Synthesis and characterization of novel dendrons bearing amino-nitro-substituted azobenzene units and oligo(ethylene glycol) spacers: Thermal, optical properties, langmuir blodgett films and liquid-crystalline behaviour

Article abstract of DOI:10.3390/molecules18021502

In this work, we report the synthesis and characterization of a novel series of first and second generation Frechet type dendrons bearing amino-nitro substituted azobenzene units and tetra(ethylene glycol) spacers. These compounds were fully characterized

Full text of DOI:10.3390/molecules18021502

Molecules 2013, 18, 1502-1527; doi:10.3390/molecules18021502
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Synthesis and Characterization of Novel Dendrons Bearing
Amino-Nitro-Substituted Azobenzene Units and
Oligo(ethylene glycol) Spacers: Thermal, Optical Properties,
Langmuir Blodgett Films and Liquid-Crystalline Behaviour
Jesús Ortíz-Palacios 1, Efraín Rodríguez-Alba 1, Mayra Avelar 1, Ana Martínez 1,
Maria del Pilar Carreón-Castro 2 and Ernesto Rivera 1,*
1
Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México,
Ciudad Universitaria, C.P. 04510 D.F., Mexico
2
Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México, Ciudad Universitaria,
C.P. 04510 D.F., Mexico
* Author to whom correspondence should be addressed; E-Mail: riverage@unam.mx;
Tel.: +52-55-5622-4733; Fax: +52-55-5616-1201.
Received: 6 December 2012; in revised form: 28 December 2012 / Accepted: 18 January 2013 /
Published: 25 January 2013
Abstract: In this work, we report the synthesis and characterization of a novel series of
first and second generation Fréchet type dendrons bearing amino-nitro substituted
azobenzene units and tetra(ethylene glycol) spacers. These compounds were fully
characterized by FTIR, 1H and 13C-NMR spectroscopies, and their molecular weights were
determined by MALDI-TOF-MS. The thermal properties of the obtained dendrons were
studied by TGA and DSC and their optical properties by absorption spectroscopy in
solution and cast film. Molecular calculations were performed in order to determine the
optimized geometries of these molecules in different environments. Besides, Langmuir and
Langmuir Blodgett films were prepared with the first generation dendrons that were shown
to be amphiphilic. Finally, some of the dendrons showed a liquid crystalline behaviour,
which was studied by light polarized microscopy as a function of the temperature in order
to determine the transition temperatures and the structure of the mesophase.
Keywords: dendron; azobenzene; optical properties; liquid crystals; Langmuir-Blodgett films

Molecules 2013, 18
1503
1. Introduction
Nowadays, dendrimers and dendrons are considered one of the most attractive research fields in
polymer chemistry, due to their sophisticated structures and potential applications [1–4]. These
molecules can be modified by introducing functional groups and specific units at different levels of
their structure: core, branches or surface [5], giving rise to well-defined and highly functionalized
molecules. Depending on the type of functional groups present in dendrimers, various properties have
been already studied such as response to light, which has a wide variety of potential applications.
Many reviews include the first examples of photo-responsive dendrimers [6–9] taking into account
various examples of azo-dendrimers. The most recent review covering the most important aspects of
azobenzene-containing dendrons and dendrimers reported until 2009 has been published by Caminade
and Deloncle [10].
In the beginning azobenzenes had been exclusively incorporated as terminal groups of dendrimers
and dendrons; the first examples were described by Vögtle and co-workers [11]. The first reported
structures were obtained from poly(propyleneimine) (PPI) dendrimers built from either
ethylenediamine [12] or 1,4-diaminobutane [1–14] cores. Except for the first example, all the terminal
groups were generally azobenzenes [15–18].
The most popular types of dendrimers: poly(amidoamine) (PAMAM) dendrimers [19] and
poly(arylether) dendrimers [20] have been rarely used as supports for azobenzene moieties. The first
example of Fréchet-type azo-dendrimers was prepared by grafting through their core poly(arylether)
dendrons bearing a single azobenzene group on the surface, leading to original dendrimers [21,22],
having azobenzene units as terminal groups. Other types of poly(ether) dendrimers having long
alkyl chains and azobenzene groups from generation 0 to generation 3 have been reported in the
literature [23].
Unlike dendrimers, dendrons have been infrequently functionalized with azobenzene units on their
surface. The first example was synthesized to be used as building block for dendrimers [21,22]. Also,
more sophisticated systems such as polyether dendrons linked to a fullerene as core have been
prepared [24].
Rau classified azobenzenes into three main groups, based on their photochemical behaviour 25.
Unsubstituted photochromic azobenzenes makes up the first group, known as “azobenzenes”. The
thermally stable trans isomer exhibits a strong -* transition at 350 nm and a weak n-* transition at
440 nm, whereas the cis isomer undergoes similar transitions but with a more intense n-* band. In
addition, “azobenzenes” have a relatively poor -* and n-* overlap. The second group, known as
“aminoazobenzenes” typically includes azobenzenes that are substituted by an electron-donor group
and are characterized by the overlapping of the -* and n-* bands. Finally, azobenzenes bearing
both electron-donor and electron-acceptor groups belong to the third category, “pseudostilbenes”,
where the -* and n-* bands are practically superimposed and inverted on the energy scale with
respect to the “azobenzenes” bands 25].
When donor-acceptor substituted azobenzenes are incorporated into a polymer backbone or side-chain,
they provide very versatile materials from the applications point of view. In particular,
“pseudostilbene” azobenzenes undergo rapid trans-cis-trans photoisomerization when they are
irradiated with linear polarized light. The use of polarized light allows the selective activation of

Molecules 2013, 18
1504
“pseudostilbenes” with polarization axis parallel to the absorbing radiation [26–32]. Azobenzene
molecules are also known to undergo chromic changes through aggregation in various media including
solution, spin-cast films and Langmuir-Blodgett layers. Both H-type and J-type aggregates have been
observed 33. On the other hand, azobenzene and poly(ethylene glycol) have been employed in the
synthesis of amphiphilic azo-dyes, copolymers [34,35], nanomaterials [36,37], cellulose derivatives [38,39]
and cyclodextrin polymers [40,41], sometimes forming supramolecular complexes with interesting
properties [42]. Poly(ethylene glycol) segments provide flexibility and water solubility to the systems
to which they are incorporated [43,44].
Previously, we have published the synthesis and characterization of four novel azo-dyes bearing
terminal hydroxyl groups (RED-PEG series), the preparation of grafted azo-polymer films containing
oligo(ethylene glycol) segments (AC-g-PE-RED-PEG series) [45], and the synthesis and
characterization of a new series of azo-polymers bearing RED-PEG units in their structure
(pnPEGMAN series) [46]. Very recently, we reported the synthesis and characterization of a series of
liquid crystalline dyes bearing two amino-nitro-substituted azobenzene units linked by well defined
oligo(ethylene glycol) spacers (DIRED-PEG series) [47].
In the last years, our research group has worked on the synthesis and characterization of amphiphilic
azo-dyes and azo-polymers bearing oligo(ethylene glycol) segments with different architectures.
Herein, we report the incorporation of the RED-PEG dyes into Fréchet type dendrons in order to
obtain new liquid crystalline materials bearing azobenzene units. The thermal and optical properties of
these dendrons were studied in detail. Some of them exhibited a liquid crystalline behaviour that was
studied by DSC and Light Polarized Microscopy as a function of the temperature. Finally, the viability
of these compounds to form Langmuir and Langmuir-Blodgett films was also investigated.
2. Results and Discussion
2.1. Synthesis of the Dendrons
Six novel Fréchet type dendrons bearing azobenzene groups have been synthesized using
3,5 dihydroxybenzylic alcohol as building unit, using the classical methodology described in the
literature [45,48]. First and second generation dendritic molecules were prepared according to the
synthetic sequences illustrated in Schemes 1 and 2, respectively.
First RED-PEG-4 (1) was treated in the presence of iodine and imidazole to give the corresponding
alkyl iodide 2. Then 3,5-dihydroxybenzylic alcohol (3) was reacted with 2 using K2CO3 as base and
DMF as solvent, in the presence of 18-crown-6 to give the first generation dendron 4G1OH. On the
other hand, when 3 was treated with 1-dodecyl bromide (1 eq) under the same reaction conditions it
gave the monoalkylated compound 5. This intermediate was reacted with 2 (1 equiv.) in the presence
of K2CO3, DMF and 18-crown-6 to give 6G1OH. Similarly, 3 was reacted with 2 equiv. of 1-dodecyl
bromide under the same reaction conditions to yield the symmetric compound 7G1OH.

Molecules 2013, 18
Scheme 1. Synthesis of the first generation dendritic molecules.
1505
First generation dendron 4G1OH and 6G1OH were treated with iodine and imidazole to give the
corresponding halogenated compounds 4G1I and 6G1I. In the case of 7G1OH, this compound was
brominated in the presence of CBr4 and triphenylphosphine to give 7G1Br. Once the activated first
generation dendrons were obtained, 3 was reacted with 2 eq of 4G1I or 6G1I in the presence of K2CO3
and 18-crown-6 using DMF as solvent, to give the corresponding symmetric second generation
dendritic molecules 8G2OH and 9G2OH, respectively. Similarly, 3 was reacted first with 7G1Br and
then with 4G1I under the same reaction conditions to give the asymmetric dendron 10G2OH.

Molecules 2013, 18
Scheme 2. Synthesis of the second generation dendritic molecules.
1506
1) Imidazole
PPh3
4G1I
4 G1OH
I2
CHCl2
1) Imidazole
PPh3
I2
6 G1OH
G I
1
6
CHCl2
1) CBr4
7
G1OH
7G1Br
PPh3
THF
OH
O
O
O
O
O
O
R1
R2 R3
R4
OH
1) K2CO3, 18 crown-6
acetone
R1, R2
R3, R4
N
O
8G2OH
9G2OH
=
N N
N N
NO2
NO2
2) 4G1I
(2 eq)
4
HO
OH
N
O
1) K2CO3, 18 crown-6
acetone
R1, R3
R2, R4
=
=
4
3
C12H25
2) 6G1I
(2 eq)
OH
OH
1) K2CO3, 18 crown-6
acetone
3) K2CO3, 18 crown-6
acetone
10G2OH
4) 4
G1I
(2 eq)
2) 7G Br
(2 eq)
1
HO
OH
HO
O
N
R1, R2
R3, R4
=
O
N N
NO2
3
4
C12H25O
OC12H25
= C12H25
7G1.5OH
2.2. Characterization of the Dendrons
1
13
All dendritic compounds were fully characterized by FTIR, H- and C-NMR spectroscopies and
their molecular weights and purity were confirmed by MALDI-TOF-MS. The spectroscopic
characterization of these compounds and that of the intermediates involved in the synthesis is included
in the Experimental part, and in this section we explain in detail the assignment of the signals only for
4G1OH and 8G2OH.

Molecules 2013, 18
1507
The FTIR spectra of the azo-dendrons (not shown) were quite similar and exhibited the signals
corresponding to the functional groups present in the molecules. For instance 4G1OH and 8G2OH
exhibited a series of signals at 3442 (O-H), 2920, 2852(C-H), 1594(CC), 1514, 1445 (NN), 1375 (C-H),
1336 (NO2), 1292, 1102, 1067 (C-O of aryl and alkyl ethers), 857 and 826 (=C-H out of plane) cm−1.
The 1H-NMR spectrum of 4G1OH is shown in Figure 1a (the assignment of the signals is indicated
in the experimental part). As we can see, in the aromatic region there are five signals corresponding to
the protons present in the azobenzene unit and the phenyl group, which appear at 8.31 ppm (H4), 7.93
ppm (H3), 7.88 ppm (H2), 6.77 ppm (H1), 6.51 ppm (H5) and 6.37 ppm (H6). In the aliphatic region, we
can observe five signals: a singlet at 4.59 ppm (CH2-OH), two triplets at 4.06 ppm and 3.82 ppm
corresponding to protons OCH2 and NCH2, followed by a multiplet at 3.68 ppm due to the protons of
all other OCH2 present in the tetra(ethylene glycol) segments. Finally, a singlet related to the methyl
groups NCH3 was observed at 3.06 ppm.
1
Figure 1. H-NMR of: (a) First generation dendron 4G1OH and (b) Second generation
dendron 8G2OH.

Molecules 2013, 18
1508
In the 13C-NMR spectrum of this dendron (not shown) (the assignment of the signals is indicated in
the experimental part), we can observe 12 signals in the aromatic region at 160.20, 157.50, 152.7,
147.50, 143.87, 143.54, 126.05, 124.56, 122.52, 111.61, 106.74, 101.10 ppm due to the 12 types of
aromatic carbons present in the structure. In the aliphatic zone, we can see a series of signals at 71.07,
71.04, 70.64, 69.75, 68.57 ppm corresponding to all OCH2 carbons present in the tetra(ethylene glycol)
segment. Finally, we can observe three more signals at 67.56 (CH2-OH), 52.16 (CH2N) and 39.10 ppm
(CH3N). The molecular weight of 4G1OH was confirmed by MALDI-TOF mass spectrometry and the
base peak was observed at m/z = 969.5 as expected.
The 1H-NMR spectrum of 8G2OH is shown in Figure 1b (the assignment of the signals is indicated
in the experimental part). In the aromatic region, one can see six signals at 8.34 ppm (H4), 7.95 ppm
(H3), 7.89 ppm (H2), 6.87 ppm (H1), 6.54 ppm (H5-H7), 6.41 ppm (H8), corresponding to the protons
present in the azobenzene units and the phenyl groups of the dendron. In the aliphatic zone, we can
observe two singlets at 5.14 ppm (CH2OH) and 4.59 ppm (CH2OPh), followed by two triplets at 4.08
and 3.81 ppm due to protons OCH2 and NCH2, respectively. Finally, a multiplet at 3.66 related to
protons OCH2 present in the tetra(ethylene glycol) segment as well as a singlet at 3.14 ppm due to the
NCH3 groups, were also seen.
13
The C-NMR spectrum of this dendron (not shown) (see experimental part) exhibits 16 signals in
the aromatic zone at 160.15, 160.12, 156.89, 152.63, 147.48, 143.95, 143.46, 143.43, 126.05, 124.89,
122.79, 111.98, 105.98, 105.90, 101.11, 100.02 ppm, due to the 16 types of aromatic carbons present
in the structure of the dendron. Moreover, in the aliphatic zone, we can observe different signals at
71.01, 70.99, 69.98, 68.82 and 67.87 ppm, due to carbons (CH2O) present in the tetra(ethylene glycol)
segments. Finally, 3 more signals were perceived at 65.98 ppm (CH2OH), 52.18 ppm (CH2N) and
39.98 ppm (CH3N). The molecular weight of 8G2OH was confirmed by MALDI-TOF mass
spectrometry and the base peak was clearly observed at m/z = 2042.2, as expected.
2.3. Thermal Properties of the Dendritic Molecules
The thermal properties of the obtained dendritic compounds were studied by Thermogravimetric
Analysis (TGA) and Differential Scanning Calorimetry (DSC), and the results are summarized in
Table 1. TGA curves of the dendrons are shown in Figure 2. All dendritic compounds exhibited good
thermal stability, with T10 values ranging from 123 and 302 °C, and showed drastic degradation
between 270 and 470 °C. As we can observe, dendrons 4G1OH and 8G2OH containing exclusively
peripheral azobenzene groups are more susceptible towards degradation than those containing also side
alkyl chains. We believe that this can be due to the presence of intramolecular H-aggregates resulting
from the proximity of the azobenzene units, which make these compounds more succeptible towards
degradation. After thermal fragmentation an azobenzene aggregate is more stabilized than a non-paired
azobenzene unit. In contrast, the other dendritic compounds showed a higher thermal stability, which
slightly decreases as the number of azobenzene units present in their structure augments. That is why
first generation dendrons were shownto be thermally more stable than those of second generation. If
we make a comparison between second generation dendrons 9G2OH and 10G2OH, it is worth
pointing out that the latter, where the azobenzene groups are located in the same branch, showed a
lower thermal stability. This fact revealed the formation of H-aggregates, which are more susceptible

Molecules 2013, 18
1509
towards degradation than azobenzene units themselves. The presence of such kind of aggregates was
further confirmed by absorption spectroscopy in solid state (vide infra).
Table 1. Thermal and optical properties of the dendrons.
Dipole Moment Dipole Moment
T10
Tm
λmax
Cut off
(nm) b
Dendrons
μ (D)
[M062x]
14.5253
9.4828
μ (D)
[BPW91]
13.9576
9.1611
(°C) a (°C) a (nm) b
6G1OH
4G1OH
9G2OH
8G2OH
10G2OH
302
172
295
124
283
36
71
29
68
51
478
482
480
480
476
628
610
615
602
624
29.9523
17.2420
17.0627
29.4126
15.5955
15.7889
(a) Under nitrogen atmosphere using a heating rate of 10 °C/min; (b) In chloroform solution at room temperature.
Figure 2. TGA of the obtained dendritic compounds.
Melting points (Tm) of the dendritic compounds were determined by DSC and the values are
included in Table 1. All compounds exhibited Tm values between 29 and 71 °C. In particular 4G1OH
(Figure 3), exhibited a well defined mesophase, which clearly indicates the presence of
liquid-crystalline domains.
DSC of 6G1OH, 9G2OH, 8G2OH and 10G2OH (not shown) exhibited broad asymmetric
endotherms at 36, 30, 68 and 51 °C, respectively. The broadness of the curves indicates the presence of
a discrete mesophase, which appears prior to the melting point. This behaviour was confirmed by
repeating the DSC experiments after cooling down at a heating rate of 10 °C/min. A similar broadness
in the DSC curves was reported in the literature for some dendrons containing azobenzene units,
triazole, alkyl chains and poly(ethylene glycol) segments in their structure [63]. In the particular case
of 4G1OH (Figure 3), a well defined endotherm can be observed at 71 °C, revealing the presence of
mesophase, followed by a second endotherm at 80 °C, due to the melting point of the dendron. In
contrast, 9G2OH only showed a symmetric endotherm at 30 °C, which is related to the melting point
of this compound. In this particular case, no mesophase was detected because despite the presence of

Molecules 2013, 18
1510
an azobenzene unit, the long oligo(ethylene glycol) spacers jointly with the alkyl chains make the
structure of this dendritic compound too flexible to favour the formation of liquid crystalline domains.
Figure 3. DSC of dendron 4G1OH.
0
-1
80.4 OC
-2
4G1OH
-3
71.51 O
C
-4
0
20
40
60
80
100
120
o
Temperature
C
2.4. Liquid Crystalline Behaviour of the Dendrons
In order to confirm the liquid-crystalline (LC) behaviour of 4G1OH, light polarized optical
microscopy studies in function of the temperature were carried out [64,65] with a heating rate of
1 °C/min. Thus, the samples were gradually heated, which allowed us to observe LC behaviour in the
range of temperatures of the mesophase. The images along the heating process are illustrated in Figure 4.
Figure 4. Liquid crystalline behaviour monitored by Light Polarized Microscopy Images
for dendron 4G1OH.
(a) 30 °C
(b) 50 °C
20 µm
(c) 71 °C
(d) 80 °C

Molecules 2013, 18
1511
Along gradual heating, the formation of structural arrangements was seen in the range of
temperatures of the mesophase. For 4G1OH (Figure 4 picture a, T = 30 °C), we can notice the presence
of a crystalline structure. After heating, the dendron starts to soften and exhibits the presence of coloured
domains (picture b, T = 50 °C). Furthermore, in the range of temperatures of the mesophase, we can
notice the appearance of a LC phase with a well defined structure (picture c, T= 71°C), which persist
until melting point was reached at 80 °C (picture d). Beyond this temperature, 4G1OH passes from the
LC to the isotropic phase, so that only darkness can be observed (not shown). During the cooling
process this dendron adopts again a LC structure and recrystallises again at 40 °C. Experiments
performed with light polarized microscopy allowed us to detect the C-LC and CL-I transitions for this
compound at 71 °C and 82 °C, respectively.
2.5. Optical Properties of the Dendrons
Absorption spectra of the obtained compounds in chloroform solution are shown in Figure 5 and
their optical properties are summarized in Table 1. All these compounds except 7G1OH showed
maxima absorption wavelength in the range between  = 476–482 nm in CHCl3 solution [32,43–47].
Since these compounds contain high dipole moment azobenzene units in their structure, they exhibit
the typical photochemical behaviour of azobenzenes belonging to the “pseudostilbenes” category.
According to Rau, these compounds exhibit a total overlap of the -* and n-* bands, which are
inverted in the energy scale, so that only one band can be observed in their absorption spectra [25,32].
In fact, the higher the generation of the dendron is the more red-shifted the absorption band appears,
which is an indication of the high conjugation degree in these molecules. On the other hand, in solid
state these dyes behave differently. The absorption spectra of all these compounds in cast film are very
similar. The UV-vis spectrum of 4G1OH in cast film is shown in Figure 5b.
Figure 5. Absorption spectra of the obtained compounds: in solution (left) and in cast film (right).
1,2
1,2
1,0
4G1OH
6G1OH
8G2OH
9G2OH
10G2OH
1,0
0,8
0,6
0,4
0,2
0,8
0,6
0,4
0,2
0,0
4G OH
1
350
400
450
500
550
600
650
350
400
450
500
550
600
650
Wavelength (nm)
Wavelength (nm)
As we can see, the absorption spectra of the dendritic molecules in cast film showed a
bathochromic shift of the maxima absorption bands. Besides, we can observe the presence of
additional blue-shifted bands at ca.  = 400 nm for both 4G1OH and 6G1OH, which reveals the
presence of H-aggregates (Figure 5b, parallel interactions) [33]. A similar behaviour was observed

Molecules 2013, 18
1512
with the second generation dendrons 9G2OH and 10G2OH. In this case, an additional blue shifted
band appears at higher wavelength values due to formation of H-aggregates between the azobenzene
chromophores [33,64]. Moreover, in the absorption spectra of these compounds in cast film, we can
notice the presence of red-shifted absorption tails due to the presence of traces of J-aggregates
(Figure 5b, head to tail interactions) [33].
2.6. Theoretical Results
Optimized structures of the dendrons in chloroform solution are shown in Figure 6. As we can see
in the case of dendrons bearing more than one azobenzene units, these chromophores stay away from
each other, which avoid the formation of aggregates in solution. This results match well with those
observed in the absorption spectra of the dendrons in chloroform solution. The dihedral angles
(C-C-N-C) are equal to 90 degrees in 4G1OH, 8G1OH, 9G2OH and 10G2OH whereas 6G1OH
presents dihedral angles close to 180 degrees. For 7G1OH all the carbon atoms of the aliphatic chains
are quasi-linear due to the sp3 hybridization.
Figure 6. Optimized geometry of the first and second generation dendritic molecules in solution.
4G1OH
7G1OH
6G1OH
8G2OH

Molecules 2013, 18
1513
Figure 6. Cont.
9G2OH
10G2OH
Theoretical UV-Vis spectra of the dendrons were calculated and the predicted max values are listed
in Table 2. Moreover, available experimental results are included for comparison. As it can be seen,
there is a very good agreement between theoretical and experimental results. With these data it is
possible to say that the methodology is appropriate and the structures correspond with the experimental
ones. Analyzing the values, the most important difference is the change in the max of the compounds
with and without nitrogen functional groups. The molecule without azobenzene (7G1OH) is colorless
and the max appears in the UV region. In contrast, azobenzene compounds present a max located in
the blue region of the visible spectrum. It is important to note that there is no effect of the azobenzene
content of the molecule since all the azobenzene units exhibit almost the same max value.
Table 2. UV-vis λmax in nm obtained at M062x/LANL2DZ level in chloroform are
reported. Oscillator strengths (f) in parenthesis are included.
Dendrons
7G1OH
4G1OH
6G1OH
8G2OH
9G2OH
10G2OH
λ (f)
λ (exp)
-
Error (%)
189 (0.83)
469 (1.86)
470 (1.40)
469 (3.07)
469 (2.14)
471 (1.48)
-
482
478
482
480
476
3
3
3
2
1
Azobenzene dendrons have a small shoulder at lower  close to the UV region with an absorption
lambda similar to the value of the compound without nitrogen functional groups. Compound 6G1OH
present the most intense shoulder, while 10G2OH present a weak intensity shoulder. Apparently, this
second signal is correlated with the number of azobenzene units, i.e. and the intensity of the shoulder
decreases as the number of azobenzene groups increases.
Table 3 reports the results of vertical ionization energy (I), vertical electron affinity (A) and the
HOMO-LUMO gap (the energy difference between the highest occupied molecular orbital (HOMO)
and the lowest unoccupied molecular orbital (LUMO)). The presence of the azobenzene groups in the
dendrons generate a -conjugated system that decreases slightly the ionization energy and increases

Molecules 2013, 18
1514
dramatically the electron affinity. As a consequence, all azo-dendrons are better electron acceptors
than compound 7G1OH. The HOMO-LUMO gap is in agreement with the absorption spectra. Small
values of the HOMO-LUMO gap indicate lower excitation energy and correspond to a greater  values.
Table 3. Ionization energy (I), electron affinity (A) and the HOMO-LUMO gap in eV.
Dendrons
7G1OH 4G1OH 6G1OH 8G2OH 9G2OH 10G2OH
I
6.8
0.2
7.9
6.2
3.4
4.1
6.1
3.4
4.1
6.1
3.4
4.1
6.1
3.4
4.1
6.1
3.4
4.1
A
HOMO-LUMO
The atomic charge of the azobenzene groups is analyzed and reported in Table 4. There is no
difference in the atomic charge between the complexes. In all compounds, the N atoms have
practically the same charge. The N atom of the tertiary amine is negative (N1), while one atom of the
azo-group is negative (N3) and the other is positive (N2). Finally de N atom of the nitro group is
positive (N4).
Table 4. Atomic charges.
Atom
N1
Atomic Charge
−1.2
0.7
N2
N3
−0.7
1.6
N4
2.7. Preparation of Langmuir and Langmuir-Blodgett Films
It has been possible to prepare Langmuir films with some of these compounds and to plot the
corresponding isotherms. Langmuir films of compounds 6G1OH and 4G1OH have been characterized
by their surface pressure versus molecular area (π/A) isotherms and Brewster angle microscopy
(BAM). The π/A isotherms obtained with 4G1OH and 6G1OH are shown in Figure 7a. The two main
parameters of these isotherms, final molecular area A0 extrapolated at zero pressure and collapse
pressure πc, have been calculated from this graph. The good quality of the obtained films was
confirmed by BAM images. In the film obtained with 4G1OH (Figure 7b), when the molecular area
reaches A ≈ 188 Å2 (πc ≈ 15 mN/m), a change of slope appears in the surface pressure curve, thereby
indicating a greater compressibility of the film. At the same time, defects can be seen in the BAM
picture showing that the film is collapsing at this point. In the case of 6G1OH (not shown), the film is
not continuous at large molecular areas and shows holes through which water can be seen. These
domains smoothly weld together when the molecular area goes below A ≈ 100 Å2, and as long as the
film does not enter the collapse regime only defectless surfaces are observed. This clearly indicates the
formation of a homogeneous monomolecular layer.

Molecules 2013, 18
Figure 7. Isotherms of the Langmuir monolayers of 4G1OH and 6G1OH with BAM images.
1515
30
25
20
15
10
5
30
25
20
15
10
5
6G1OH
4G1OH
0
0
0
50
100 150 200 250
0
50
100
150
200
250
2
2
Molecular Area [ ]
Molecular Area [
]
Finally Langmuir-Blodgett films were prepared with 4G1OH and 6G1OH by transferring the
monolayer on quartz slides. Film transfers were performed at surface pressures close to the collapse
point, corresponding to the most condensed phase in the monolayer. The vertical dipping method with
a dipping speed of 5 mm/min allowed us to get 2, 10, 14 and 24 multilayer films (Figure 8). The
obtained multilayers films exhibited good quality and were analyzed by absorption spectroscopy.
Figure 8 shows the UV-vis spectra of 4G1OH in solution and LB film (24 layers). As we can observe
the LB film of this dendron shows a maximum absorption wavelength at 468 nm. Moreover, we can
perceive a blue shifted shoulder at  = 425 nm, which reveals the formation of H-aggregates with the
azobenzene units after the compression process. The amphiphilicity of 4G1OH and 6G1OH allows the
preparation of LB films which can be useful for the future elaboration of NLO devices.
Figure 8. UV-Visible spectra of 4G1OH.
--- dissolution and LB films 24 layers.
3. Experimental
3.1. General Conditions
All reagents used in the synthesis of the dendrons were purchased from Aldrich and used as
received without further purification. Acetone and dichloromethane were dried by distillation over

Molecules 2013, 18
1516
calcium hydride. Precursor dye RED-PEG-4 was synthesized according to the method previously
reported by us [45] and the poly(aryl ether) dendrons were prepared as described in the literature [48].
FTIR spectra of the compounds were carried out on a Spectrum 100 (Perkin Elmer PRECISELY)
1
spectrometer in solid state. H and 13C-NMR spectra of these compounds were recorded in CDCl3
solution at room temperature on a Bruker Avance 400 MHz spectrometer, operating at 400 MHz and
100 MHz for 1H and 13C, respectively.
Thermal properties of the obtained azo-dendrons were studied by determining T10 (10% weight loss
temperature) and Tm (melting point). Thermogravimetric Analysis (TGA) was conducted on a Hi-Res
TGA 2950 Instrument (from 20 to 800 °C) under inert atmosphere and Differential Scanning
Calorimetry (DSC) was carried out in a DSC 2910 Instrument (from 20 to 250 °C), in both cases with
a heating rate of 10 °C/min.
All dendritic compounds were dissolved in spectral quality solvents purchased from Aldrich,
and their absorption spectra were recorded on a Varian Cary 1 Bio UV-vis (model 8452A)
spectrophotometer at room temperature, using 1 cm quartz cuvettes. Absorption spectra of these
compounds were also recorded in cast films, using the same instrument. Cast films were prepared from
a saturated solution of the compounds in CHCl3, which was deposited over a glass substrate with
further evaporation of the solvent.
3.2. Synthesis of the Dendrons
Precursor azo-dye RED-PEG-4 (1) was prepared according to the method previously reported by
us 45. The synthesis of first and second generation dendritic molecules is shown in Schemes 1 and 2,
respectively.
3-Dodecyloxy-5-hydroxybenzyl alcohol (5). A mixture of 1-bromododecane (1.12 g, 8.02 mmol),
3,5-dihydroxybenzyl alcohol (3, 1 g, 4.01 mmol), K2CO3 (4.43 g, 32 mmol) and 18-crown-6 in
500 mL of acetone was heated to reflux with vigorous stirring for 48 h. The reaction mixture was
filtrated and concentrated at reduced pressure. The resulting product was purified by column
chromatography using ethyl acetate/hexane 4:6, 5:5, and 6:4 as eluent to yield (5). Yield: 68%.
1H-NMR (CDCl3): δ (ppm) = 6.35 (s, 2H, H1-H3), 6.28 (s, 1H, H2), 4.46 (s, 2H, CH2OH), 3.80 (t, 2H,
CH2OPh), 1.69 (m, 2H, CH2CH2OPh), 1.26 (m, 18H, other CH2 of the aliphatic chain), 0.88 (t, 3H,
CH3) ppm. 13C-NMR (CDCl3): δ = 160.41 (1C, Cg), 157.13 (1C, Cc), 142.67 (1C, Ca), 106.32 (1C, Cf),
105.49 (1C, Cb), 101.28 (1C, Cd), 68.13 (1C, Ce), 64.91 (1C, CH2OPh), 31.88 (1C, CH2CH2OPh),
29.61, 29.41, 29.32, 29.18, 25.98, 22.64 (9C, other CH2 of the aliphatic chain), 14.05 (1C, CH3) ppm.
H1
O
b
c
HO
a
H2
e
d
f
g
H3
OH

Molecules 2013, 18
1517
(2-{2-[2-(2-Iodo-ethoxy)-ethoxy]-ethoxy}-ethyl)-methyl-[4-(4-nitro-phenylazo)-phenyl]-amine (2). The
intermediate 1 was treated with imidazole (0.89 g, 13.1 mmol), triphenylphosphine (3.44 g,
13.1 mmol) and iodine (3.34 g, 13.1 mmol) in anhydrous dichloromethane (50 mL) at room
temperature. The resulting solution was stirred for 6 h, filtered and concentrated at reduced pressure.
The crude product was purified by column chromatography over silica gel using mixtures of ethyl
acetate/hexane 4:6, 5:5 and 6:4 as eluent. Since this intermediate is very unstable it has to be
immediately employed in the next reaction. Relative yield: 80% [the formation of this compound was
confirmed by FTIR spectroscopy, due to the absence of the band at 3400 cm−1 (OH) and the
appearance of a new band at 600 cm−1(C-I)].
[3-Dodecyloxy-5-(2-{2-[2-(2-{methyl-[4-(4-nitro-phenylazo)-phenyl]-amino}-ethoxy)-ethoxy]-ethoxy}-
ethoxy)-phenyl]-methanol (6G1OH). A mixture of 5 (0.706 g, 229 mmol), 2 (1.14 g, 210 mmol),
K2CO3 (1.58 g, 149 mmol) and 18-crown-6 dissolved in anhydrous DMF (50 mL) was heated at
80 °C with vigorous stirring for 48 h. The reaction mixture was filtered and evaporated at reduced
pressure. The crude product was purified by column chromatography in silica gel using mixtures ethyl
acetate/hexane 5:5 and 6:4 as eluent, to yield 6G1OH. Yield: 62%. 1H-NMR (CDCl3): δ (ppm) = 8.24
(d, J = 9.02 Hz, 2H, H4), 7.84 (d, J = 9.05 Hz, 2H, H3), 7.81 (d, J = 9.21 Hz, 2H, H2), 6.70 (d, J = 9.23 Hz,
2H, H1), 6.44 (s, 2H, H5), 6.31 (s, 1H, H6), 4.53 (s, 2H, CH2OH), 4.03 (t, 2H, CH2-OPh), 3.84 (t, 2H,
CH2N), 3.75 (t, 2H, OCH2R), 3.64-3.57 (m, 12H, all other OCH2), 3.06 (s, 3H, CH3N), 1.68 (m, 2H,
OCH2CH2), 1.35 (m, 2H, OCH2CH2CH2), 1.30–1.19 (m, 16H, other CH2 groups of the dodecyl chain),
0.81 (t, 3H, CH3 of the aliphatic chain) ppm. 13C-NMR (CDCl3): δ = 160.63 (1C, Ck), 160.22 (1C, Cm),
156.89 (1C, Ce), 152.67 (1C, Ca), 147.53 (1C, Ch), 143.92 (1C, Cd), 143.37(1C, Ci), 126.08 (2C, Cc),
124.62 (2C, Cg), 122.59 (2C, Cf), 111.60 (2C, Cb), 105.60 (1C, Cj), 105.26 (1C, Cn), 101.04 (1C, Cl),
70.88, 70.77, 69.80, 68.68, 68.22 (7C, all CH2O), 67.63 (1C, Co), 65.35 (1C, OCH2R), 52.31 (1C,
CH2N), 39.26 (1C, CH3N), 31.91, 29.65, 29.62, 29.59, 29.39, 29.31, 26.06, 22.65 (10C, other CH2 of
the aliphatic chain) 14.03 (1C, CH3 of the aliphatic chain) ppm. MALDITOF: C40H58N4O8 Calcd:
722.91 Found: (m/z = 722.47).
H3
e
H4
h
g
f
H1
a
H2
d
H3C
b c
N
NO2
N
N
H5
g
O
f
n
j
m
H3
H4
c
HO
b
H1
H2
H6
o
l
i
k
O
H5
[2-(2-{2-[2-(3-Dodecyloxy-5-iodomethyl-phenoxy)-ethoxy]-ethoxy}-ethoxy)-ethyl]-methyl-[4-(4-nitro-
phenylazo)-phenyl]-amine (6G1I). 6G1OH (0.22 g, 0.44 mmol) was treated with imidazole (0.055 g,
0.80 mmol), triphenylphosphine (0.21 g, 0.80 mmol) and iodine (0.20 g, 0.80 mmol) in anhydrous
dichloromethane (50 mL) at room temperature. The resulting solution was stirred for 6 h, filtered and
concentrated at reduced pressure. The crude product was purified by column chromatography in silica

Molecules 2013, 18
1518
gel using a mixture of ethyl acetate/hexane (5:5 and 6:4) as eluent to give 6G1I. Because of its
instability this intermediate was immediately used in the next reaction. Relative yield: 65% (the
formation of this compound was confirmed by FTIR spectroscopy).
3,5-Bis-[3-dodecyloxy-5-(2-{2-[2-(2-{methyl-[4-(4-nitro-phenylazo)-phenyl]-amino}-ethoxy)-ethoxy]-
ethoxy}-ethoxy)-benzyloxy]-phenyl}-methanol (9G2OH). 3,5-Dihydroxybenzyl alcohol (3, 0.043 g,
0.31 mmol) was reacted with 6G1I (0.13 g, 0.015 mmol), K2CO3 (0.18 g, 1.2 mmol) and a catalytic
amount of 18-crown-6 in dry acetone (50 mL). The reaction mixture was heated to reflux for 48 h,
cooled to room temperature, filtered and concentrated at reduced pressure. The crude product was
purified by column chromatography in silica gel using mixtures of ethyl acetate/hexane 8:2, 9:1 and
pure ethyl acetate as eluent, to give 9G2OH. Yield: 45%. 1H-NMR (CDCl3): δ = 8.28 (d, J = 9.10 Hz,
4H, H4), 7.89 (d, J = 9.01 Hz, 4H, H3), 7.87 (d, J = 9.30 Hz, 4H, H2), 6.75 (d, J = 9.30 Hz, 4H, H1),
6.57 (s, 2H, H9), 6.54 (s, 4H, H7-H8), 6.49 (s, 1H, H6), 6.40 (s, 2H, H5), 4.92 (s, 4H, PhOCH2Ph),
4.59(s, 2H, CH2OH), 4.08 (t, 4H, OCH2), 3.90 (t, 4H, OCH2R), 3.81 (t, 4H, OCH2), 3.70–3.60 (m,
24H, other OCH2 and CH2N), 3.11 (s, 6H, CH3N), 1.74 (m, 4H, OCH2CH2R), 1.41 (m, 4H,
OCH2CH2CH2R), 1.33–1.25 (m, 32H, other CH2 of the aliphatic chains), 0.87 (t, 6H, CH3 of the
aliphatic chains) ppm. 13C-NMR (CDCl3): δ = 160.51 (2C, Ck), 160.08 (2C, Co), 156.09 (2C, Cq),
152.98 (2C, Ce), 147.29 (2C, Ca), 143.74 (2C, Ch), 143.41 (2C, Cd), 130.12 (2C, Cm), 126.50 (4C, Cg),
124.63 (4C, Cf), 122.34 (4C, Cc), 111.85 (4C, Cb) , 106.03 (4C, Cn-Cr), 105.71 (2C, Cj), 101.31 (2C,
Cp), 100.98 (1C, Cl), 70.82 (2C, Cs), 70.73, 70.69, 69.99 (6C, OCH2), 68.57 (1C, Ct), 68.14, 67.63,
67.49, 65.51 (10C, all other OCH2), 52.37 (2C, CH2N), 39.43 (2C, CH3N), 31.88, 30.57, 29.63, 29.59,
29.58, 29.38, 29.30, 29.25, 26.03, 22.64 (22C, CH2 of the aliphatic chain), 14.06 (2C, CH3 of the
aliphatic chain) ppm. MALDITOF: C87H120N8O17 Calcd: 1549.93 Found: (m/z = 1549.91).
H3
H4
g
f
H1
a
H2
d
b
b
c
c
e
h
H3C
N
NO2
N
N
g
f
H3
H4
O
p
H7
m
H1
H2
o
q
n
r
H9
s
O
O
H5
i
j
k
k
H8
O
O
HO
l
H6
t
j
H5
O
N
H3C
N
N
NO2
(3,5-Bis-dodecyloxy-phenyl)-methanol (7G1OH). A mixture of 1-bromododecane (1 g, 4.01 mmol),
3,5-dihydroxybenzyl alcohol (3, 0.28 g, 2.0 mmol), K2CO3 (1.10 g, 8.0 mmol) and 18-crown-6
dissolved in 50 mL of acetone was heated to reflux with vigorous stirring for 48 h. The reaction
mixture was filtered and concentrated at reduced pressure. The resulting product was purified by flash
column chromatography in silica gel using mixtures ethyl acetate/hexane (1:9 and 2:8) as eluent to

Molecules 2013, 18
1519
1
give 7G1OH. Yield: 75%. H-NMR (CDCl3):  = 6.49 (s, 2H, H1), 6.37 (s, 1H, H2), 4.61 (s, 2H,
CH2OH), 3.93 (t, 4H, OCH2), 1.76 (m, 4H, OCH2CH2), 1.26 (s, 36H, other CH2 of the aliphatic
chains), 0.88 (t, 6H, CH3) ppm. 13C-NMR (CDCl3):  = 160.52 (2C, Cc), 143.16 (1C, Ca), 105.02 (2C,
Cb), 100.52 (1C, Cd), 68.03 (1C, Ce), 65.44 (2C, OCH2), 63.08 (2C, OCH2CH2), 32.79, 31.90, 29.58,
29.36, 29.25, 26.02, 25.7, 22.87, (18C, all other CH2), 14.09 (2C, CH3) ppm.
H1
O
c
b
HO
a
d
H2
e
c
b
H1
O
1,3-Bis-dodecyloxy-5-iodomethyl-benzene (7G1Br). 7G1OH was treated with carbon tetrabromide
(1.33 g, 4 mmol) and triphenylphosphine (1.07 g, 4 mmol) in dry THF (50 mL) at 0 °C and the
resulting solution was stirred for 6 h at room temperature. The crude product was purified by column
chromatography in silica gel using mixtures acetate/hexane 1:9 and 2:8 as eluent to give 7G1Br.
Because of its instability, this intermediate was used in the next step. Relative yield: 60% (the
formation of this compound was confirmed by FTIR spectroscopy).
3-(3,5-Bis-dodecyloxy-benzyloxy)-5-hydroxymethyl-phenol (7G1.5OH). 7G1Br (0.235 g, 0.43 mmol),
3,5-dihydroxybenzyl alcohol (3, 0.122 g, 0.87 mmol), K2CO3 (0.602 g, 4.3 mmol) and 18-crown-6
dissolved in 500 mL of acetone were heated to reflux with vigorous stirring for 48 h. The reaction
mixture was filtered and concentrated at reduced pressure. The resulting product was purified by flash
column chromatography in silica gel using mixtures ethyl acetate/hexane 3:7 and 4:6 as eluent to give
7G1.5OH. Yield: 60%. 1H-NMR (CDCl3):  = 6.39 (s, 1H, H2), 6.28 (s, 2H, H3), 6.26 (s, 2H, H1), 6.22
(s, 1H, H4), 5.17 (s, 2H, OCH2Ph), 4.70 (s, 2H, CH2OH), 4.35 (s, 4H, PhOCH2R), 3.77 (t, 4H,
PhOCH2CH2R), 1.16 (m, 4H, CH2 of the aliphatic chain), 1.28 (m, 4H, CH2 of the aliphatic chain),
1.14 (m, 28H, other CH2 of the aliphatic chain), 0.76 (t, 6H, CH3) ppm. 13C-NMR (CDCl3):  = 160.43
(1C, Ch), 160.11 (1C, Ci), 157.30 (2C,Cc), 143.03 (1C, Ce), 139.04 (1C, Ca), 106.76 (1C, Cf), 105.81
(1C, Cg), 105.66 (2C, Cb), 101.65 (1C, Cj), 100.79 (1C, Cd), 70.01 (1C, Ck), 68.16 (2C, OCH2R), 64.96
(1C, Cl), 53.43, 31.96, 29.72, 29.68, 29.64, 29.47, 29.39, 29.30, 26.09, 22.72 (20C, other CH2 of the
aliphatic chains), 14.14 (2C, CH3) ppm.
OH
H3
f
i
HO
j
e
H4
k
l
H1
a
O
d
g
c
c
h
b
b
H3
O
H2
H1
O
[3,5-Bis-(2-{2-[2-(2-{methyl-[4-(4-nitro-phenylazo)-phenyl]-amino}-ethoxy)-ethoxy]-ethoxy}-ethoxy)-
phenyl]-methanol (4G1OH). A mixture of 2 (1.3 g, 2.3 mmol), 3,5-dihydroxybenzyl alcohol (3, 0.11 g,
0.79 mmol), K2CO3 (0.44 g, 3.1 mmol) and 18-crown-6 dissolved in 50 mL of anhydrous DMF was
heated at 80 °C with vigorous stirring for 48 h. Then, the reaction mixture was filtered and
concentrated under vacuum. The crude product was purified by flash column chromatography in silica

Molecules 2013, 18
1520
gel eluting first with a mixture ethyl acetate/hexane 9:1, and then with ethyl acetate 100% to give the
1
first generation dendron 4G1OH. Yield: 45%. H-NMR (CDCl3): δ = 8.31 (d, 4H, J = 9.06 Hz, H4),
7.93 (d, 4H, J = 9.06 Hz, H3), 7.88 (d, 4H, J = 9.32 Hz, H2), 6.77 (d, 4H, J = 9.32 Hz, H1), 6.51 (s, 2H,
H5), 6.37 (s, 1H, H6), 4.59 (s, 2H, CH2OH), 4.06 (t, 4H, OCH2), 3.82 (t, 4H, NCH2), 3.68 (m, 24H, all
OCH2), 3.06 (s, 6H, CH3N) ppm. 13C RMN (CDCl3): δ = 160.27(2C, Ck), 157.44 (2C, Ce), 152.70 (2C,
Ca), 147.55 (2C, Ch), 143.91 (2C, Cd), 143.65 (1C, Ci), 126.05 (4C, Cc), 124.56 (4C, Cg), 122.52 (4C,
Cf), 111.61 (4C, Cb), 106.74 (2C, Cj), 101.10 (1C, Cl), 70.90, 70.77, 70.53, 69.74, 68.57 (14C, all
OCH2), 67.56 (1C, Cm), 52.19 (2C, CH2N), 39.13 (2C, CH3N) ppm. MALDITOF: C49H60N8O13 Calcd:
969.05 Found: (m/z = 969.5).
H3
e
H4
h
g
f
H1
a
H2
d
H3C
b
b
c
c
N
NO2
N
N
H5
g
j
O
f
k
H3
H4
HO
H1
H2
H6
m
l
i
j
k
H5
O
4
N
N
N
NO2
H3C
(E)-N,N'-(2,2'-(2,2'-(2,2'-(2,2'-(5-(iodomethyl)-1,3-phenylene)bis(oxy) bis(ethane-2,1-diyl))bis(oxy)bis
(ethane-2,1-diyl))bis(oxy)bis(ethane-2,1-diyl))bis(oxy) bis(ethane-2,1-diyl))bis(N-methyl-4-((E)-(4-
nitrophenyl)diazenyl)aniline) (4G1I). 4G1OH (0.22 g, 0.44 mmol), imidazole (0.055 g, 0.80 mmol),
triphenylphosphine (0.21 g, 0.80 mmol) and iodine (0.20 g, 0.80 mmol) were dissolved in 50 mL of
anhydrous dichloromethane at room temperature. The resulting solution was stirred for 6 h, filtered
and concentrated at reduced pressure. The crude product was purified by column chromatography in
silica gel using a mixture of ethyl acetate/hexane 8:2 as eluent to give 4G1I. Because of its instability
this intermediate was immediately used in the next reaction. Relative yield: 60% (the formation of this
compound was confirmed by FTIR spectroscopy).
{3-(3,5-Bis-dodecyloxy-benzyloxy)-5-[3,5-bis-(2-{2-[2-(2-{methyl-[4-(4-nitro-phenylazo)-phenyl]-
amino}-ethoxy)-ethoxy]-ethoxy}-ethoxy)-benzyloxy]-phenyl}-methanol (10G2OH). 7G1.5OH (0.123 g,
0.02 mmol), 4G1I (0.224 g, 0.022 mmol), K2CO3 (0.11 g, 0.082 mmol) and 18-crown-6 were dissolved
in 50 mL of acetone and heated to reflux for 48 h. The reaction mixture was filtered and concentrated
at reduced pressure. The crude product was purified by column chromatography in silica gel first
eluting with a mixture ethyl acetate/hexane 8:2 and then with ethyl acetate 100%, to yield 10G2OH.
Yield: 70%. 1H-NMR (CDCl3):  = 8.17 (d, J = 8.94 Hz, 4H, H4), 7.77 (d, J = 7.02 Hz, 4H, H3), 7.75
(d, J = 7.08 Hz, 4H, H2), 6.63 (d, J = 9.2 Hz, 4H, H1), 6.44 (s, 4H, H8-H10), 6.41 (s, 2H, H5-H6), 6.37
(s, 1H, H7), 6.27 (s, 2H, H9-H11), 4.80 (s, 4H, PhOCH2Ph), 4.47 (s, 2H, CH2OH), 3.95 (t, 4H, OCH2),
3.79 (t, 4H, OCH2R), 3.68 (t, 4H, NCH2), 3.57–3.49 (m, 24H, OCH2), 2.99 (s, 6H, CH3N), 1.63 (m,
4H, OCH2CH2R), 1.30 (m, 4H, CH2), 1.13 (m, 32H, other CH2 of the aliphatic chains), 0.75 (t, 6H,

Molecules 2013, 18
1521
CH3) ppm. 13C-NMR (CDCl3):  = 160.52 (2C, Ck-Cm), 160.08 (2C, Cw), 160.01 (2C, Cq), 152.65 (2c,
Ce), 147.34 (1C, Ca), 143.68 (2C, Ch), 143.54 (1C, Cs), 139.27 (1C, Co), 138.94 (2C, Cd), 126.22 (4C,
Cg), 124.66 (4C, Cc), 122.54 (4C, Cf), 111.59 (4C, Cb), 106.00 (1C, Cj), 105.76 (1C, Cn), 105.67 (2C,
Ct), 101.33 (2C, Cp), 101.12 (1C, Cu), 100.76 (1C, Cr), 100.00 (1C, Cl), 70.85 (1C, Cv), 70.74 (1C, Cx),
70.70, 70.12, 69.89, 69.67 (8C, OCH2), 68.59(1C, Cy), 68.10, 67.50, 65.21 (8C, all other OCH2), 52.25
(2C, CH2N), 39.38 (2C, CH3N), 31.93, 29.68, 29.65, 29.42, 29.36, 29.29, 26.08, 22.70 (20C, all other
CH2 of the aliphatic chains), 14.13 (2C, CH3 of the aliphatic chains) ppm. MALDITOF: C87H120N8O17
Calcd: 1549.93 Found: (m/z = 1549.91).
H3
H4
g
f
H1
a
H2
d
c
c
b
b
e
h
H3C
N
NO2
N
N
g
f
H3
H4
H1
H2
O
O
H8
t
t
w
w
s
u
v
H9
O
H5
i
H8
j
k
HO
N
N
l
H7
y
N
NO2
CH3
m
n
H6
H10
o
O
q
q
O
p
r
H11
x
p
H10
O
(3,5-bis(3,5-bis(2-(4-((E)-(4-nitropheyl)diazenyl)phenyl)-5,8,11,14,17-pentaoxa-2-azanonadecan-19-
yloxy)benzyloxy)phenyl)methanol (8G2OH). 3 (0.0704 g, 0.503 mmol) was reacted in the presence of
4G1I (1.091 g, 1.106 mmol), K2CO3 (0.556 g, 4.02 mmol), DMF (10 mL) and 18-crown-6 as catalyst.
The reaction was heated at 100 °C with vigorous stirring for 4 days. The crude product was dried
under vacuum and purified by column chromatography in silica gel using chloroform 100% as eluent,
increasing gradually the polarity until chloroform: acetone 97:3 to give 8G2OH as a red powder.
Yield: 15%. 1H-NMR (CDCl3):  = 8.34 (d, 8H, J = 8.81 Hz, H4), 7.95 (d, 8H, J = 8.81 Hz, H3), 7.89
(d, 8H, J = 9.32 Hz, H2), 6.87 (d, 8H, J = 9.32 Hz, H1), 6.54 (s, 6H, H5-H7), 6.41 (s, 3H,
H6-H8), 5.14 (s, 2H, CH2OH), 4.59 (s, 4H, PhCH2OPh), 4.18 (t, 8H, CH2O), 3.81 (t, 8H, CH2N), 3.64
(m, 48H, OCH2), 3.16 (s, 12H, CH3N) ppm. 13C-NMR (CDCl3):  = 160.15 (4C, Co), 160.12 (2C, Ck),
156.89 (4C, Ce), 152.63 (4C, Ca), 147.48 (4C, Ch), 143.95 (4C, Cd), 143.46 (1C, Ci), 143.43 (2C, Cm),
126.05 (8C, Cc), 124.89 (8C, Cg), 122.79 (8C, Cf), 111.98 (8C, Cb), 105.98 (2C, Cj), 105.90 (4C, Cn),
101.11 (1C, Cl), 100.02 (2C, Cp), 71.01 (2C, Cq), 70.99 (1C, Cr), 69.98, 68.82, 67.87, 65.98 (28C, all
OCH2), 52.18 (4C, CH2N), 39.98 (4C, CH3N) ppm. MALDITOF: Calcd (C105H124N16O27): 2042.20
Found (m/z = 2042.20).

Molecules 2013, 18
1522
H3
e
H4
h
g
g
f
H1
a
H2
d
c
c
b
b
H3C
N
NO2
N
N
f
4
H3
H4
H5
m
H1
H2
O
n o
n o
p
q
H6
N
N
O
O
H7
i
H5
j
k
k
CH3
N
NO2
NO2
4
HO
l
H8
r
N
j
H7
N
N
O
O
O
CH3
CH3
N
N
N
NO2
3.3. Computational Details
Density functional theory [49–51] as implemented in Gaussian09 [52] was used for all calculations.
Full geometry optimizations of the obtained compounds without symmetry constraints and frequency
analysis were carried out for all the stationary points, using the BPW91 density functional [53–55],
and the LANL2DZ basis set [56–58]. Local minima were identified by the number of imaginary
frequencies (NIMAG = 0). The absorption spectra of the dendritic molecules have been computed with
time-dependent density functional theory (TD-DFT) using M062X functional [59] and the same basis
sets. Theoretically, the intensity of the bands was expressed in terms of the oscillator strengths (f).
Stationary points were first modeled in gas phase (vacuum), and solvent effects were included a
posteriori, applying single point calculations at the same level of theory, using a polarizable continuum
model, specifically the integral-equation-formalism (IEF-PCM) [60–62] with chloroform as solvent, in
order to make a comparison with available experimental results. Vertical Ionization Energy (I) was
calculated as the difference between the energy of the cation and the neutral molecule, assuming that
both of these have the ground-state nuclear configuration of the neutral molecule. Vertical Electron
Affinity (A) represents the energy difference between the neutral molecule and the anion, calculated
with the ground-state nuclear configuration of the former.
3.4. Preparation of Langmuir and Langmuir-Blodgett Films
Solutions of the dendritic molecules were prepared by dissolving these compounds in chloroform,
using concentrations of 1 mg/mL. These solutions were spread on the water surface with a
microsyringe, and the film was left to equilibrate for 15–20 min before the compression started. Data
were collected with a KSV 5000 system 3 using a Teflon trough and barriers in a dust-free
environment, the isotherms were recorded at 20 °C and temperature was controlled to ± 0.1 °C.
Ultrapure water (ρ = 18.2 M•cm) obtained from a Milli-DIPAK/ Milli-Q185 ultrapurification system
from Millipore was used as subphase. The monolayers were compressed at 5 mm/min. Langmuir-

Molecules 2013, 18
1523
Blodgett films were obtained by transfer on quartz slides. The vertical dipping method with a dipping
speed of 5 mm/min was used to obtain 2, 10, 14 and 24 multilayer films. Film transfers were
preformed at surface pressures close to the collapse point, corresponding to the most condensed phase
in the monolayer. The phase transitions of the spread monolayer were monitored using a Nanofilm
Technologie Brewster Angle Microscope (BAM) fitted with a Teflon trough. Along the compression
process, some images on the air-solution interface were taken with a CCD camera. The image contrast
consisted of 256 gray levels.
4. Conclusions
Two novel series of first and second generation Frechet type dendritic compounds bearing
amino-nitro-substituted azobenzene units and tetra(ethylene glycol) spacers were synthesized and
characterized. These materials exhibited a good thermal stability with T10 values between 124–302 °C
Showing drastical degradation between 270 and 470 °C All compounds bearing pseudostilbene type
azobenzene units in CHCl3 solution exhibited maxima absorption wavelengths in the range of
 = 476–482 nm and no aggregation was observed, which is in agreement with the results and the
optimized geometries obtained by molecular modelling. However, in cast film these molecules showed
an additional absorption band at  = 400 nm that indicates the formation of H-aggregates. Differential
Scanning Calorimetry jointly with Light Polarized Microscopy studies revealed that dendron 4G1OH
exhibited a liquid crystalline behaviour, with the formation of a mesophase. Besides, 4G1OH and
6G1OH are able to form Langmuir films and Langmuir Blodgett multilayers due to their amphiphilic
character and good compressibility.
Acknowledgements
We are grateful to Miguel Angel Canseco for his assistance with UV-vis spectroscopy and Gerardo
13
Cedillo for his help with 1H and C-NMR spectroscopy. We thank Esteban Fregoso for his help with
TGA and DSC measurements. We acknowledge José Reyes Gasga and Pedro Mexía Hernández for
their assistance recording microscopy images. This project was financially supported by PAPIIT-DGAPA
(Projects IN-105610, IN-118808), CONACYT (Project 128788) and resources provided by the
Instituto de Investigaciones en Materiales IIM. JOP, ERA and MA acknowledge CONACYT for
scholarships. Theoretical work was carried out using a KanBalam supercomputer, provided by
DGTIC, UNAM. We also thank DGTIC of Universidad Nacional Autónoma de México for his
excellent and free supercomputing services. The authors acknowledge Oralia L Jiménez A. and María
Teresa Vázquez for their technical support.
References
1. Fréchet, J.M.J.; Tomalia, D. Dendrimers and Other Dendritric Polymers; Wiley: New York, NY,
USA, 2002.
2. Newkome, G.R.; Vögtle, F. Moorefield, C.N. Dendrimers and Dendrons; Wiley-VCH:
Weinheim, Germany, 2001.

Molecules 2013, 18
1524
3. Newkome, G.R.; Shreiner, C.D. Poly(amidoamine), polypropylenimine, and related dendrimers
and dendrons possessing different 1/2 branching motifs: An overview of the divergent procedures.
Polymer 2008, 49, 1–173.
4. Majoral, J.P.; Caminade, A.M. Dendrimers Containing Heteroatoms (Si, P, B, Ge, or Bi).
Chem. Rev. 1999, 99, 845–880.
5. Vögtle, F.; Richardt, G.; Werner, N. Dendrimer Chemistry; Wiley-VCH, Weinheim, Germany,
2009.
6. Momotake, A.; Arai, T. Synthesis, excited state properties, and dynamic structural change of
photoresponsive dendrimers. Polymer 2004, 45, 5369–5390.
7. Momotake, A.; Arai, T. Photochemistry and photophysics of stilbene dendrimers and related
compounds. J. Photochem. Photobiol. C. Photochem. Rev. 2004, 5, 1–25.
8. Shibaev, V.; Bobrovsky, A.; Boiko, N. Photoactive liquid crystalline polymer systems with
light-controllable structure and optical properties. Prog. Polym. Sci. 2003, 28, 729–836.
9. Villavicencio, O.; McGrath, D.V. Advances in dendritic. Macromolecules 2002, 5, 1–44.
10. Deloncle, R.; Caminade, A.M. Stimuli-responsive dendritic structures: The case of light-driven
azobenzene-containing dendrimers and dendrons. J. Photochem. Photobiol. C. Photochem. Rev.
2010, 11, 25–45.
11. Mekelburger, H.B.; Rissanen, K.; Vögtle, F. Repetitive-Synthesis of Bulky Dendrimers A
reversibly Photoactive Dendrimers with six Azobenzene Side Chains. Chem. Ber. 1993, 126,
1161–1169.
12. Buhleier, E.W.; Wehner, W.; Vögtle, F. Cascade and Nonskid-Chain-like Syntheses of molecular
Cavity Topologies. Synthesis 1978, 2, 155–158.
13. Wörner, C.; Mülhaupt, R. Polynitrile and polyamine funtional poly(trimethylene imine)
dendrimers. Angew. Chem. Int. Ed. Engl. 1993, 32, 1306–1308.
14. De Brabander-van den Berg, E.M.M.; Meijer, E.W. Poly(propylene imine) dendrimers: Large-Scale
synthesis by Hetereogeneously catalyzed Hydrogenations. Angew. Chem. Int. Ed. Engl. 1993, 32,
1308–1311.
15. Archut, A.; Vögtle, F.; De Cola, L.; Azzellini, G.C.; Balzani, V.; Ramanujam, P.S.; Berg, R.H.
Azobenzene-Functionalized Cascade Molecules: Photoswitchable Supramolecular Systems.
Chem. Eur. J. 1998, 4, 699–706.
16. Dirksen, A.; Zuidema, E.; Williams, R.M.; De Cola, L.; Kauffmann, C.; Vögtle, F.; Roque, A.;
Pina, F. Photoactivity and pH Sensitivity of Methyl Orange Functionalized Poly(Propyleneamine)
Dendrimers. Macromolecules 2002, 35, 2743–2747.
17. Schenning, A.P.; Elissen-Roman, C.; Weener, J.W.; Baars, M.W.; Van der Gaast, S.J.;
Meijer, E.W. Amphiphilic Dendrimers as Building Blocks in Supramolecular Assemblies.
J. Am. Chem. Soc. 1998, 120, 8199–8208.
18. Alcala, R.; Gimenez, R.; Oriol, L.; Pinol, M.; Serrano, J.L.; Villacampa, B.; Vinuales, A.I. Synthesis,
Characterization, and Induction of Stable Anisotropy in Liquid Crystalline Photo-addressable PPI
Dendrimers. Chem. Mater. 2007, 19, 235–246.
19. Tomalia, D.A.; Baker, H.; Dewald, J.; Hall, M.; Kallos, G.; Martin, S.; Roeck, J.; Ryder, J.;
Smith, P. A New Class of Polymers: Starburts-Dendritic Macromolecules. Polym. J. 1985, 17,
117–132.

Molecules 2013, 18
1525
20. Hawker, C.J.; Fréchet, J.M.J. Preparation of Polymers with Controlled Molecular Architecture. A
New Convergent Approach to Dendritic Macromolecules. J. Am. Chem. Soc. 1990, 112, 7638–7647.
21. Li, S.; McGrath, D.V. Macromolecular isomers of azobenzene-containing photochromic
dendrimers. Polym. Prep. 2000, 41, 861–864.
22. Li, S.; McGrath, D.V. Effect of Macromolecular Isomerism on the Photomodulation of Dendrimer
Properties. J. Am. Chem. Soc. 2000, 122, 6795–6796.
23. Nithyanandhan, J.; Jayaraman, N.; Davis, R.; Das, S. Synthesis, Fluorescence and
Photoisomerization Studies of Azobenzene-Functionalized Poly(alkyl aryl ether) Dendrimers.
Chem. Eur. J. 2004, 10, 689–698.
24. Kay, K.Y.; Han, K.J.; Yu, Y.J.; Park, Y.D. Dendritic fullerenes (C60) with photoresponsive
azobenzene groups. Tetrahedron Lett. 2002, 43, 5053–5056.
25. Rau H. Photochemistry and Photophysics; Rabek, J.K., Ed.; CRC Press: Boca Raton, FL, USA,
1990; Volume 2, p. 119.
26. Natansohn, A.; Rochon, P. Macromolecular Science and Engineering Award lecture The
Versatility of azobenzene Polymers. Can. J. Chem. 2001, 79, 1093–1100.
27. Todorov, T.; Nikalova, L.; Tomova, N. Polarization Holography. 1: A new high-efficiency
organic material with reversible photoinduced birefringence. Appl. Opt. 1984, 23, 4309–4312.
28. Xie, S.; Natansohn, A.; Rochon, P. Recent Developments in Aromatic Azo Polymers Research.
Chem. Mater. 1993, 5, 403–411.
29. Viswanathan, N.K.; Kim, D.Y.; Bian, S.; Williams, J.; Liu, W.; Li, L.; Samuelson, L.; Kumar, J.;
Tripathy, S.K. Surface relief structures on azo polymer films. J. Mater. Chem. 1999, 9, 1941–1955.
30. Ichimura, K. Photoalignment of Liquid-Crystal Systems. Chem. Rev. 2000, 100, 1847–1874.
31. Delaire, J.A.; Nakatani, K. Linear and Nonlinear Optical Properties of Photochromic Molecules
and Materials. Chem. Rev. 2000, 100, 1817–1846.
32. Natansohn, A.; Rochon, P. Photoinduced Motions in Azo Containing Polymers. Chem. Rev. 2002,
102, 4139–4175.
33. Kasha, M. Energy Transfer Mechanisms and the Molecular Exciton Model for Molecular
Aggregates. Radiat. Res. 1963, 20, 55–71.
34. He, X.; Zhang, H.L.; Yan, D.L.; Wang, X. Synthesis of Side-Chain Liquid-Crystalline
Homopolymers and Triblock Copolymers with p-Methoxyazobenzene Moieties and Poly(ethylene
glycol) as Coil Segments by Atom Transfer Radical Polymerization and Their Thermotropic
Phase Behavior. J. Polym. Sci. Part A Polym. Chem. 2003, 41, 2854–2864.
35. Tian, Y.Q.; Watanabe, K.; Kong, X.X.; Abe, J.; Iyoda, T. Synthesis, Nanostructures, and
Functionality of Amphiphilic Liquid Crystalline Block Copolymers with Azobenzene Moieties.
Macromolecules 2002, 35, 3739–3747.
36. Saito, M.; Shimomura, T.; Okumura, Y.; Ito, K.; Hayakawa, R. Temperature dependence of
inclusion-dissociation behavior between molecular nanotubes and linear polymers. J. Chem. Phys.
2001, 114, 1–3.
37. Shimomura, T.; Funaki, T.; Ito, K. Circular Dichroism Study of the Inclusion-Dissociation
Behavior of Complexes between a Molecular Nanotube and Azobenzene Substituted Linear
Polymers. J. Incl. Phenom. Macroc. Chem. 2002, 44, 275–278.

Molecules 2013, 18
1526
38. Zheng, P.J.; Wang, C.; Hu, X.; Tam, K.C.; Li, L. Supramolecular Complexes of Azocellulose and
R-Cyclodextrin: Isothermal Titration Calorimetric and Spectroscopic Studies. Macromolecules
2005, 38, 2859–2864.
39. Hu, X.; Zheng, P.J.; Zhao, X.Y.; Li, L.; Tam, K.C.; Gan, L.H. Preparation, characterization and
novel photoregulated rheological properties of azobenzene functionalized cellulose derivatives
and their α-CD complexes. Polymer 2004, 45, 6219–6225.
40. Takashima, Y.; Nakayama, T.; Miyauchi, M.; Kawaguchi, Y. Complex Formation and Gelation
between Copolymers Containing Pendant Azobenzene Groups and Cyclodextrin Polymers.
Chem. Lett. 2004, 33, 890–891.
41. Ikeda, T.; Ooya, T.; Yui, N. Regulation of Pseudo-Polyrotaxane Formation between
α-Cyclodetrins and Azobenzene-terminated poly(ethylene glycol). Polym. J. 1999, 31, 658–663.
42. Tung, C.H.; Wu, L.Z.; Zhang, L.P.; Chen, B. Supramolecular Systems as Microreactors: Control
of Product Selectivity in Organic Phototransformation. Acc. Chem. Res. 2003, 36, 39–47.
43. Rivera, E.; Belletête, M.; Natansohn, A.; Durocher, G. Synthesis characterization and optical
properties of a novel azo-dye bearing an oligo(ethylene glycol) methyl ether side chain in solution
and in the solid state. Can. J. Chem. 2003, 81, 1076–1082.
44. Rivera, E.; Carreón-Castro, M.P.; Buendía, I.; Cedillo, G. Optical properties and aggregation of
novel azo-dyes bearing an end-capped oligo(ethylene glycol) side chain in solution, solid state and
LangmuireBlodgett films. Dyes Pigm. 2006, 68, 217–226.
45. Rivera, E.; Carreón-Castro, M.P.; Salazar, R.; Huerta, G.; Becerril, C.; Rivera, L. Preparation and
characterization of novel grafted polyethylene based azo-polymers bearing oligo(ethylene glycol)
spacers. Polymer 2007, 48, 3420–3428.
46. García, T.; Carreón-Castro, M.P.; Gelover-Santiago, A.; Ponce, P.; Romero, M.; Rivera, E.
Synthesis and Characterization of Novel Amphiphilic Azo-Polymers Bearing Well-Defined
Oligo(Ethylene Glycol) Spacers. Des. Monomers Polym. 2012, 15, 159–174.
47. Caicedo, C.; Rivera, E.; Valdez-Hernández, Y.; Carreón-Castro, M.P. Synthesis and
characterization of novel liquid-crystalline azo-dyes bearing two amino-nitro substituted
azobenzene units and a well-defined, oligo(ethylene glycol) spacer. Mater. Chem. Phys. 2011,
130, 471–480.
48. Alvarez-Venicio, V.; Jiménez-Nava, B.; Carreón-Castro, M.P.; Rivera, E.; Méndez, I.A.;
Huerta, A.A.; Gutiérrez-Nava, M. Synthesis and incorporation in Langmuir films of
oligophenylenevinylene dendrimers bearing a polar head group and different dendritic
poly(benzyl ether) branches. Polymer 2008, 49, 3911–3922.
49. Kohn, W.; Becke, A.D.; Parr, R.G. Density Functional Theory of Electronic Structure.
J. Phys. Chem. 1996, 100, 12974–12980.
50. Hohenberg, P.; Kohn, W. inhomogeneous electron gas. Phys. Rev. 1964, 136, B864–B871.
51. Kohn, W. Sham, L. self-consistent equations including exchange and correlations effects.
Phys. Rev. 1965, 140, A1133–A1138.
52. Frisch, M.J.; Trucks, G.W.; Schlegel, H.B.; Scuseria, G.E.; Robb, M.A.; Cheeseman, J.R.;
Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G.A.; et al. Gaussian 09, Revision A.02.
Gaussian, Inc. Wallingford, CT, USA, 2009.

Molecules 2013, 18
1527
53. Becke, A.D. Density-functional exchange-energy approximation with correct asymptotic
behavior. Phys. Rev. A 1988, 38, 3098–3100.
54. Perdew, J.P. Electronic Structure of Solids; Ziesche, P., Eschrig, H., Eds.; Akademie Verlag:
Berlin, Germany, 1991, p. 11.
55. Perdew, J.P.; Chevary, J.A.; Vosko, S.H.; Jackson, K.A.; Pederson, M.R.; Singh, D.J.; Fiolhais, C.
Atoms, molecules, solids, and surfaces: Applications of the generalized gradient approximation
for exchange and correlation. Phys. Rev. B 1992, 46, 6671–6687.
56. Hay, P.J.; Wadt, W.R. Ab Initio effective core potentials for molecular calculations. Potentials for
the transition metal atoms Sc to Hg. J. Chem. Phys. 1985, 82, 270–283.
57. Hay, P.J.; Wadt, W.R. Ab initio effective core potentials for molecular calculations. Potentials for
K to Au including the outermost core orbitals. J. Chem. Phys. 1985, 82, 299–310.
58. Wadt, W.R.; Hay, P.J. Ab initio effective core potentials for molecular calculations. Potentials for
main group elements Na to Bi. J. Chem. Phys. 1985, 82, 284–298.
59. Dunning, T.H.; Hay, P.J. Modern Theoretical Chemistry; Schaefer, H.F., III, Ed.; Plenum:
New York, NY, USA, 1976, pp. 1–28.
60. Zhao, Y.; Truhlar, D.G. The M06 suite of density functionals for main group
thermochemistry,thermochemical kinetics, noncovalent interactions, excited states, and transition
elements: two new functionals and systematic testing of four M06-class functionals and 12 other
functionals. Theor. Chem. Acc. 2008, 120, 215–241.
61. Cances, M.T.; Mennucci, B.; Tomasi, J. A new integral equation formalism for the polarizable
continuum model: Theoretical background and applications to isotropic and anisotropic
Dielectrics. J. Chem. Phys. 1997, 107, 3032–3037.
62. Mennucci, B.; Tomasi, J. Continuum solvation models: A new approach to the problem of
solute’s charge distribution and cavity boundaries J. Chem. Phys. 1997, 106, 5151–5158.
63. Del Barrio, J.; Oriol, L.; Alcalá, R.; Sánchez, C. Azobenzene-Containing Linear-Dendritic
Diblock Copolymers by Click Chemistry: Synthesis, Characterization, Morphological Study, and
Photoinduction of Optical Anisotropy. Macromolecules 2009, 42, 5752–5760.
64. Iftime, G.; Lagugné-Labarthet, F.; Natansohn, A.; Rochon, P. Control of Chirality of an
Azobenzene Liquid Crystalline Polymer with Circularly Polarized Light. J. Am. Chem. Soc. 2000,
122, 12646–12650.
65. Lagugne-labarthet, F.; Freiberg, S.; Pellerin, C.; Pezolet, M.; Natansohn, A.; Rochon, P.
Spectroscopic and Optical Characterization of a Series of Azobenzene-Containing Side-Chain
Liquid Crystalline Polymers. Macromolecules 2000, 33, 6815–6823.
Sample Availability: Samples of the compounds 4G1OH and 6G1OH are available from the authors.
© 2013 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article
distributed under the terms and conditions of the Creative Commons Attribution license
(http://creativecommons.org/licenses/by/3.0/).