Liquid-crystalline methanofullerodendrimers which display columnar mesomorphism

Article abstract of DOI:10.1039/b717105f

Liquid-crystalline methanofullerodendrimers were synthesized via the Bingel addition reaction of mesomorphic malonate derivatives and C₆₀. Second- and third-generation poly(benzyl ether) dendrons were selected as liquid-crystalline promoters to induce columnar mesomorphism. Based on a convergent and modular synthetic methodology, symmetrical (two identical dendrons) and non-symmetrical (two different dendrons) dendrimers were prepared, as well as hemidendrimers (only one dendron). The liquid-crystalline properties of the malonates and fullerodendrimers were investigated by polarized optical microscopy, differential scanning calorimetry, and X-ray diffraction. All the malonates give rise to hexagonal columnar phases of p6mm symmetry. As for the fullerodendrimers, the second-generation hemidendrimer shows a rectangular columnar phase of c2mm symmetry, while the other materials give rise to hexagonal columnar phases of p6mm symmetry. The Royal Society of Chemistry.

Full text of DOI:10.1039/b717105f

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www.rsc.org/materials | Journal of Materials Chemistry
Liquid-crystalline methanofullerodendrimers which display columnar
mesomorphism†‡
a
a
b
b
Natacha Maringa, Julie Lenoble, Bertrand Donnio, Daniel Guillon and Robert Deschenaux
a
*
*
Received 5th November 2007, Accepted 14th January 2008
First published as an Advance Article on the web 14th February 2008
DOI: 10.1039/b717105f
Liquid-crystalline methanofullerodendrimers were synthesized via the Bingel addition reaction of
mesomorphic malonate derivatives and C60. Second- and third-generation poly(benzyl ether) dendrons
were selected as liquid-crystalline promoters to induce columnar mesomorphism. Based on
a convergent and modular synthetic methodology, symmetrical (two identical dendrons) and
non-symmetrical (two different dendrons) dendrimers were prepared, as well as hemidendrimers
(only one dendron). The liquid-crystalline properties of the malonates and fullerodendrimers were
investigated by polarized optical microscopy, differential scanning calorimetry, and X-ray diffraction.
All the malonates give rise to hexagonal columnar phases of p6mm symmetry. As for the
fullerodendrimers, the second-generation hemidendrimer shows a rectangular columnar phase of c2mm
symmetry, while the other materials give rise to hexagonal columnar phases of p6mm symmetry.
was functionalized with second-generation poly(benzyl ether)
dendrons; rectangular columnar phases (c2mm symmetry) were
obtained. Within the rectangular columnar phases, the columns
were formed by a hexagonal close compact packing of C60,
surrounded by the dendrons, the alkyl chains forming the outer
layer of the columns. In a second study,5b Janus-type fullero-
pyrrolidines bearing a poly(benzyl ether) dendron functionalized
with alkyl chains and a poly(aryl ester) dendron functionalized
with cyanobiphenyl units were synthesized. The generation of
each dendron was varied, and, depending on the generation,
smectic (C and/or A) or rectangular columnar (c2mm symmetry
or p2gg symmetry) phases were obtained. We have demonstrated
that the supramolecular organization of the liquid-crystalline
fullero(codendrimers) within the mesophases is governed by (1)
the ‘‘aliphatic terminal chains/mesogenic groups’’ ratio, (2) effec-
tive lateral interactions between the cyanobiphenyl mesogenic
groups, (3) microsegregation of the dendrons, and (4) deforma-
tion of the dendritic core. In a third study,9 addition of two
poly(benzyl ether) dendrons onto C60 led to fulleropyrrolidines
which were found to be non-mesomorphic. The absence of
liquid-crystalline properties for those materials is the conse-
quence of the formation of materials which lack shape specificity
due to conformations induced by C60.
Introduction
[60]Fullerene-containing liquid crystals1 displaying columnar
phases are of interest for electronic and optoelectronic applica-
tions (e.g., one-dimensional electron transportation). So far,
only a few examples have been reported in the literature: colum-
nar phases were obtained by (1) attaching five aromatic groups
around one pentagon of [60]fullerene (C60),2 (2) complexing
C
60 with liquid-crystalline dendritic porphyrins,3 (3) mixing two
non-mesomorphic compounds, one of which is a C60-triphe-
nylene derivative,4 and (4) functionalizing C60 with mesomorphic
dendrimers.5
The use of liquid-crystalline dendrimers as mesomorphic
promoters to functionalize C60 has two advantages: firstly, the
fullerene cores are isolated from each other, and thus, C60–C60
interactions (responsible for the formation of aggregates), which
may be detrimental to the formation of mesophases, are reduced
or even suppressed,6 and secondly, the supramolecular organiza-
tion within the liquid crystal state can be controlled owing to the
numerous possibilities which can be used to modify the structure
of the dendrimers (generation, polarity and stiffness of the core,
number of branching units, nature of the end-groups).7
With a view to designing liquid-crystalline fullerenes which
display columnar phases, we selected poly(benzyl ether)
dendrons8 as a source of mesomorphism. In a first study,5a C60
Obviously, the formation of columnar phases for fullerene-
containing liquid crystals is not yet fully understood. An
important step to better our understanding of the ‘‘structure–
supramolecular organization’’ relationship for liquid-crystalline
fullerodendrimers could be achieved by investigating the proper-
ties of methanofullerenes and of their corresponding malonates.
Such a study would emphasize the role played by C60 in the
formation, structure, and stability of the columnar phases.
We report, herein, thesynthesis, characterization, mesomorphic
properties, and supramolecular organization of methanofullero-
dendrimers 1–5 (Charts 1 and 2) and compare their properties
with those of the corresponding malonates.
a
´
Institut de Chimie, Universite de Neuchatel, Avenue de Bellevaux 51, Case
Postale 158, 2009 Neuchatel, Switzerland. E-mail: robert.deschenaux@
ˆ
ˆ
unine.ch
b
´
Institut de Physique et Chimie des Materiaux de Strasbourg, Groupe des
Materiaux Organiques, 23 Rue du Loess, BP 43, 67034 Strasbourg Cedex
´
´
2, France. E-mail: daniel.guillon@ipcms.u-strasbg.fr
† This paper is part of a Journal of Materials Chemistry theme issue on
carbon nanostructures.
‡ Electronic supplementary information (ESI) available: Further
experimental details. See DOI: 10.1039/b717105f
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Chart 1
ether) dendron; G3C60 (5): third-generation poly(benzyl ether)
Results and discussion
dendron] (Chart 2).
Design
This study is based on symmetrical [(G2)2C60 (1): two second-
generation poly(benzyl ether) dendrons; (G3)2C60 (2): two
third-generation poly(benzyl ether) dendrons] and non-symmet-
rical [G2G3C60 (3): mixed second- and third-generation poly
(benzyl ether) dendrons] fullerodendrimers (Chart 1) and fullero-
hemidendrimers [G2C60 (4): second-generation poly(benzyl
Synthesis
The dendrons were synthesized via a convergent approach.10
The malonates were added onto C60 via the Bingel reaction.11
The syntheses of 6,5a 8,12 and 98b have already been described
elsewhere.
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The structure and purity of all compounds were confirmed by
1H NMR spectroscopy, GPC (all compounds were found to be
monodisperse), UV-vis spectroscopy, and elemental analysis.
Liquid-crystalline properties
The liquid-crystalline and thermal properties of the malonate
and fullerene derivatives were investigated by polarized optical
microscopy (POM), differential scanning calorimetry (DSC),
and X-ray diffraction (XRD). The liquid-crystalline and thermal
properties of the intermediates were investigated by POM and
DSC. The phase transition temperatures and enthalpies are
reported in Table 1. The XRD data are collected in Table 2.
With the exception of (G2)2C60 (1), which is non-mesomorphic,
all methanofullerenes display columnar mesomorphism. For
(G3)2C60 (2), G2G3C60 (3), and G3C60 (5), hexagonal columnar
phases (p6mm symmetry) were obtained, while for G2C60 (4) a
rectangular columnar phase (c2mm symmetry) was detected. A
typical texture was observed by POM only for G3C60 (5) (Fig. 1).
The clearing points indicate that the stability of the mesophase
increases with the dendrimer generation [e.g. 68 ꢀC for G2C60
(4), 85 ꢀC for G3C60 (5)] in agreement with previous results.14
All the malonates give rise to hexagonal columnar phases
(p6mm symmetry). Typical textures were observed by POM
(Fig. 2 and 3). As expected, the stability of the mesophases
ꢀ
increases with the dendrimer generation [clearing points: 88 C
for (G2)2Mal (7), 105 ꢀC for G2G3Mal (14), and 109 ꢀC for
(G3)2Mal (12); 86 ꢀC for G2Mal (15) and 114 ꢀC for G3Mal
(16)]. As already observed for other compounds,14 grafting of
C60 onto malonates lowered the isotropization temperature, or
even suppressed the liquid-crystalline properties [i.e. for
(G2)2C60 (1)].
Chart 2
X-Ray investigations
The synthesis of methanofullerene (G2)2C60 (1) is presented in
Scheme 1. Condensation of 6 with malonyl chloride gave
malonate derivative (G2)2Mal (7), which was added to C60 to
give (G2)2C60 (1).
Compounds (G2)2C60 (1) and (G2)2Mal (7). Whereas com-
pound (G2)2C60 (1) is not mesomorphic, its malonate precursor
(G2)2Mal (7) shows complicated thermal behavior. The sample
melts into an amorphous solid, which then crystallizes, and
finally melts into the mesophase which was identified as a hexa-
gonal columnar phase. X-Ray diffraction patterns recorded every
5 ꢀC from 40 to 90 ꢀC correspond systematically to a mixture of
amorphous, crystalline, columnar and isotropic phases depending
upon temperature. This behavior highlights some kinetic and
thermodynamic features associated with the phases transforma-
tion. This mesophase is thermodynamically unstable.
The synthesis of fullerodendrimer (G3)2C60 (2) is illustrated
in Scheme 2. Esterification of 8 and 9 in the presence of N,N0-
dicyclohexylcarbodiimide (DCC), 4-(dimethylamino)pyridinium
toluene-p-sulfonate (DPTS), and 4-pyrrolidinopyridine (4-Ppy)
led to alcohol intermediate 10, which was reacted with Meldrum
acid (2,2-dimethyl-1,3-dioxane-4,6-dione) to furnish carboxylic
acid 11. Esterification of the latter with 10 gave malonate
(G3)2Mal (12), which was used in the addition reaction with
C60 [/ (G3)2C60 (2)].
Compounds (G3)2C60 (2) and (G3)2Mal (12). Above ca. 40 ꢀC,
both compounds exhibit a hexagonal columnar phase character-
ized by a series of three sharp and intense diffraction peaks in the
small-angle region, in the ratio 1 : O3 : O4. The X-ray diffraction
The synthesis of methanofullerene G2G3C60 (3) is reported in
Scheme 3. Condensation of 6 with Meldrum acid gave 13, which
was esterified with 10 to yield malonate G2G3Mal (14). Addition
of G2G3Mal (14) with C60 furnished G2G3C60 (3).
˚
patterns contain also two diffuse signals at ca. 8.5 and 4.6 A,
The synthesis of fullerohemidendrimers G2C60 (4) (Scheme 4)
and G3C60 (5) (Scheme 5) required the preparation of G2Mal
(15) and G3Mal (16), respectively, which were obtained by
condensation of ethyl malonyl chloride with the appropriate
alcohol derivative [6 / G2Mal (15), and 10 / G3Mal (16)].
Addition of G2Mal (15) or G3Mal (16) to C60 led to G2C60
(4) or G3C60 (5), respectively.
which are attributed to an intracolumnar periodicity along the
columnar axis and to the liquid-like arrangement of the terminal
aliphatic chains of the dendrons, respectively. The hexagonal cell
parameters of both compounds are almost the same, and do not
vary significantly as a function of temperature. Therefore, the
lateral and stacking arrangements of (G3)2C60 (2) and
(G3)2Mal (12) are essentially governed by the dendritic parts.
1526 | J. Mater. Chem., 2008, 18, 1524–1534
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Scheme 1 (i) Malonyl chloride, Et3N, CH2Cl2, r.t., overnight, 27%; (ii) C60, DBU, I2, toluene, r.t., overnight, 26%. For abbreviations, see ref. 13.
The C60 unit is encapsulated in the dendritic matrix and has no
significant influence on the supramolecular organization.
phase) and G2Mal (15) (hexagonal columnar phase) is due to the
presence or not of C60, the attachment of which onto the spacer
strongly limits the possibility for the dendritic part to self-
organize into a hexagonal symmetry.
Compounds G2G3C60 (3) and G2G3Mal (14). Both compounds
display an amorphous structure at room temperature. A
ꢀ
hexagonal columnar phase is obtained above 55 C. The latter
Compounds G3C60 (5) and G3Mal (16). Hemidendrꢀimer G3C60
(5) exhibits a hexagonal columnar phase above 70 C deduced
from the presence of two sharp reflections in the small-angle
region of the X-ray diffraction patterns. These signals can be
indexed as the (10) and (11) reflections of a two-dimensional
mesophase is characterized by two [for G2G3C60 (3)] or three
[for G2G3Mal (14)] sharp diffraction signals in the small-angle
region in the ratio 1 : O3 : O4; two diffuse signals at ca. 8–8.5
˚
and 4.6 A are also detected. The hexagonal cell parameters of
˚
hexagonal lattice. In addition, a diffuse band at 8.5 A indicates
G2G3C60 (3) and G2G3Mal (14) are similar and do not vary
with temperature, confirming that C60 does not influence the
molecular organization of the mesophase.
the presence of an average periodicity along the columnar axis.
For the corresponding malonate G3Mal (16), the presence of
a hexagonal columnar phase from room temperature up to 110
ꢀC is characterized by three reflections indexed as (10), (11)
and (20) of a two-dimensional arrangement. The hexagonal
cell parameters are similar for both compounds, indicating the
primary role of the dendrimer in the supramolecular organiza-
tion. The C60 unit is effectively encapsulated in G3C60 (5) as it
is for (G3)2C60 (2) and G2G3C60 (3).
Compounds G2C60 (4) and G2Mal (15). For hemidendrimer
G2C60 (4), a rectangular columnar phase forms above 55 ꢀC.
A series of nine sharp diffraction signals in the small-angle region
has been indexed according to a two-dimensional centered rect-
angular lattice of c2mm symmetry. For precursor G2Mal (15),
no clear organization is revealed at room temperature (only
diffuse signals are observed). However, above 40 ꢀC, a sharp
and intense reflection in the small-angle region is detected,
reminiscent of some of the thermal events observed for other
compounds described above, and consisting of a crystallization
process with slow kinetics, resultiꢀng in a mixture of amorphous
and crystalline solids. Above 80 C, the X-ray patterns exhibit
three small-angle reflections which can be indexed according to
a two-dimensional lattice of a hexagonal columnar phase. The
difference of behavior between G2C60 (4) (rectangular columnar
Supramolecular organization
Liquid-crystalline malonates. Overall, the self-organization
behavior of dendritic (7, 12, 14) and hemidendritic (15, 16)
malonates and fullerodendrimers (2–5) within the columnar
phases retains the general characteristics of the methyl ester
dendrons of second (derived from 6) and third (derived from
9) generation8b used here as building blocks. In particular,
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Scheme 2 (i) DCC, DPTS, 4-Ppy, CH2Cl2, r.t., overnight, 48%; (ii) Meldrum acid, toluene, 65 ꢀC, 24 h, 89%; (iii) DCC, DPTS, 4-Ppy, CH2Cl2, r.t.,
overnight, 80%; (iv) C60, DBU, I2, toluene, r.t., overnight, 14%. For abbreviations, see ref. 13.
1528 | J. Mater. Chem., 2008, 18, 1524–1534
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Scheme 3 (i) Meldrum acid, toluene, 65 ꢀC, 24 h, 92%; (ii) DCC, DPTS, 4-Ppy, CH2Cl2, r.t., overnight, 76%; (iii) C60, DBU, I2, toluene, r.t., overnight,
28%. For abbreviations, see ref. 13.
hemidendrimers G2Mal (15) and G3Mal (16) exhibit comparable
mesomorphism (mesophase stability and mesophase parameters)
to that of the related esters derived from 6 and 9, respectively,
indicative that a similar packing mode within the hexagonal
columnar phases is reproduced. In the flat conformation,
G2Mal (15) approximates an open fan-shape with a planar angle
a at the focal point of ca. 120ꢀ, whereas G3Mal (16) resembles
more a semi-disc (a ꢁ 180ꢀ), as shown in Scheme 6. Using
a straightforward geometrical approach, which takes into
account the relationships between the planar angle a, that repre-
sents the projection of the tapered dendrons solid angle u (a ¼
u/2) and ND (number of monodendrons or dendritic branches
covering the columnar cross-section area, thus per repeat unit)
linked through the equation a ¼ 2p/ND,8b it was found that 3
molecules of G2Mal (15) and 2 molecules of G3Mal (16), respec-
tively (here Z ¼ ND), self-associate into an elementary slice of
cylindrical column, with calculated thicknesses h00 of 8.6 and
accommodates the aliphatic malonate moiety; the columns are
surrounded by the molten aliphatic chains.
The connection of two dendrons by a long spacer (leading to
dendritic dimers) does not modify the nature of the mesophase,
and hexagonal columnar phases are observed for (G2)2Mal (7),
(G3)2Mal (12) and G2G3Mal (14). However, the mesophase
stability, and to some extent the thermal behavior, can be
affected upon dimerization as shown by (G2)2Mal (7), for which
an unstable thermal phase sequence is observed. In contrast, the
behavior of (G3)2Mal (12) is comparable to that of the third-
generation methyl ester precursor.8b The mixed derivative
G2G3Mal (14) also behaves like (G3)2Mal (12) suggesting that
the self-organization properties of G2G3Mal (14) are dominated
by its large side. In both cases, intercolumnar distances are close
to each other and to that of G3Mal (16) (and also to the mono-
meric methyl ester precursor8b); similarly, the dimensions of the
hexagonal cell of (G2)2Mal (7) and G2Mal (15) are identical.
As a consequence, the same type of aggregation found for
G2Mal (15) and G3Mal (16) can be deduced for their symmetri-
cal and mixed dimeric structures. Applying the retrostructural
analysis8 used above, although with ND ¼ 2 ꢂ Z, 1.5 molecular
equivalents of (G2)2Mal (7) [equivalent to three (ND) dendritic
parts as in G2Mal (15), a ¼ 120ꢀ for each dendritic branch],
˚
7.9 A, respectively. The latter values are in good agreement
with h0, i.e. the average periodicities of the broad halos measured
on the diffractograms. In other words, the repeating distance
along the columnar axis is defined by the thickness of the disc
formed by the dendritic branches. The hard part of the column
is filled with the poly(benzyl ether) dendron, and the inner part
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Table 1 Phase transitiona temperatures and enthalpies of dendrons 10,
11, and 13, malonates 7, 12, and 14–16, and methanofullerenes 1–5
Compound
Tg/ꢀC Transition
Temperature/ꢀC DH/kJ molꢃ1
10
11
63
65
Col / I
Col / I
Cr / I
Cr / Colh-p6mm 73
Colh-p6mm / I
Cr / I
Colh-p6mm / I 109
Colh-p6mm / I 93
Colh-p6mm / I 105
Colh-p6mm / I 74
Cr / Colh-p6mm 56
115
112
11.3
11.6
1.9
7.8
—
18.8
20.7
15.8
18.2
10.9
5.1
13
(G2)2Mal (7)
72b
88c
52b
(G2)2C60 (1)
(G3)2Mal (12)
(G3)2C60 (2)
G2G3Mal (14) 45
G2G3C60 (3) 64
d
44
G2Mal (15)
40
Colh-p6mm / I
Colr-c2mm / I
Colh-p6mm / I 114
Colh-p6mm / I 85
86
68
8.1
6.8
11.1
7.9
G2C60 (4)
G3Mal (16)
G3C60 (5)
36
51
35
a
Cr ¼ crystalline or semicrystalline solid, Tg ¼ glass transition
temperature determined during the first cooling run, Col ¼ columnar
phase, Colh-p6mm ¼ hexagonal columnar phase of p6mm symmetry,
Colr-c2mm ¼ rectangular columnar phase of c2mm symmetry, I ¼
isotropic liquid. Temperatures are given as the onset values taken from
b
the second heating run. Temperature determined during the first
heating run. Determined by POM and XRD. Not detected.
c
d
Scheme 4 (i) Ethyl malonyl chloride, Et3N, CH2Cl2, r.t., overnight,
52%; (ii) C60, DBU, I2, toluene, r.t., overnight, 23%. For abbreviations,
see ref. 13.
1 molecule of (G3)2Mal (12) [equivalent to two (ND) dendritic
parts as in G3Mal (16), a ¼ 180ꢀ for each dendron] and 1
molecule of G2G3Mal (14) [as for (G3)2Mal (12), ND ¼ 2] fitting
the volume of the elemental stratum with a thickness h00 of 8.0,
˚
8.3, and 8.3 A, respectively. Again, a good agreement is found
between h0 and h00, and this periodicity is assigned to the mean
stacking distance between discs. Similarly to the hemiden-
drimers, the column formation results from the stacking of the
supramolecular discs, the hard part of the column is filled with
the poly(benzyl ether) dendron, and the inner part accommo-
dates the malonate moiety; the columns are surrounded by the
molten aliphatic chains. Note that in the supramolecular organi-
zation of (G2)2Mal (7), one of the dimer shares its dendritic
branches with two consecutive slices (discs) of the columns,
which is permitted due to the great flexibility of the malonate
spacer.
Liquid-crystalline fullerodendrimers. The non-mesomorphic
character of (G2)2C60 (1) confirms that mesophase induction in
such bulky materials requires the connection of strong liquid-
crystalline promoters.1 As for fullerodendrimers (G3)2C60 (2)
and G2G3C60 (3), derived from malonates (G3)2Mal (12) and
G2G3Mal (14), respectively, they both exhibit a hexagonal
columnar phase. The dimensions of the hexagonal lattices were
not greatly modified upon addition of C60, indicating a similar
paving of the two-dimensional network of these species
[(G3)2C60 (2), G2G3C60 (3), (G3)2Mal (12), and G2G3Mal
(14)]. The same retrostructural analysis can thus be applied here
for the paving of the hexagonal lattices. However, to accommo-
date the bulky C60 unit, the thickness of an elementary stratum
˚
of the column needs to increase to a minimum of 10 A, i.e. the
diameter of C60. For (G3)2C60 (2), the planar angle of the
dendritic branches is ca. 180ꢀ, which suggests one molecule
(Z ¼ 1, ND ¼ 2) to cover the hexagonal lattice and forming one
Scheme 5 (i) Ethyl malonyl chloride, Et3N, CH2Cl2, r.t., overnight,
91%; (ii) C60, DBU, I2, toluene, r.t., overnight, 16%. For abbreviations,
see ref. 13.
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Table 2 X-Ray characterization of the mesophases
a
ad
e
[hk]b
Ic
Parametersd
ND
˚
dexp/A
˚
dtheo/A
Compound
(G2)2Mal (7)
35.6
4.6
10
hch
VS (sh)
VS (br)
35.6
T ¼ 80 ꢀC
3
˚
a ¼ 41.1 A
2
˚
S ¼ 1460 A
3
˚
Vmol ¼ 7960 A
h00 ꢁ 8.0 A, Z ¼ 1.5
˚
(G3)2Mal (12)
(G3)2C60 (2)
G2G3Mal (14)
G2G3C60 (3)
G2Mal (15)
G2C60 (4)
40.3
23.25
20.15
8.5
10
11
20
h0
hch
10
11
20
h0
hch
10
11
20
h0
hch
10
11
h0
VS (sh)
M (sh)
M (sh)
VW (br)
VS (br)
VS (sh)
M (sh)
M (sh)
VW (br)
VS (br)
VS (sh)
M (sh)
M (sh)
VW (br)
VS (br)
VS (sh)
M (sh)
W (br)
VS (br)
40.3
23.27
20.15
T ¼ 90 ꢀC
2
˚
a ¼ 46.5 A
2
˚
S ¼ 1875 A
3
˚
Vmol ¼ 15570 A
h00 ꢁ 8.3 A, Z ¼ 1
˚
4.5
40.75
23.55
20.35
8.5
40.75
23.52
20.37
T ¼ 80 ꢀC
4
˚
˚
a ¼ 47.05 A
2
S ¼ 1920 A
3
˚
Vmol ¼ 16160 A
h00 ꢁ 16.8 A, Z ¼ 2
˚
4.6
39.75
22.95
19.9
8.0
39.8
22.97
19.9
T ¼ 60 ꢀC
2
˚
˚
a ¼ 45.95 A
2
S ¼ 1830 A
3
˚
Vmol ¼ 11540 A
h00 ꢁ 8.3 A, Z ¼ 1
˚
4.6
40.45
23.3
8.5
40.4
23.3
T ¼ 80 ꢀC
4
˚
˚
a ¼ 46.65 A
2
S ¼ 1885 A
3
˚
4.6
hch
Vmol ¼ 12410 A
h00 ꢁ 13.2 A, Z ¼ 2
˚
35.25
20.5
17.5
8.0
10
11
20
h0
VS (sh)
M (sh)
M (sh)
VW (br)
VS (br)
VS (sh)
VS (sh)
S (sh)
35.25
20.35
17.6
T ¼ 80 ꢀC
3
˚
a ¼ 40.7 A
2
˚
S ¼ 1435 A
3
˚
Vmol ¼ 4120 A
h00 ꢁ 8.6 A, Z ¼ 3
˚
4.5
hch
11
20
02
31
22
40
13
33
60
h0
hch
10
11
20
h0
75.15
66.85
45.5
39.9
37.5
33.35
29.6
25.1
22.3
8.8
75.15
66.85
45.6
40.0
37.7
33.4
29.65
25.1
22.3
T ¼ 60 ꢀC
12
˚
a ¼ 133.7 A
˚
b ¼ 91.2 A
2
˚
VW (sh)
S (sh)
S ¼ 6100 A
3
˚
Vmol ¼ 4760 A
h00 ꢁ 9.4 A, Z ¼ 12
˚
M (sh)
M (sh)
M (sh)
W (sh)
W (br)
VS (br)
VS (sh)
S (sh)
4.5
G3Mal (16)
G3C60 (5)
41.3
23.9
20.6
8.0
41.3
23.85
20.65
T ¼ 60 ꢀC
2
2
˚
a ¼ 47.7 A
2
˚
S (sh)
S ¼ 1970 A
3
˚
VW (br)
VS (br)
VS (sh)
S (sh)
VW (br)
VS (br)
Vmol ¼ 7750 A
h00 ꢁ 7.9 A, Z ¼ 2
˚
4.6
hch
10
11
h0
39.8
22.8
8.5
39.65
22.9
T ¼ 80 ꢀC
˚
a ¼ 45.8 A
2
˚
S ¼ 1820 A
3
˚
4.6
hch
Vmol ¼ 8570 A
h00 ꢁ 9.4 A, Z ¼ 2
˚
a
b
dexp and dtheo are the experimental and theoretical diffraction spacings, respectively. [hk] are the indexation of the reflections; h0 and hch are short
˚
range order periodicities determined by XRD corresponding to some weak liquid-like correlations (accuracy: ca. ꢄ1 A) and to the liquid-like order of
c
the molten chains, respectively. Intensity of the reflections: VS: very strong, S: strong, M: medium, W: weak, VW: very weak; br: broad, sh: sharp.
d
dtheo is deduced from the lattice parameters a (Colh) or a and b (Colr) from the following mathematical expressions: i) for Colh, a ¼ 2 ꢂ [Shkdhk.(h2 + k2
+ hk) ]/O3Nhk where Nhk is the number of hk reflections. S is the lattice area: S ¼ a2O3/2; ii) for Colr, S ¼ a ꢂ b/2 and <dhk> ¼ 1/[(h2/a2 + k2/b2) ]. Z is the
½
½
aggregation number or the number of molecular equivalents per stratum of column. Vmol is the molecular volume: Vmol ¼ VC60 + Vmalonate, where VC60
¼ 700 A , Vmalonate ¼ (MW/0.6022) ꢂ (VCH2(T)/VCH2(T0)), MW the molecular weight of the malonate and VCH2 ¼ 26.5616 + 0.02023T. h00 is the
3
˚
e
theoretical intracolumnar repeating distance, deduced from the measured molecular volume and the columnar cross-section, h00 ¼ Z ꢂ Vmol/S. ND:
number of dendritic branches per stratum.
00
˚
stratum (h ¼ 8.4 A) as in (G3)2Mal (12). However, such a height
of column is not compatible with the molecular thickness imposed
by C60. Consequently, two molecules of (G3)2C60 (2) were consid-
ered to define one slice of column (h00 ¼ 2 ꢂ h0), with h0 corres-
ponding to the average distance between two consecutive discs
(i.e. the thickness of the dendritic branches). Similarly, for the
mixed fullerodendrimer G2G3C60 (3), one molecule was first
considered, by analogy to its malonate precursor, leading to a
˚
columnar slice thickness of ca. 6.6 A, incompatible with the size
of C60. As above, two molecules were thus considered to match
the structural requirements. Note that for fullerodendrimers
bearing two dendritic branches, non-integer Z values are unlikely
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Scheme 6 Schematic representation of the formation of supramolecular
discs with hemidendritic and dendritic systems. The supramolecular
organization of the fullerodendrimers is derived from that of the
malonate precursors. For details, see Table 2 and main text.
(the molecules are no longer dimers, C60 is indeed indivisible). For
this compound, the discrepancy between the two periodicities is
likely due to a deficit of dendritic branches, not large enough to
fully embed C60, with h0 representing the interdisc distance over
a short range correlation length. Thus, the larger hexagonal cells
Fig. 1 Thermal polarized optical micrograph of the texture displayed by
G3C60 (5) in the hexagonal columnar phase at 84 ꢀC.
00
˚
with a thickness h of 16.8 and 13.2 A, respectively, incorporate 2
molecular equivalents of (G3)2C60 (2) and G2G3C60 (3). In both
arrangements, the C60 units are located towards the interior of the
column, and loosely stacked along the columnar axis, surrounded
by the dendritic part.
Rectangular and hexagonal columnar phases are obtained for
hemidendrimers G2C60 (4) and G3C60 (5), respectively. The
lattice parameter of the hexagonal columnar phase on going
from G3Mal (16) to G3C60 (5) remains almost constant, so
that the same packing mode is likely reproduced, and for an
equivalent surface to pave, two molecules per lattice were
considered leading to a cell thickness h00 of 9.4 A. In this case,
˚
the thickness is compatible with aggregates of C60 if some kind
of close compact-like (hexagonal or cubic) arrangement of C60
in the centre of the column is considered (h00 ꢁ fO3/2, f ꢁ 10
˚
A ¼ diameter of C60), which is allowed since only one dendritic
Fig. 2 Thermal polarized optical micrograph of the texture displayed by
G2Mal (15) in the hexagonal columnar phase at 85 ꢀC.
branch is attached to C60, encouraging C60–C60 interactions
through the ‘‘unprotected face’’.5a However, for G2C60 (4), the
situation is slightly more complicated since the expansion of
the cell occurs in the symmetry plane as well as along the column.
The section area S of the column is almost quadrupled on going
from G2Mal (15) to G2C60 (4). Considering that the dendritic
parts adopt in both cases a flat conformation and that each
tapered dendron paves a similar surface area, the number of
fullerodrendrimers should also be quadrupled. Thus, at least
twelve molecules of G2C60 (4) are considered in the elementary
columnar slice, with a thickness h00 of 9.4 A, which is identical
˚
to the value found for G3C60 (5). The average stacking distance,
h0, between consecutive disc-like [G3C60 (5)] or discoid-like
[G2C60 (4)] assemblies remains the same. Such a non-circular
arrangement was also found in related systems,5a and the rather
low thickness value was attributed to the close packing of C60
into aggregates forming the centre of the column. Thus, the pack-
ing of G2C60 (4) and G3C60 (5) is different to that of (G3)2C60 (2)
and G2G3C60 (3) in the sense that for (G3)2C60 (2) and G2G3C60
(3) the packing and the C60 interactions are rather loose whilst for
G2C60 (4) and G3C60 (5) the C60 units self-assemble into aggre-
gates, which grow along the columnar axis.
Fig. 3 Thermal polarized optical micrograph of the texture displayed by
(G2)2Mal (7) in the hexagonal columnar phase at 79 ꢀC.
1532 | J. Mater. Chem., 2008, 18, 1524–1534
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In order to confirm the above arrangements, molecular
dynamics experiments on compound G2G3C60 (3) were per-
formed. Two minimized structures of G2G3C60 (3) were placed
The results of the dynamics experiments are in agreement with
the experimental data: a lattice parameter comparable to that
of the X-ray experiments and a density close to unity were found.
Moreover, a perfect paving of the surface and a dense occupa-
tion of the available volume were confirmed (Fig. 4 and 5).
in a hexagonal cell, where only the thickness was fixed (h00
¼
˚
13 A), and the molecules were only allowed to expand laterally.
Conclusion
In order to design fullerodendrimers which display columnar
mesomorphism, C60 was functionalized via the Bingel reaction
with symmetrical and non-symmetrical malonates which carry
poly(benzyl ether) dendrons as liquid-crystalline promoters.
The liquid-crystalline malonates were prepared by applying
a modular synthetic approach. Hemidendrimers were also
synthesized. All the malonate and fullerene derivatives give rise
to columnar phases. The supramolecular organization of the
malonate and fullerene derivatives within the columnar phases
is governed by the dendrimer. Indeed, for the fullerodendrimers,
C
60 has no (or little) influence on the structure of the mesophases
providing the size of the dendritic addends is large enough, i.e.
when two third-generation dendrons [compound (G3)2C60 (2)],
or one third-generation and one second-generation [compound
G2G3C60 (3)] dendron are associated, or for a third-generation
hemidendrimer [compound G3C60 (5)]. If the dendron is not
large enough to encapsulate C60, the latter influences the organi-
zation as observed for compound G2C60 (4) (second-generation
hemidendrimer) which gives rise to a rectangular columnar phase
whereas its malonate precursor shows a hexagonal columnar
phase. The design of fullerene-containing liquid crystals with
tailor-made properties requires that the influence of C60 is dras-
tically minimized and even suppressed. This work and former
studies1,5b confirm that functionalization of C60 with liquid-
crystalline dendrimers is an elegant strategy for the design of
fullerene-containing liquid crystals for which the mesomorphism
and supramolecular organization can be controlled by design.
Fig. 4 Top view of the supramolecular organization of G2G3C60 (3)
within the hexagonal columnar phase of p6mm symmetry.
Acknowledgements
RD thanks the Swiss National Science Foundation (Grant No.
200020-111681) for financial support. BD and DG thank
Dr C. Bourgogne for modelling experiments and CNRS-Univer-
´
site Louis Pasteur for constant support.
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