Liquid crystalline oligo(p-phenylene vinylene)-terminated poly(propylene imine) dendrimers. Synthesis and characterization

Article abstract of DOI:10.1021/ma021338b

Different generations of poly(propylene imine) dendrimers modified peripherically with urea oligo(p-phenylene vinylene)s (OPV) having a 3,4,5-tri(dodecyloxy)phenyl mesogenic unit (OPV-dendrimers) were successfully synthesized and fully characterized. Although intramolecular H-bonding exist between the urea linkages of the end groups, only weak interactions between the π-conjugated segments were observed in solution. The hydrogen-bonding interactions increased for higher generation dendrimers due to the closer proximity of the end groups, indicative of a more dense-shell packing. All generations of OPV-dendrimers showed liquid crystalline behavior in which the OPV-dendrimers have a homeotropic orientation. The temperature range of the liquid crystalline phase increased with higher generation.

Full text of DOI:10.1021/ma021338b

Macromolecules 2003, 36, 565-572
565
Liquid Crystalline Oligo(p-phenylene vinylene)-Terminated
Poly(propylene imine) Dendrimers. Synthesis and Characterization
F . S. P r ecu p -Bla ga , A. P . H. J . Sch en n in g,* a n d E. W. Meijer *
Laboratory for Macromolecular and Organic Chemistry, Eindhoven University of Technology,
P.O. Box 513, 5600 MB Eindhoven, The Netherlands
Received August 15, 2002; Revised Manuscript Received October 28, 2002
ABSTRACT: Different generations of poly(propylene imine) dendrimers modified peripherically with urea
oligo(p-phenylene vinylene)s (OPV) having a 3,4,5-tri(dodecyloxy)phenyl mesogenic unit (OPV-dendrim-
ers) were successfully synthesized and fully characterized. Although intramolecular H-bonding exist
between the urea linkages of the end groups, only weak interactions between the π-conjugated segments
were observed in solution. The hydrogen-bonding interactions increased for higher generation dendrimers
due to the closer proximity of the end groups, indicative of a more dense-shell packing. All generations
of OPV-dendrimers showed liquid crystalline behavior in which the OPV-dendrimers have a homeotropic
orientation. The temperature range of the liquid crystalline phase increased with higher generation.
In tr od u ction
and differential scanning calorimetry have been used
to study the thermotropic properties in the solid state.
One of the most interesting features of dendrimers is
the existence of a high number of functional groups at
the periphery, which modulate the properties of these
macromolecules.1 Therefore, dendrimers can easily been
used as polymeric materials in fields such as catalysis,2
chiral recognition,3 unimolecular micelles,4 host-guest
chemistry,5 and light-harvesting antennas.6 Modifica-
tion of the dendritic macromolecules with mesogenic end
groups has resulted in liquid crystalline (LC) materials.7
One of the interesting properties of these materials is
their supramolecular organization giving rise to long-
range order. Moreover, as π-functional materials, they
can be applied in electrooptical switches8 and liquid
crystal displays (LCDs).9
Dendrimers with π-conjugated oligomers at the pe-
riphery have also been synthesized.10 Branching of
π-conjugated systems by attaching them to dendrimers
can be an attractive tool to avoid highly ordered
(crystalline) domains resulting in low quantum ef-
ficiency of luminescence.11 For example, high quality
amorphous thin films can be prepared by conventional
spin-coating from dendrimers.12 Moreover, dendrimers
modified with π-conjugated oligomers can be used as
hosts for dye molecules13 showing efficient energy
transfer from the π-conjugated units to the encapsulated
dye.
Dendrimers in which π-conjugated oligomers are
combined with liquid crystalline behavior are quite
rare.14 Dendritic macromolecules modified peripheri-
cally with π-conjugated oligomers can bridge the gap
between π-conjugated polymers (easy to process) and
oligomers (well-defined). Substitution of the terminal
π-conjugated end groups of the dendrimers with me-
sogenic units will lead to liquid crystalline behavior
resulting in order on a supramolecular level. In this
paper, we report the synthesis and characterization of
liquid crystalline poly(propylene imine) dendrimers
modified peripherically with π-conjugated oligo(p-phen-
ylene vinylene)s. Physical properties of these function-
alized dendrimers have been investigated in detail by
IR and 1H NMR spectroscopy, T1 and T2 relaxation
measurements, UV-vis, fluorescence, and circular dichro-
ism. Polarizing microscopy, thermogravimetric analysis,
Exp er im en ta l Section
Gen er a l Meth od s. 1H NMR and 13C NMR spectra were
recorded at room temperature on a Varian Gemini 300 or 400
MHz spectrometer in deuterated chloroform or 1,1,2,2-tetra-
chloroethane-d2 (TCE-d2) and tetramethylsilane (TMS) was
used as internal reference. Abbreviations used are s ) singlet,
d ) doublet, t ) triplet, m ) multiplet, and b ) broad. For
samples recorded in TCE-d2, the solvent resonance was set to
5.9 ppm and all other signals were referred with respect to
this peak. For the variable-temperature 1H NMR measure-
ments in TCE-d2, a minimum of 64 scans were signal averaged
for each measurement, and the samples were allowed to
equilibrate at the experimental temperature for 15 min prior
to each measurement. The NMR relaxation time experiments
were carried out on a Varian Inova 500 spectrometer operating
at 500.618 MHz and equipped with a 5 mm 500 SW/PFG
probe. For preparative size exclusion chromatography, Bio-
Beads S-X1 (mesh size 200-400, MW operational range 600-
14000) was used. Infrared (FT-IR) spectra were recorded on a
Perkin-Elmer 1605 FT-IR spectrometer. UV-vis spectra and
fluorescence spectra were recorded on a Perkin-Elmer Lambda
40 spectrometer and a Perkin-Elmer luminescence spectrom-
eter LS 50 B instrument, respectively. CD spectra were
recorded on a J asco J -600 spectrometer. Spectra at different
temperatures were not corrected for volume changes of the
solvents. Matrix assisted laser desorption ionization time-of-
flight (MALDI-TOF) mass spectrometry were measured on a
Perspective Biosystems Voyager-DE PRO instrument in reflec-
tor mode, using R-cyano-4-hydroxycinnamic acid as the matrix.
Thermal optical properties of LC materials were studied using
a J enaval polarization microscope equipped with crossed
polarizers and a Linkam THMS 600 hot stage. Differential
scanning calorimetry was performed on a Perkin-Elmer DSC
Pyris 1 at a heating rate of 10 °C/min. Thermal gravimetric
analysis (TGA) data was collected under nitrogen on a Perkin-
Elmer TGA 7 apparatus. Elemental analyses were performed
on a Perkin-Elmer instrument, Series II, 2400. Spin-coated
thin films were prepared by spin-casting 2 wt % toluene
solutions using a Headway Research Spin-coat apparatus.
Low-angle X-ray powder diffraction measurements were per-
formed with a self-made instrument (Netherlands Institute
for Sea Research, NIOZ, Texel) at room temperature. This
high-accuracy φ-φ diffractometer is equipped with a Cu tube,
variable divergence and antiscatter slits, and an energy
dispersive Si/Li Kevex detector, which enables a high peak-
10.1021/ma021338b CCC: $25.00 © 2003 American Chemical Society
Published on Web 01/07/2003

566 Precup-Blaga et al.
Macromolecules, Vol. 36, No. 3, 2003
Sch em e 1
to-background ratio. Special Si single-crystal wafers, cut along
the (501) plane, were used for low-angle measurements. The
compounds were spin-coated and subsequently annealed at 100
°C for 1 h.
mixture was extracted with 1 N NaOH (2×). The organic layer
was dried with Na2SO4, filtered, and dried in vacuo. The
product was further purified with Bio-Beads column chroma-
tography (CH2Cl2) yielding OP VG3 (80%) as a yellow solid.
1H NMR (TCE, 25 °C): δ 8.45 (s, 16H, ArNHCONH), 7.40
(s, 64H, ArH), 7.20-7.47 (d, 96H, ArCHdCH), 7.30 (s, 16H,
ArH), 7.15 (s, 16H, ArH), 7.10 (s, 16H, ArH), 7.00 (s, 16H,
ArH), 6.90 (s, 16H, ArH), 6.70 (s, 16H, ArH), 6.40 (t, 16H,
ArNHCONHCH2CH2CH2), 3.80-4.00 (m, 224H, OCH2), 3.40
(q, br, 32 H, ArNHCONHCH2CH2CH2N), 2.80 (t, br, 84H, 32H,
NHCONHCH2CH2CH2N + 4H NHCH2CH2CH2CH2N + 48H
NCH2CH2CH2N), 1.70-2.00 (m, 64H, CH), 1.6-1.79 (m, 320H,
OCH2CH(CH3)CH2CH3 + OCH2CH2CH2(CH2)8CH3), 1.50-1.60
(m, 60H, 4H NHCH2CH2CH2CH2N + 56H NHCONHCH2CH2-
CH2N)), 1.30-1.46 (m, 768H, O(CH2)3(CH2)8CH3), 0.95-1.30
(m, 528H, CCH3). 13C NMR (300 MHz, TCE, 25 °C): 154.90,
153.34, 153.30, 151.29, 151.26, 151.16, 151.10, 138.20, 138.04,
133.56, 133.50, 128.65, 128.17, 127.65, 127.54, 127.44, 127.31,
126.94, 126.87, 122.80, 122.76, 122.69, 122.56, 120.94, 120.73,
110.92, 110.65, 110.05, 109.81, 105.19, 75.75, 75.39, 75.37,
75.02, 36.74, 33.23, 31.82, 31.10, 31.02, 30.99, 30.95, 30.79,
30.61, 27.90, 27.56, 23.94, 18.18, 15.27, 12.71. IR (KBr): ν
(cm-1) 3326.7, 2959.5, 2875.1, 1645.2. UV/vis (CHCl3): λmax (ꢀ)
) 330 nm (321 850), 432 nm (621 175). Anal. Calcd. for:
Ma ter ia ls. Poly(propylene imine)dendrimers were kindly
provided by DSM, The Netherlands. (E,E,E)-4-[4-{3,4,5-Tri-
dodecyloxystyryl)-2,5-bis[(S)-2-methylbutoxy]styryl}-2,5-bis-
[(S)-2-methylbutoxy]styryl]aniline (1) was synthesized accord-
ing a literature procedure.15 N,N-Dimethylformamide was
stored over BaO or 4.0 Å molsieves. THF (Biosolve, p.a.) was
distilled over Na/benzophenone under an argon atmosphere.
Dichloromethane was freshly distilled from P2O5. Diethyl ether
was freshly distilled over potassium/sodium. All other reagents
were purchased from Acros and Aldrich and used without
further purification. For column chromatography Merck silica
gel 60 (particle size 0.063-0.200 mm) was used.
(E,E,E)-4-[4-{3,4,5-Tr id od ecyloxyst yr yl)-2,5-b is[(S)-2-
m eth ylbu toxy]styr yl}-2,5-bis[(S)-2-m eth ylbu toxy]styr yl]-
p h en ylisocya n a te (2). Under an argon atmosphere, (E,E,E)-
4-[4-{3,4,5-Tridodecyloxystyryl)-2,5-bis[(S)-2-methylbutoxy]-
styryl}-2,5-bis[(S)-2-methylbutoxy]styryl]aniline (1)15 (0.5 g,
0.385 mmol) was suspended in 31 mL of a 20% phosgene
solution in toluene. The reaction mixture was heated to reflux
temperature. After 16 h the mixture was cooled to room
temperature and the toluene was removed in vacuo to dryness
and flushed with CHCl3. Compound 2 was used without
further purification. IR (KBr): ν (cm-1) 2261 (isocyanate).
Typ ica l P r oced u r e for OP V Den d r im er s (OP VG3).
DAB-dendr-(NH2)16 (0.018 g, 0.0107 mmol; 1/16 equiv) dis-
solved in dichloromethane was added to the isocyanate 2. The
mixture was stirred for 48 h at room temperature. The reaction
C
1480H2368O128N46 (Mr ) 22 854.8): C, 77.83; H, 10.38; N, 2.82.
Found: C, 78.10; H, 10.02; N, 2.89.
OP VG0. 1H NMR (TCE, 25 °C): δ 7.53 (s, 2H), 7.41 (d, 1H),
7.34 (d, 2H), 7.31 (d, 1H), 7.21 (s, 1H), 7.19 (s, 1H), 7.11 (s,
1H), 7.10 (s, 1H), 7.07 (d, 1H), 7.04 (d, 1H), 6.75 (s, 2H), 6.64
(d,2H), 6.29 (s,1H, ArNHCONHCH2CH2CH3), 4.62 (t,1H,
NHCONHCH2CH2CH3), 4.08-3.78 (m, 14H), 3.20 (q, 2H,

Macromolecules, Vol. 36, No. 3, 2003
Liquid Crystalline Dendrimers 567
F igu r e 1. IR spectra of the OPV-dendrimers in dichloromethane at 298 K (end group concentration ) 5 mM).
F igu r e 2. 1H NMR spectra of OP VG3 in TCE-d2 at 293 and 393 K.
CH2N), 2.05-1.92 (m, 4H), 1.92-1.18 (m, 73H), 1.18-0.80 (m,
33H). 13C NMR (400 MHz, CDCl3): δ 155.98, 153.34, 153.30,
151.29, 151.26, 151.16, 151.10, 138.20, 138.04, 133.56, 133.50,
128.65, 128.17, 127.65, 127.54, 127.44, 127.31, 126.94, 126.87,
122.80, 122.76, 122.69, 122.56, 120.94, 120.73, 110.92, 110.05,
109.81, 105.19, 74.96, 74.72, 74.46, 74.34, 73.93, 69.45, 69.36,
42.45, 35.53, 35.46, 35.35, 32.33, 32.31, 30.72, 30.14, 30.12,
30.10, 30.08, 30.05, 30.01, 29.83, 29.78, 29.76, 26.81, 26.79,
26.76, 26.53, 23.78, 23.09, 17.28, 17.26, 17.20, 14.53, 11.92,
11.83, 11.77. MALDI-TOF MS: 1381.14 [M]+; calcd, 1381.09.

568 Precup-Blaga et al.
Macromolecules, Vol. 36, No. 3, 2003
IR (KBr): ν (cm-1) 3335, 2957.8, 2877.1, 1641.2. UV/vis
(CHCl3): λmax (ꢀ) ) 330 nm (17 900), 434 nm (25 570). Anal.
Calcd. for: C90H144O8N2 (Mr ) 1382.1): C, 78.26; H, 10.43; N,
2.03. Found: C, 78.38; H, 10.56; N, 2.05.
OP VG1. 1H NMR (TCE, 25 °C): δ 7.95 (s, 4H, ArNH-
CONH), 7.40 (s, 16H, ArH), 7.00-7.45 (m, 24H, ArCHdCH),
7.20 (s, 4H, ArH), 7.10 (s, 4H, ArH), 6.90 (s, 4H, ArH), 6.80 (s,
4H, ArH), 6.65 (s, 4H, ArH), 6.55 (s, 4H, ArH), 5.90 (t, 4H,
ArNHCONHCH2CH2CH2), 3.80-4.00 (m, 56H, OCH2), 3.25 (q,
br, 8 H, ArNHCONHCH2CH2CH2), 2.56 (t, br, 8H, NHCONH-
CH2CH2CH2N), 2.40 (t, br, 4H, NHCH2CH2CH2CH2N), 1.81-
2.00 (m, 40H, CH), 1.79 (m, 8H, NHCONHCH2CH2CH2N),
1.60-1.70(m,80H,OCH2CH(CH3)CH2CH3 +OCH2CH2CH2(CH2)8-
CH3), 1.46 (m, 4H, NHCH2CH2CH2CH2N), 1.30-1.46 (m,
192H, O(CH2)3(CH2)8CH3), 0.95-1.30 (m, 132H, CCH3). 13C
NMR (300 MHz, TCE, 120 °C): 154.90, 153.34, 153.30, 151.29,
151.26, 151.16, 151.10, 138.20, 138.04, 133.56, 133.50, 128.65,
128.17, 127.65, 127.54, 127.44, 127.31, 126.94, 126.87, 122.80,
122.76, 122.69, 122.56, 120.94, 120.73, 110.92, 110.65, 110.05,
109.81, 105.19, 75.76, 75.39, 75.03, 71.18, 36.75, 36.67, 33.26,
33.24, 31.82, 31.11, 31.03, 31.01, 30.99, 30.96, 30.79, 30.64,
30.61, 27.91, 27.57, 23.94, 18.18, 18.11, 15.28, 12.73, 12.64.
MALDI-TOF MS: 5627.5 [M+ Na]+; calcd, 5627.4. IR (KBr):
ν (cm-1) 3317.8, 2958.6, 2876.4, 1645.4. UV/vis (CHCl3): λmax
(ꢀ) ) 329 nm (92 400), 433 nm (192 490). Anal. Calcd. for:
C
364H580O32N10 (Mr ) 5608.7): C, 77.95; H, 10.42; N, 2.50.
Found: C, 77.92; H, 10.24; N, 2.54.
OP VG5. 1H NMR (TCE, 25 °C): δ 8.45 (s, 64H, ArNH-
CONH), 7.40 (s, 256H, ArH), 7.20-7.47 (d, 384H, ArCHdCH),
7.30 (s, 64H, ArH), 7.15 (s, 64H, ArH), 7.10 (s, 64H, ArH), 7.00
(s, 64H, ArH), 6.90 (s, 64H, ArH), 6.70 (s, 64H, ArH), 6.40 (t,
64H, ArNHCONHCH2CH2CH2), 3.80-4.00 (m, 896H, OCH2),
3.40 (q, br, 128 H, ArNHCONHCH2CH2CH2N), 2.80 (t, br,
500H, 128H NHCONHCH2CH2CH2N + 4H NHCH2CH2-
CH2CH2N + 368H NCH2CH2CH2N), 1.81-2.00 (m, 256H, CH),
1.60-1.79 (m, 1280H, OCH2CH(CH3)CH2CH3 + OCH2CH2CH2-
(CH2)8CH3), 1.50-1.60 (m, 252H, 4H NHCH2CH2CH2CH2N +
248H NHCONHCH2CH2CH2N)), 1.30-1.46 (m, 3072H, O(CH2)3-
(CH2)8CH3), 0.95-1.30 (m, 2112H, CCH3). 13C NMR (300 MHz,
TCE, 120 °C): 154.90, 153.34, 153.30, 151.29, 151.26, 151.16,
151.10, 138.20, 138.04, 133.56, 133.50, 128.65, 128.17, 127.65,
127.54, 127.44, 127.31, 126.94, 126.87, 122.80, 122.76, 122.69,
122.56, 120.94, 120.73, 110.92, 110.65, 110.05, 109.81, 105.19,
75.74, 75.37, 75.01, 33.25, 31.05, 30.97, 30.80, 30.64, 27.87,
27.56, 23.97, 18.21, 15.35, 12.78. IR (KBr): ν (cm-1) 3326.8,
2959.5, 2879.2, 1640.1. UV/vis (CHCl3): λmax (ꢀ) ) 330 nm
F igu r e 3. Chemical shift as a function of the number of end
groups of the OPV-dendrimers at 293 and 393 K.
solution between the urea linkages in the dendrimers.
Propylamine reference compound OP VG0 showed a
non-hydrogen-bonded vibration (3440 cm-1) in the NH
stretching vibration region while the dendritic macro-
molecules OP VG1, OP VG3, and OP VG5 revealed only
the hydrogen-bonded urea absorption (3320 cm-1, Fig-
ure 1). Similar results were found earlier for urea-linked
adamantyl- and palmitoyl-functionalized poly(prop-
ylene imine) dendrimers.17,18 Remarkably, the carbonyl
stretching vibration does not show the same trend (see
Experimental Section).
(1156 300), 432 nm (2204 305). Anal. Calcd. for: C5944H9520O512
-
N190 (Mr ) 91 840): C, 77.78; H, 10.38; N, 2.90. Found: C,
78.02; H, 9.67; N, 2.74.
Resu lts a n d Discu ssion
Syn t h esis a n d Ch a r a ct er iza t ion . The oligo(p-
phenylene vinylene) (OPV) containing dendrimers were
synthesized from (E,E,E)-4-[4-{3,4,5-Tridodecyloxystyryl)-
2,5-bis[(S)-2-methylbutoxy]styryl}-2,5-bis[(S)-2-methyl-
butoxy]styryl]aniline (1) (Scheme 1).15 Reaction of 1 with
phosgene in toluene yielded isocyanate derivative 2.
Reaction of 2 with n-propylamine and the first, third
and fifth generation poly(propyleneimine) dendrimer5
yielded OP VG0, OP VG1, OP VG3, and OP VG5, re-
spectively. Efficient purification of the dendrimers was
achieved by repeated preparative size exclusion chro-
matography, using dichloromethane as eluent. The
dendrimers were fully characterized with infrared spec-
troscopy, elemental analysis and NMR spectroscopy.
Mass spectrometry showed fully reacted dendrimer, in
the case of OP VG0 and OP VG1. It proved to be
impossible to yield a considerable ionization for OP VG3
and OP VG5, despite variations in the experimental
conditions.16
The hydrogen-bonding interactions between the urea
moieties were further investigated by 1H NMR spec-
troscopy at room temperature19 (Figures 2 and 3) by
monitoring the chemical shift of the two N-H reso-
nances of OPV-dendrimers. Deuterium exchange mea-
surements in CD3OD showed that the NH resonances
are positioned at 4.62 and 6.29 ppm for OP VG0, 5.9
and 7.95 ppm for OP VG1, and 6.4 and 8.45 ppm for
OP VG3 and OP VG5 (Figure 2). These results indicate
an increase of the hydrogen-bonding interactions in the
higher generation dendrimer. Moreover, a constant
value of the chemical shift for both N-H protons is
already reached at the third generation (n ) 16),
suggesting that all end groups are hydrogen-bonded at
the third generation stage.17,20
Infrared spectroscopy was used to gain qualitative
information on the hydrogen-bonding interactions in
1H NMR spin-lattice (T1) and spin-spin relaxation
(T2) measurements were performed for all generations

Macromolecules, Vol. 36, No. 3, 2003
Liquid Crystalline Dendrimers 569
F igu r e 4. Relaxation time as a function of the number of end groups of the OPV-dendrimers.
Tem p er a tu r e-Dep en d en t Beh a vior in Solu tion .
A temperature-dependent proton NMR measurement
was carried out for OP VG0, OP VG1, OP VG3, and
OP VG5 in order to study in more detail the hydrogen
bonding interactions (Figure 2). An increase in temper-
ature from 293 to 393 K produces an upfield shift of
the δ value of N-H protons of all generations of OPV-
dendrimers. Upfield shifts from 4.62 to 4.56 ppm and
from 6.29 to 6.21 ppm (∆δ ) 0.08) for OP VG0, from 5.9
to 5.52 ppm and from 7.95 to 7.23 ppm (∆δ ) 0.38 and
0.72) for OP VG1, and from 6.4 to 5.9 ppm and from
8.45 to 7.75 ppm (∆δ ) 0.50 and 1.20) for both OP VG3
and OP VG5 were observed. For the propylamine model
compond, the N-H shift is smaller compared with those
of the dendrimers. This is in a good agreement with the
IR measurements, where propylamine reference com-
pound OP VG0 shows an almost completely non-
hydrogen-bonded species at room temperature while the
dendritic macromolecules revealed only the hydrogen-
bonded urea absorption. These changes in chemical
shifts indicate that upon increasing the temperature the
hydrogen bonding interactions decrease.17 This behavior
is further supported by the sharpening of the signals
at higher temperatures.
F igu r e 5. Extinction coefficient vs the number of end groups
of each generation dendrimer.
OPV-dendrimers in order to investigate restricted
motions in the dendrimer.21 The changes in T1 relax-
ations upon going from OP VG0 to OP VG1 is due to
the hindered mobility (Figure 4).22 The dipoles become
closer in space and start to interact with each other.
Moreover, an increase in relaxation times is observed
passing from OP VG1 to OP VG5 due to segmental
mobilities and steric hindrance. The motion in the
dendrimers decreases upon increasing the number of
end groups. T2 is decreasing by increasing the number
of end groups, which can be observed by line broadening
of the signals. These results suggest a denser shell
packing of the dendrimer by increasing generation.23
The variable-temperature UV-vis measurements in
orthodichlorobenzene (ODCB) for all generations of
OPV-dendrimers were also carried out (Figure 6). Upon
decreasing the temperature from 353 to 283 K, a
gradual red shift of λmax was observed for all generations
(λmax ) 434 ( 2 to 442 ( 2 nm). Remarkably, when the
absorption maxima of the OPV-dendrimers in ODCB
with chloroform solutions at room temperature are
compared, a hypsochromic shift of 4-7 nm is observed.
If the wavelength of the absorption maximum is plotted
against the temperature a linear dependence is observed
for all generations (Figure 7). This relation demon-
strates that this behavior is not due to hydrogen bond
interactions (which are generation dependent). Isos-
bestic points (420 nm) were observed in the tempera-
ture-dependent absorption spectra, indicating the ex-
istence of two phases. These phases can be the result
of backbone conformational changes, i.e., a planar and
twisted conformation.26 To have a better understanding
of this behavior the fluorescence spectra of all genera-
tions of OPV-dendrimers were recorded in ODCB at
different temperatures (from 263 to 353 K). At low
temperature (263 K), photoexcitation (λex ) 434 ( 2 nm)
of OP VG0, OP VG1, OP VG3, and OP VG5 result in an
Absorption spectra of the OPV-dendrimers in chlo-
roform show the typical π-π* transition of the OPV
tetramer units.24 The absorption of the main band is
located at λmax ) 434 ( 2 nm for all generations. The
extinction coefficient shows a linear dependence with
the number of dendrimer end groups (Figure 5). This
behavior indicate that only weak interchromophoric
interactions exist between the π-conjugated segments.25
The fluorescence spectrum of all OPV-dendrimers
(λem,max ) 434 nm) has a maximum at 500 nm and shows
the vibronic fine structure typical of molecular dissolved
chromophores. The lack of a CD (circular dichroism)
signal (optical density of 1), despite the stereocenters
present in the side chains of the OPV units, also
supports the absence of strong interchromophoric in-
teractions.

570 Precup-Blaga et al.
Macromolecules, Vol. 36, No. 3, 2003
Ta ble 1. Tem p er a tu r e Ra n ges a t Wh ich th e OP V
Den d r im er s Ar e Liqu id Cr ysta llin e
LC temp ranges, °C
OP VG0
OP VG1
OP VG3
OP VG5
90-120
70-170
55-180
50-160
but to conformational changes in the oligomer back-
bone.24,26
Th er m otr op ic P r op er ties. The thermal stabilities
of OP VG0, OP VG1, OP VG3, and OP VG5 have been
studied with thermal gravimetric analysis (TGA). For
OP VG0 the degradation starts at about 170 °C, while
OP VG1, OP VG3, and OP VG5 were thermally stable
to about 200 °C.
Polarizing microscopy revealed birefringence for all
compounds (temperature ranges summarized in Table
1). Dropletlike textures (Maltese crosses) were observed
during slow cooling typical for homeotropic orientated
molecules while the surface of the glass substrate was
not treated to bring about a homeotropic orientation for
the mesogens (Figure 8). Homeotropic orientation of
poly(propylene imine) dendrimers peripherically modi-
fied with cyanobiphenylene mesogens has recently been
observed by the Yonetake and co-workers.27
DSC measurements, however, did not give phase
transition temperatures, probably due to the high
viscosity of the polymeric phases and hence to the
subsequent slow transition from one phase to the other.
Similar behavior has been reported in the literature for
polypropylene dendrimers containing trialkoxybenzene
wedges as mesogenic units.28
The liquid crystalline behavior was further investi-
gated by X-ray powder diffraction. No reflections were
observed in the case of OP VG0. However OP VG1,
OP VG3, and OP VG5 gave in all cases one reflection
corresponding to a distance of 55.9, 59.0, and 53.9 Å,
respectively (Figure 9). According to CPK models, such
a distance could correspond to an interdigitated bilayer
in which the mesogenic units and the dendritic interior
are separated. The arrangement of the mesogenic end
groups is perpendicular with respect to the dendritic
backbone. The behavior is similar to poly(propylene-
imine) dendrimers with cyanobiphenyl or mesogenic
ester units.27,29,30
F igu r e 6. (a) Temperature dependence UV-vis spectra of
OP VG3 and (b) temperature dependence fluorescence spectra
of OP VG3 in ODCB.
emission maximum at λem,max ) 500 nm attributed to
the emission of the molecularly dissolved chromophores.
At higher temperatures the fluorescence emission spec-
tra matched well with the one at low temperature.
When the excitation wavelength was changed similar
fluorescence spectra were obtained. The fluorescence
data show that the observed red shift in the absorption
spectra is not due to aggregation of the chromophores
F igu r e 7. Absorption maximum of OP VG0, OP VG1, OP VG3, and OP VG5 in ODCB as a function of the temperature.

Macromolecules, Vol. 36, No. 3, 2003
Liquid Crystalline Dendrimers 571
F igu r e 8. Polarizing optical textures of OP VG3 during cooling at 1 °C min-1 at 170 °C (bar represents 400 µm).
F igu r e 9. X-ray powder diffraction pattern observed for
OP VG5.
F igu r e 10. UV-vis spectra of OP VG3 in spin-coated film
before and after annealing.
Con clu sion s
Op tica l Stu d ies in Solid Sta te. The thermotropic
Different generations of dendrimers with peripheral
liquid crystalline OPV units can be synthesized. In these
systems, intramolecular H-bonds exist between the urea
linkages, and a gradual increase in hydrogen bond
interactions with increasing dendrimer generation is
observed. Remarkably, only weak interactions between
the attached OPV chromophores exist. All generations
of OPV-dendrimers show liquid crystalline behavior.
A homeotropic orientation was observed for all OPV-
dendrimers on a glass plate. Currently we are investi-
gating if these materials can be applied in electrooptical
switches and light-emitting diodes.
behavior in the solid state was investigated by UV-vis
(Figure 10) and fluorescence spectroscopy. OP VG1,
OP VG3, and OP VG5 could be spin-coated from a
toluene solution giving good quality thin films.13
A
nonhomogeneous very thin film was obtained in the case
of OP VG0. The absorption spectra of the reference
compound OP VG0 is hypsochromically shifted com-
pared with the behavior in CHCl3 solution (λsolution
)
434 nm in CHCl3; λfilm ) 419 nm). In the dendrimer
films (OP VG1, OP VG3, and OP VG5) small blue-shifts
(∆λ ) 4 nm, λfilm ) 430 ( 2 nm) of the absorption
maxima were observed in comparison with the den-
drimer solution. After annealing at 120 °C, the absorp-
tion decreases for all generations, which is presumably
due to the homeotropic oriention of the OPV moieties.
The chromophores are oriented parallel to the incoming
light beam resulting in a lower absorption. The remain-
ing absorption is probably due to nonoriented chromo-
phores, e.g., defects in the film or slightly tilted orienta-
tion.
Ack n ow led gm en t. We would like to thank S. van
der Gaast, Dr. Annika J . Karlsson, and Dr. Stefan
Meskers for stimulating discussions and assistance with
the relaxation data measurements and X-ray powder
diffraction measurements. We would like to acknowl-
edge J oost van Dongen for performing the MALDI TOF
measurements. The research of A.P.H.J .S. has been
made possible by a fellowship of the Royal Netherlands

572 Precup-Blaga et al.
Macromolecules, Vol. 36, No. 3, 2003
(11) Schenning, A. P. H. J .; Weener, J . W.; Meijer, E. W. In
Conjugated Polymers and Molecular Interfaces; Salaneck, W.
R., Seki, K., Kahn, A., Pireaux, J .-J ., Eds.; Marcel Dekker:
New York and Basel, Switzerland, 2002; pp 1-21.
(12) Wang, P.-W.; Liu, Y.-J .; Devadoss, C.; Barathi, P.; Moore, J .
S. Adv. Mater. 1996, 8, 237.
Academy of Arts and Sciences. The Dutch Science
Foundation (NWO) is acknowledged for financial sup-
port (F.S.P.-B.).
Refer en ces a n d Notes
(13) Schenning, A. P. H. J .; Peeters, E.; Meijer, E. W. J . Am. Chem.
Soc. 2000, 122, 4489.
(14) (a) Meier, H.; Lehman, M. Adv. Mater. 1998, 37, 643. (b)
Meier, H.; Lehmann, M.; Kolb, U. Chem.sEur. J . 2000, 6,
2462.
(15) (a) Peeters, E.; van Hal, P. A.; Meskers, S. C. J .; J anssen, R.
A. J .; Meijer, E. W. Chem.sEur. J . 2002, 8, 4470. (b)
Syamakumari, A.; Schenning, A. P. H. J .; Meijer, E. W.
Chem.sEur. J . 2002, 8, 3353.
(16) Schenning, A. P. H. J .; Elissen-Roma´n, C.; Weener, J .-W.;
Baars, M. W. P. L.; van der Gaast, S. J .; Meijer, E. W. J .
Am. Chem. Soc. 1996, 118, 8199.
(17) IR and 1H NMR data on adamantyl and palmitoyl urea-
functionalized dendrimers: Baars, M. W. P. L. Ph.D. Thesis,
Eindhoven University of Technology, 2000.
(1) (a) Tomalia, D. A.; Naylor, A.; Goddard, W. A. Angew. Chem.,
Int. Ed. 1990, 29, 138. (b) Newkome, G. R.; Moorefield, C.
N.; Vo¨gtle, F. Dendritic Molecules: Concepts, Synthesis and
Perspectives; VCH: Weinheim, Germany, 1996. (c) Fre´chet,
J . M. J . Science 1994, 263, 1710. (d) Dvornic, P. R.; Tomalia,
D. A. Macromol. Symp. 1994, 88, 123. (e) Bosman, A. W.;
J anssen, H. M.; Meijer, E. W. Chem. Rev. 1999, 99, 1665.
(2) Zeng, F.; Zimmerman, S. C. Chem. Rev. 1997, 97, 1681.
(3) Stevelmans, S.; van Hest, J . C. M.; J ansen, 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.
(4) Sayed-Sweet, Y.; Hedstrand, D. M.; Spinder, R.; Tomalia, D.
A. J . Mater. Chem. 1997, 7, 1199.
(5) Baars, M. W. P. L.; Meijer, E. W. Top. Curr. Chem. 2000,
210, 131.
(18) Boas, U.; Karlsson, A. J .; de Waal, B. F. M.; Meijer, E. W. J .
Org. Chem. 2001, 66, 2136.
(19) ,1,2,2-Tetrachloroethane-d2 (TCE) was used as solvent for the
variable-temperature measurement based on the fact that
the solubility of all OPV-dendrimers described here was
comparable to that in CDCl3 and one could go up to much
higher temperatures.
(20) Bosman, A. W.; Bruining, M. J .; Kooijman, H.; Spek, A. L.;
J anssen, R. A. J .; Meijer, E. W. J . Am. Chem. Soc. 1998, 120,
8547.
(21) Friebolin, H. Basic One-and Two-Dimensional NMR Spec-
troscopy, 2nd ed.; VCH: Weinheim, Germany, 1993.
(22) Baars, M. W. P. L.; Karlsson, A. J .; Sorokin, V.; de Waal, B.;
Meijer, E. W. Angew. Chem., Int. Ed. 2000, 39, 4262.
(23) van Genderen, M. H. P.; de Brabander-van den Berg, E. M.
M.; Meijer, E. W. Adv. Dendritic Macromol. 1971, 4, 61.
(24) Peeters, E.; J anssen, R. A. J .; Meskers, S. C. J .; Meijer, E.
W. Polym. Prepr. 1999, 40, 519.
(25) Meskers, S. C. J .; Bender, M.; Hu¨bner, J .; Romanovskii, Y.
V.; Oestreich, M.; Schenning, A. P. H. J .; Meijer, E. W.;
Ba¨ssler, H. J . Phys. Chem. A 2001, 105, 10220.
(26) ten Hoeve, W.; Wynberg, H.; Havinga, E. E.; Meijer, E. W.
J . Am. Chem. Soc. 1991, 113, 5887.
(6) (a) Matthews, O. A.; Shipwa, A. N.; Stoddart, J . F. Prog.
Polym. Sci. 1998, 23, 1. (b) Stewart, G. M.; Fox, M. A. J . Am.
Chem. Soc. 1996, 118, 4354. (c) Devadoss, C.; Bharathi, P.;
Moore, J . S. J . Am. Chem. Soc. 1996, 118, 9635. (d) Bar-Haim,
A.; Klafter, J .; Kopelman, R. J . Am. Chem. Soc. 1997, 119,
6197. (e) J iang, D. L.; Aida, T. Nature (London) 1997, 388,
454. (f) Tretiak, S.; Chernyak, V.; Mukamel, S. J . Phys. Chem.
B 1998, 102, 3310. (g) Kawa, M.; Fre´chet, J . M. J . Chem.
Mater. 1998, 10, 286. (h) Balzani, V.; Campagna, S.; Denti,
G.; J uris, A.; Serroni, S.; Venturi, M. Acc. Chem. Res. 1998,
31, 26. (i) Sylvain, L.; Gilat, A. A.; Fre´chet, J . M. J . Angew.
Chem., Int. Ed. Engl. 1999, 38, 1422. (j) Sato, T.; J iang, D.-
L.; Aida, T. J . Am. Chem. Soc. 1999, 121, 10658. (k) Adronov,
A.; Gilat, S. L.; Fre´chet, J . M. J .; Ohta, K.; Neuwahl, F. V.
R.; Fleming, G. R. J . Am. Chem. Soc. 2000, 122, 1175.
(7) For example: (a) Percec, V.; Chu, P.; Kawasumi, M. Macro-
molecules 1994, 27, 4441. (b) Bauer, S.; Fischer, H.; Ringsdorf,
H. Angew. Chem., Int. Ed. Engl. 1993, 32, 1589. (c) Pono-
marenko, S. A.; Rebrov, E. A.; Bobrovsky, A. Y.; Boiko, N. I.;
Muzafarov, A. M.; Shibaev, V. P. Liq. Cryst. 1996, 21, 1. (d)
Chen, F. L.; J amieson, A. M.; Kawasumi, M.; Percec, V. J .
Polym. Sci. B 1995, 33, 1213. (e) Tsiourvas, D.; Felekis, T.;
Sideratou, Z.; Paleos, C. M. Macromolecules 2002, 35, 6466.
(8) Baars, M. W. P. L.; van Boxtel, M. C. W.; Bastiaansen, C. W.
M.; Broer, D. J .; So¨ntjens, S. H. M.; Meijer, E. W. Adv. Mater.
2000, 12, 715.
(27) Yonetake, K.; Masuko, T.; Suzuki, K.; Ueda, M.; Nagahata,
R. Macromolecules 1999, 32, 6578.
(9) Scheffer, T.; Nehring, J . Liquid Crystals, Applications and
Uses; Bahadur, B., Ed.; World Scientific: Singapore, 1990; Vol.
1.
(10) (a) Wang, F.; Rauh, R. D.; Rose, T. L. J . Am. Chem. Soc. 1997,
119, 11106. (b) Miller, L. L.; Kunugi, Y.; Canavesi, A.; Rigaut,
S.; Moorefield, C. N.; Newkome, G. R. Chem. Mater. 1998,
10, 1751. (c) Sebastian, R. M.; Caminade, A. M.; Majoral, J .
P.; Levillain, E.; Huchet, L.; Roncali, J . Chem. Commun.
2000, 507.
(28) Cameron, J . H.; Facher, A.; Lattermann, G.; Diele, S. Adv.
Mater. 1997, 9, 398.
(29) Baars, M. W. P. L.; Sontjens, S. H. M.; Fisher, H. M.;
Peerlings, H. W. I.; Meijer, E. W. Chem.sEur. J . 1998, 4,
2456.
(30) Lorenz, K.; Holter, D.; Stuhn, B.; Mulhaupt, R.; Frey, H. Adv.
Mater. 1996, 8, 414.
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