Liquid crystalline dendrimers containing photoactive cinnamate units

Article abstract of DOI:10.1002/pola.24785

Cinnamate-containing dendrimers have been prepared by peripheral functionalization of the amine groups of a poly(propyleneimine) dendrimer with 4-methoxycinnamate- or 4-(N,N-dimethylamino)cinnamate-derived units and/or 4-cyanobiphenyl units in different p

Full text of DOI:10.1002/pola.24785

Liquid Crystalline Dendrimers Containing Photoactive Cinnamate Units
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ANA I. VINUALES, JOSE LUIS SERRANO,2 RAQUEL GIMENEZ, MILAGROS PINOL, JAROSLAW TOMCZYK,3 JOACHIM STUMPE3
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1Quı´mica Orga´nica, Facultad de Ciencias-Instituto de Ciencia de Materiales de Arago´ n, Universidad de Zaragoza-CSIC,
Pedro Cerbuna, 12, 50009 Zaragoza, Spain
2Quı´mica Orga´nica, Facultad de Ciencias-Instituto de Nanociencia de Arago´ n, Universidad de Zaragoza,
Pedro Cerbuna, 12, 50009 Zaragoza, Spain
3Fraunhofer Institute for Applied Polymer Research, Geiselbergstrasse 69, 14476 Potsdam, Germany
Received 7 April 2011; accepted 20 May 2011
DOI: 10.1002/pola.24785
Published online 14 June 2011 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Cinnamate-containing dendrimers have been pre-
pared by peripheral functionalization of the amine groups of a
poly(propyleneimine) dendrimer with 4-methoxycinnamate- or
4-(N,N-dimethylamino)cinnamate-derived units and/or 4-cya-
nobiphenyl units in different proportions. The synthesis, full
characterization in solution, thermal properties and optical
properties of the novel monomers, homodendrimers and
codendrimers are reported. The composition of the molecular
structure of the codendrimers has been elucidated by matrix
assisted laser desorption/ionization time-of-flight mass spec-
trometry (MALDI-TOF MS). These liquid crystalline den-
drimers display lamellar SmA mesophases. The codendrimers
have been tailored in such a way that the photoactive units
and the liquid crystal units absorb in different regions in order
to allow better control over the processes induced by light.
Linearly polarized UV light irradiation studies performed on
thin films of the cinnamate codendrimers show that they are
photoresponsive. A photoinduced anisotropy is generated
with increasing exposure time, but in-plane amplification of
anisotropy by thermal annealing in the mesophase was not
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observed.
2011 Wiley Periodicals, Inc. J Polym Sci Part A:
Polym Chem 49: 3499–3498, 2011
KEYWORDS: copolymerization; dendrimers; liquid crystal;
MALDI; photoselection
INTRODUCTION Dendrimers are hyperbranched molecules that
can be considered as intermediate between low molecular
weight compounds and polymers. The defined chemical struc-
tures of these systems have unique characteristics depending on
the type of nucleus, number and length of branches, and type of
functional groups.1,2 The possibility of designing dendrimers that
are able to perform specific functions by chemical modification of
their molecular structure has attracted enormous interest in
many fields of science as they are seen as transport systems for
biological or technological applications.3–6 Among these proper-
ties, those concerning the controlled response to light have
numerous potential applications due to the possibility of induc-
ing macroscopically ordered materials by noncontact methods.7,8
homopolymers but in copolymers with small percentages
(10–20 mol %) of photoactive units.16–18
As part of our work on functional polymers for optical applica-
tions, we were interested in transferring the concept outlined
above to dendrimers by preparing novel liquid crystalline cinna-
mate-containing materials. Cinnamates are interesting photoac-
tive units as they are able to yield angular-selective E/Z photoiso-
merization and [2þ2] photocycloaddition processes. Side chain
liquid crystalline cinnamate polymers have been shown to give
good orientational properties under LPUV light irradia-
tion,13,14,19–21 and these materials are also more compatible with
luminophores than azobenzene units,22,23 thus allowing multi-
functional properties in a single layer.24 Furthermore, although
some cinnamate dendrimers have been reported,25,26 cinnamate
liquid crystal (LC) dendrimers have barely been investigated.27
Up to now, mainly photoactive side chain liquid crystalline
polymers have been intensely investigated for this purpose.
Among them, azobenzene or cinnamate polymers are able to
generate films with high and stable photoinduced optical an-
isotropy based on axis-selective photoreactions that occur on
irradiation with linearly polarized (LP) light, a process that
leads to a self-organization in a preferred direction.7,9–11 In
some cases, the photoinduced anisotropy can be amplified
significantly with subsequent thermal annealing in the meso-
phase.12–15 This effect has been observed not only in
One of the possible strategies to obtain liquid crystalline proper-
ties with dendrimers involves the covalent modification of the
periphery of a presynthesized dendritic core with liquid crystal-
line units.28,29 In this work, we introduced liquid crystalline cya-
nobiphenyl-derived units and/or photoactive cinnamate-derived
units into the third generation of poly(propyleneimine) (PPI)
dendrimer, which contains 16 functionalizable terminal amino
Correspondence to: M. Pin˜ ol (E-mail: mpinol@unizar.es)
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Journal of Polymer Science Part A: Polymer Chemistry, Vol. 49, 3499–3512 (2011) 2011 Wiley Periodicals, Inc.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 1 Homodendrimers and
codendrimers based on PPI-
(NH2)16 (x and y represent the
monomer feed in mol
their sum equals 16).
% and
groups. PPI dendrimers act as polar hyperbranched cores and
these tend to stabilize liquid crystal phases by microsegrega-
tion.30 In addition, it is expected that PPI systems will show
lower transition temperatures than other polar dendritic cores
such as poly(amido amine) (PAMAM) on peripheral functionali-
zation with mesogenic units.31 As photoactive units, we prepared
two cinnamates with different optical properties. These systems
are derived from 4-methoxycinnamate and 4-(N,N-dimethylami-
no)cinnamate (Fig. 1). These moieties were grafted onto the den-
dritic matrix through a flexible decamethylene spacer to decou-
ple the matrix from the functional units in a similar way to
photoactive side chain LC copolymers (Scheme 1).
ester-terminated monomers (Scheme 1). Dendrimer [1] has
been described previously.30,32 The liquid crystalline unit
pentafluorophenyl 11-[40-cyano-4-biphenyloxy]undecanoate,
denoted as CNB, was synthesized as reported.30 The route
for the novel pentafluorophenyl ester-terminated cinnamate
monomers MOMe and MNMe2 is shown in Scheme 2.
Synthesis of the Monomers
40-Hydroxy-4-biphenyl 4-methoxycinnamate (P1)
A solution of N,N0-dicyclohexylcarbodiimide (DCC) (22.5 g, 109
mmol) in DMF (75 mL) was added dropwise to a cold (0 ꢀC)
N,N-dimethylformamide (DMF) solution (250 mL) of 4,40-dihy-
droxybiphenyl (20 g, 107 mmol), N,N-dimethyl-4-aminopyridine
(DMAP) (1.31 g, 10.7 mmol), and 4-methoxycinnamic acid
(17.2 g, 96 mmol). The mixture was stirred at r.t. for 3 days and
filtered. The clear solution was diluted with water (300 mL)
and extracted with dichloromethane (3 ꢁ 150 mL). The organic
layer was dried, evaporated to dryness, and the residue was
purified by column chromatography (silica gel, dichlorome-
thane/ethyl acetate 9/1).
In this work, we report the synthesis and characterization, as
well as the thermal and optical properties, of the novel cinna-
mate promesogenic units and also the synthesis of homoden-
drimers and codendrimers which contain 80/20 and 90/10
mol %, respectively, of cyanobiphenyl/cinnamate units (Fig. 1).
The compositions of the codendrimers were fully characterized
by a combination of analytical techniques such as 1H NMR,
UV–vis, and MALDI-TOF MS to gain structural information. The
synthesis and full characterization of these molecules was a
prerequisite to the study of their photoactive properties.
1
Yield: 13%. H NMR (300 MHz, (CD3)2CO, d, ppm): 8.48 (br s,
1H), 7.84 (d, J ¼ 15.9 Hz, 1H), 7.75 (d, J ¼ 8.7 Hz, 2H), 7.63 (d, J ¼
8.9 Hz, 2H), 7.52 (d, J ¼ 8.6 Hz, 2H), 7.24 (d, J ¼ 8.6 Hz, 2H), 7.04
(d, J ¼ 9.0, 2H), 6.94 (d, J ¼ 8.6 Hz, 2H), 6.64 (d, J ¼ 16.0 Hz, 1H),
3.88 (s, 3H). IR (nujol, cmꢂ1): 3437–3353 (OH), 1728 (C¼¼O),
1625 (C¼¼C), 1591, 1549, 1509, 1493 (C-Car), 1257, 1214 (CAO).
EXPERIMENTAL
Dendrimers were prepared by an amidation reaction
between the amine groups of PPI and pentafluorophenyl
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SCHEME 1 Synthesis of the
dendrimers.
2-[40-(Tetrahydropyranyl-2-oxy)-4-biphenyloxy]
ethanol (P2)
dihydroxybiphenyl. The organic layer was separated, dried
over MgSO4, and evaporated. The product, 40-(tetrahydropyr-
anyl-2-oxy)-4-hydroxybiphenyl, was purified by column chro-
matography (silica gel, hexane/ethyl acetate 10/1).
Step 1. 3,4-Dihydro-2H-pyran (DHP) (4.47 mL, 49 mmol) was
added dropwise to a flask containing 4,40-dihydroxybiphenyl
(10 g, 54 mmol), p-toluensulfonic acid (PTSA) (0.46 g), and
dry tetrahydrofuran (THF) (250 mL). The solution was stirred
at r.t. for 2 h and evaporated to dryness. The solid was dis-
solved in diethyl ether and washed with an aqueous solution
of NaOH (54 mmol) to remove most of the unreacted 4,40-
1
Yield: 37%. H NMR (300 MHz, CDCl3, d, ppm): 7.41 (m, 4H),
7.07 (d, J ¼ 8.5 Hz, 2H), 6.85 (d, J ¼ 8.6 Hz, 2H), 5.43 (m,
1H), 4.79 (s, 1H), 4.00–3.86 (m, 1H), 3.65–3.56 (m, 1H),
2.10–1.50 (m, 6H). IR (nujol, cmꢂ1): 3350 (OH), 1609, 1592,
1499 (C-Car), 1266, 1229 (CAO), 1101, 827 (cyclic CAOAC).
SCHEME 2 Synthetic pathways followed for the preparation of the monomers.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
Step 2. In Step 2, 2-bromoethanol (1.63 mL, 22 mmol) was
added dropwise to a solution of 40-(tetrahydropyranyl-2-
oxy)-4-hydroxybiphenyl (5.40 g, 20 mmol), K2CO3 (4.15 g,
30 mmol), and KI (0.66 g, 4 mmol) in dry DMF (125 mL).
The mixture was stirred at 140 ꢀC for 24 h. After cooling,
H2O (100 mL) was added, and the product was extracted
with a mixture of hexane/ethyl acetate 1/2 (5 ꢁ 75 mL).
The organic layer was washed with H2O (4 ꢁ 75 mL), dried,
and evaporated to dryness. The resulting white solid was
purified by washing the crushed solid in boiling hexane.
dichloromethane (200 mL). The solution was stirred at r.t.
for 3 days. The solid was filtered off, and the filtrate was
washed with saturated aqueous NaHCO3 (3 ꢁ 150 mL),
dried over MgSO4, filtered, and evaporated to dryness. The
product was purified by column chromatography (silica gel,
hexane/ethyl acetate 20/1).
Yield 31%. 1H NMR (300 MHz, CDCl3, d, ppm): 3.38 (t, J ¼
6.8 Hz, 2H), 2.18 (t, J ¼ 7.3 Hz, 2H), 1.86–1.78 (m, 2H),
1.58–1.52 (m, 2H), 1.42 (s, 9H), 1.27 (m, 12H). IR (neat,
cmꢂ1): 1729 (C¼¼O), 1252 (CAO).
Yield: 79%. 1H NMR (300 MHz, CDCl3, d, ppm): 7.47–7.42
(m, 4H), 7.08 (d, J ¼ 8.8 Hz, 2H), 6.95 (d, J ¼ 8.5 Hz, 2H),
5.44 (s, 1H), 4.20–4.05 (m, 2H), 4.06–3.85 (m, 3H), 3.70–
3.57 (m, 1H), 2.15–1.80 (m, 3H), 1.79–1.50 (m, 4H). IR
(nujol, cmꢂ1): 3452 (OH), 1606, 1498 (C-Car), 1267, 1238
(CAO), 1108, 813 (cyclic CAOAC).
40-[10-(Pentafluorophenyloxycarbonyl)decyloxy]-4-
biphenyl 4-methoxycinnamate (MOMe
)
Step 1. A mixture of P4 (3.6 g, 11.3 mmol), P1 (3 g. 8.6
mmol), K2CO3 (2.15 g, 15.6 mmol), and 18-crown-6 ether
(0.18 g, 0.7 mmol) in acetone (120 mL) was heated under
reflux for 2 days. The mixture was evaporated to dryness
and the residue was dissolved in hexane/ethyl acetate 1/2,
washed with water (5 ꢁ 100 mL), dried, and evaporated to
dryness. The resulting solid was purified by column chroma-
tography (silica gel, hexane/ethyl acetate 9/1).
2-(40-Hydroxy-4-biphenyloxy)ethyl 4-(N,N-dimethylamino)-
cinnamate (P3)
Step 1. Triphenylphosphine (7.32 g, 27.9 mmol), 4-dimethy-
laminocinnamic acid (18.6 mmol), and dry dichloromethane
(100 mL) were stirred for 30 min at r.t. P2 (5.86 g, 18.6
mmol) and diisopropyl azodicarboxylate (DIAD) (5.48 mL,
27.9 mmol diluted in dry dichloromethane) were added. The
mixture was stirred for 48 h and was evaporated to dryness.
The residue was purified on a silica gel column (eluent hex-
ane/ethyl acetate 8/2).
Yield of 40-[10-(tert-butoxycarbonyl)decyloxy]-4-biphenyl 4-
1
methoxycinnamate: 39%. H NMR (300 MHz, CDCl3, d, ppm):
7.82 (d, J ¼ 15.9 Hz, 1H), 7.54 (d, J ¼ 8.5 Hz, 2H), 7.53 (d, J
¼ 8.8 Hz, 2H), 7.48 (d, J ¼ 8.4 Hz, 2H), 7.18 (d, J ¼ 8.7 Hz,
2H), 6.94 (d, J ¼ 8.4 Hz, 2H), 6.92 (d, J ¼ 8.8 Hz, 2H), 6.50
(d, J ¼ 15.9 Hz, 1H), 3.97 (t, J ¼ 6.5 Hz, 2H), 3.84 (s, 3H),
2.18 (t, J ¼ 7.3 Hz, 2H), 1.84–1.74 (m, 2H), 1.60–1.49 (m,
2H), 1.42 (s, 9H), 1.28 (s broad, 12H). IR (nujol, cmꢂ1): 1718
(C¼¼O), 1627 (C¼¼C), 1599, 1571, 1508, 1493 (C-Car), 1286,
1269, 1249, 1209 (CAO).
Yield of 2-[40-(tetrahydropyranyl-2-oxy)-4-biphenyloxy]ethyl
4-(N,N-dimethylamino)cinnamate: 50%. 1H NMR (300 MHz,
CDCl3, d, ppm): 7.63 (d, J ¼ 16.2 Hz, 1H), 7.47–7.38 (m, 6H),
7.07 (d, J ¼ 8.9 Hz, 2H), 6.97 (d, J ¼ 8.8 Hz, 2H), 6.64 (d, J
¼ 8.7 Hz, 2H), 6.25 (d, J ¼ 15.9 Hz, 1H), 5.43 (t, J ¼ 3.1 Hz,
1H), 4.53 (t, J ¼ 4.9 Hz, 2H), 4.25 (t, J ¼ 4.6 Hz, 2H), 3.96–
3.87 (m, 1H), 3.63–3.57 (m, 1H), 2.99 (s, 6H), 1.90–1.82 (m,
2H), 1.70–1.57 (m, 4H). IR (nujol, cmꢂ1): 1700 (C¼¼O), 1626
(C¼¼C), 1607, 1528, 1499 (C-Car), 1283,1240 (CAO), 1109,
818 (cyclic CAOAC).
Step 2. The product from Step 1 (2.50 g, 4.7 mmol), formic
acid (45 mL), and the minimum volume of dichloromethane
to dissolve the solid were stirred at room temperature for
5 h. The mixture was evaporated to dryness, and the result-
ing solid was dissolved in hot ethyl acetate (300 mL). The
solution was washed with H2O (5 ꢁ 200 mL), dried, and
evaporated to dryness.
Step 2. In Step 2, the product from Step 1 (3.98 mmol) and
a mixture of THF (150 mL), ethanol (75 mL), and pyridinium
p-toluensulfonate (PPTS) (1.02 g, 3.98 mmol) was stirred for
5 h at 50 ꢀC and evaporated to dryness. The solid was dis-
solved in dichloromethane, and the solution was washed
with H2O (4 ꢁ 75 mL), dried over MgSO4, filtered, and
evaporated to dryness. The solid was purified by washing
the crushed solid with hexane.
Yield of 40-[10-(carboxy)decyloxy]-4-biphenyl 4-methoxycin-
namate: 60%. 1H NMR (400 MHz, DMSO-d6, d, ppm): 12.5–
11.7 (m, 1H), 7.83 (d, J ¼ 15.9 Hz, 1H), 7.78 (d, J ¼ 8.5 Hz,
2H), 7.66 (d, J ¼ 8.6 Hz, 2H), 7.60 (d, J ¼ 8.8 Hz, 2H), 7.24
(d, J ¼ 8.6 Hz, 2H), 7.02 (d, J ¼ 8.9 Hz, 2H), 7.01 (d, J ¼ 8.8
Hz, 2H), 6.74 (d, J ¼ 15.9 Hz, 1H), 4.00 (t, J ¼ 6.4 Hz, 2H),
3.82 (s, 3H), 2.19 (t, J ¼ 7.3 Hz, 2H), 1.77–1.68 (m, 2H),
1.53–1.37 (m, 2H), 1.34–1.23 (s broad, 12H). IR (nujol,
cmꢂ1): 3097–3010 (OH), 1719 (C¼¼O), 1633 (C¼¼C), 1604,
1598, 1573 (C-Car), 1287, 1256, 1223 (CAO).
Yield: 87%. 1H NMR (300 MHz, CDCl3, d, ppm): 7.63 (d, J ¼
15.9 Hz, 1H), 7.46–7.38 (m, 6H), 6.97 (d, J ¼ 9.1 Hz, 2H),
6.86 (d, J ¼ 8.5 Hz, 2H), 6.64 (d, J ¼ 9.0 Hz, 2H), 6.24 (d, J
¼ 15.9 Hz, 1H), 4.73 (s, 1H), 4.53 (t, J ¼ 4.6 Hz, 2H), 4.26 (t,
J ¼ 4.7 Hz, 2H), 2.99 (s, 6H). IR (nujol, cmꢂ1): 3409 (OH),
1686 (C¼¼O), 1604, 1528, 1499 (C-Car).
Step 3. DCC (0.39 g, 1.9 mmol) dissolved in DMF (20 mL)
was added dropwise to a cold solution (0 ꢀC) of the acid
obtained in the previous step (1 g, 1.9 mmol), pentafluoro-
phenol (0.384 g, 2.1 mmol), and DMAP (0.023 g, 0.2 mmol)
in DMF (65 mL). The reaction mixture was stirred for 3 days
at r.t. The work-up was performed by adding H2O (80 mL),
and the product was extracted with hexane/ethyl acetate 1/
2 (4 ꢁ 100 mL). The organic layer was washed with water
tert-Butyl 11-bromoundecanoate (P4)
A solution of DCC (11.7 g, 57 mmol) in dry dichlorꢀomethane
(40 mL) was added dropwise to a cold solution (0 C) of 11-
bromoundecanoic acid (15 g, 56 mmol), tert-butanol
(5.9 mL, 62 mmol), and DMAP (0.68 g, 5.6 mmol) in dry
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(3 ꢁ 150 mL), dried over MgSO4, filtered, and evaporated to
dryness. The solid was purified by column chromatography
(silica gel, dichloromethane as eluent).
J ¼ 8.3 Hz, 2H), 6.95 (d, J ¼ 8.6 Hz, 2H); 6.66 (d, J ¼ 8.8 Hz,
2H), 6.27 (d, J ¼ 15.6 Hz, 1H), 4.56 (t, J ¼ 4.3 Hz, 2H); 4.28
(t, J ¼ 4.5 Hz, 2H), 3.99 (t, J ¼ 6.6 Hz, 2H), 3.02 (s, 6H), 2.66
(t, J ¼ 7.5 Hz, 2H), 1.84–1.73 (m, 4H), 1.51–1.31 (m, 12H).
13C{1H} NMR (100 MHz, CDCl3, d, ppm): 169.6, 167.8, 158.3,
157.7, 151.8, 145.9, 142.0–136.0 (several broad and weak
signals, C6F5), 134.0, 133.2, 129.8, 127.7, 125.1 (broad,
C6F5), 122.2, 115.0, 114.7, 111.8, 100.0, 68.1, 66.4, 62.6,
40.1, 33.3, 29.4, 29.3, 29.1, 28.8, 26.0, 24.8. 19F{1H} NMR
(282 MHz, CDCl3, d, ppm): ꢂ152.79 (d, J ¼ 17.1 Hz, 2F),
ꢂ158.19 (t, J ¼ 21.3 Hz, 1F), ꢂ162.41 (dd, J1 ¼ 17.0 Hz, J2
¼ 21.4, 2F). IR (KBr, cmꢂ1): 1791, 1699 (C¼¼O), 1609, 1521,
1499 (C-Car), 1271, 1248, 1223 (CAO). Anal. calcd for
Yield: 66%. 1H NMR (400 MHz, CDCl3, d, ppm): 7.84 (d, J ¼
16.0 Hz, 1H), 7.56 (d, J ¼ 8.9 Hz, 2H), 7.55 (d, J ¼ 9.0 Hz, 2H),
7.50 (d, J ¼ 8.9 Hz, 2H), 7.20 (d, J ¼ 8.7 Hz, 2H), 6.96 (d, J ¼
8.9 Hz, 2H), 6.94 (d, J ¼ 8.8 Hz, 2H), 6.51 (d, J ¼ 15.9 Hz, 1H),
3.99 (t, J ¼ 6.6 Hz, 2H), 3.86 (s, 3H), 2.66 (t, J ¼ 7.4 Hz, 2H),
1.83–1.74 (m, 4H), 1.50–1.25 (m, 12H). 13C{1H} NMR (100
MHz, CDCl3, d, ppm): 169.6, 165.8, 161.7, 158.7, 149.8, 146.3,
143.0–136.0 (several broad and weak signals, C6F5), 138.5,
132.8, 130.0, 128.1, 127.7, 126.9, 121.8, 114.8, 114.6, 114.4,
68.0, 55.4, 33.3, 29.4, 29.3, 29.1, 28.8, 26.0, 24.7. 19F{1H} NMR
(282 MHz, CDCl3, d, ppm): –152.79 (d, J ¼ 17.0 Hz, 2F),
ꢂ158.20 (t, J ¼ 21.3 Hz, 1F), ꢂ162.41 (dd, J1 ¼ 21.7 Hz, J2 ¼
17.1 Hz, 2F). IR (KBr, cmꢂ1): 1786, 1727 (C¼¼O), 1634 (C¼¼C),
1602, 1573, 1518, 1495 (C-Car), 1288, 1252, 1216 (CAO).
Anal. calcd for C39H37O6F5: C, 67.23; H, 5.35. Found: C, 67.57;
H, 5.19. MS (FAB, m/z): 665 [M-OCH3]þ.
C
42H44F5NO6: C, 66.95; H, 5.84; N, 1.86. Found: C, 66.75; H,
5.61; N, 1.95%. MS (FAB, m/z): 753 [MþH]þ.
Synthesis of the Dendrimers
Dendrimer [2]
To a solution of MOMe (0.35 g, 0.5 mmol) in dry dichlorome-
thane (20 mL), a solution of PPI-(NH2)16 (0.048 g, 0.028
mmol) was added dropwise in dichloromethane (2 mL). The
mixture was heated under reflux for 3 days. The solid was
filtered off, washed with dichloromethane and methanol, and
2-[40-[10-(Pentafluorophenyloxycarbonyl)decyloxy]-4-
biphenyloxy]ethyl 4-(N,N-dimethylamino)cinnamate
ꢀ
dried in a vacuum oven at 90 C for 12 h.
(MNMe2
)
Step 1. A mixture of P3 (2.27 g, 5.6 mmol), K2CO3 (1.20 g,
8.5 mmol), potassium iodide (KI) (0.187 mg, 1.1 mmol), dry
DMF (80 mL), and P4 (2 g, 6.2 mmol) was stirred at 90 ꢀC
for 24 h. The mixture was cooled to r.t., H2O (80 mL) was
added and the solution was extracted with hexane/ethyl ace-
tate 1/2 (4 ꢁ 100 mL). The organic layer was washed with
water (4 ꢁ 200 mL), dried and evaporated to dryness. The
product was purified by recrystallization from ethanol.
Yield: 90%. The compound is not soluble in common organic
solvents which restricts its full characterization in solution.
IR (KBr, cmꢂ1): 3295, 3074 (NH), 2925, 2850 (CH2), 1724
(C¼¼O),1631 (C¼¼O, C¼¼C), 1602, 1570, 1547, 1495 (C-Car),
1288, 1250, 1207 (CAO). Anal. calcd for C616H784N30O80: C,
74.82; H, 7.99; N, 4.25. Found: C, 74.16; H, 7.76; N, 5.01.
Dendrimer [3]
To a solution of MNMe2 (0.5 g, 0.66 mmol) in dry dichlorome-
thane (25 mL), a solution of PPI-(NH2)16 (0.063 g, 0.037
mmol) was added dropwise in dichloromethane (2 mL). The
mixture was heated under reflux for 4 days. The mixture
was diluted with dichloromethane, washed with saturated
aqueous Na2CO3 (3 ꢁ 30 mL), dried, and evaporated to dry-
ness. The resulting solid was purified by dissolving in
dichloromethane and reprecipitation in hexane.
Yield: 98%. 1H NMR (400 MHz, CDCl3, d, ppm): 7.65 (d, J ¼
15.8 Hz, 1H), 7.47 (m, 4H), 7.42 (d, J ¼ 8.8 Hz, 2H), 6.99 (d, J
¼ 8.8 Hz, 2H), 6.94 (d, J ¼ 8.8 Hz, 2H), 6.66 (d, J ¼ 8.7 Hz,
2H), 6.27 (d, J ¼ 15.8 Hz, 1H), 4.56 (t, J ¼ 4.8 Hz, 2H), 4.28
(t, J ¼ 4.7 Hz, 2H), 3.98 (t, J ¼ 6.7 Hz, 2H), 3.02 (s, 6H), 2.20
(t, J ¼ 7.7 Hz, 2H), 1.84–1.74 (m, 2H), 1.60–1.55 (m, 2H), 1.44
(s, 9H), 1.37–1.29 (m, 12H). IR (nujol, cmꢂ1): 1725, 1698
(C¼¼O), 1608, 1527, 1498 (C-Car), 1270, 1246, 1220 (CAO).
Yield: 58%. 1H NMR (400 MHz, CDCl3, d, ppm): 7.64 (d, J ¼
15.9 Hz, 16H), 7.46–7.38 (m, 96H), 7.17–7.11 (m, 16H), 6.95
(d, J ¼ 8.6 Hz, 32H), 6.90 (d, J ¼ 8.5 Hz, 32H), 6.63 (d, J ¼ 8.9
Hz, 32H), 6.25 (d, J ¼ 15.6 Hz, 16H), 4.52 (t, J ¼ 4.6 Hz, 32H),
4.23 (t, J ¼ 4.6 Hz, 32H), 3.93 (t, J ¼ 6.4 Hz, 32H), 3.25–3.19
(m, 32H), 2.99 (s, 96H), 2.28–2.52 (m, 84H), 2.17 (t, J ¼ 7.6
Hz, 32H), 1.80–1.70 (m, 32H), 1.68–1.50 (m, 64H), 1.48–1.20
(m, 220H). 13C{1H} NMR (100 MHz, CDCl3, d, ppm): 173.9,
165.5, 157.7, 151.8, 154.9, 133.1, 129.9, 127.7, 122.1, 114.9,
114.7, 111.7, 68.0, 66.3, 62.5, 40.1, 36.6, 29.6, 29.5, 29.4, 29.3,
26.1, 25.9. IR (KBr, cmꢂ1): 3284, 3073 (NH), 2921, 2849 (CH2),
1703 (C¼¼O), 1637 (C¼¼O, C¼¼C), 1598, 1549, 1524, 1494 (C-
Car), 1263, 1239 (CAO). Anal. calcd for C664H896N46O80: C,
73.83; H, 8.36; N, 5.96. Found: C, 72.46; H, 8.30; N, 5.38.
Step 2. This acid was obtained by a similar procedure to
that reported for MOMe
.
Yield of 2-[40-[10-(carboxy)decyloxy]biphenyloxy]ethyl 4-
(N,N-dimethylamino)cinnamate: 92%. 1H NMR (400 MHz,
CDCl3, d, ppm): 7.66 (d, J ¼ 15.6 Hz, 1H), 7.47 (m, 4H), 7.41
(d, J ¼ 8.5 Hz, 2H), 6.99 (d, J ¼ 8.6 Hz, 2H), 6.94 (d, J ¼ 8.4
Hz, 2H), 6.65 (d, J ¼ 8.5 Hz, 2H), 6.27 (d, J ¼ 15.9 Hz, 1H),
4.56 (t, J ¼ 4.5 Hz, 2H), 4.28 (t, J ¼ 4.5 Hz, 2H), 3.98 (t, J ¼
6.5 Hz, 2H), 3.01 (s, 6H), 2.35 (t, J ¼ 7.7 Hz, 2H), 1.81–1.76
(m, 2H), 1.66–1.60 (m, 2H), 1.48–1.43 (m, 2H), 1.31 (m,
10H). IR (nujol, cmꢂ1): 3150–2923 (OH), 1710 (C¼¼O), 1606,
1525, 1500 (C-Car), 1273, 1246 (CAO).
Step 3. The compound was obtained by a similar procedure
to that reported for MOMe
.
Dendrimer [4]
Yield: 64%. 1H NMR (400 MHz, CDCl3, d, ppm): 7.66 (d, J ¼
15.8 Hz, 1H), 7.47 (m, 4H), 7.42 (d, J ¼ 9.1 Hz, 2H), 6.99 (d,
To a solution containing MCNB (0.23 g, 0.42 mmol) and MOMe
(0.033 g, 0.047 mmol) in dichloromethane (20 mL), a
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
solution of PPI-(NH2)16 (0.045 g, 0.027 mmol) was added
dropwise in dichloromethane (2 mL). The mixture was
heated under reflux for 4 days. The mixture was diluted
with dichloromethane, washed with saturated aqueous
Na2CO3 (3 ꢁ 30 mL), dried, and evaporated to dryness. The
resulting solid was purified by dissolving in dichloromethane
and precipitation in hexane. This reprecipitation must be
repeated if free monomers are observed.
(NH), 2923, 2850 (CH2), 2224 (C[tbond]N), 1706 (C¼¼O),
1641 (C¼¼O, C¼¼C), 1602, 1549, 1526, 1494 (C-Car), 1289,
1249 (CAO). Anal. calcd for C496H672N46O38: C, 75.53; H,
8.59; N, 8.17. Found: C, 74.96; H, 7.99; N, 7.96.
Dendrimer [7]
This codendrimer was prepared by a similar procedure as
for [4] with MCNB (0.6 g, 1.10 mmol), MNMe2 (0.207 g, 0.27
mmol), and PPI-(NH2)16 (0.131 g, 0.078 mmol).
Yield: 66%. 1H NMR (400 MHz, CDCl3, d, ppm): 7.85 (d, J ¼
15.9 Hz, 2H), 7.67 (d, J ¼ 8.1 Hz, 28H), 7.62 (d, J ¼ 8.3 Hz,
28H), 7.57–7.46 (m, 36H), 7.19 (d, J ¼ 7.2 Hz, 4H), 7.14–
7.04 (br m, 16H), 6.96–6.93 (d, J ¼ 8.7; m, 40H) Hz, 6.51 (d,
J ¼ 15.9 Hz, 2H), 3.98 (br t, J ¼ 6.4 Hz, 32H), 3.88 (s, 6H),
3.31–3.25 (m, 32H), 2.43–2.33 (m, 84H), 2.20 (m, 32H),
1.82–1.30 (m, 316H). 13C{1H} NMR (100 MHz, CDCl3, d,
ppm): 173.7, 165.8, 161.8, 159.7, 158.7, 146.4, 145.1, 132.5,
131.2, 130.1, 128.3, 127.6, 127.0, 121.9, 119.1, 115.0, 114.5,
110.0, 68.1, 55.4, 52.0, 51.3, 37.7, 36.6, 29.6, 29.2, 27.0, 26.0,
24.2, 22.6. IR (KBr, cmꢂ1): 3296, 3073 (NH), 2924, 2851
(CH2), 2224 (C[tbond]N), 1728 (C¼¼O), 1641 (C¼¼O, C¼¼C),
1603, 1549, 1526, 1495 (C-Car), 1290, 1251, 1211 (CAO).
Anal. calcd for C490H658N44O38: C, 75.72; H, 8.53; N, 7.93.
Found: C, 73.06; H, 8.47; N 7.90.
Yield: 69%. 1H NMR (400 MHz, CDCl3, d, ppm): 7.68–7.57
(m, 55H), 7.51 (d, J ¼ 8.4 Hz, 32H), 7.46–7.38 (m, 12H),
7.16–7.08 (m, 16H), 6.99–6.92 (m, 32H), 6.91 (d, J ¼ 8.5 Hz,
6H), 6.66 (d, J ¼ 8.9 Hz, 6H), 6.27 (d, J ¼ 15.9 Hz, 3H), 4.55
(t, J ¼ 4.8 Hz, 6H), 4.27 (t, J ¼ 4.8 Hz, 6H), 3.97 (t, J ¼ 6.4
Hz, 32H), 3.30–3.20 (m, 32H), 3.02 (s, 18H), 2.50–2.24 (m,
84H), 2.20 (t, J ¼ 7.7 Hz, 32H), 1.83–1.70 (m, 32H), 1.68–1.20
(284H). 13C{1H} NMR (100 MHz, CDCl3, d, ppm): 171.4, 165.4,
164.3, 157.4, 155.9, 155.3, 149.5, 143.6, 142.8, 131.4, 130.8,
130.5, 130.2, 128.8, 127.5, 125.9, 125.3, 124.6, 119.7, 116.7,
112.7, 112.3, 109.4, 108.7, 107.6, 65.7, 65.6, 63.9, 60.1, 59.9,
49.7, 49.0, 37.7, 35.4, 34.3, 27.3, 27.1, 26.9, 24.7, 23.7, 23.6. IR
(KBr, cmꢂ1): 3300, 3076 (NH), 2923, 2851 (CH2), 2224
(C[tbond]N), 1705 (C¼¼O), 1641 (C¼¼O, C¼¼C), 1601, 1549, 1525,
1495 (C-Car), 1290, 1249 (CAO). Anal. calcd for C508H688N46O41:
C, 75.37; H, 8.57; N, 7.96. Found: C, 74.55; H, 8.41; N, 7.69.
Dendrimer [5]
This codendrimer was prepared by a similar procedure as
for [4] with MCNB (0.25 g, 0.46 mmol), MOMe (0.08 g, 0.115
mmol), and PPI-(NH2)16 (0.054 g, 0.032 mmol).
Characterization Techniques
Instrumentation for the molecular characterization of the
monomers and dendrimers was similar to that reported previ-
ously.32 Thin films were prepared by spin coating (Laurell, WS-
400B-6TFM-LITE) on quartz glass substrates. The frequency
of the rotation was 2000 rpm, and the time of rotation was
2 min. A dynamic method was used with 30 lL volume of
solution. The concentration of the dendrimer was 10 mg mLꢂ1
in 1,1,2-trichloroethane or THF (HPLC grade). The film thick-
ness was measured by a Dektak 150 profilometer (Veeco).
Yield: 67%. 1H NMR (400 MHz, CDCl3, d, ppm): 7.83 (d, J ¼
16.3 Hz, 3H), 7.64 (d, J ¼ 8.2 Hz, 26H), 7.61 (d, J ¼ 8.1 Hz,
26H), 7.55–7.44 (m, 38H), 7.19–7.12 (m, 28H), 6.96–6.90 (m,
38H), 6.50 (d, J ¼ 15.7 Hz, 3H), 3.95 (t, J ¼ 6.5 Hz, 32H),
3.85 (s, 9H), 2.50–2.25 (m, 84H), 2.18 (t, J ¼ 7.0 Hz, 32H),
1.82–1.15 (m, 316H) . 13C{1H} NMR (100 MHz, CDCl3, d,
ppm): 173.7, 159.7, 145.2, 132.6, 130.1, 128.3, 127.6, 127.0,
121.9, 119.1, 115.0, 114.5, 110.1, 68.1, 55.4, 52.2, 51.4, 37.8,
36.7, 29.5, 29.3, 27.2, 26.1, 26.0. IR (KBr, cmꢂ1): 3296, 3073
(NH), 2923, 2851 (CH2), 2225 (C[tbond]N), 1730 (C¼¼O),
1641 (C¼¼O, C¼¼C), 1603, 1549, 1527, 1495 (C-Car), 1290,
1252, 1211 (CAO). Anal. calcd for C499H667N43O41: C, 75.64;
H, 8.48; N, 7.60. Found: C, 75.05; H, 8.93; N, 8.05.
The anisotropy in the films was induced by irradiation with
linearly polarized light of an Argon (Arþ) laser (Coherent,
Innova 90-4) at k ¼ 364 nm with a power density of 10 mW
cmꢂ2, a Mercury lamp with an interference filter for k ¼ 365
nm equipped with a polarizer resulting in a power density
of 6.3 mW cmꢂ2, or, alternatively, a Helium-Cadmium (He-
Cd) laser (Kimmon Electronics) at k ¼ 325 nm with a power
density of 20 mW cmꢂ2
.
Dendrimer [6]
This codendrimer was prepared by a similar procedure as
for [4] with MCNB (0.6 g, 1.10 mmol), MNMe2 (0.092 g, 0.12
mmol), and PPI-(NH2)16 (0.118 g, 0.07 mmol).
The angular-dependent UV–vis absorbance spectra were
measured using a diode array XDAP Spectrometer (Polytec)
equipped with a polarizer. The spectra were measured every
5ꢀ. The dichroism was calculated from the angular-dependent
absorbance spectra at 375 nm or at 310 nm that correspond
to the absorption bands of 4-(N,N-dimethylamino)cinnamate
and 4-methoxycinnamate units, respectively. The dichroism
was calculated according to the following equation:
Yield: 74%. 1H NMR (400 MHz, CDCl3, d, ppm): 7.69–7.56
(m, 58H), 7.49 (d, J ¼ 8.5 Hz, 32H), 7.46–7.38 (m, 8H), 7.15–
7.07 (br, 16H), 6.95 (d, J ¼ 8.5 Hz, 32H), 6.90 (d, J ¼ 8.6 Hz,
4H), 6.64 (d, J ¼ 8.6 Hz, 4H), 6.25 (d, J ¼ 15.8 Hz, 2H), 4.54
(m, 4H), 4.25 (m, 4H), 3.96 (t, J ¼ 6.4 Hz, 32H), 3.32–3.20
(m, 32H), 3.01 (s, 12H), 2.48–2.26 (m, 84H), 2.18 (t, J ¼ 7.6
Hz, 32H), 1.83–1.71 (m, 32H), 1.70–1.21 (284H). 13C{1H}
NMR (100 MHz, CDCl3, d, ppm): 173.7, 165.5, 159.7, 157.7,
145.1, 132.5, 131.2, 129.8, 128.3, 127.6, 127.0, 119.0, 115.0,
114.7, 111.8, 110.0, 68.1, 52.1, 51.4, 40.0, 37.7, 36.6, 29.6,
29.5, 29.4, 29.2, 27.1, 26.0, 25.9. IR (KBr, cmꢂ1): 3230, 3074
D ¼ ðAmax ꢂ AminÞ=ðAmax þ Amin
Þ
where D is the dichroism at a defined wavelength, Amax is
the maximal, and Amin the minimal absorbance values at the
defined wavelength.
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ARTICLE
RESULTS AND DISCUSSION
Synthesis and Molecular Structure Characterization of
the Monomers
Photoactive monomers MOMe and MNMe2 possess a bipheny-
loxy group covalently joined, either directly or through an
ethylene spacer, to the cinnamate moiety (Scheme 2). This
extended cinnamate design is favorable for mesogenic rod-
like behavior and has proven to exhibit large photoinduced
anisotropy values in side chain polymers, as the photoprod-
ucts obtained on irradiation with light are more anisometric
than those obtained without the biphenyloxy moiety.33 The
cinnamate moiety is substituted in the para-position with a
methoxy group (in MOMe) or a N,N-dimethylamino group (in
M
NMe2). MOMe does not have an ethylene spacer between the
biphenyloxy and the cinnamate moiety, which yields a more
rigid and favorable system for mesomorphic properties than
M
NMe2. The combination of a different substitution at the
FIGURE 2 UV–vis spectra of the monomers in chloroform solu-
tion. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
cinnamate group and the presence/absence of the ethylene
spacer provide two monomers with different absorption
properties.
For the synthesis of MOMe
,
4,40-dihydroxybiphenyl was
helped to improve the solubility of the final materials com-
pared to systems with shorter spacers. All of the dendrimers
were soluble in common organic solvents except for [2],
which precipitated out of the reaction medium as an insolu-
ble solid. This drawback precluded all characterization of
this material in solution.
monoesterified with 4-methoxycinnamic acid in the first
step. In the case of MNMe2, it was necessary to monoprotect
the 4,40-dihydroxybiphenyl with tetrahydropyran before al-
kylation with 2-bromoethanol and esterification with 4-(N,N-
dimethylamino)cinnamic acid. The decamethylenic spacer
was introduced into the photoactive units at the end of the
synthetic route by a Williamson reaction with tert-butyl 11-
bromoundecanoate. The protection of the carboxylic acid as
a tert-butyl ester allowed the subsequent deprotection under
weakly acidic conditions without any interference from the
cinnamate functional group. Finally, the carboxylic acids
were transformed into their pentafluorophenyl esters. The
monomers were characterized by elemental analysis, MS,
FTIR, 1H NMR, 13C NMR, and 19F NMR, and the data are in
accordance with the proposed structures.
All of the soluble dendrimers were characterized by elemental
1
analysis, FTIR, H NMR, 13C NMR, gel permeation chromatog-
raphy (GPC), MALDI-TOF, and UV–vis. In the FTIR spectra,
N¼¼H and C¼¼O stretching bands corresponding to the amide
group were observed. The compounds were free of precursor
materials, as evidenced by the absence of NAH amine bands
corresponding to PPI at 3370 cmꢂ1 or the C¼¼O band from the
pentafluorophenyl ester monomer at around 1790 cmꢂ1. For
the cinnamate dendrimers, the C¼¼C stretching band over-
lapped the amide band at 1640 cmꢂ1 in all cases. The C¼¼O
band of the cinnamic ester appears at 1729 cmꢂ1 for the 4-
methoxycinnamate-derived dendrimers and at 1705 cmꢂ1 for
the 4-(N,N-dimethyl)amino-derived dendrimers. The function-
alization of the PPI dendrimer was confirmed by the presence
of a broad singlet in the 6.5–7.2 ppm region (corresponding to
the amide protons) in the 1H NMR spectra and a signal at
175 ppm due to the amide carbon in the 13C NMR spectra.
The UV–vis absorption spectra in chloroform solutions
(10ꢂ5 M) show a band at 298 nm for the CNB monomer, a
broad band centered at 317 nm for MOMe, and two bands at
263 and 367 nm for MNMe2—thus showing the bathochromic
effect induced by the N,N-dimethylamino group. It can be
observed that the three monomers absorb at different wave-
lengths. Therefore, these compounds could be influenced and
studied separately in photoirradiation experiments per-
formed on the codendrimers (Fig. 2).
1
The H NMR spectra also proved useful in estimating the av-
erage compositions of the codendrimers. Proton resonances
corresponding to the functional unit (CNB or cinnamate)
were compared to determine the average relative concentra-
tion of each unit in a given codendrimer. The 1H NMR spec-
trum of dendrimer [5] is shown in Figure 3 and the values,
which are in fair agreement with the theoretical ones, are
shown in Table 1. For codendrimers [6] and [7], the different
absorption regions of the two constitutive monomers, CNB
and MNMe2, also allowed us to calculate the percentage com-
position by UV–vis spectroscopy, as the spectra of the den-
drimers in solution were similar to the sum of the spectra of
the separate monomers (Fig. 4 and Table 1).
Synthesis and Molecular Structure Characterization
of the Dendrimers
Dendrimers were synthesized by grafting the pentafluoro-
phenyl ester monomers onto the periphery of the amine-ter-
minated PPI core, with a molar feed ratio as outlined in Fig-
ure 1. The amidation reaction proceeded quantitatively as
described for previous PPI functionalizations.30,32,34,35 The
third generation PPI was selected to avoid solubility prob-
lems associated with higher generations as low solubility
would hinder the characterization and processability of the
materials. The long decamethylenic spacer in the monomer
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FIGURE 3 1H NMR spectra for [5] and peak integration for the calculation of the comonomer composition.
GPC analysis of the dendrimers was performed using as elu-
ent dichloromethane with a 1% v/v of triethylamine to
improve the elution of the PPI-derived dendrimers through
the column and try to avoid partial adsorption processes due
to the amine groups, which lead to high polydispersity val-
ues.36 In general, PI values were low (between 1.2 and 1.08),
but the molecular weight was overestimated in all cases.
MALDI-TOF mass spectrometry enabled us to ascertain the
extent of substitution at the periphery. For the homoden-
drimers [1] and [3], only the parent peak was observed,
indicating that all 16 amine end groups of PPI had been
modified. The mass spectra of the codendrimers [4]–[7]
showed several peaks, indicating that the material consisted
of mixtures of fully functionalized dendrimers (16 in total)
TABLE 1 Comparative Data for the Calculated Cinnamate Contents (mol %) in Dendrimers from 1H NMR or UV–vis Spectra, UV–vis
Data in Chloroform Solution, and GPC Data Analysis
Dendrimers
Theoretical Values
1H NMR
UV–vis
kmax (nm)
Mw
Mn
PI
[1]
[3]
[4]
[5]
[6]
[7]
a
–
–
–
–
299
11,554
11,838
11,890
11,580
11,549
11,624
10,679
9,905
1.08
1.20
1.08
1.07
1.09
1.12
100
10
20
10
20
–
264, 367
298
a
9
–
10,962
10,768
10,595
10,378
a
20
7
–
298
9
297, 368
294, 367
18
17
Not determined because of partial overlap of the absorption bands from cyanobiphenyl and cinnamate units.
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ARTICLE
Thermal Properties of the Dendrimers
Thermogravimetric analysis (TGA) of the dendrimers showed
the excellent stability toward volatile decomposition, with
ꢀ
onset temperatures for weight loss above 360 C and a first
derivative of the weight loss (DTGA) over 396 ꢀC (Table 4).
It is known that dendrimers tend to retain small molecules
within their inner cavity, and, for this reason, the dendrimers
were dried at 80 ꢀC under vacuum for 2 days before per-
forming the analysis. It is interesting to note that these cin-
namate dendrimers are much more stable in TGA than simi-
lar cyanoazobenzene-containing dendrimers.32
The phase behavior of the dendrimers was investigated by
DSC and POM. The transition temperatures and enthalpies
determined both on heating and cooling are summarized in
Table 4. All dendrimers were liquid crystalline except for [3],
which lacks of mesomorphic peripheral units, and this again
provides evidence of the influence of the ethylene spacer
and the terminal group of the cinnamate units on the meso-
morphic properties of the final dendritic macromolecule.
Compounds [2] and [4]–[7] display at the POM focal-conic
textures that coexist with a homeotropic texture and these
are characteristic of a SmA mesophase [Fig. 6(b–d)]. In par-
ticular, Compounds [6] and [7] show a high tendency toward
FIGURE 4 UV–vis spectra in chloroform solution of the 4-(N,N-
dimethylamino)cinnamate-containing dendrimers. [Color figure
can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
but with different numbers of cinnamate or cyanobiphenyl
monomers (Table 2 and Fig. 5). Assuming that all the species
have the same response to the mass detector, we can assess
the relative abundance of the different species from the
intensities of the peaks.36 Correlation of the calculated
weights of the possible structures with the peaks observed
in the spectrum allowed us to determine the molecular com-
position. As an example, codendrimer [4] essentially con-
tained 29% of molecules with 14 CNB units and two cinna-
mate units, and 39% of molecules with 15 CNB units and
one cinnamate monomer, together with minor amounts of
other dendrimeric compositions.
TABLE 2 MALDI-TOF MS Data and Peak Assignments
Calculated MS
Peak Relative
Dendrimers Mass
Peaks (m/z) Abundance (%) x/ya
[1]
[3]
[4]
7470.4
7470.4
10803
8076.0
7924.5
7773.4
7622.3
7471.3
8227.7
8076.6
7925.5
7773.5
7622.4
8304.9
8095.8
7888.7
7679.6
7471.4
8514.1
8304.9
8096.8
7887.7
7678.6
100
100
3
16/0
16/0
12/4
13/3
14/2
15/1
16/0
11/5
12/4
13/3
14/2
15/1
12/4
13/3
14/2
15/1
16/0
11/5
12/4
13/3
14/2
15/1
10802.5
7772.7b
12
29
39
17
8
Thermal Properties of the Monomers
Pentafluorophenyl ester monomers were characterized by
polarizing optical microscopy (POM) and differential scan-
ning calorimetry (DSC). Phase transitions and enthalpy val-
ues are collected in Table 3.
[5]
[6]
[7]
a
7923.9c
7886.9b
8095.2c
19
31
28
14
3
CNB is a liquid crystal with two monotropic mesophases. The
compound is a crystalline solid that melts directly on heating
to the isotropic ꢀliquid. However, on cooling, a schlieren texture
develops at 41 C, and this is typical of a nematic mesophase.
On further cooling, the phase transforms into a SmA phase,
10
30
39
18
8
ꢀ
with homeotropic and fan-shaped textures observed at 36 C.
Cinnamate monomers have higher transition temperatures as
consequence of their extended aromatic core. MNMe2 does
not show liquid crystalline properties. In contrast, MOMe is
more rigid and conjugated than MNMe2, and this allows stabi-
lization of a SmA phase between 110 and 166 ꢀC and a
21
28
29
14
ꢀ
nematic mesophase up to 204 C. The SmA mesophase has a
strong tendency toward homeotropic alignment [Fig. 6(a)].
We observed poor reproducibility in DSC cycles, which has
been attributed to partial decomposition of the pentafluoro-
phenyl ester and the conjugated cinnamate ester on heating
at high temperatures.
Number of cyanobiphenyl units (x) and cinnamate units (y) in a mole-
cule, given in relation to the 16 free amino groups of PPI.
b
Calculated molecular weight of a codendrimer with x/y ¼ 14/2.
c
Calculated molecular weight of a codendrimer with x/y ¼ 13/3.
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DENDRIMERS CONTAINING PHOTOACTIVE CINNAMATE UNITS, VINUALES ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 5 MALDI-TOF MS of codendrimer
[5] showing the molecular composition.
homeotropic orientation. In general, these cinnamate den-
drimers have higher tendency toward crystallization than
analogous cyanoazobenzene dendrimers.32
homodendrimer [2] was a crystalline material that melted at
157 ꢀC and exhibited a SmA mesophase up to 239 ꢀC. DSC
curves were not reproducible in subsequent cycles, and, on
cooling, the isotropic liquid a Schlieren texture could be
observed at the POM, which may belong to the thermal
behavior of a mixture containing decomposed species. Never-
theless, the transitions and associated enthalpies are repro-
ducible as far as thermally induced reactions of cinnamates
are avoided, that is, isotropization is not reached. In this
case, DSC traces recorded on heating show several endo-
therms and even exotherms, which are associated to melting
of different crystalline forms. From our experience, this is
not uncommon for dendrimers and dendrons.
Dendrimers with 4-methoxycinnamate-derived monomers
[2], [4], and [5] exhibit the mesophase SmA in a broad
range. Codendrimers [4] and [5] have lower isotropization
temperatures than the corresponding homodendrimer [2], as
well as broader mesophase interval. On cooling, from the
SmA mesophase, the crystallization is observed for all the
ꢀ
ꢀ
compounds, at 125 C for [2] and at about 40 C for [4] and
[5]. Also, the three compounds showed low reproducibility
in the DSC cycles if the isotropization point was reached.
This is probably associated with a thermal reaction of the
biphenyl 4-methoxycinnamate unit, in a similar way to the
found for the cinnamate monomer MOMe. As an example,
As a consequence of the more flexible structure of the cinna-
mate, dendrimers with 4-(N,N-dimethylamino)cinnamate units
[6] and [7] show the lowest isotropization points and conse-
quently narrower mesophase interval than the methoxy coun-
terparts. The decrease of the isotropization temperatures also
restricts thermally induced reactions that are not observed
from the DSC traces. Both compounds crystallize on cooling;
however, the DSC thermogram of [7] show, in the second
heating cycle, a glass transition at 29 ꢀC pointing to a lesser
extent of crystalline degree. On further heating above the Tg,
several overlapped melting peaks between 97 and 130 ꢀC
coexist with the mesophase.ꢀOn cooling, the SmA mesphase is
stable between 127 and 71 C when crystallization occurs.
TABLE 3 Phase Transitions and Thermodynamic Data for the
Monomers
Monomers
Transition
T (ꢀC)
DH (kJ molꢂ1
)
CNBa
Cr-I
73
41
55.8
0.5
I-N
N-SmA
SmA-Cr
Cr-SmA
SmA-N
N-I
36
0.5
26
30.3
38.3
MOMe
110
166
204
127
b
–
Photoinduced Optical Anisotropy and Its Thermal
Amplification
The investigated dendrimers are functionalized with two
units: photosensitive cinnamoyl and mesogenic cyanobi-
phenyl. The irradiation with LPUV light leads to the
1.3
MNMe2
Cr-I
65.9
a
Pentafluorophenyl 11-[4’-cyano-4-biphenyloxy]undecanoate.
Transition only observed by polarizing optical microscopy.
b
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FIGURE 6 POM microphotographs of MOMe at 112 ꢀC after rubbing stress (a), [2] at 179 ꢀC (b), [4] at 135 ꢀC (c), and [7] at 120 ꢀC (d).
TABLE 4 Thermal Stability and Thermodynamic Data for the
generation of optical anisotropy as result of axis-selective
[2þ2] cycloaddition, as well as E–Z photoisomerization.37,38
Dendrimers
(Fig. 7). As shown for cinnamate containing LC side chain poly-
TGA/
Tonset (ꢀC)a
Thermal Transitions
(ꢀC) [DH (J gꢂ1)]b
mers, annealing in the LC phase may lead to bulk alignment
causing a significant amplification of the initially photoinduced
optical anisotropy generated in the amorphous state.14,24,39,40
Dendrimers
[1]32
367
361
368
371
361
362
364
Cr 104 [49.8] SmA 125 [9.3] I
I 121 [8.8] SmA 43 [23.0] Cr
Cr 158 [40.9] SmA 239 [–] Ic
SmA 125 [20.25] Cr
The cinnamate codendrimers [4]–[7] were spin coated onto
quartz glass plates and irradiated with LP UV light. The film
forming properties, using good solvents like 1,1,2-trichloro-
ethane or THF, were rather poor, always showing some scat-
tering. These could be related to their liquid crystallinity and
the tendency of crystallization rather than to form an amor-
phous glassy film at room temperature. The thicknesses of
the films were between 25 and 35 nm. Apart from the struc-
tural characteristics of the materials, it is known that there
are a lot of factors, which may have an impact on the amplifi-
cation of the photoinduced anisotropy. We checked for various
ones: irradiation wavelength (we chose 324 and 364 nm as
they were in the absorption band of the 4-methoxycinnamate
dendrimers or the 4-(N,N-dimethylamino)cinnamate den-
drimers, respectively), irradiation time (from 10 s to 30 min),
[2]
[3]
[4]
[5]
[6]
[7]
a
Cr 142 [38.2] I
I 130 [37.1] Cr
Cr 99 [40.0] SmA 149 [–] Ic
SmA 37 [19.3] Cr
Cr 95 [21.4] SmA 174 [–] Ic
SmA 43 [18.3] Cr
Cr 98 [30.1] SmA 128 [8.4] I
I 124 [8.2] SmA 47 [21.9] Cr
g 29 Cr 97 [–] SmA 130 [–]d
I
I 126 [8.5] SmA 71 [23.0] Cr
ꢀ
Decomposition temperature determined by thermogravimetric analy-
sis (TGA).
temperature of annealing (from 5 C above the melting point
ꢀ
and 5 C below the clearing point), and thickness of the films.
b
Thermal transitions determined by ꢂD1 SC from the peak maximum
upon heating and cooling at 10 ꢀC min
c
.
A typical experiment for the codendrimers containing the 4-
methoxycinnamate (Fig. 8) and 4-(N,N-dimethylamino)cinna-
mate units (Fig. 9) is described below. A thin films series of
Thermal reaction was observed associated to the isotropization. Data on
cooling is given for a sample heated just above the the SmA-I transition.
d
Several overlapped peaks are observed in the 97–130 ꢀC interval.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 7 [2þ2] Cycloaddition of 4-(N,N-dimethylamino)cinnamate PPI dendrimers [6] and [7] under irradiation with UV light (k ¼
364/365 nm). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
[4] and [5], with 10 and 20 mol % of 4-methoxycinnamate,
were irradiated at 324 nm from 10 s to 30 min. The samples
were stored for 24 h before further investigations. The
absorption change for the irradiation time below 1 min was
barely observed. For longer exposure time, with a photocon-
version of cinnamate esters groups above 10%, the photoin-
duced anisotropy was noticeable by UV–vis spectroscopy. A
decrease of angular absorbance is observed in the direction
of the electric field vector, whereas in the direction perpen-
dicular to it remains constant. All films were subsequently
groups (Fig. 10) and J-aggregation.14 In contrast to glassy LC
polymers, we could not find a planar amplification; the tend-
ency of homeotropic alignment of the smectic dendrimers
dominates. Interestingly and in contrast to the polymers, the
maximum of absorbance is perpendicular to the electric field
vector indicating the alignment by remaining cinnamate
groups. The best dichroism after annealing obtained with
these samples was 0.1.
The same result was observed for the codendrimers [6] and
[7] (Fig. 9), with 10 and 20 mol % of 4-(N,N-dimethylami-
no)cinnamate units, where the absorption band of the photo-
active cinnamate units is separated from that of the meso-
gen. In that case, irradiation at 364 nm for 6 min led to the
ꢀ
annealed in the SmA mesophase (105 C, 48 h) and a signifi-
cant drop in the absorbance was observed in the whole
spectrum together with a slight red shift of the band. This
indicates the out-of-plane orientation of the mesogenic side
FIGURE 9 Changes of polarized absorbance spectra in a film of
[7], under irradiation with linearly polarized light (k ¼ 364 nm,
P ¼ 10 mW cmꢂ2, t ¼ 6 min). [Color figure can be viewed in
the online issue, which is available at wileyonlinelibrary.com.]
FIGURE 8 Changes of polarized absorbance spectra in a film of
[4], under irradiation with linearly polarized light (k ¼ 324 nm,
P ¼ 20 mW cmꢂ2, t ¼ 3 min). [Color figure can be viewed in
the online issue, which is available at wileyonlinelibrary.com.]
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ARTICLE
chromophores in the irradiation experiments. Further work
is in progress to optimize the structure of the photoactive
cinnamate unit trying to gain amorphous materials with
improved film forming properties.
The authors thank the Gobierno de Arago´n (project PI109/09,
research group E04), MICINN-Spain and FSE-UE (projects
MAT2008-06522-CO2-01 and MAT2009-14636-CO3-01), and
UE project (7th FP—THE PEOPLE PROGRAMME. The Marie
Curie Actions—ITN, No. 215884-2).
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