Angewandte
Chemie
compounds. The disappearance of the bands in the IR
spectrum at about 1700 cm corresponding to carbonyl
temperature as the percentage of semifluorinated moiety
À1
increases. Surprisingly, X-ray results indicated some more
complicated phase behavior, because despite the fan-shaped
texture observed by POM, two SmA phases with different
spacing layers were detected. Both homo-dendrimers present
a SmA mesophase, but the layer thicknesses are different (see
Table 1). The layer spacing of the homodendrimer PPI-(ac)16
groups in the carboxylic acids and the presence of the bands
À1
around 1560 and 1400 cm corresponding to the asymmetric
and symmetric stretching modes of the carboxylate group
observed indicated the formation of the complexes. The same
1
conclusion is drawn by H NMR, because of the appearance
of the signal at circa d = 2.95 ppm that corresponds to
+
CH NH3 and the absence of signal at d = 2.70 ppm related
2
Table 1: Phases and X-ray data of the ionic compounds.
[8d,12]
to CH NH of PPI.
2
2
[
a]
[b]
[c]
Co-dendrimer
Phase
T [8C]
Layer spacing [ꢂ]
NMR studies also allow the proportion of the perhydro-
genated and the semifluorinated chains in the co-dendrimers
to be studied (see Scheme 1 and the Supporting Information).
The ratio of the integrals corresponding to the protons of the
methylene groups linked to the carboxylate group of the ac
PPI-(ac)16
PPI-(ac)14-(af)
SmAH
SmA
H
RT
RT
RT
50
RT
RT
RT
RT
RT
110
24.3
24.7
23.5 /27.4
–
–
2
4
4
6
[
d]
[e]
PPI-(ac) -(af)
SmA /SmA
12
H
F
+
PPI-(ac) -(af)
SmA
SmA
SmA
1
2
+
À
PPI-(ac)10-(af)
PPI-(ac) -(af)
moiety, CH (CH ) CH COO , that appear at d = 2.15 ppm
+
3
2
8
2
–
8
8
and the protons of the methylene groups linked to the
ammonium group coming from the terminal protonated
PPI-(ac) -(af)
SmAF
SmAF
SmAF
27.4
27.7
27.6
28.5
6
10
12
14
PPI-(ac) -(af)
4
+
amine groups of the PPI dendrimer, NCH CH CH NH , at
PPI-(ac) -(af)
2
2
2
3
2
d = 3.00 ppm confirmed the correspondence between the
theoretical and experimental composition of the ac moiety in
each co-dendrimer.
PPI-(af)16
SmA
F
[a] Mesophases exhibited by the compounds. [b] Temperature of the X-
ray study of these mesophases. [c] Layer spacing of the smectic phase
determined by X-ray diffraction. [d,e] Layer spacings of the two coexisting
smectic phases obtained for PPI-(ac) -(af) .
The mesomorphic behavior of these dendrimers was
analyzed by combining different techniques, including polar-
izing optical microscopy (POM), differential scanning calo-
rimetry (DSC), and X-ray diffraction (XRD). Despite the fact
that neither the acids nor commercial dendrimer are meso-
gens, the ionic interaction forces the phase segregation
responsible of the mesomorphism of these compounds. All
of them were liquid crystals at room temperature, except for
12
4
(SmA , 24.3 ꢀ) is significatly lower than the analogous totally
H
semifluorinated PPI-(af)16 (SmA , 28.5 ꢀ). This difference is
because the perhydrogenated chains possess more flexibility
F
[
14]
and may adopt a curly conformation, whereas the stiffness
arising from fluorinated chains produces a more elongated
conformation of the molecule. The layer spacing slightly
increases by 0.4 ꢀ in PPI-(ac) -(af) (24.7 ꢀ) relative to the
homodendrimer PPI-(ac)16 owing to the introduction of two
semifluorinated units. PPI-(ac) -(af) , PPI-(ac) -(af) , and
[8a]
the fully semifluorinated dendrimer [PPI-(af) ], which has
1
6
a melting point at 478C (Figure 1).
Upon observation by POM, textures characteristic of the
smectic A mesophase were detected. Fluorinated chains
1
4
2
[
13]
stabilize liquid crystal behavior in the molecules; therefore,
these materials show a tendency to increase the isotropization
2
14
4
12
PPI-(ac) -(af) present a SmA phase with a similar layer
6
10
spacing, indicating the predominance of the semifluori-
nated chains.
PPI-(ac) -(af) , PPI-(ac) -(af) , and PPI-(ac) -(af)
4
8
8
10
6
12
present textures observed by POM typical of a SmA
mesophase, but they exhibit a singular X-ray behavior,
because only diffuse scattering in the small-angle region
appeared, which suggests the existence of a frustrated
+
[15]
smectic A phase (SmA ). Furthermore, in case of PPI-
ac) -(af) the coexistence of two SmA phases with
(
1
2
4
different parameters (23.5 ꢀ and 27.4 ꢀ) is detected at
room temperature. These values agree with those coming
from dendrimers with only perhydrogenated acids (SmAH,
2
3.5 ꢀ) and only semifluorinated acids (SmA , 27.4 ꢀ).
F
The composition necessary to avoid the less-organized
+
SmA phase is clearly dominated by semifluorinated
moieties, because SmAF phase is maintained even with
six ac moieties; in contrast, a SmA phase is only obtained
H
when the content of ac is higher than twelve. The most
plausible explanation of this phenomenon is the different
nature and geometry of the semifluorinated and perhy-
drogenated chains. Taking into account the cross-sectional
+
Figure 1. Mesomorphic behavior. SmA =frustrated smectic A phase;
SmA =smectic A phase with dimensions similar to those for PPI-(ac) ;
SmA =smectic A phase with dimensions similar to those for PPI-(af) ;
C=crystalline behavior. x=average number of undecanoic acid units that
H
16
F
16
[
16]
2
2
area that each chain takes up (0.28 nm and 0.18 nm
contain the dendrimer.
respectively), the relative percentage that each moiety
Angew. Chem. Int. Ed. 2010, 49, 1990 –1994
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1991