Self-assembly of semifluorinated Janus-dendritic benzamides into bilayered pyramidal columns

Article abstract of DOI:10.1002/anie.200501254

(Figure Presented) Two faces: Semifluorinated Janus-dendritic benzamides self-assemble into supramolecular bilayered pyramidal columns with diameters over twofold greater than those of columns generated from twin-dendritic benzamides (see model). The Janu

Full text of DOI:10.1002/anie.200501254

Angewandte
Chemie
Liquid Crystal Dendrimers
Self-Assembly of Semifluorinated Janus-
Dendritic Benzamides into Bilayered Pyramidal
Columns**
Virgil Percec,* Mohammad R. Imam, Tushar K. Bera,
Venkatachalapathy S. K. Balagurusamy, Mihai Peterca,
and Paul A. Heiney
Amphiphilic dendrons that adopt the shape of a disk or disk
fragment can self-assemble into supramolecular columns
which self-organize into p6mm hexagonal or p2mm and
c2mm rectangular 2D periodic assemblies.[1,2] The apex of
such dendrons generates the hollow[1l] or occupied[1a–k] func-
tional core of the supramolecular column (Figure 1a). Twin-
tapered dendritic benzamides self-assemble into supramolec-
Figure 1. Self-assembly of building blocks based on a) flat, tapered
dendrons into hollow and occupied supramolecular columns; b) twin
dendrons into cylindrical columns; and c) crownlike dendrons into
pyramidal columns and their subsequent self-organization into p6mm
hexagonal columnar (Fh), p2mm simple rectangular columnar (Fr-s),
or c2mm centered rectangular columnar (Fr-c) lattices.
[*] Prof. V. Percec, M. R. Imam, Dr. T. K. Bera, Dr. V. S. K. Balagurusamy
Roy & Diana Vagelos Laboratories, Department of Chemistry
Laboratory for Research on the Structure of Matter
University of Pennsylvania
ular columns generated by an orthogonal hydrogen-bonded
stack of dendritic benzamide units (Figure 1b).[3] These
supramolecular columns self-organize into periodically
ordered p6mm hexagonal assemblies or co-assemble into
more complex hexagonal superlattices.[3] Amphiphilic den-
drons that adopt the shape of a sphere fragment can self-
assemble into spherical supramolecular dendrimers, which
Philadelphia, PA 19104-6323 (USA)
Fax: (+1)215-573-7888
E-mail: percec@sas.upenn.edu
Dr. V. S. K. Balagurusamy, M. Peterca, Prof. P. A. Heiney
Department of Physics and Astronomy
Laboratory for Research on the Structure of Matter
University of Pennsylvania
[1d,h,4a–4c]
[4d–f]
¯
¯
self-organize into periodic Pm3n,
Im3m
3D cubic
and P42/mnm 3D tetragonal,[4g] and quasiperiodic 3D 12-fold
symmetry-forbidden liquid quasicrystals.[4h] It was recently
reported that semifluorination of the conical dendron
(3,4,5)212G2-CO2CH3 yields the crownlike dendron
(3,4,5)212F8G2-CO2CH3 as one of the two products.[5] This
Philadelphia, PA 19104-6396 (USA)
[**] Financial support by the National Science Foundation (DMR-
9996288 and DMR-0102459) is gratefully acknowledged.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2005, 44, 4739 –4745
DOI: 10.1002/anie.200501254
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4739

Communications
process changes the mode of self-
assembly from a supramolecular
sphere (for (3,4,5)212G2-CO2CH3)
to
column
a
supramolecular pyramidal
for
(3,4,5)212F8G2-
CO2CH3 (Figure 1c). Herein, we
report that semifluorination of one
of the two dendrons of the twin-
dendritic benzamides (that is, a
benzamide containing two identi-
cal dendrons)[3b,c] changes the mode
of self-assembly from a single-layer
supramolecular column to an
unprecedented bilayered pyrami-
dal supramolecular column. The
supramolecular pyramidal columns
generated from these semifluori-
nated
Janus-dendritic
benz-
Table 1: Thermal analysis of the supramolecular dendrimers self-assembled from twin-dendritic and
amides (that is, a benzamide con-
taining two different dendrons)
exhibit diameters more than twice
as large as their nonfluorinated or
Janus-dendritic benzamides.[a]
Compound
Heating
T
[8C]
Cooling
T
[8C]
Phase
Transition
DH
Phase
Transition
DH
[kcalmolꢀ1
]
[kcalmolꢀ1
]
semifluorinated
twin-dendritic
1
k!Fh
Fh!i
k!Fh
Fh!i
76.9
121.3
73.5
35.04
0.63
14.60
0.42
i!Fh
112.7
47.1
ꢀ0.77
benzamide counterparts. Non-
fluorinated Janus-dendritic benza-
mides continue to self-assemble
like twin-dendritic benzamides.
Our study involved two non-
Fh!k(Fr-c)
ꢀ13.94
s.h.[b]
120.6
2
k!Fh
Fh!i
k1!k2
k2!Fh
Fh!i
134.9
150.1
104.6
134.8
149.8
16.28
2.38
ꢀ0.64
16.65
2.29
i!Fh
Fh!k
146.5
49.9
ꢀ2.26
fluorinated
(3,4,5)12G1-(3,4,5)12G1-BzA
benzamides
ꢀ10.06
s.h.
1
and (4-3,4,5)12G1-(4-3,4,5)12G1-
BzA 3, one fluorinated twin-den-
dritic benzamide (3,4,5)12F8G1-
(3,4,5)12F8G1-BzA 2, and a non-
fluorinated Janus-dendritic benza-
mide (3,4,5)12G1-(4-3,4,5)12G1-
BzA 4. The synthesis, structural
analysis, and mechanism of self-
assembly of the two nonfluorinated
twin-dendritic benzamides 1 and 3
were reported previously.[3b,c] Both
1 and 3 self-assemble into supra-
molecular columns by the orthog-
onal hydrogen-bonded stack mech-
anism outlined in Figure 1b. The
fluorinated twin-dendritic benza-
mide 2 was synthesized with the
procedure used for the correspond-
ing nonfluorinated twin-dendritic
benzamides[3b] (see Supporting
Information). The semifluorinated
twin-dendritic benzamides self-
assemble into supramolecular col-
umns by a mechanism similar to
that of the nonfluorinated twin-
dendritic benzamides (Figure 1b).
The phase transitions for these
three twin-dendritic benzamides,
identified by a combination of
3
k1!k2
k2!Fh
Fh!i
k1!k2
k2!Fh
Fh!i
66.3
101.0
182.8
ꢀ5.9
101.5
181.9
20.34
11.16
2.43
2.37
12.41
2.76
i!Fh
Fh!k
179.5
78.3
ꢀ2.54
ꢀ10.72
s.h.
4
k!Nc
Nc!Fh
Fh!i
k!Fh
Fh!i
45.7
56.3
141.6
48.5
8.63
2.81
1.62
0.91
1.61
i!Fh
Fh!k
138.3
34.9
ꢀ1.41
ꢀ0.74
s.h.
141.6
5
k!Fh
Fh!i
k!Fh
Fh!i
106.1
156.9
105.8
156.5
9.40
1.82
9.95
2.03
i!Fh
153.1
69.8
ꢀ1.88
ꢀ9.16
Fh!k(Fr-c)
s.h.
6
k1!k2
62.6
105.6
94.7
101.7
39.1
88.4
137.3
137.3
0.57
23.11
8.92
0.89
1.44
3.14
3.16
3.70
i!Nc
72.2
ꢀ15.33
ꢀ3.15
k1!i
s.h.
7
k!Nc
Nc!i
k1!k2
Fh dec !k
131.2
k2!Fr-s
Fr-s!Fh dec[c]
k!Fh dec
s.h.
[a] k: Crystalline; Fh: p6mm hexagonal columnar LC phase; i: isotropic; k(Fr-c): c2mm centered
rectangular columnar crystalline; Nc: columnar nematic LC phase; Fr-s: p2mm simple rectangular
columnar LC phase. [b] s.h.: Data from the second heating. [c] The compound exhibits a decomposition
temperature at 240.98C.
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differential scanning calorimetry (DSC) and
X-ray diffraction (XRD) experiments, are
reported in Table 1. Their structural and
retrostructural analysis by a combination of
XRD, thermal optical polarized microscopy
(TOPM), and molecular modeling experi-
ments is reported in Table 2. Surprisingly,
the nonfluorinated Janus-dendritic benz-
amide 4 also self-assembles into a supra-
molecular column by a mechanism similar
to that of the twin-dendritic benzamides
(Figure 1b).
The semifluorinated Janus-dendritic
benzamides
(3,4,5)12G1-(3,4,5)12F8G1-
BzA 5 and (4-3,4,5)12G1-(4-3,4,5)12F8G1-
BzA 7 were synthesized from dendrons with
identical structures, except one is semi-
fluorinated and the other has hydrogenated
alkyl groups on the periphery. Both den-
dritic benzamides self-assemble into supra-
molecular columns that self-organize in a
p6mm columnar hexagonal (Fh) liquid
crystal (LC) lattice (Table 1 and Table 2).
However, the diameter of the supramolec-
ular column self-assembled from 5 is twice
that of the column self-assembled from the
twin-dendritic benzamides 1 and 2. A sim-
ilar result was observed for the diameter of
the supramolecular column self-assembled
from 7. The benzamide (3,4)12G1-
(3,4,5)12F8G1-BzA 6 self-assembles into a
less perfect supramolecular column with a
diameter approximately twice as large as
Table 2: Structural and retrostructural analysis of 2D lattices self-organized from twin- and Janus-
dendritic benzamides.
Compound T [8C] LC Phase
d Spacings [Š] and Indices
Lattice
1
m[j]
Dimension [gcmꢀ3
or Column
Diameter
]
[i]
[Š][h]
[a]
[b]
[c]
[d]
[e]
[f]
d20
d51
d10
d30
d10
d10
d04
d11
d42
d11
d22
d11
d11
d30
d31
d13
d20
d40
d40
d33
d21
d22
d53
d12
d31
d21
d15
[g]
that of the column generated from
2
22.8[a] 18.6
8.3[b]
7.6
21.7[c] 12.8
–
6.8
–
11.3
–
–
–
–
–
–
a=45.5
b=20.5
–
–
–
–
1.8
(Table 1 and Table 2). The less perfect
supramolecular column produced by 6 self-
organizes in a columnar nematic meso-
phase.
1
21
Fr-c
Fr-c
Fh
Fh
Fh
Nc
Fh
Fr-c
Fr-c
Fh
Fh
Nc
90
78
94
50
70
19
25.3 ꢁ 0.4 0.98
2
3
4
28.8[c] 16.7 14.3 10.9
33.2 ꢁ 0.2–
–
29.2[c] 17.2–
42.5[e] 23.9
27.4[c] 15.9
–
–
34.1
ꢁ 0.5 0.93
2.1
–
2.3
–
–
9.2
–
–
–
–
9.6
–
The XRD and TOPM textures of the
columnar hexagonal lattices generated from
nonfluorinated twin-dendritic benzamides
are compared with those of semifluorinated
Janus-dendritic benzamides in Figure 2 and
Figure 3. Electron density calculations were
performed to elucidate the origin of the
difference between the diameters of the
supramolecular columns self-organized
from twin- and Janus-dendritic benzamides.
The Janus-dendritic benzamide 4, which
contains hydrogenated aliphatic chains,
shows a strong and a very weak reflection
in the XRD pattern recorded at 708C. The
d spacings of these reflections are 27.4 Š
and 15.9 Š, respectively (Table 2). The ratio
–
–
24.8
–
25.4 19.2
–
–
–
–
–
–
–
48.4
31.7
–
0.99
–
–
5
45.3[a]
–
20.4 a=90.3
16.6[b] 15.6
13.1 11.7 b=44.7
120
51.9[c]
16.9[d]
45.9[e]
–
–
–
–
–
–
–
–
–
–
59.0 ꢁ 1.0 1.57
–
–
–
–
–
–
–
1.30
–
6
7
50
100
53.0
Fr-s
Fr-s
Fh
Fh
66.1[f] 56.1 41.3 32.4
a=66.9
b=106.1
75.0
22.6[g] 21
64.8[c]
21.7 20.1
32.6 24.5
150
–
21.6[d] 18.8 16.3
–
–
[a] d Spacings with indices for a c2mm simple rectangular columnar lattice (Fr-c): (20), (11), (31), (40),
and (22); [b] for indices (51), (42), (13), (33), and (53). [c] d-Spacings with indices for a p6mm hexagonal
columnar lattice (Fh): (10), (11), (20), and (21); [d] for indices (30), (22), and (40); the d-spacing ratios
pffiffi
pffiffi
are as expected for the 2D hexagonal columnar lattice: d10/d11/d20/d21/d30/d22/d40 =1:(1/ 3):(1/ 4):(1/
pffiffi
pffiffi
pffiffiffiffiffi
pffiffiffiffiffi
7):(1/ 9):(1/ 12):(1/ 16). [e] d Spacings with indices for a nematic columnar lattice (Nc).
[f] dSpacings with indices for a p2mm simple rectangular columnar lattice (Fr-s): (10), (11), (12), and
(21); [g] For indices (04), (30), (31), and (15). [h] For the Fh LC phase, lattice dimension a is the same as
the column diameter; for Fr-c and Fr-s phases, lattice dimensions a and b are given. [i] Experimental
of these d spacings is 1.72:1, which is very
pffiffiffi
close to 3. This ratio demonstrates a Fh
phase that exhibits only (10) and (11)
reflections. Furthermore, these reflections
give a column diameter of 31.7 Š. This is
density of the molecule in the LC phase. [j] Number of monodendrons per cylindrical stratum of the
pffiffi
supramolecular column: m=( 3NA a2 t1)/2M; (NA =6.022045”1023 molꢀ1, t=7.2Š =average column
stratum height, 1=average density of the molecule in the LC phase, a=lattice dimension, M=molar
mass of the monodendron).
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the column diameter of the Janus benzamide 4 (a = 31.7 Š) is
larger than that of the supramolecular column generated from
the twin benzamide 1 (a = 25.3 Š).[3b] This indicates that the
nonfluorinated Janus-dendritic benzamide 4 forms an orthog-
onal stack of dendrons in the column of its Fh phase that is
similar to those of the columns self-assembled from non-
fluorinated twin-dendritic benzamides 3 and 1.
The twin-dendritic benzamide 2, which contains only
semifluorinated aliphatic chains, shows four reflections in the
XRD pattern at 788C. They are indexed to the (10), (11), (20),
and (21) peaks of a 2D Fh phase generated from columns with
a diameter of 33.2 Š (Table 2). This diameter is substantially
larger than that of the nonfluorinated twin-dendritic benza-
mide 1. However, the column diameter is close to that of the
supramolecular column of the nonfluorinated Janus-dendritic
benzamide 4, evidence that its corresponding supramolecular
column is constructed from an orthogonal stack of building
blocks.
The semifluorinated Janus-dendritic benzamide 7, which
contains hydrogenated aliphatic chains in one dendron and
semifluorinated aliphatic chains in the other, forms a Fh
phase above 137.38C. The X-ray diffraction pattern at
1508C shows the reflections (10), (20), (21), (30), (22), and
(40) that index to a 2D Fh lattice with a column diameter of
75.0 Š (Figure 2b). This column diameter is about 2.2-fold
larger than that of the twin-based column of 3 (a = 34.1 Š),
suggesting that in the semifluorinated Janus column, the
fluorinated aliphatic regions do not mix with the hydro-
genated aliphatic regions as a result of a fluorophobic effect.[6]
If they mixed, they would generate only an orthogonal stack
with a column diameter expected to be very close to that of
the nonfluorinated Janus-dendritic benzamide 4. Similarly,
the semifluorinated Janus-dendritic benzamide 5 self-assem-
bles into a supramolecular column with a column diameter of
59 Š (Table 2), which is 2.3-fold larger than that of the
corresponding nonfluorinated twin-dendritic benzamide, 1
(a = 25.3 Š). These results suggest that the semifluorinated
Janus-dendritic benzamides 5 and 7 self-assemble into a
bilayered disc or crown in which the semifluorinated regions
are segregated from hydrogenated aliphatic regions, with the
aromatic part of the molecules occupying the intermediate
region between the core and the periphery. The disc or
crownlike aggregate generates the supramolecular column.
This arrangement contrasts with the orthogonal stack formed
by nonfluorinated or semifluorinated twin-dendritic benza-
mides and by nonfluorinated Janus-dendritic benzamides.
Electron density profiles were computed with the aid of
the integrated intensity data calculated from the X-ray
diffraction peaks obtained in the Fh phase to visualize the
molecular organization of the dendrons in various supra-
molecular columns (Figure 4).[4a] The XRD of the nonfluori-
nated Janus benzamide 4 in the Fh phase shows a very strong
(10) peak and a very weak (11) peak that are expected from
the orthogonal stacks formed by these molecules.[3c] The
electron density profile computed with the phase choice “ +
ꢀ” for these two reflections exhibits a maximum in the core.
The maximum electron density of the core decreases towards
the periphery of the column, becoming almost constant on the
periphery (Figure 4a). This variation of the electron density
Figure 2. XRD pattern in the Fh phase of a) 3 and b) 7. In b), the
observed peaks shift to larger d spacings, which leads to a greater
than twofold increase in column diameter.
Figure 3. Representative TOPM textures of the hexagonal columnar
(Fh) mesophase, obtained by cooling at a rate of 18Cminꢀ1 from
isotropic phase of a) 1; b) 2; c) 3; d) 4; e) 5; and f) 7.
smaller than that of the twin-dendritic benzamide 3 (a =
34.1 Š, Figure 2a and Table 2), which forms an orthogonal
stack of dendrons in its supramolecular column.[3c] However,
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hydrogenated aliphatic part of the molecule. Notably, the
electron density profile is similar to that observed in the Fh
phase of a pyramidal columnar supramolecular dendrimer
with semifluorinated aliphatic chains that was reported
previously.[5a]
The electron density profiles of the twin-dendritic benza-
mide 3 is shown in Figure 4c. These profiles show maximum
electron density in the center of the columns corresponding to
the aromatic regions and low electron density corresponding
to the aliphatic region in the periphery. The semifluorinated
Janus-dendritic benzamide 7 shows the reflections (10), (20),
(21), (30), (22), and (40). Whereas the peak with the strongest
intensity is (10), the intensities of (20) and (30) peaks are
higher than that of the (21) peak (Figure 2b). This is
consistent with the semifluorinated aliphatic chains occupying
the peripheral region of the column. Accordingly, the electron
density profiles for the phases “ꢀ + ꢀ ꢀ + ꢀ” has high
electron density in the peripheral region of the column and
low electron density in the central region of the column
(Figure 4d). Starting from the center of the column, the
electron density increases from the lowest value to a small
local maximum that corresponds to the aromatic part of the
molecules. As one moves closer to the periphery from the
aromatic region, the electron density decreases slightly owing
to the hydrogenated part of the semifluorinated aliphatic
chains. Therefore, the electron density profiles are consistent
with the bilayered columnar structure containing the hydro-
genated aliphatic chains in the core and the semifluorinated
aliphatic chains in the periphery with the aromatic part
occupying the intermediate region. Molecular models of the
bilayered supramolecular columns obtained from small- and
wide-angle XRD of oriented fibers are shown in Figure 5,
Figure 4. Electron density profiles and contour plots of the twin- and
Janus-dendritic benzamides a) 4 showing the center of the supra-
molecular columns with high electron density (red) due to the aro-
matic parts; b) 2 with high electron density (red) in the periphery of
the columns which corresponds to the semifluorinated aliphatic
chains, and the low electron density (black), which corresponds to the
hydrogenated aliphatic chains; c) 3 showing the center of the supra-
molecular columns with high electron density (red), which corre-
sponds to the aromatic parts; and d) 7 with high electron density (red)
in the periphery of the columns which corresponds to the semifluori-
nated aliphatic chains.
profiles is consistent with the nanophase segregation expected
in these systems.[1c,d,4a] The maximum and the surrounding
high electron density regions correspond to the aromatic
portion of the molecules, and the near-constant peripheral
regions represent the aliphatic part of the molecules from the
orthogonal stack of the twin-dendritic benzamides. Similar
electron density profiles were observed for the supramolec-
ular columns self-assembled from the twin-dendritic benz-
amide 1.
The semifluorinated twin-dendritic benzamide 2 shows
additional reflections in the Fh phase that correspond to a
strong (10) peak, a moderately strong (21) peak, a very weak
(20) peak, and a weak (11) peak. The observation of a
moderately strong (21) peak rather than a sequence of higher
order peaks with decreasing intensity suggests that the
semifluorinated aliphatic chains of the molecules occupy the
peripheral regions of the columns. The electron density
profiles computed with the phases “ꢀ ꢀ + + ” for the peaks
(10), (11), (20), and (21), respectively, show high and nearly
constant electron density regions on the periphery of the
columns (Figure 4b). They also show an intermediate elec-
tron density region near the core of the columns that
corresponds to the aromatic portions. As one moves toward
the periphery from the aromatic regions of the column, the
electron density reaches a minimum and then rises toward the
semifluorinated regions on the periphery. The regions sur-
rounding the minima in the electron density correspond to the
Figure 5. Structures of a) 5 and b) 7. Top left: single dendron with the
semifluorinated tails in blue; bottom left: space-filling side view of one
crownlike layer; right: top view of two consecutive crownlike layers
(i indicates the hydrogen bonding between layers).
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Figure 6, and Figure 7. These models illustrate the location of
crownlike stratum of the pyramidal column (Figure 5 and
ꢀ
ꢀ
the nonfluorinated alkyl chains in the central region of the
column and the perfluorinated regions in the periphery of the
column. Nonorthogonal hydrogen bonding is available in the
Figure 6). N-methylation of the NH groups of the Janus-
dendritic benzamides almost destroys the pyramidal colum-
nar assembly. The N-methylated Janus-benzamide derivative
5 displays a monotropic crystalline p2mm simple rectangular
phase before melting at 678C (see Supporting Information).
The crystals of its precursor benzamide 5 melt into a Fh phase
at 1068C, followed by isotropization at 1578C (Table 1). The
behavior of the latter demonstrates that hydrogen bonding is
responsible for the formation of the LC phases of the Janus-
dendritic benzamides.
In conclusion, the selective attachment of nonfluorinated
aliphatic chains on one dendron and semifluorinated aliphatic
chains on the other dendron of the twin-tapered dendritic
benzamides generates semifluorinated Janus-dendritic ben-
zamides. These, in turn, self-assemble into supramolecular
pyramidal columns that exhibit diameters more than double
those of the columns generated from nonfluorinated or
semifluorinated twin-dendritic benzamides. This new mode
of self-assembly of the semifluorinated Janus-dendritic benz-
amides results from the tendency of the semifluorinated
aliphatic chains to segregate from the hydrogenated aliphatic
chains. Aside from providing an unusual example of the
fluorophobic effect in self-assembly[8,9] and the second
example of supramolecular pyramidal dendrimers[5a,b] self-
assembled from supramolecular crowns, this novel bilayered
supramolecular pyramidal columnar structure opens numer-
ous new strategies for the construction of complex supra-
molecular nanosystems.[2m,n,10] This initiates a new direction in
the area of Janus-derived supramolecular liquid crystals,[2h,i]
complements pyramidal mesophases generated from covalent
crownlike molecules,[11] and raises the question about the
generality of this concept.
Figure 6. Hydrogen-bonding details for Janus benzamides a) 5 and
b) 7; only the aromatic regions are shown, with each dendron molecule
colored differently. The formation of hydrogen bonds (i) requires a
benzamide torsion angle (b) of about 308, as the layer spacing is
approximately 4.5–5.0 Š. Top left: dihedral angles between planes I
and II, approximately 308; between planes II and III, approximately 308
(Ref. [7]); and between planes I and III, 08. The dihedral angle between
planes I and III was chosen to be 08 instead of 62.68, as reported in
Ref. [7], to decrease the steric constrains and to facilitate better
packing.
Received: April 10, 2005
Published online: July 1, 2005
Keywords: amides · dendrimers · liquid crystals ·
.
self-assembly · supramolecular chemistry
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Figure 7. Supramolecular assembly models of a) 5 and b) 7, indicating
the higher electron density on the semifluorinated periphery and the
short-range helical order of the supramolecular assembly.
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