Discotic liquid crystalline poly(propylene imine) dendrimers based on triphenylene

Article abstract of DOI:10.1021/ja0455906

The design, synthesis, and mesomorphic properties of a new series of homodendrimers consisting of the commercially available poly(propylene imine) (PPI) dendrimers (G = 1-5), PPI-(NH₂)_n (n = 4, 8, 16, 32, 64), functionalized with a discotic triphenylene moiety are reported. The liquid crystalline behavior was investigated by means of differential scanning calorimetry (DSC), polarizing-light optical microscopy (POM), and X-ray diffractometry (XRD). All of the homodendrimers showed mesomorphic properties, with the second to fifth generations giving a hexagonal columnar mesophase (Col_h) and the first generation a rectangular columnar mesophase (Col_r). The X-ray study reveals that these mesophases show a highly ordered structure with segregation of triphenylenes and dendrimers into separate columns and a regular stacking distance inside the triphenylene columns. GPC analysis showed that the dendrimers had good monodispersity and MALDI-TOF studies of the first three generations gave good evidence that all of the terminal amino groups of the dendrimers were functionalized with a discotic unit.

Full text of DOI:10.1021/ja0455906

Published on Web 12/10/2004
Discotic Liquid Crystalline Poly(propylene imine) Dendrimers
Based on Triphenylene
Mark D. McKenna, Joaqu´ın Barbera´, Mercedes Marcos, and Jose´ Luis Serrano*
Contribution from Qu´ımica Orga´nica, Facultad de Ciencias - ICMA,
UniVersidad de Zaragoza-CSIC, E-50009, Zaragoza, Spain
Received July 22, 2004; E-mail: joseluis@unizar.es
Abstract: The design, synthesis, and mesomorphic properties of a new series of homodendrimers consisting
of the commercially available poly(propylene imine) (PPI) dendrimers (G ) 1-5), PPI-(NH2)n (n ) 4, 8,
16, 32, 64), functionalized with a discotic triphenylene moiety are reported. The liquid crystalline behavior
was investigated by means of differential scanning calorimetry (DSC), polarizing-light optical microscopy
(POM), and X-ray diffractometry (XRD). All of the homodendrimers showed mesomorphic properties, with
the second to fifth generations giving a hexagonal columnar mesophase (Colh) and the first generation a
rectangular columnar mesophase (Colr). The X-ray study reveals that these mesophases show a highly
ordered structure with segregation of triphenylenes and dendrimers into separate columns and a regular
stacking distance inside the triphenylene columns. GPC analysis showed that the dendrimers had good
monodispersity and MALDI-TOF studies of the first three generations gave good evidence that all of the
terminal amino groups of the dendrimers were functionalized with a discotic unit.
Introduction
the struggle between the preferred arrangements of the den-
drimer core and the peripheral mesogenic units. To minimize
In the past decade or so, dendrimers1-3 have been attracted
a great deal of attention because of their suitability for use as
building blocks in supramolecular chemistry and molecular
scaffolding in nanotechnology.
their free energy, dendrimers tend to adopt a 3D spherical
structure, but the attachment of anisotropic mesogenic units
causes this globular shape to be distorted because of the strong
interactions between such units. Indeed, liquid crystalline phases
can arise in this type of system. Percec et al. showed that the
attachment of rodlike units gives both nematic and smectic
phases17-19 which occur because of the parallel alignment of
the mesogenic units with respect to one another. Such an
arrangement causes the dendrimer core to be distorted from its
ideal conformation. Our group has recently shown that it is
possible to evolve from a SmA phase to a Colh phase, via SmC
and Colr phases, by changing from a homodendrimer function-
alized with a terminally attached mesogenic unit bearing a single
alkyl chain to a homodendrimer with a terminally attached unit
bearing two alkyl chains, passing through co-dendrimers
consisting of both units.20 Columnar hexagonal liquid crystalline
phases were also observed by Lattermann et al.21,22 in poly-
This high level of interest has been generated because of the
structural properties of these materials. The molecules have a
monodisperse globular shape and bear a large number of end
groups that play an important role in determining the properties
of the system. Various examples are available of dendrimers
that behave as unimolecular micelles, whether it be with a
hydrophobic core and hydrophilic periphery or vice versa, and
the use of these systems in selective extraction has been
described.4-6 Diverse applications include templates for the
formation of mesoporous silicas,7,8 the harvesting of light,9
catalytic nanoreactors,10,11 drug delivery systems,12,13 and liquid
crystalline dendrimers.14-16
The incorporation of mesogenic units at the periphery of
dendrimers gives rise to an interesting phenomenon, namely,
(10) Piotti, M. E.; Rivera, F., Jr.; Bond, R.; Hawker, C. J.; Fre´chet, J. M. J. J.
Am. Chem. Soc. 1999, 121, 9471-9472.
(1) Tomalia, D. A.; Naylor, A. M.; Goddard, W. A., III. Angew. Chem., Int.
Ed. Engl. 1990, 29, 138-175.
(2) Newkome, G. R. Dendrimers and Dendrons: Concepts, Syntheses, Ap-
plications; Wiley-VCH: Weinheim, Germany, 2001.
(11) Hecht, S.; Fre´chet, J. M. J. J. Am. Chem. Soc. 2001, 123, 6959-6960.
(12) de Groot, F. M. H.; Albrecht, C.; Koekkoek, R.; Beusker, P. H.; Scheeren,
H. W. Angew. Chem., Int. Ed. 2003, 42, 4490-4494.
(13) Meijer, E. W.; van Genderen, M. H. P. Nature, 2003, 426, 128-129.
(14) Ponomarenko S. A.; Boiko, N. I.; Shibaev, V. P. Polym. Sci. Ser. C 2001,
43, 1601-1650.
(3) Dendrimers and Other Dendritic Polymers; Fre´chet, J. M. J., Tomalia, D.
A. Eds.; John Wiley and Sons Ltd: Chichester, U.K., 2001.
(4) Newkome, G. R.; Moorefield, C. N.; Baker, G. R.; Saunders: M. J.;
Grossman, S. H. Angew. Chem., Int. Ed. Engl. 1991, 30, 1178-1180.
(5) Stevelmans, S.; van Hest, J. C. M.; Jansen, J. F. G. A.; van Boxtel, D. A.
F. J.; de Brabander-van den Berg, E. M. M.; Meijer, E. W. J. Am. Chem.
Soc. 1996, 118, 7398-7399.
(15) Guillon, D.; Deschenaux, R. Curr. Opin. Solid State Mater. Sci. 2002, 6,
515-525.
(16) Marcos, M.; Omenat, A.; Serrano, J. L. C. R. Chimie 2003, 6, 947-957.
(17) Percec, V.; Chu, P.; Kawasumi, M. Macromolecules 1994, 27, 4441-4453.
(18) Percec, V. Pure Appl. Chem. 1995, 67, 2031-2038.
(19) Percec, V.; Chu, P.; Ungar, G.; Zhou, J. J. Am. Chem. Soc. 1995, 117,
11441-11454.
(6) Baars, M. W. P. L.; Froehling, P. E.; Meijer, E. W. Chem. Commun. 1997,
1959-1960.
(7) Larsen, G.; Lotero, E.; Marquez, M. Chem. Mater. 2000, 12, 15113-15115.
(8) Hukkama¨ki J.; Pakkanen, T. T. Microporous Mesoporous Mater. 2003,
65, 189-196.
(20) Rueff, J. M.; Barbera´, J.; Donnio, B.; Guillon, D.; Marcos, M.; Serrano, J.
L. Macromolecules 2003, 36, 8368-8375.
(21) Usol’tseva, N.; Bykova, V.; Smimova, A.; Grusdev, M.; Lattermann, G.;
Facher, A. Mol. Cryst. Liq. Cryst. 2004, 409, 29-42.
(9) Adronov, A.; Fre´chet, J. M. J. Chem. Commun. 2000, 1701-1710.
9
10.1021/ja0455906 CCC: $30.25 © 2005 American Chemical Society
J. AM. CHEM. SOC. 2005, 127, 619-625
619

A R T I C L E S
McKenna et al.
Scheme 1. Synthetic Route to Mesogenic Unit
(propylene imine) (PPI) dendrimers acylated with simple
carboxylic acids. A great deal of work has been carried out on
LC dendrimers containing calamitic units but, to the best of
our knowledge, very little work has been carried out on discotic
units.
systems give columnar phases. A lot of attention has recently
been focused on these compounds because of their photocon-
ductive properties.27,28 The discs tend to assemble into columns
through face-face packing of the aromatic units, thereby
allowing a pathway for energy or charge transfer (because of
Discotic liquid crystals were discovered just over 25 years
ago by Chandrasekhar, with his work on the hexaesters of
benzene.23 Hexasubstituted derivatives of triphenylene were
among the first examples to be discovered24-26 and these
(24) Tinh, N. H.; Dubois, J. C.; Maltheˆte, J.; Destrade, C. C. R. Acad. Sci. 1978,
C286, 463-464.
(25) Billard, J.; Dubois J. C.; Tinh, N. H.; Zann A. NouV. J. Chim. 1978, 2,
535-540.
(26) Destrade, C.; Mondon, M. C.; Maltheˆte, J. J. Phys. Colloq. 1979, C3, 17-
21.
(27) Bengs, H.; Closs, F.; Frey, T.; Funhoff, D.; Ringsdorf, H.; Siemensmeyer,
K. Liq. Cryst. 1993, 15, 565-574.
(28) Simmerer, J.; Glusen, B.; Paulus, W.; Kettner, A.; Schuhmacher, P.; Adam,
D.; Etzbach, K. H.; Siemensmeyer, K.; Wendorff, J. H.; Ringsdorf, H.;
Haarer, D. AdV. Mater. 1996, 8, 815-819.
(22) Cameron, J. H.; Facher, A.; Lattermann, G.; Diele, S. AdV. Mater. 1997,
9, 398-403.
(23) Chandrasekhar, S.; Sadashiva, B. K.; Suresh, K. A. Pramana 1977, 7, 471-
480.
9
620 J. AM. CHEM. SOC. VOL. 127, NO. 2, 2005

Highly Ordered Discotic PPI Dendrimers
A R T I C L E S
Table 1. GPC and MALDI-TOF Data for Dendrimers
dendrimer
Ip (GPC)
theoretical Mw
MALDI-TOF
Mwexp/Mwtheor
PPI-(NHCO-DiscC10)4
PPI-(NHCO-DiscC10)8
PPI-(NHCO-DiscC10)16
PPI-(NHCO-DiscC10)32
PPI-(NHCO-DiscC10)64
1.12
1.11
1.10
1.16
1.23
5084.3
10293.1
20746.3
41653.8
83447.8
5084.0
10308.3
1.00
1.00
1.00
20756.8
a
a
a The high Mw of these compounds precludes the measurement of this
value.
analysis. The polydispersity index was obtained from the GPC
measurements (mobile phase: CH2Cl2, calibration standard:
polystyrene) and this information, combined with the results of
the MALDI-TOF analysis (see Table 1), demonstrated that the
dendrimers did not contain significant amounts of free primary
amines. Indeed, in the dendrimers obtained for generations 1-3,
there was no evidence for the presence of any free amine and
all of the dendrimer terminal groups were bound to a mesogenic
unit.
Differential Scanning Calorimetry (DSC). The phase tran-
sitions of the functionalized dendrimers PPI-(NHCO-DiscC10)4
to PPI-(NHCO-DiscC10)64 were measured using DSC at a
heating rate of 10 °C min-1. All samples were dried in a vacuum
desiccator prior to analysis.
Figure 1. PPI-(NHCO-DiscC10)n functionalized dendrimers with discotic
mesogen.
the large π-π overlap of the discotic units), with the alkyl
chains acting as insulators.29 The low stability and poor
processibility of the low molar mass discotic molecules has led
to the search for ways to improve these properties. One way
that has been suggested is polymerization and, as a result, much
work has been carried out on creating polymer LCs that
incorporate triphenylenes.30-32 These materials can be processed
more easily to give ordered macrodomains. To date, however,
studies concerning the use of dendrimers as the support have
not been carried out.
In this paper, we report the synthesis of a series of homo-
dendrimers of the first five generations of PPI that incorporate
discotic triphenylene mesogenic units. The characterization of
both the chemical and mesogenic properties of these materials
is described.
All of the functionalized dendrimers exhibited liquid crystal-
line behavior. DCS traces (second scan) for all the functionalized
dendrimers are shown in Figure 2 and the phase-transition
temperatures and enthalpic data for the compounds are given
in Table 2.
It can be seen from the results in Table 2 that there is only
a small variation in the transition temperatures for dendrimer
generations 2-5 in terms of transitions from crystal to meso-
phase and mesophase to isotropic liquid. The transitions for the
generation 1 dendrimer (bottom trace) occur at a higher tem-
perature, both on heating and cooling, and the mesophase range
is smaller. The similarities in temperature and enthalpy (in terms
of J.g-1 and for ∆H per mesogenic unit) of the crystal-to-
mesophase transition show that mesophase formation is inde-
pendent of the dendrimer generation, a situation also found by
various other groups.33,36,37 The corresponding low molecular
weight compound, hexadecyloxytriphenylene, passes to the
liquid crystalline state at a similar temperature to the first
generation dendrimer and at a higher temperature than the higher
generation dendrimers. Furthermore, the parent compound is
mesogenic over a shorter temperature range than the dendrimeric
Results and Discussion
Synthesis and Characterization. A series of PPI dendrimers
modified with a reactive triphenylene unit was synthesized.
These dendrimers were synthesized by reaction of the activated
pentafluorophenol ester of the acid group of the mesogenic unit
(see Scheme 1) with the terminal primary amine groups of the
corresponding PPI dendrimer (generations 1-5).33-35 These
reactions gave PPI-(NHCO-DiscC10)n, where n ) 4, 8, 16, 32,
or 64 (see Figure 1). All of the compounds were isolated as
yellowish air-stable solids that were soluble in dichloromethane
and chloroform.
26,38
systems (C 58 °C Colho 69 °C I).
Polarizing Optical Microscopy (POM). Samples of the five
dendrimers were introduced between glass slides for study by
POM. On heating, the changes observed for all samples were
consistent with the DSC traces obtained. Generation 2 passed
to an isotropic state at approximately 106 °C on heating and on
subsequent cooling (2 °C min-1) the mesophase began to form
at approximately 105 °C (see Figure 3a). Further cooling led to
the growth of a pseudo-focal-conic fan texture (see Figure 3b).
This texture is indicative of a hexagonal columnar (Colh)
mesophase,39 which is not unexpected given the fact that
The structures of the target compounds were established using
1H and 13C NMR spectroscopy, gel permeation chromatography
(GPC), mass spectrometry (MALDI-TOF), and elemental
(29) van Nostrum, C. F. AdV. Mater. 1996, 8, 1027-1030.
(30) Kreuder, W.; Ringsdorf, H. Makromol. Chem., Rapid Commun. 1983, 4,
807-815.
(31) Disch, S.; Finkelmann, H.; Ringsdorf, H.; Schuhmacher, P. Macromolecules
1995, 28, 2424-2428.
(32) Kumar, S.; Schuhmacher, P.; Henderson, P.; Rego, J.; Ringsdorf, H. Mol.
Cryst. Liq. Cryst. 1996, 288, 211-222.
(33) Baars, M. W. P. L.; So¨ntjens, S. H. M.; Fischer, H. M.; Peerlings, H. W.
I.; Meijer, E. W. Chem. Eur. J. 1998, 4, 2456-2466.
(34) Schenning, A. P. H. J.; Elissen-Roman, C.; Weener, J.-W.; Baars, M. W.
P. L.; van der Gaast, S. J.; Meijer, E. W. J. Am. Chem. Soc. 1998, 120,
8199-8208.
(35) Baars, M. W. P. L. PhD Dissertation, Eindhoven University of Technology,
The Netherlands, 2000.
(36) Marcos, M.; Gime´nez, R.; Serrano, J. L.; Donnio, B.; Heinrich, B.; Guillon,
D. Chem. Eur. J. 2001, 7, 1006-1013.
(37) Barbera´, J.; Gime´nez, R.; Marcos, M.; Serrano, J. L. Liq. Cryst. 2002, 29,
309-314.
(38) Destrade, C.; Tinh, N. H.; Gasparoux, H.; Maltheˆte, J.; Levelut A. M. Mol.
Cryst. Liq. Cryst. 1981, 71, 111-135.
9
J. AM. CHEM. SOC. VOL. 127, NO. 2, 2005 621

A R T I C L E S
McKenna et al.
Figure 2. DCS traces for PPI-(NHCO-DiscC10)n; second heating and cooling cycles, rate of 10 °C min-1
Table 2. Thermal Data Determined by DSC for
first generation compound does not have hexagonal symmetry
and the diffraction maxima observed in the X-ray patterns are
consistent with a rectangular columnar structure. On the other
hand, the diffractograms recorded at room temperature for all
compounds are typical of crystalline phases.
PPI-(NHCO-DiscC10
)
(Second Scan)
n
na
transitionsc
°
C, (
∆
H) Jg-
mesophase range
°
C
∆ΗCol-I/m.u.d kJ
1
4hb
4cb
8h
C 61 (37.1) Colr 79 (20.7) I
I 70 (3.1) Colr 32 (33.7) C
C 49 (28.7) Colh 107 (6.3) I
I 96 (6.2) Colh 19 (28.0) C
18
38
58
77
68
91
70
95
69
92
3.9
8.0
8.4
7.8
6.9
The data obtained for the five compounds are shown in Table
3. The patterns for generations 2-5 contain a large number of
reflections with a reciprocal spacing ratio 1:x3:x4:x7..., which
is typical of a two-dimensional hexagonal lattice. In addition,
two diffuse haloes are observed. The first halo corresponds to
a distance of about 4.5 Å and is characteristic of the molten
hydrocarbon chains. This type of halo is typically observed in
all kinds of rodlike and disklike liquid crystals. The second halo,
which corresponds to a distance of about 3.53-3.55 Å, is not
as broad as the first and arises from stacking of the triphenylene
rings. The mesophases of these compounds (including the first
generation) can therefore be described as ordered columnar.
The X-ray patterns are practically identical for the four
generations between 2 and 5. The measured spaces (Table 3)
are very similar and, consequently, the lattice parameters a and
c deduced from these spaces are practically the same.
8c
16h C 46 (26.7) Colh 114 (7.0) I
16c I 106 (6.5) Colh 15 (21.2) C
32h C 44 (27.4) Colh 114 (6.3) I
32c I 107 (6.0) Colh 12 (19.8) C
64h C 42 (24.9) Colh 111 (5.1) I
64c I 103 (5.3) Colh 11 (17.5) C
a n: number of discotic units in the dendrimer. b h: heating process, c:
cooling process. c C ) crystal, Colr ) rectangular columnar mesophase,
Colh ) hexagonal columnar mesophase, I ) isotropic liquid. d Enthalpy of
the columnar-to-isotropic liquid transition per mesogenic unit.
hexaalkoxytriphenylenes form both ordered and disordered Colh
phases.38,40 The other generations (3-5) showed similar be-
havior, although the mesophase forms at slightly higher tem-
peratures on cooling. The texture of the mesophase of the gen-
eration 1 dendrimer is somewhat different (see Figure 3c) and
it was not possible to assign a mesophase from the texture alone.
The relative intensities of the different reflections are
qualitatively compared in Table 3 and from these data it can be
seen that the strongest reflection in all the patterns is (2 1 0).
This means that there is a strong modulation of the electronic
density wave with a period x7 times shorter than the (1 0 0)
spacing. This is consistent with the existence in the hexagonal
columnar mesophase of a hexagonal sublattice generated by
segregated columns containing either the stacked triphenylenes
or the dendrimer. This model is schematically represented in
Figure 4 and is supported by the occurrence of scattering at
3.53-3.55 Å in the X-ray patterns. Such scattering reflects the
tendency of the triphenylene cores to stack on top of each other.
In the proposed structure, each dendrimer column is surrounded
by six columns of triphenylene molecules. The resulting
supercolumns consist of seven individual columns that form a
X-ray Diffraction (XRD). The five compounds in this series
were investigated by X-ray diffraction in their liquid-crystal
phases. For the sake of comparison, the low-molecular mass
analogue hexakis(decyloxy)triphenylene (DiscC10), which shows
a hexagonal columnar mesophase, was also studied. The mea-
surements performed on the second, third, fourth, and fifth
generations (n ) 8, 16, 32, and 64, respectively) confirm that
the mesophase, as expected from their optical textures (Figure
3b), is hexagonal columnar. However, the mesophase of the
(39) Dierking, I. Textures of Liquid Crystals; Wiley-VCH: Weinheim, Germany,
2003.
(40) Cammidge, A. N.; Bushby, R. J. In Handbook of Liquid Crystals, Vol. 2B,
Low Molecular Weight Liquid Crystals II; Demus, D., Goodby, J., Gray,
G. W., Spiess, H. W., Vill, V., Eds.; Wiley-VCH: Weinheim, Germany,
1998; Chapter VII, pp 694-748.
9
622 J. AM. CHEM. SOC. VOL. 127, NO. 2, 2005

Highly Ordered Discotic PPI Dendrimers
A R T I C L E S
Figure 3. Optical texture of (a) PPI-(NHCO-DiscC10
PPI-(NHCO-DiscC10)4 Colr mesophase at 73 °C.
)
I-Colh transition at 95 °C. (b) PPI-(NHCO-DiscC10
)
Colh mesophase at 89 °C (c)
8
8
Table 3. X-ray Results for the Mesophases of the Dendrimers and the Model Compounda
compound
h k l
dobs (Å)
dcalc (Å)
intensityb
mesophase type and lattice constants (Å)
PPI-(NHCO-DiscC10)4
2 0 0
4 0 0
4 1 0
1 3 0
4 2 0
3 3 0
51.8
26.1
24.2
22.5
21.9
19.5
4.5
52.0
26.0
24.35
22.6
21.8
19.3
s
Colr
m
vw
s
s
w
d
a ) 104
b ) 69.5
c ) 3.8
0 0 1
1 0 0
1 1 0
2 0 0
2 1 0
3 0 0
3 1 0
4 0 0
3 2 0
4 1 0
3.54
57.8
33.7
28.6
21.8
19.0
15.9
3.53
13.2
12.5
4.5
d
s
PPI-(NHCO-DiscC10)8
57.7
33.3
28.8
21.8
19.2
16.0
14.4
13.2
12.6
Colh
a ) 66.6
c ) 3.53
vw
m
vs
w
m
w
w
w
d
0 0 1
1 0 0
1 1 0
2 0 0
2 1 0
3 0 0
3 1 0
4 0 0
3 2 0
4 1 0
14.4
58.1
33.4
28.9
21.7
19.1
16.0
14.4
13.2
12.7
4.5
d
s
PPI-(NHCO-DiscC10)16
57.8
33.35
28.9
21.8
19.25
16.0
14.4
13.2
12.6
Colh
a ) 66.7
c ) 3.55
vw
m
vs
w
m
w
w
w
d
0 0 1
1 0 0
2 0 0
2 1 0
3 0 0
3 1 0
4 1 0
3.55
58.4
28.7
22.0
18.7
16.1
12.7
4.5
d
s
DAB-(NHCO-DiscC10)32
57.85
28.9
21.9
19.3
16.0
12.6
Colh
a ) 66.8
c ) 3.54
m
vs
w
m
w
d
0 0 1
1 0 0
1 1 0
2 0 0
2 1 0
3 0 0
3 1 0
4 0 0
3 2 0
4 1 0
3.54
58.0
33.2
29.2
22.0
19.2
16.0
14.5
13.3
12.8
4.5
d
s
DAB(NHCODiscC10)64
58.0
33.5
29.0
21.9
19.3
16.1
14.5
13.3
12.7
Colh
a ) 67.0
c ) 3.55
vw
m
vs
w
m
w
vw
w
d
0 0 1
1 0 0
1 1 0
3.55
21.95
12.8
4.5
d
vs
d
d
w
DiscC10
22.0
12.7
Colh
a ) 25.3
c ) 3.56
0 0 1
3.56
a The temperature of the experiments was 70 °C for all the dendrimers and 65 °C for hexakis(decyloxy)triphenylene (DiscC10). b vs: very strong; s:
strong; m: medium; w: weak; vw: very weak; d: diffuse.
hexagonal lattice with an a parameter of about 67 Å. This value
is x7 times larger than the parameter of the sublattice, which
consists of the individual columns (ca. 25.3 Å). The sublattice
parameter corresponds roughly to the diameter of the hexa-
substituted triphenylene (containing conformationally disordered
substituents) and is in good agreement with the hexagonal lattice
9
J. AM. CHEM. SOC. VOL. 127, NO. 2, 2005 623

A R T I C L E S
McKenna et al.
Figure 5. Structural model for generation 1.
Figure 4. Structural model for generations 2-5.
determine the exact organization of the individual columns in
the resulting sublattice, the fact that reflections (1 3 0) and (4
2 0) are strong suggests that the triphenylene columns lie on
these planes and the probable structure is represented in Figure
5. In a similar way to the structure proposed for generations
2-5, there are six triphenylene columns around each dendrimer
column. This arrangement, in contrast to that represented in
Figure 4 for the other generations, has a distorted hexagonal
structure. The distortion is probably related to the tilt of the
triphenylene discs, which in turn accounts for the smaller cross
section of the columns. It is clear that only four out of the six
triphenylene rings that surround the dendrimer can belong to
the same molecule. One and a half molecules are therefore
necessary to fill each columnar slice. In this situation, and owing
to the tilt of molecule, the separation between two adjacent
triphenylenes along the column axis must be slightly higher than
the experimentally measured face-to-face separation of 3.54 Å.
It seems reasonable that the tilt can be estimated bearing in
mind that the packing density must be preserved while the cross-
sectional area has decreased compared to that in the hexagonal
packing arrangement. Thus, the tilt angle can be estimated as
cos-1 (104 × 69.5)/(672x3). This gives a tilt angle of ca. 21.5°
and the deduced c constant for generation 1 is 3.8 Å, which
gives a packing density for this dendrimer of 0.92 g/cm3.
constant of the columnar mesophase of the analogous low
molecular mass compound hexakis(decyloxy)triphenylene
(DiscC10). The diffraction maxima and the lattice constants
deduced for this compound are included in Table 3. The stacking
distance (c constant) for this compound is practically identical
to those observed in the dendrimers.
The proposed structural model is valid for generations 2-5.
The difference between the generations concerns the length of
column required to accommodate a complete molecule. In each
columnar “slice”, which is ca. 3.54 Å thick, there are six
triphenylenes surrounding a portion of dendrimer. Given this
situation, it is clear that the molecules of generation 2 (contain-
ing 8 triphenylene units) fill one and one-third of a slice. Sim-
ilarly, molecules of generation 3 (containing 16 triphenylenes)
fill two and two-thirds of a slice, molecules of generation 4
(containing 32 triphenylenes) fill five and one-third of a slice,
and molecules of generation 5 (containing 64 triphenylenes) fill
10 and two-thirds of a slice. Using this model, the calculated
density for the mesophase of each of the four compounds is ca.
0.94 g/cm3. In this model, the flexible dendrimeric part adopts
a cylindrical arrangement with the four generations having a
similar diameter but their height increasing from ca. 4.7 Å for
the generation bearing 8 triphenylenes up to 38 Å for the gen-
eration bearing 64. This behavior is usual for terminal func-
tionalized liquid crystalline dendrimers, in which the interactions
between the promesogenic units determine the supramolecular
structure and the dendrimeric central core adopts the geometry
necessary to allow this molecular arrangment.16,20,31
As mentioned above, the X-ray patterns of the mesophase of
the first generation dendrimer are indexed in terms of a two-
dimensional rectangular packing of columns. In this case, there
is also evidence of segregation of triphenylenes and dendrimer
into separate columns. This evidence is based on the following
points:
There is scattering that corresponds to a distance of 3.55 Å,
which is characteristic of the stacking of triphenylene rings.
The second reflection (2 0 0) is weak, whereas the two
subsequent reflections [(1 3 0) and (4 2 0)] are much stronger.
This again reveals the existence of a strong modulation of the
electronic density wave at distances that correspond to the
separations between the segregated columns. In this case, there
is no single intercolumnar distance as the symmetry of the
packing is no longer hexagonal. Although it is difficult to
Conclusions
We have shown that the attachment of discotic triphenylene
moieties to the commercially available PPI-(NH2)n dendrimers
produces highly ordered columnar mesophases, which are
hexagonal in the second to fifth generations and rectangular
for the first generation. These materials show certain improve-
ments over the analogous low molecular weight unit, hexakis-
(decyloxy)triphenylene. For example, the mesophase tempera-
ture range of the dendrimers is larger and the materials also
become liquid crystalline at lower temperatures. Compared to
the columnar dendrimers previously described in the litera-
ture,22,36 these compounds show a highly ordered structure in
two respects: (1) A regular stacking distance is measured by
X-ray diffraction in the mesophase (i.e., the mesophases are
ordered columnar) and (2) there is segregation of triphenylenes
and dendrimers into separate columns. These segregated col-
umns pack into a two-dimensional lattice, the symmetry of
which depends on the generation number: the mesophase is
Colr for the first generation and Colh for the other dendrimers.
The work described here opens up possibilities for the synthesis
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624 J. AM. CHEM. SOC. VOL. 127, NO. 2, 2005

Highly Ordered Discotic PPI Dendrimers
A R T I C L E S
of discotic dendrimers as interesting new materials for applica-
tions such as, for example, molecular electronics.41
of Spain (MAT 2002-04118-C02-01 and MAT 2003-07806-
C01), and FEDER (EU).
Supporting Information Available: Experimental section,
techniques, synthesis of discotic mesogen, synthesis of discotic
LC dendrimers, and general procedure. This material is available
free of charge via the Internet at http://pubs.acs.org.
Acknowledgment. This article is dedicated to Sivaramakrish-
na Chandrasekhar (1930-2004). This research was supported
by the EU through the RTN Project Supramolecular Liquid
Crystal Dendrimers (LCDD; HPRN-CT-2000-00016), the CICYT
(41) Kumar, S. Liq. Cryst. 2004, 31, 1037-1059.
JA0455906
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