
Compounds in series II present only one maximum at 365−366
nm (see Figure 6).
The quantum yields of luminescence have been measured
taking the 2-phenyl-5-(4-biphenylyl)-1,3,4-oxadiazole as stand-
ard (Φ = 0.80, benzene),23 and they fluctuate between 0.46 and
0.74. Lower quantum yields are observed for series II, as it was
expected because 1,2,4-oxadiazole molecules have been
previously reported to produce lower quantum yield values
compared with their 1,3,4-isomers.4e In general, in series I the
quantum yields decrease as the generation increases. However,
in series II, efficiency is not dependent on the generation. The
covalent compound (PPI-(1,3,4-OXA10)16-cov) presents a
lower value than the analogous ionic compound (PPI-(1,3,4-
OXA10)16).
The emission in solid film has also been analyzed for these
compounds by casting of a solution of ∼1 mg/mL in CHCl3 on
a quartz plate. Interestingly, as observed in Figure 6, emission
bands appear broader respect to those in solution because of
the stronger intermolecular interactions produced in the solid
state. The Stokes shifts are remarkably higher in film than in
solution.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: jbarbera@unizar.es (J.B.); mmarcos@unizar.es
(M.M.).
ACKNOWLEDGMENTS
■
This work was supported by the MICINN, Spain, under Project
CTQ2009-09030, and FEDER funding, EU, by the seventh FP-
THE PEOPLE PROGRAMME, The Marie Curie Actions;
́
ITN, No. 215884-2, and by the Gobierno de Aragon (Project
PI109/09, Research Group E04). S.H.-A. thanks the MICINN
(Spain) for a FPU grant.
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We have successfully synthesized liquid crystal dendrimers
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