Incorporation of novel azobenzene dyes bearing oligo(ethylene glycol) spacers into first generation dendrimers

Article abstract of DOI:10.1016/j.dyepig.2014.11.023

Herein, we report the synthesis and inclusion of a new series of azo-dyes into first generation Fréchet type dendrimers. The incorporated dyes are amino-nitro, amino-methoxy and amino-butyl substituted azobenzenes bearing a well defined oligo(ethylene gly

Full text of DOI:10.1016/j.dyepig.2014.11.023

Dyes and Pigments 116 (2015) 1e12
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Incorporation of novel azobenzene dyes bearing oligo(ethylene glycol)
spacers into first generation dendrimers
ꢀ
Jesús Ortíz-Palacios, Efraín Rodríguez-Alba, Gerardo Zaragoza-Galan,
Jorge Rafael Leon-Carmona, Ana Martínez, Ernesto Rivera
*
ꢀ
ꢀ
ꢀ
ꢀ
Instituto de Investigaciones en Materiales, Universidad Nacional Autonoma de Mexico, Ciudad Universitaria, C.P. 04510 D.F. Mexico, Mexico
a r t i c l e i n f o
a b s t r a c t
Article history:
ꢀ
Herein, we report the synthesis and inclusion of a new series of azo-dyes into first generation Frechet
type dendrimers. The incorporated dyes are amino-nitro, amino-methoxy and amino-butyl substituted
azobenzenes bearing a well defined oligo(ethylene glycol) side chain. The optical properties of the
dendrimers were studied by absorption spectroscopy. Dendrimer bearing amino-nitro substituted azo-
Received 16 May 2014
Received in revised form
10 September 2014
Accepted 25 November 2014
Available online 13 January 2015
benzenes (
l
¼ 480 nm in CHCl3) behaved as a pseudostilbene type azobenzene whereas the others
(
l
¼ 409 nm in CHCl3) showed the typical behaviour of aminoazobenzenes. Moreover, the transecis
photoisomerization of the dendrimers was studied by UVevis spectroscopy by irradiating at two
different wavelengths (254 and 365 nm). In chloroform, the appearance of an intense red-shifted band
revealed the presence of a photoprotonation effect in the azobenzene moiety. This phenomenon was also
studied by absorption spectroscopy in function of time. The results were compared to those predicted by
molecular modelling using Density Functional Theory calculations.
Keywords:
Azobenzene
Dedrimers
Synthesis
Optical properties
Photoisomerization
Photoprotonation
© 2015 Elsevier Ltd. All rights reserved.
1. Introduction
ethylenediamine [12] or 1,4-diaminobutane [1e14] as core. In most
of the cases, all the terminal groups were azobenzenes [15e18].
Nowadays, dendrimers and dendrons have been considered one
of the most attractive research fields in polymer chemistry due to
their well-defined structures, versatility and potential applications
[1e4]. These molecules can be modified by introducing functional
groups and specific units at different levels of their structure: core,
branches or periphery [5], giving rise to well-structured and highly
functionalized molecules. Depending on the type of functional
groups present in dendrimers, different properties have been
already investigated, such as response to light. Some reviews
include the first reports of photo-responsive dendrimers [6e9]
giving many examples of azo-dendrimers. The most recent re-
view covering the most important aspects of azobenzene contain-
ing dendrons and dendrimers, has been published by Caminade
and Deloncle [10].
The most popular types of dendrimers, poly(amidoamine) [19]
and poly(arylether) [20], have been rarely used as support of azo-
ꢀ
benzene moieties. The first example of Frechet type azo-
dendrimers was synthesized by grafting through their core poly(-
arylether) dendrons bearing a single azobenzene group on the
surface, leading to original dendrimers [21e23] with azobenzene as
terminal groups. Moreover, dendrons have not been frequently
functionalized with azobenzenes on their periphery. The first
example was used as building block for dendrimers [21,22]. More
sophisticated systems, such as polyether dendrons linked to ful-
lerenes, had been also prepared [24].
Rau classified azobenzenes into three main categories based on
their photochemical behaviour [25]. Unsubstituted photochromic
azobenzene makes up the first category, known as “azobenzenes”.
Azobenzenes had been used as terminal groups of dendrimers
The thermally stable trans isomer exhibits a strong
at 350 nm and a weak ne * transition at 440 nm, whereas the cis
isomer undergoes similar transitions but with a more intense ne
band. Moreover, “azobenzenes” have a relatively poor * and
ne * overlap. The second category, known as “aminoazobenzenes”
typically includes azobenzenes that are substituted by an electron-
donor group and are characterized by the overlapping of the
and ne * bands. Finally, azobenzenes bearing both electron-donor
pep* transition
€
and dendrons, being the first examples those described by Vogtle
p
and co-workers [11]. The first structures were prepared from
poly(propyleneimine) (PPI) dendrimers built from either
p*
pep
p
pep*
* Corresponding author. Tel.: þ52 55 5622 4733; fax: þ52 55 5616 1201.
E-mail address: riverage@unam.mx (E. Rivera).
p
http://dx.doi.org/10.1016/j.dyepig.2014.11.023
0143-7208/© 2015 Elsevier Ltd. All rights reserved.

2
J. Ortíz-Palacios et al. / Dyes and Pigments 116 (2015) 1e12
and electron-acceptor groups belong to the third category, “pseu-
dostilbenes”, where the * and ne * bands are practically
superimposed and inverted on the energy scale with respect to the
“azobenzenes” bands [25].
without further purification. Acetone and dichloromethane were
dried by distillation over calcium hydride. Precursor dyes (E)-2-(4-
((4-nitrophenyl)diazenyl)phenyl)-5,8,11-trioxa-2-azatridecan-13-
ol (1), (E)-2-(4-((4-methoxyphenyl)diazenyl)phenyl)-5,8,11-trioxa-
2-azatridecan-13-ol (2) and (E)-2-(4-((4-butylphenyl)diazenyl)
phenyl)-5,8,11-trioxa-2-azatridecan-13-ol (3) were synthesized
according to the method previously reported by us [45], and the
poly(aryl ether) dendrons were prepared as described in the liter-
ature [58]. FTIR spectra of the compounds were carried out on a
Spectrum 100 (Perkin Elmer PRECISELY) spectrometer in solid
state. 1H and 13C NMR spectra of these compounds in CDCl3 solution
were recorded at room temperature on a Bruker Avance 400 MHz
spectrometer operating at 400 MHz and 100 MHz for 1H and 13C,
respectively.
All dendrons were dissolved in spectral quality solvents pur-
chased from Aldrich, and their absorption spectra were recorded on
a Varian Cary 1 Bio UVevis (model 8452A) spectrophotometer at
room temperature, using 1 cm quartz cuvettes.
Photoisomerization and photoprotonation experiments of azo-
dendrimers 14G1 and 15G1 were carried out in DMF and CHCl3
solutions (2.5 ꢀ 10ꢁ5 M) at room temperature. The samples were
irradiated with UV light using a Compact UV lamp model UVGL-25,
254/365 nm (6W). Each solution was irradiated at 254 and 365 nm
for 5 min. The spectral changes were monitored by absorption
spectroscopy with intervals of 5 s.
p
ep
p
When donoreacceptor substituted azobenzenes are incorpo-
rated into a polymer backbone or side-chain, they constitute ver-
satile materials for several applications. In particular, irradiation
with linear polarized light produces rapid transecisetrans photo-
isomerization of “pseudostilbene” azobenzenes. As a consequence,
polarized light allows the selective activation of “pseudostilbenes”
with polarization axis parallel to the absorbing radiation [26e32].
Azobenzene molecules can also undergo chromic changes
through aggregation in various media including solution, spin-
coated films and LangmuireBlodgett multilayers. In these media,
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]. In fact, poly(ethylene
glycol) segments provide flexibility and water solubility to the
systems to which they are incorporated [43,44].
In the last ten 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. We reported the synthesis and characterization of
four novel azo-dyes bearing terminal hydroxyl groups, the prepa-
ration of grafted azo-polymer films containing oligo(ethylene gly-
col) segments [45], and the synthesis and characterization of a new
series of polymethacrylates bearing amino-nitro azobenzene units
and oligo(ethylene glycol) chains their structure [46]. More
recently, we published the synthesis and characterization of a se-
ries of liquid crystalline dyes bearing two amino-nitro substituted
azobenzene units linked by well defined oligo(ethylene glycol)
spacers [47].
2.2. Computational and theoretical details
All calculations were carried out by using the Gaussian 09
implementation [59]. Calculations involving atomic geometry and
electronic structure were performed by applying the Density
Functional Theory (DFT) framework for all the stationary points,
using the B3LYP [60] functional and the basis 6-31G [61e65]. In
order to verify optimized minima, harmonic analyses were ach-
ieved and local minima were identified with zero imaginary fre-
quencies. In order to simulate the UVevisible spectra, time-
dependent DFT (TD-DFT) method was employed using the same
methodology. The UVevisible spectra were obtained for half singlet
and half triplet states, effective for closed-shell systems.
Many articles about the preparation of new dendritic molecules
containing azobenzene have been reported in the literature. Some
of these materials exhibited outstanding optical properties, NLO
response [48e54] or a liquid crystalline behaviour [55,56]. Last
year, we reported the incorporation of amino-nitro substituted
azobenzenes containing a tetra(ethylene glycol) side chain and
other related dyes into dendritic structures, in order to get new
liquid crystalline materials bearing azobenzene units. The thermal
and optical properties of such dendrons were studied in detail, and
some of them exhibited a liquid crystalline behaviour [57]. Herein,
we report the incorporation of novel azo-dyes bearing amino-nitro,
amino-methoxy and amino-butyl substituted azobenzenes into
2.3. Synthesis
2.3.1. Synthesis of the precursor azo-dyes
Precursor azo-dyes (E)-2-(4-((4-nitrophenyl)diazenyl)phenyl)-
5,8,11-trioxa-2-azatridecan-13-ol (1), (E)-2-(4-((4-methoxyphenyl)
diazenyl)phenyl)-5,8,11-trioxa-2-azatridecan-13-ol (2) and (E)-2-
(4-((4-butylphenyl)diazenyl)phenyl)-5,8,11-trioxa-2-azatridecan-
13-ol (3) were prepared according to the method previously re-
ported by us [45].
ꢀ
first generation Frechet type dendrimers. The optical properties of
these compounds, as well as the transecis photoisomerization and
the photoprotonation effect, were studied by absorption spectros-
copy. The results were compared to those predicted by molecular
modelling using Density Functional Theory calculations. These new
azo-dendrimers can be used for optical switching and storage as
other azo-polymers previously reported in the literature. In addi-
tion, they can act as photochromic sensors since they exhibit
noticeable colour changes arising from the photoinduced proton-
ation which occur in CHCl3 solution when they are irradiated with
UV light at 254 nm.
2.4. Synthesis of the dendrons
2.4.1. Synthesis of the 3-dodecyloxy-5-hydroxybenzyl alcohol (8)
The synthesis and characterization of the dendron (8) has been
previously reported by us [57]. Yield: 68%.
FTIR (Film) y
/cmꢁ1: 3375 (OH), 2913, 2847 (CH3, CH2),1595,1469
(C]C, Ar), 1378, 1328 (CH3, CH2), 1305 (ArOCH), 1160 (COC) and
1037 (ArOCH).
1H NMR (400 MHz, CDCl3):
d
¼ 6.35 (s, J ¼ 2 Hz, 2H, H1eH3), 6.28
2. Experimental
(s, J ¼ 2 Hz, 1H, H2), 4.46 (s, 2H, PhCH2OH), 3.80 (t, J ¼ 6.62 Hz, 2H,
PhOCH2), 1.69 (m, 2H, PhOCH2CH2), 1.26 (m, 18H, all CH2 of the
aliphatic chain), 0.88 (t, J ¼ 6.56 Hz, 3H, CH3) ppm.
2.1. General conditions
13C NMR (100 MHz, CDCl3):
d
¼ 160.41 (1C, Cb), 157.13 (1C, Cd),
All reagents used in the synthesis of the azo-dyes, dendrons and
dendrimers were purchased from Aldrich and used as received
142.67 (1C, Cf), 106.32 (1C, Ce), 105.49 (1C, Ca), 101.28 (1C, Cc), 68.13
(1C, PhOCH2Ce), 64.91 (1C, PhCH2OH), 31.88 (1C, PhOCH2CH2),

J. Ortíz-Palacios et al. / Dyes and Pigments 116 (2015) 1e12
3
29.61, 29.41, 29.32, 29.18, 25.98, 22.64 (9C, all CH2 of the aliphatic
chain), 14.05 (1C, CH3) ppm.
13C NMR (100 MHz, CDCl3) (Scheme 1a):
d
¼ 160.63 (1C, Ce),
160.22 (1C, Cc), 156.89 (1C, Ck), 152.67 (1C, Cg), 147.53 (1C, Cn),
143.92 (1C, Cj), 143.37 (1C, Ca), 126.08 (2C, Ci), 124.62 (2C, Cm),
122.59 (2C, Cl), 111.60 (2C, Ch), 105.60 (1C, Cf), 105.26 (1C, Cb), 101.04
(1C, Cd), 70.88e70.77, 69.80, 67.63 (6C, OCH2), 68.68 (1C, PhOCH2
aliphatic chain), 68.22 (1C, PhOCH2 of the tetra(ethylene glycol)
chain), 65.35 (1C, Co), 52.31 (1C, NCH2), 39.26 (1C, NCH3), 31.91,
29.65e29.31, 26.06, 22.65 (10C, all CH2 of the aliphatic chain), 14.03
(1C, CH3) ppm.
2.4.2. Synthesis of the iodided intermediates
Intermediates (4), (5) and (6) were obtained using the meth-
odology reported by us [57] which is described only for (4).
(2-{2-[2-(2-Iodo-ethoxy)-ethoxy]-ethoxy}-ethyl)-methyl-[4-(4-
nitro-phenylazo)-phenyl]-amine (4)
Compound (1) (4.37 g, 10.11 mmol)was reacted with imidazole
(0.89 g, 13.1 mmol), triphenylphosphine (3.44 g, 13.1 mmol) and
iodine (3.34 g, 13.1 mmol) in 50 mL 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 mix-
tures of ethyl acetate/hexane (4:6, 5:5, and 6:4) as eluent. Since this
intermediate is very instable it was immediately used in the next
reaction without further purification. Relative yield: 80%.
2.4.4. Synthesis of the [3-dodecyloxy-5-(2-{2-[2-(2-{[4-(4-
methoxy-phenylazo)-phenyl]-methyl-amino}-ethoxy)-ethoxy]-
ethoxy}-ethoxy)-phenyl]-methanol (10G1OH)
Yield: 64%.
FTIR (Film) y
/cmꢁ1: 3426 (OH), 2920, 2851 (CH3, CH2),1594,1514
(C]C, Ar), 1447 (N]N), 1376(CH), 1244 (ArOCH) and 1147 (ArOCH).
MALDITOF:
C
41H61N3O7 Calcd: [MþH]þ 707.96 Found: (m/
z):[MþH]þ 707.41.
For
(2-{2-[2-(2-Iodo-ethoxy)-ethoxy]-ethoxy}-ethyl)-[4-(4-
1H NMR (400 MHz, CDCl3) (Scheme 1b):
d
¼ 7.83 (d, J ¼ 9.1 Hz,
methoxy-phenylazo)-phenyl]-methyl-amine (5): Relative yield:
85%.
For [4-(4-Butyl-phenylazo)-phenyl]-(2-{2-[2-(2-iodo-ethoxy)-
ethoxy]-ethoxy}-ethyl)-methyl-amine (6): Relative yield: 85%.
2H, H6), 7.73 (d, J ¼ 8.7 Hz, 2H, H5), 7.26 (d, J ¼ 8.2 Hz, 2H, H7), 6.76
(d, J ¼ 9.1 Hz, 2H, H4), 6.48 (d, J ¼ 2 Hz, 2H, H1eH3), 6.37 (d, J ¼ 2 Hz,
1H, H2), 4.60 (s, 2H, PhCH2OH), 4.08 (t, J ¼ 4.54 Hz, 2H, PhOCH2 of
the tetra(ethylene glycol) chain), 3.90 (t, J ¼ 6.57 Hz, 2H, PhOCH2 of
the aliphatic chain), 3.87 (s, 3H, PhOCH3), 3.82 (t, J ¼ 4.81 Hz, 2H,
CH2N), 3.69e3.62 (m, 12H, OCH2), 3.11 (s, 3H, CH3N), 1.77e1.70 (m,
2H, PhOCH2CH2), 1.45e1.38 (m, 2H, PhO(CH2)2CH2), 1.34e1.26 (m,
16H, all CH2 of the aliphatic chain), 0.884 (t, J ¼ 6.56 Hz, 3H, CH3)
ppm.
2.4.3. Synthesis of [3-dodecyloxy-5-(2-{2-[2-(2-{methyl-[4-(4-
nitro-phenylazo)-phenyl]-amino}-ethoxy)-ethoxy]-ethoxy}-
ethoxy)-phenyl]-methanol (9G1OH)
The dendrons 9G1OH, 10G1OH and 11G1OH were obtained
employing the procedure previously reported by us [57]. For
9G1OH: Yield: 62%.
13C NMR (100 MHz, CDCl3) (Scheme 1b):
d
¼ 160.72 (1C, Cn),
160.34 (1C, Ce), 159.96 (1C, Cc), 150.90 (1C, Cg), 147.34 (1C, Ck),
143.51 (1C, Cj), 143.35 (1C, Ca), 124.49 (2C, Ci), 123.71 (2C, Cl), 113.99
(2C, Cm), 111.36 (2C, Ch), 105.30 (1C, Cf), 104.86 (1C, Cb), 100.56 (1C,
Cd), 70.67e70.59, 69.62, 67.33 (6C, OCH2), 68.46 (1C, PhOCH2 of the
aliphatic chain), 67.97 (1C, PhOCH2 tetra(ethylene glycol) chain),
65.04 (1C, Co), 55.39 (1C, NCH2), 52.08 (1C, OCH3), 39.10 (1C, NCH3),
31.83, 29.58e29.16, 25.96, 22.60 (10C, all CH2 of the aliphatic chain),
14.05 (1C, CH3) ppm.
FTIR (Film) y
/cmꢁ1: 3452 (OH), 2921, 2852 (CH3, CH2),1602,1514
(C]C, Ar), 1455 (N]N), 1380 (CH), 1340 (NO2), 1137 (ArOCH), 1102
(COC) and 1070 (ArOCH). MALDITOF: C40H58N4O8 Calcd: [MþH]þ
722.91 Found (m/z): [MþH]þ 722.47.
1H NMR (400 MHz, CDCl3) (Scheme 1a):
d
¼ 8.24 (d, J ¼ 9.02 Hz,
2H, H7), 7.84 (d, J ¼ 9.05 Hz, 2H, H6), 7.81 (d, J ¼ 9.21 Hz, 2H, H5),
6.70 (d, J ¼ 9.23 Hz, 2H, H4), 6.49 (d, J ¼ 2 Hz, 2H, H1eH3), 6.37 (t,
J ¼ 2 Hz, 1H, H2), 4.53 (s, 2H, PhCH2OH), 4.03 (t, J ¼ 4.99 Hz, 2H,
PhOCH2 of the tetra(ethylene glycol) chain), 3.84 (t, J ¼ 6.58 Hz, 2H,
PhOCH2 of the aliphatic chain), 3.75 (t, J ¼ 4.62 Hz, 2H, CH2N),
3.64e3.57 (m, 12H, OCH2 of the tetra(ethylene glycol) chain), 3.06
(s, 3H, CH3N), 1.77e1.70 (m, 2H, PhOCH2CH2), 1.40e1.38 (m, 2H,
PhO(CH2)2CH2), 1.34e1.25 (m, 18H, all CH2 of the aliphatic chain),
0.81 (t, J ¼ 6.67 Hz, 3H, CH3) ppm.
2.4.5. Synthesis of the [3-(2-{2-[2-(2-{[4-(4-butyl-phenylazo)-
phenyl]-methyl-amino}-ethoxy)ethoxy]-ethoxy}-ethoxy)-5-
dodecyloxy-phenyl]-methanol (11G1OH)
Yield: 63%.
FTIR (Film) y
/cmꢁ1: 3420 (OH), 2921, 2852 (CH3, CH2), 1597, 1514
(C]C, Ar), 1447 (N]N), 1376(CH), 1155 (ArOCH), 1138 (COC), 1068
(ArOCH). MALDITOF: C44H67N3O6 Calcd: [MþH]þ 733.50 Found:
(m/z):[MþH]þ 733.54.
1H NMR (400 MHz, CDCl3) (Scheme 1c):
d
¼ 7.75 (d, J ¼ 9.05 Hz,
2H, H6), 7.67 (d, J ¼ 8.29 Hz, 2H, H5), 7.19 (d, J ¼ 8.66 Hz, 2H, H7),
6.67 (d, J ¼ 9.14 Hz, 2H, H4), 6.42 (t, J ¼ 2 Hz, 2H, H1eH3), 6.31 (t,
J ¼ 2 Hz, 1H, H2), 4.48 (s, 2H, PhCH2OH), 4.08 (t, J ¼ 4.39 Hz, 2H,
PhOCH2 of the tetra(ethylene glycol) chain), 3.91 (t, J ¼ 6.55 Hz, 2H,
PhOCH2 of the aliphatic chain), 3.81 (t, J ¼ 4.78 Hz, 2H, CH2N),
3.69e3.60 (m, 12H, OCH2), 3.08 (s, 3H, CH3N), 2.65 (t, J ¼ 7.7 Hz, 2H,
PhCH2CH2), 1.78e1.71 (m, 2H, PhOCH2CH2), 1.66e1.59 (m, 2H,
PhCH2),1.43e1.26 (m, 20H, Ph(CH2)2CH2 and all CH2 of the aliphatic
chain), 0.93 (t, J ¼ 7.33 Hz, 3H, CH3), 0.87 (t, J ¼ 6.71 Hz, 3H,
Ph(CH2)3CH3) ppm.
13C NMR (100 MHz, CDCl3) (Scheme 1c):
d
¼ 160.45 (1C, Ce),
160.08 (1C, Cc), 151.37 (1C, Cg),151.17 (1C, Ck), 144.66 (1C, Cn), 143.65
(1C, Cj), 143.29 (1C, Ca), 128.91 (2C, Cm), 124.79 (2C, Ci), 122.09 (2C,
Cl), 111.39 (2C, Ch), 105.40 (2C, Cf), 104.98 (1C, Cb), 100.70 (1C, Cd),
70.78e70.69, 69.70, 68.54 (6C, OCH2), 68.06 (1C, PhOCH2 of the
aliphatic chain), 67.44 (1C, PhOCH2 of the tetra(ethylene glycol)
chain), 65.27 (1C, Co), 52.17 (1C, NCH2), 39.21 (1C, NCH3), 35.48 (1C,
PhCH2), 33.49 (1C, PhCH2CH2), 31.89, 29.60e29.31, 26.02, 22.64
Scheme 1. Assignment of the signals for dendrons 9G1OH, 10G1OH and 11G1OH.

4
J. Ortíz-Palacios et al. / Dyes and Pigments 116 (2015) 1e12
(10C, all CH2 of the aliphatic chain), 22.31 (1C, Ph(CH2)2CH2), 14.07
(1C, CH3), 13.89 (1C, Ph(CH2)3CH3) ppm.
CH3N), 1.76e1.69 (m, 6H, PhOCH2CH2), 1.44e1.39 (m, 6H,
Ph(CH2)2CH2), 1.32e1.24 (m, 54H, all CH2 of the aliphatic chain),
0.87 (t, J ¼ 6.24 Hz, 9H, CH3) ppm.
13C NMR (100 MHz, CDCl3) (Scheme 2a):
d
¼ 164.66 (3C, Cr),
2.5. Synthesis of the dendrimers
160.57 (3C, Ce), 160.14 (3C, Cc), 156.81 (3C, Ck), 152.58 (3C, Cg),
147.42 (3C, Cn), 143.81 (3C, Cj), 137.58 (3C, Ca), 134.84 (3C, Cp), 131.26
(3C, Cq), 126.07 (6C, Ci), 124.62 (6C, Cm), 122.57 (6C, Cl), 111.52 (6C,
Ch), 107.09 (3C, Cf), 106.70 (3C, Cb), 101.49 (3C, Cd), 70.85e70.71,
68.21 (18C, OCH2), 69.68 (3C, PhOCH2 of the aliphatic chain), 68.61
(3C, PhOCH2 of the tetra(ethylene glycol) chain), 67.56 (3C, Co),
52.23 (3C, NCH2), 39.30 (3C, NCH3), 31.90, 29.66e29.26, 26.05,
22.66 (30C, all CH2 of the aliphatic chain), 14.08 (3C, CH3) ppm.
The synthesis of the dendrimers 13G1, 14G1 and 15G1 is
described only for 13G1 and the spectroscopical characterization is
given for all the compounds.
2.5.1. Tris(3-(dodecyloxy)-5-(2-(methyl(4-((E)-(4nitrophenyl)
diazenyl)phenyl)amino) ethoxy) benzyl) benzene-1,3,5-
tricarboxylate (13G1)
The dendron (9G1OH) (0.1476 g, 0.17 mmol) and triethylamine
(0.023 mL, 0.51 mmol) were dissolved in 10 mL anhydrous CH2Cl2.
2.5.2. Tris(3-(dodecyloxy)-5-(2-((4-((E)-(4 methoxyphenyl)
diazenyl)phenyl) (methyl)amino) ethoxy)benzyl) benzene-1,3,5-
tricarboxylate (14G1)
A
solution of 1,3,5-benzenetricarboxyl trichloride (0.016 g,
0.063 mmol) in 5 mL anhydrous CH2Cl2 was added dropwise. The
reaction mixture was stirred out at room temperature for 4 days.
Then, it was filtrated and evaporated at reduced pressure. The
crude product was washed with water, extracted with chloroform
and dried with anhydrous MgSO4. The final product was evapo-
rated at reduced pressure and purified by column chromatography
in silica gel using mixtures ethyl acetate/hexane (5:5, 6:4 and 7:3)
as eluent, to yield the first generation dendrimer 13G1. Yield: 31%.
Yield: 29%.
FTIR (Film) y
/cmꢁ1: 2921, 2852 (CH3, CH2), 1726 (C]O), 1595,
1513 (C]C, Ar) and 1448 (N]N). MALDITOF: C132H183N9O24 Calcd:
[MþH]þ 2279.91 Found: (m/z): [MþH]þ 2280.17.
1H NMR (400 MHz, CDCl3) (Scheme 2b):
d
¼ 8.87 (s, 3H, H8), 7.81
(d, J ¼ 9.05 Hz, 6H, H6), 7.79 (d, J ¼ 9.05 Hz, 6H, H5), 6.96 (d,
J ¼ 9.05 Hz, 6H, H7), 6.73 (d, J ¼ 9.05 Hz, 6H, H4), 6.56 (t, J ¼ 2 Hz, 6H,
H1eH3), 6.43 (t, J ¼ 2 Hz, 3H, H2), 5.29 (s, 6H, PhCOOCH2Ph), 4.08 (t,
J ¼ 4.54 Hz, 6H, PhOCH2 of the tetra(ethylene glycol) chain), 3.91 (t,
J ¼ 6.54 Hz, 6H, PhOCH2 of the aliphatic chain), 3.85 (s, 9H,
PhOCH3), 3.81 (t, J ¼ 4.94 Hz, 6H, CH2N), 3.69e3.58 (m, 36H, OCH2),
3.06 (s, 9H, CH3N), 1.77e1.70 (m, 6H, PhOCH2CH2), 1.45e1.37 (m,
6H, PhO(CH2)2CH2), 1.33e1.25 (m, 48H, all CH2 of the aliphatic
chain), 0.88 (t, J ¼ 6.72 Hz, 9H, CH3) ppm.
FTIR (Film) y
/cmꢁ1: 2922, 2853 (CH3, CH2) 1727 (C]O), 1599,
1517 (C]C, Ar) and 1450 (N]N). MALDITOF: C129H174N12O27 Calcd:
[MþH]þ 2324.83. Found: (m/z): [MþH]þ 2324.92.
1H NMR (400 MHz, CDCl3) (Scheme 2a):
d
¼ 8.85 (s, 3H, H8), 8.29
(d, J ¼ 8.87 Hz, 6H, H7), 7.88 (d, J ¼ 8.96 Hz, 6H, H6), 7.86 (d,
J ¼ 9.28 Hz, 6H, H5), 6.74 (d, J ¼ 9.04 Hz, 6H, H4), 6.55 (d, J ¼ 2 Hz,
6H, H1eH3), 6.42 (d, J ¼ 2 Hz, 3H, H2), 5.28 (s, 6H, PhCOOCH2Ph),
4.08 (t, J ¼ 3.55 Hz, 6H, PhOCH2 of the tetra(ethylene glycol) chain),
3.90 (t, J ¼ 6.37 Hz, 6H, PhOCH2 of the aliphatic chain), 3.81 (t,
J ¼ 4.54 Hz, 6H, CH2N), 3.68e3.61 (m, 36H, OCH2), 3.10 (s, 9H,
13C NMR (100 MHz, CDCl3) (Scheme 2b):
d
¼ 164.72 (3C, Cr),
160.82 (3C, Cn), 160.50 (3C, Ce), 160.11 (3C, Cc), 150.99 (3C, Cg),
147.38 (3C, Ck), 143.60 (3C, Cj), 137.55 (3C, Ca), 134.91 (3C, Cp), 131.21
Scheme 2. Assignment of the signals for dendrimers 13G1, 14G1 and 15G1.

J. Ortíz-Palacios et al. / Dyes and Pigments 116 (2015) 1e12
5
(3C, Cq), 124.65 (6C, Ci), 123.82 (6C, Cl), 114.09 (6C, Cm), 111.49 (6C,
Ch), 107.08 (3C, Cf), 106.62 (3C, Cb), 101.36 (3C, Cd), 70.75e70.62,
69.60, 67.41 (18C, OCH2), 68.49 (3C, PhOCH2 of the aliphatic chain),
68.09 (3C, PhOCH2 of the tetra(ethylene glycol) chain), 67.24 (3C,
Co), 55.45 (3C, NCH2), 52.15 (3C, OCH3), 39.20 (3C, NCH3), 31.88,
29.64e29.20, 26.01, 22.65 (30C, all CH2 of the aliphatic chain), 14.10
(3C, CH3) ppm.
dendrimers were prepared from (5) and (6) under the same reac-
tion conditions described above to give the corresponding den-
drons 14G1 and 15G1, respectively (Fig. 1).
3.2. Characterization of the dendrimers
The precursor azo-dyes, dendrons and dendrimers were fully
characterized by 1H and 13C NMR spectroscopies, and the molecular
weights and purity of the final products were confirmed by MALDI-
TOF mass spectrometry using dithranol as matrix. The 1H NMR and
13C NMR spectra of the dendrimer bearing amino-methoxy
substituted azobenzene units (14G1) are shown in Fig. 2, but the
spectroscopic data of all compounds are given in the experimental
section. The 1H NMR and 13C NMR of all dendrimers can be found in
the Supporting Information (SI).
2.5.3. Tris(3-(2-((4-((E)-(4-butylphenyl)diazenyl)phenyl)(methyl)
amino)ethoxy)-5 (dodecylo-xy)benzyl)benzene-1,3,5-tricarboxylate
(15G1)
Yield: 29%.
FTIR (Film) y
/cmꢁ1: 2923, 2853 (CH3, CH2), 1726 (C]O), 1596,
1514 (C]C, Ar) and 1446 (N]N). MALDITOF: C132H183N9O24 Calcd:
[MþH]þ 2358.15. Found: (m/z):[MþH]þ 2359.14.
1H NMR (400 MHz, CDCl3) (Scheme 1c):
d
¼ 8.87 (s, 3H, H8), 7.82
In the 1H NMR spectrum of 10G1OH (SI, Fig. S2), there are six
signals in the aromatic region at 7.87, 7.73, 7.26, 6.76, 6.50 and
6.37 ppm due to the aromatic protons present in the azobenzene
unit and phenyl group H6, H7, H4, H1eH3 and H2, respectively. In the
aliphatic zone, we can observe a singlet at 4.60 ppm (PhCH2OH),
two triplets at 4.08 (PhOCH2) and 3.90 ppm (PhOCH2) due to the
tetra(ethylene glycol) chain and aliphatic chain, respectively. In
addition, we can perceive a singlet at 3.87 ppm (PhOCH3) and a
(d, J ¼ 8.98 Hz, 6H, H6), 7.74 (d, J ¼ 8.23 Hz, 6H, H5), 7.26 (d,
J ¼ 8.23 Hz, 6H, H7), 6.73 (d, J ¼ 9.03 Hz, 6H, H4), 6.56 (d, J ¼ 2 Hz,
6H, H1eH3), 6.44 (t, J ¼ 2 Hz, 3H, H2), 5.29 (s, 6H, PhCOOCH2Ph),
4.09 (t, J ¼ 4.25, 6H, PhOCH2 of the tetra(ethylene glycol) chain),
3.91 (t, J ¼ 6.44 Hz, 6H, PhOCH2 of the aliphatic chain), 3.82 (t,
J ¼ 4.46 Hz, 6H, CH2N), 3.68e3.61 (m, 36H, OCH2), 3.06 (s, 6H,
CH3N), 2.66 (t, J ¼ 7.62 Hz, 2H, PhCH2CH2), 1.76e1.70 (m, 6H,
PhOCH2CH2), 1.66e1.58 (m, 6H, PhCH2), 1.41e1.25 (m, 60H,
Ph(CH2)2CH2 and all CH2 of the aliphatic chain), 0.94 (t, J ¼ 7.33 Hz,
6H, CH3), 0.88 (t, J ¼ 6.48 Hz, 3H, Ph(CH2)3CH3) ppm.
triplet at 3.82 ppm (CH2N), followed by
a multiplet at
3.69e3.62 ppm related to the protons OCH2 of the oligo(ethylene
glycol) segments as well as a singlet at 3.11 ppm (NCH3). Finally, the
protons corresponding to methylenes (CH2) present in the aliphatic
chain appear at 1.74, 1.42, 1.34e1.36 and 0.88 ppm.
13C NMR (100 MHz, CDCl3) (Scheme 1c):
d
¼ 164.79 (3C, Cr),
160.61 (3C, Ce), 160.21 (3C, Cc),151.51 (3C, Cg),151.28 (3C, Ck), 144.75
(3C, Cn), 143.79 (3C, Cj), 137.66 (3C, Ca), 134.99 (3C, Cp), 131.33 (3C,
Cq), 129.03 (6C, Cm), 124.91 (6C, Cl), 122.24 (6C, Ci), 111.50 (6C, Ch),
107.18 (6C, Cf), 106.72 (3C, Cb), 101.49 (3C, Cd), 70.53e70.35, 69.37,
67.21 (18C, OCH2), 68.28 (3C, PhOCH2 of the aliphatic chain), 67.87
(3C, PhOCH2 of the tetra(ethylene glycol) chain), 67.01 (3C, Co),
51.89 (3C, NCH2), 38.93 (3C, NCH3), 35.22 (3C, PhCH2), 33.24 (3C,
PhCH2CH2), 31.64, 29.40e29.08, 28.08, 25.78, 22.41 (30C, all CH2 of
the aliphatic chain), 22.06 (3C, Ph(CH2)2CH2), 13.86 (3C, CH3), 13.68
(3C, Ph(CH2)3CH3) ppm.
The 1H NMR spectrum of dendrimer 14G1 is very similar, we can
observe two signals at 8.87 and 5.29 ppm in the aromatic region,
due to the protons H8 and Ho respectively (Fig. 2a). The signals
corresponding to the protons present in the azobenzene unit and
the phenyl group appear at the same shift as those of 10G1OH.
Finally in the aliphatic region we can observe the same signals
perceived for the aliphatic protons between 1.75 and 0.88 ppm (SI,
Fig. S6).
The 13C spectrum of 14G1 is illustrated in the Fig. 2b. As we can
see, there are 17 signals in aromatic region at 164.72,160.82,160.50,
160.11, 150.99, 147.38, 143.60, 137.55, 134.91, 131.21, 124.65, 123.82,
114.09, 111.49, 107.08, 106.62, 101.36 ppm due to the 17 types of
aromatic carbons present in the structure of the dendrimer.
Moreover, in the aliphatic region we can observe various peaks at
70.75e70.62, 69.60, 67.41 ppm, due to the methylenes present in
the tera(ethylene glycol) spacers. The carbons PhOCH2 of the oli-
go(ethylene glycol) and the aliphatic chain appear at 68.49 and
68.09 ppm, respectively. Four more signals can be perceived at
67.24, 55.45, 52.15 and 39.20 ppm, corresponding to carbons Co,
NCH2, OCH3 and NCH3, respectively. Finally, the signals due to all
the CH2 and CH3 present in the alphatic chain appear at 31.88,
29.64e29.20, 26.01, 22.65, 14.10 ppm (SI, Fig. S7). The structure and
purity of these dendrimers were confirmed by MALDI-TOF mass
spectrometry. All dendrimers showed a molecular ion peak which
is in agreement with their calculated molecular weight.
3. Results and discussion
3.1. Synthesis of the dendrimers
The synthesis of the dendrimers was achieved using the
convergent approach and is illustrated in Fig. 1. Three different
ꢀ
series of first generation Frechet type dendrimers bearing azo-
benzene units in the periphery were prepared employing 3,5-
dihydroxy benzylic alcohol as built unit. The dendrimers were ob-
tained by coupling 3.2 eq of the precursor dendrons (G1OH) in the
presence of 1 eq of the reactive 1,3,5-benzenetricarboxyl tri-
chloride. The first series of dendrimers were funcionalized with
(amino-nitro), (amino-methoxy) and (amino-butyl) substituted
azobenzenes.
Precursor azobenzenes (1), (2) and (3) have been prepared ac-
cording to the method previously reported by us [45]. These
compounds were treated in the presence of iodine, imidazole and
PPh3 to give the corresponding alkyl iodides (4), (5) and (6). On the
other hand, 3,5-dihydroxy benzylic alcohol (7) (1 eq) was reacted in
the presence of 1-dodecyl bromide (1 eq) using K2CO3 as base and
acetone as solvent, with a catalytic amount of 18-crown-6 to give
the asymmetric dendron (8). This compound was further reacted
with intermediate (4), using K2CO3 as base and DMF as solvent in
the presence of 18-crown-6 to give the first generation dendron
9G1OH. Finally, 9G1OH was reacted with 1,3,5-benzenetricarboxyl
trichloride (3.2 eq) in the presence of Et3N to give first generation
dendrimer(13G1). Similarly, other two series of first generation
4. Optical properties of the dendrimers
Optical properties of the azodendrimers in CHCl3 and DMF so-
lution were studied by absorption spectroscopy in the UVevis re-
gion and the results are summarized in Table 1. The optical data of
the dendrons employed as intermediates in the synthesis have
been previously reported. The absorption spectra were normalized
for a better comparison. In general, the absorption bands of the
dendrimers did not show any significant shift with respect to those
of the precursor dendrons. However, we can observe a sol-
vatochromic effect; for instance azodendrimer 13G1, which is high

6
J. Ortíz-Palacios et al. / Dyes and Pigments 116 (2015) 1e12
Fig. 1. Synthesis of the first-generations dendrimers.
dipole moment dendrimer bearing amino-nitro substituted azo-
benzenes, showed a maximum absorption band at lmax ¼ 480 nm
in CHCl3 solution, whereas in DMF solution the absorption band
was red-shifted to lmax ¼ 498 nm (Fig. 3). This compound belong to
the pseudostilbenes category, so that it exhibits a total overlap of
at 418 nm and the shoulder at 453 nm (Fig. 3). Since these com-
pounds belong to the aminoazobenzenes category they show a
partial overlap of the
spectra.
pep* and nep* bands in their UVevis
the
pep
* and ne
p
* bands, so that only one band can be observed in
5. Protonation effect upon irradiation
the UVevis spectra. In contrast, low dipole moment dendrimers
14G1 (amino-methoxy) and 15G1 (amino-butyl), exhibited blue-
shifted absorption bands with respect to 13G1. They exhibited
maximum absorption band at lmax ¼ 409 nm followed by a
When all the dendrimers were irradiated for 5 min with UV light
at 365 nm in chloroform solution, they did not show any changes in
their spectra absorption. However, when they were exposed to the
UV lamp at 254 nm the absorption band at 409 nm decreased
drastically in intensity, whereas a very intense band appeared at
560 nm. This photochromic effect was monitored by recording
shoulder at lmax ¼ 423 nm due to the
pep* and nep* transitions,
respectively. In DMF solution, a significant red-shift was observed
for 14G1 and 15G1 so that the maximum absorption band appeared

J. Ortíz-Palacios et al. / Dyes and Pigments 116 (2015) 1e12
7
Fig. 2. a) 1H NMR spectral. b) 13C NMR spectral of the dendrimer 14G1 in CDCl3 disolvent.
absorption spectra every 5 s. Since the extinction coefficient of the
ne * band is very low the appearance of this new band cannot be
p
attributed to transecis photoisomerization itself. Moreover, this
band can be also observed in azodendrimer 13G1 (amino-nitro)
Table 1
Optical properties of the dendrimers.
which behaves as a pseudostilbene, where both the pep* and nep*
Dendrimers
In CHCl3 solution
In DMF solution
bands are totally overlapped so that it is not possible to monitor a
transecis photoisomerization for this kind of azo-compounds.
The absorption spectrum of azodendrimer 13G1 (amino-nitro)
(not shown) exhibited these chromic changes reaching a photo-
stationary state (PSS) after 200 s of irradiation. After this time two
absorption bands were observed: the first one at 523 nm and the
lmax
Cut off
lmax
Cut off
(nm)
(nm)
(nm)
(nm)
13G1
14G1
15G1
480
409
409
600
525
525
493
418
418
621
549
549

8
J. Ortíz-Palacios et al. / Dyes and Pigments 116 (2015) 1e12
exhibited an efficient photoprotonation in the presence of a slightly
acidic solvents such as CHCl3 (pKa ¼ 15.5). In contrast, when the
same irradiation experiments were carried out in the presence of a
non acidic solvent such as DMF transecis occurred in the absence of
protonation as expected.
6. Transecis photoisomerization
Photoisomerization experiments were performed with all den-
drimers in DMF solution by irradiating with UV light at 365 and
254 nm for 5 min. Low dipole moment azodendrimers 14G1 and
15G1 exhibited a trans (E) to cis (Z) isomerization. This phenomenon
was monitored by absorption spectroscopy every 5 s. In the case of
azodendrimer 13G1 the sample was not monitored since kind of
azobenzenes show a total overlap of the pep* and nep* bands. In
contrast, low dipole moment azodendrons showed also transecis
photoisomerization when they were irradiated with UV light. The
maximum absorption band of 14G1 gradually decreases in intensity
upon irradiation since the extinction coefficient of the cis isomer is
significantly lower (Fig. 5a). This is an indication of the configura-
tion change from the trans (E) to the cis (Z) isomer. A photosta-
tionary state (PSS) was after 45 s of irradiation when all the
azobenzenes completely switched to the Z form.
Fig. 3. Normalized absorption spectral of the azodendrimers in CHCl3 and DMF
solution.
second one (new) at 550 nm. In contrast, for azodendrimer 14G1
(amino-methoxy) (Fig. 4) the new absorption band due to photo-
protonation appears at 560 nm, reaching a photostationary state
(PSS) after 45 s. Finally, azodendrimer 15G1 (amino-butyl) behaved
similarly, the absorption band at 409 nm decreased in intensity
whereas a new absorption band due to photoprotonation increased
gradually in intensity at 560 nm; in this case the PSS was attained
after 55 s. Additionally these compounds exhibited drastic colour
changes along the photoprotonation process. For low dipole
moment dendrimers 14G1 and 15G1 a colour change from yellow to
pink was observed, whereas for the highly polar dendrimer 13G1 a
colour change from red to pink was seen. This colour change can be
attributed to the azo-hydrazone tautomerism showed by the
amino-azobenzenes derivatives. A similar behaviour was observed
in the absorption spectra of some fullerene containing azobenzene
derivatives after protonation, which confirmed our hypothesis [66].
In fact, the transecis photoisomerization and the photoprotonation
can occur simultaneously but the effect of the second is more
visible in the absorption spectra. Therefore, our dendrimers
Fig. 5. UVevis changes spectral in trans to cis conversion of azodendrimer 14G1. a)
Fig. 4. Photoprotonation effect in 14G1 upon irradiation in function of time.
Upon irradiation at 365 nm. b) At 254 nm, for 60 s.

J. Ortíz-Palacios et al. / Dyes and Pigments 116 (2015) 1e12
9
The same behaviour was observed upon irradiation at 254 nm,
the maximum absorption band decreases continuously in intensity
and the PPS was achieved in 55 s (Fig. 5b). In the plot Ln (abs) versus
time for the azodendrimer bearing amino-methoxy substituted
azobenzenes has similar characteristics when was irradiated at
these two wavelengths (Fig. 6). The decay of the conversion from E
to Z isomer in the first 20 s exhibited an exponential increase. In the
first time interval the conversion from E to the Z form was faster,
furthermore the conversion resulted to be slower. However, the
opposite behaviour was observed upon irradiation at 254 nm. In
the first 5 s the conversion from the E to the Z isomer occurred
much faster.
Finally, the azodendrimer 15G1 behaved as 14G1 (Fig. 7). The
transecis photoisomerization of the dendrimer bearing amino-
butyl substituted azobenzene was totally achieved in 60 s by irra-
diating at 365 nm, whereas at 254 nm the conversion occurred in
only 5 s. Since 14G1 and 15G1 belong to the aminoazobenzenes
category a partial overlap of the
observed, where the first one is more intense in intensity. Upon
irradiation a significant decrease in intensity of the * band was
observed thereby indicating that transecis isomerization occurred.
Changes in the ne * band were not noticeable since this band
pep* and nep* bands can be
pep
p
appears as a shoulder.
7. Molecular modelling of the dendrimers
Compound 13G1 was selected as an example for a theoretical
study. Fig. 8 shows the most stable optimized structures for trans
and cis isomers of 13G1. The planar reported structures were used
as initial geometries to analyse the transecis isomerization. Initial
geometries present well defined cis or trans azobenzene groups.
Optimized structures of cis and trans isomers of the dendrimers
are quite similar. In fact, the main difference is on the side chain
that is twisted in the trans isomers in order to reduce the steric
hindrance. Both are not planar molecules, with the chains sepa-
rated and forming a windmill-like structure. The centre of the
molecule is planar in both isomers, and in both structures the
chains are separated to avoid any steric hindrances.
Fig. 7. Absorption spectral changes after irradiation with UV lamp of the azodendrimer
15G1. a) Upon irradiation at 365 nm. b) Upon irradiation at 254 nm, for 60 s.
The cisetrans energy difference of 13G1 is about 40 kcal/mol,
being the trans isomer the most stable. There is a remarkable dif-
ference between the dipole moments of both isomers. For 13G1,
trans isomer, it is 4.10 D whereas for the cis isomer it is 15.64 D. This
difference is due to the orientation of the substituents in the
azobenzene unit. In the trans isomer of 13G1 the amino-nitro
substituted azobenzenes are oriented up and down while in the
cis isomer all azobenzenes are oriented in the same direction.
The theoretical UVevis spectra of the trans and cis isomers of
13G1 are illustrated in Fig. 9. As we can see, there is a significant
overlap between both spectra but there is also a significant shift in
the maximum absorption bands. For the trans isomer, the
maximum absorption appears at
at
¼ 540 nm. In the experimental UVevis spectrum of 13G1 we
can observe a broad absorption band centred at 480e500 nm, since
the * and the ne * are very close to each other and practically
l
¼ 475 nm, and for the cis isomer
l
pep
p
superimposed. Thus, the theoretical results are in good agreement
with those obtained experimentally.
8. Conclusions
Three novel Frechet type dendrimers bearing a well defined
oligo(ethylene glycol) side chain and amino-nitro (13G1), amino-
methoxy (14G1) and amino-butyl (15G1) substituted azobenzenes
were successfully synthesized and characterized. The optical
properties of the azodendrimers were studied in CHCl3 and DMF
solution by absorption spectroscopy in the UVevis region. 13G1
which has amino-nitro substituted azobenzenes, showed
a
Fig. 6. Ln(abs) versus time plot for change absorbance of the azodendrimer 14G1.
maximum absorption band at lmax ¼ 480 nm in CHCl3 solution,

10
J. Ortíz-Palacios et al. / Dyes and Pigments 116 (2015) 1e12
Fig. 8. Optimized structures for the all azodendrimers 13G1, 14G1 and 15G1.
which was red-shifted to lmax ¼ 498 nm in DMF solution. This
compound exhibits a total overlap of the * and ne * bands. In
Acknowledgements
pep
p
contrast, 14G1 (amino-butyl) and 15G1 (amino-methoxy) exhibited
maximum absorption band at lmax ¼ 409 nm followed by a
We are grateful to Miguel Angel Canseco for his assistance with
UVevis spectroscopy and Gerardo Cedillo for his help with 1H and
13C NMR spectroscopy. This project was financially supported by
PAPIIT (Project IN-100513). This study was also funded by Consejo
Nacional de Ciencia y Tecnología (CONACyT Project 128788) and
resources provided by the Instituto de Investigaciones en Materi-
ales (IIM). Theoretical calculations were carried out using a NES
shoulder at lmax ¼ 423 nm due to the
pep* and nep* transitions.
These compounds belong to the aminoazobenzenes category they
show a partial overlap of the * and ne * bands in their UVevis
pe
p
p
spectra. The obtained results fitted well with those predicted by
molecular modelling.
ꢀ ꢀ
Photoisomerization experiments were performed with all den-
drimers in DMF solution by irradiating with UV light at 365 and
254 nm. Low dipole moment azodendrimers 14G1 and 15G1
exhibited a trans (E) to cis (Z) isomerization. This phenomenon was
monitored by absorption spectroscopy. A photostationary state
(PSS) was after 45 s of irradiation when all the azobenzenes
completely switched to the Z form. The same behaviour was
observed upon irradiation at 254 nm, the maximum absorption
band decreases continuously in intensity and the PPS was achieved
in 55 s.
(Miztli) supercomputer, provided by Direccion General de Computo
ꢀ
ꢀ
y Tecnologías de Informacion y Comunicacion (DGTIC).
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2014.11.023.
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